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Please check this box if you have no corrections to make to the PDF file Thank you for your assistance. SAA 13648 No. of Pages 12, Model 5G 2 May 2015 Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 1 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa 6 7 5 Q1 Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3 8 Q2 Diego M. Gil, María Eugenia Tuttolomondo, Aída Ben Altabef ⇑,1 3 4 9 10 INQUINOA – CONICET – UNT, Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, San Lorenzo 456, T4000CAN Tucumán, Argentina 12 11 13 1 5 216 9 17 18 19 20 21 22 23 24 25 26 27 28 h i g h l i g h t s g r a p h i c a l a b s t r a c t  The molecular structure of S-methyl thiobutanoate was determined by abinitio and DFT calculations.  The experimental and theoretical results confirm the presence of two stable conformations.  The infrared and Raman spectra were recorded and the bands observed were assigned to the vibrational normal modes.  Global and local reactivity descriptors were computed to predict reactivity. 31 a r t i c l e 3 6 3 4 34 35 36 37 38 39 40 41 42 43 44 45 i n f o Article history: Received 12 November 2014 Received in revised form 9 March 2015 Accepted 22 April 2015 Available online xxxx Keywords: S-methyl thiobutanoate DFT calculations Internal barrier to rotation Infrared and Raman spectroscopy Global and local descriptors a b s t r a c t In the present article, the molecular structure of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3 was determined by ab initio (MP2) and DFT calculations using different basis sets. The infrared and Raman spectra for the liquid phase were also recorded and the bands observed were assigned to the vibrational normal modes. The experimental and calculations confirm the presence of two most stable conformers, one with pseudo anti–syn conformation and another with gauche–syn conformation. The study was completed using natural bond orbital (NBO) and AIM analysis. The molecular properties like dipole moment, molecular electrostatic potential surface (MEP) and HOMO–LUMO molecular orbitals were calculated to get a better insight of the properties of the title molecule. Global and local reactivity descriptors were computed in order to predict reactivity and reactive sites on the molecule for nucleophilic, electrophilic and radical attacks. Ó 2015 Published by Elsevier B.V. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 Introduction 62 Thioesters, compounds with general formula RCOSR’, are an obligatory intermediate in many key biosynthetic reactions, including the formation and degradation of fatty acids and complex lipids, and the regeneration and dehydration of adenosine 63 64 65 ⇑ Corresponding author. Tel.: +54 381 4311044; fax: +54 381 4248169. 1 E-mail address: altabef@fbqf.unt.edu.ar (A. Ben Altabef). Member of the Research Career of CONICET, Argentina. triphosphate [1,2]. They also participate in the synthesis of a large number of cellular components such as peptides, sterols, and others. In addition, thioesters also play an important role in protein tagging with ubiquitin, which tags the protein for degradation [1]. The structural and conformational properties of thioesters are of great interest because of their close relation to many biomolecules where they constitute their most important property because these molecules must adopt definite forms to carry out specific biological functions. For example, coenzyme A plays an important role in metabolic energy production and is recognized as a http://dx.doi.org/10.1016/j.saa.2015.04.097 1386-1425/Ó 2015 Published by Elsevier B.V. Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 66 67 68 69 70 71 72 73 74 75 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 2 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 116 universal carrier of activated acyle groups. The HS-CoA reacts with acyle groups to form RCO-SCoA acylcoenzyme A which is due to the high reactivity of thioesters in nucleophilic acyl transfer reactions as compared with oxoesters. S-methyl thioesters with an acyl chain length of 2-10 carbons are of great interest because of their very powerful odors often associated with cheese flavor and their low perception thresholds [3]. S-methyl thioacetate is quantitatively the most important in cheeses. Previous studies suggested that S-methyl thioesters were probably produced through a reaction involving methanethiol and acyl-CoAs during cheese ripening. This process is produced by different bacteria like Brevibacterium antiquum, Brevibacterium aurantiacum and Brevibacterium linens [3]. The conformational study of several methyl thioesters has been reported by different groups [4–7]. The general tendency found in these compounds indicates that the syn conformation (C@O double bond syn with respect to S–CH3 single bond) prevails over the anti one. A microwave study for S-methyl thioformate demonstrated that this molecule exists only in the syn conformation [6]. The compound S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, is commercially available, but its molecular structure has not been studied. Our group have studied various esters with the general formula CF3CO2R (R = –CH3, –CH2CH3, –CH2CF3) [8–10] including the related thioester, CF3COSCH2CH3 [11]. In this article, we have performed a conformational analysis to determine the most stable conformation of the title compound. Infrared and Raman spectra were recorded in liquid phase and these experimental measurements were complemented by quantum chemical calculations to obtain an optimized molecular structure and a scaled quantum mechanical force field. The spectral features were assigned to the different normal modes of vibration. The conformational study was complemented by natural bond orbital (NBO) analysis to assess the significance of hyperconjugative interactions which would favor one conformation over another and the study of the reactivity was performed by AIM approach. HOMO–LUMO analysis was performed to determine some molecular properties like ionization potential, electron affinity, electronegativity, chemical potential, hardness, softness and global electrophilicity index. Local reactivity descriptors were calculated to identify the preferred sites for electrophilic, nucleophilic and radical attacks. 117 Experimental 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 (NSCCS), running the Gaussian 03 suite of programs [12]. Geometry optimizations were performed at the MP2 [13] and DFT levels using a variety of basis sets. Electron correlation was then considered using the MP2 approach with the 6-311G(d,p), 6-311++G(d,p) and 6-311++G(3df,3pd) basis set [14–17]. DFT calculations were performed using Becke’s three-parameter hybrid exchange functional [18] (B3) combined with both the Lee–Yang– Parr gradient-corrected correlation functional [19] (LYP) and the same basis sets as for the MP2 calculations. The second DFT method used, mPW1PW91 [20] applies a modified Perdew–Wang exchange functional and Perdew–Wang 91 correlation functional [20]. All calculations were performed using standard gradient techniques and default convergence criteria. The potential energies associated with the SCCC, CCCC and CSCC dihedral angles were calculated at MP2, B3LYP and mPW1PW91 levels using the 6-311++G(d,p) basis sets, with that torsional angle frozen and all other parameters allowed to relax. The total energy curves were prepared in steps of 10° using default convergence criteria as implemented in the Gaussian program. A natural bond orbital (NBO) calculation was performed at the B3LYP/6-311++G(d,p) level using the program NBO 3.1 [21] as implemented in the Gaussian 03 package. This analysis was performed to understand various second order interactions between the filled orbitals of one subsystem and the vacant orbitals of another in order to have a measure of intra-molecular delocalization of hyper-conjugation. In addition, reactivity was analyzed with Bader’s atoms in molecules theory (AIM) by using the AIM2000 code [22,23]. Molecular properties such as ionization potential (IP), electron affinity (EA), electronegativity (v), chemical potential (l), hardness (g), softness (s) and global electrophilicity index (x) were deduced from HOMO–LUMO analysis employing B3LYP/6-311++G(d,p) level. Raman activities (SRa) calculated with the Gaussian 03 program were converted to relative Raman intensities (IRa) using the following relationship derived from the intensity theory of Raman scattering [24]: 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 f ðm0  mi Þ4 Si Ii ¼ mi ½1  expðhcmi =kTÞ ð1Þ 175 176 121 Samples of CH3CH2CH2C(O)SCH3 for spectroscopy measurements were purchased from Sigma–Aldrich and used without further purification. The purity of S-methyl thiobutanoate was checked by FTIR spectra. where m0 is the laser exciting wavenumber in cm1 (in this work, we used the excitation wavenumber m0 = 12820.5 cm1, which corresponds to the wavelength of 780 nm of the solid state laser), mi is the vibrational wavenumber of the ith normal mode (cm1) and Si is the Raman scattering activity of the normal mode mi. f (it is a constant equal to 1012). This is a suitably chosen common scaling factor for all peak intensities. 122 Infrared and Raman spectroscopy Results and discussion 183 123 Quantum chemical calculations 184 133 Computational methods 134 Calculations were performed using the resources of the United Kingdom National Service for Computational Chemistry Software Conformational analysis The potential energy scans around the SCCC, CSCC and CCCC dihedral angles calculated at B3LYP/6-311++G(d,p) level are shown in Fig. 1(a–c), respectively. Total energies (E), differences in total energies (DE), free energies (G) and differences in free energies for the possible conformations found for CH3CH2CH2C(O)SCH3 calculated at B3LYP/6-311++G(d,p) approximation are presented in Table 1. Fig. 2 shows the possible conformations predicted for the title compound. When the SCCC dihedral angle was varied (Fig. 1(a)), conformers I and II were observed. Conformer I shows an anti–syn orientation (CCCC dihedral angle is 180° and syn between the C@O double bond and the C–S single bond) and conformer II shows a pseudo anti–syn orientation (CCCC dihedral angle 185 132 Infrared spectra for CH3CH2CH2C(O)SCH3 in the liquid phase were recorded in the 4000–400 cm1 range at room temperature (RT) using a Perkin-Elmer GX1 Fourier transform infrared instrument. The Raman spectrum of the liquid at RT between 3500 and 50 cm1 was measured on a Thermoscientific DXR Smart Raman instrument. Data were collected using a diode-pump, solid-state laser of 780 nm (5 cm1 spectral resolution). A confocal aperture of 25 lm pinhole was used. In order to achieve a sufficient signal to noise, 100 expositions of 2 s were accumulated for the sample. The laser power was maintained at 5 mW when collecting data. 118 119 120 124 125 126 127 128 129 130 131 135 Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 177 178 179 180 181 182 186 187 188 189 190 191 192 193 194 195 196 197 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 3 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Fig. 1. Potential energy curves for CH3CH2CH2C(O)SCH3 as a function of the (a) S(3)–C(1)–C(8)–C(11), (b) C(4)–S(3)–C(1)–C(8) and (c) C(1)–C(8)–C(11)–C(14) dihedral angles calculated at B3LYP/6-311++G(d,p) approximation. Table 1 Total energies (E), differences in total energies (DE), free energies (G) and differences in free energies for the possible conformations of CH3CH2CH2C(O)SCH3 calculated at B3LYP/ 6-311++G(d,p) approximation. Ea Ga DE b DG b a b 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 I II III IV V 670.091318 669.983842 1.19 5.23 670.091770 669.985837 0 0 670.091488 669.985636 0.75 0.53 670.083934 669.976532 20.55 24.40 670.082235 669.974477 25.00 29.79 Absolute energies in Hartrees. Relative energies in kJ mol1; DE = E(conformer)  E(II); DG = G(conformer)  G(II). is 177° and syn between the C@O double bond and the C–S single bond). The differences in total energy between the two minima is 5.23 kJ mol1 calculated at B3LYP/6-311++G(d,p) approximation indicating that conformers II is more stable. When we varied the CSCC dihedral angle (Fig. 1(b)), two additional minima were observed, namely III and IV. Conformer III presents a gauche–syn orientation (CCCC dihedral angle is 68° and syn between the C@O double bond and the C–S single bond) and the orientation observed for conformer IV was gauche–anti (CCCC dihedral angle is 68° and anti between the C@O double bond and the C–S single bond). The difference in total energy between both conformers is high (20 kJ mol1) indicating that only conformer III could be observed in experimental data (see below). The scans corresponding to the variation of CCCC dihedral angle (Fig. 1(c)) shows three minima, two for conformers II and III and a third conformation namely V. Conformer V presents a gauche–anti conformation (gauche between C(8)–C(11) and anti between the C@O double bond and the C–S single bond). There was an important difference in total energy between the three conformations indicating that conformers II and III were more stable than conformer V. All the minima observed in the curves were optimized. From these calculations, the free energy values for each conformer could be obtained. Table 1 summarizes the values of free energies and the differences of free energies relative to the conformer II that was predicted to be the most stable conformer. For conformers II and III, the free energy calculated using the B3LYP/6-311++G(d,p) approximation was used along with the average temperature of the experiment to estimate the population of each conformer that should be observed in gas phase. As the difference in free energy was calculated to be 0.53 kJ mol1 (conformer II lower in energy), the ratio of II to III conformers was predicted to be 0.45:0.59. 218 Molecular structure Taking into account the high flexibility of this compound and the several possible conformations, most of which were undetectable at room temperature, full optimizations of the two lowest energy conformers on the potential energy surface of the title compound were carried out using the B3LYP/6-311++G(d,p) approximation. The calculated structural parameters for conformers II and III of the title compound are given in Table 2. The calculated 229 Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 219 220 221 222 223 224 225 226 227 228 230 231 232 233 234 235 236 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 4 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Fig. 2. Molecular structure for the conformers of CH3CH2CH2C(O)SCH3. The complete description is presented in the body of the text. 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 geometrical parameters were compared with those experimental values reported for related compounds [4,11]. Conformer II was predicted to be the most stable form (Fig. 2). It exhibits a C(1)–C(8)–C(11)–C(14) torsion angle of 177° and the CH3 group was on the same side of the carbonyl group. In this structure, one of the methyl C–H bonds coexists in the plane of the ester group. Fig. S1 shows the optimized molecular structure of the pseudo anti–syn conformer of the title compound calculated at B3LYP/6-311++G(d,p) level. According to the Della Védova et al., the syn form for the CH3C(O)SCH3 is more stable than the anti one. This affirmation was supported by gas electron diffraction (GED) measurements [4]. The geometrical parameters calculated at the B3LYP and MP2 levels of approximations using 6-311++G(3df,3pd) basis sets for conformers I, II and III of the title compound are shown in Table S1. The theoretical description of molecules containing C–S bonds requires the use of highly polarized basis functions. As was found for different molecules containing C–S bonds such as CH3SO2SCH3 [25] and CF3C(O)SCH2CH3 [11], the inclusion of extra polarization functions (beyond a single d-function) is necessary to predict the bond distances in these molecules accurately. The most sensitive parameter to this orbital description was the C–S bond length, which was shortened 0.012 Å at B3LYP when we replaced the 6-311++G(d,p) basis sets with 6-311++G(3df,3pd) basis sets. All bonds involving the sulfur atom were shortened, but the remaining bond lengths were relatively unchanged. This produced a calculated geometry close to the experimental structure determined by GED measurements [4,11]. The MP2/6-311++G(3df,3pd) level of theory estimates matches well all the bond distances and angles reported using GED data for related molecules [4,11]. The longer bond length observed at the B3LYP method is due to the over-estimation of electron–electron repulsions. Internal barrier decomposition schemes The study of the nature of the barrier to rotation of the C(1)–S and C(1)–C (sp2) bonds in terms of hyper-conjugative, steric and electrostatic interactions will give us an insight into the reasons for the relative stability of conformers III and IV. The Table 2 Selected calculated and experimental (taken from relative compounds) structural parameters of the two lowest energy conformers of CH3CH2CH2C(O)SCH3. Parametera II III Experimental Bond distances (Å) C–H (mean) C(4)–S(3) S(3)–C(1) C(1)@O(2) C(1)–C(8) C(8)–C(11) C(11)–C(14) 1.093 1.824 1.804 1.206 1.517 1.538 1.530 1.093 1.825 1.804 1.206 1.521 1.536 1.531 1.092b 1.805b 1.781b 1.216c 1.546c 1.533c 1.533c Angles (°) C(1)–S(3)–C(4) S(3)–C(1)@O(2) O(2)@C(1)–C(8) C(1)–C(8)–C(11) C(8)–C(11)–C(14) 101.1 123.4 123.7 111.7 112.4 101.0 123.2 123.9 113.0 113.4 99.20b 122.8b 123.4b – – Dihedral angles (°) C(4)–S(3)–C(1)–C(8) S(3)–C(1)–C(8)–C(11) C(1)–C(8)–C(11)–C(14) 179.5 120.4 177.1 176.5 139.6 68.8 – – – a Calculated at the B3LYP/6-311++G(d,p) level of approximation. See Figure S1 for atoms numbering. b Taken from Ref. [4]. c Taken from Ref. [11]. potential-energy surface for the target torsion angle was calculated in 10° steps in the range 0–180°, allowing all other geometrical parameters to relax. The energy profiles were fitted to a sixth-order Fourier expansion: X6 1 VðhÞ ¼ V iN ð1  cos iNhÞ i¼1 2 274 275 276 277 278 ð2Þ where N, the symmetry number, equals 1. No contributions to torsional energies from zero-point energy were taken into account. The decomposition of the total energy function and the analysis of the different Vi terms has previously been shown to be an effective method of analyzing the stabilization of different conformations in molecular systems [8–11,26]. The six Vi terms calculated Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 280 281 282 283 284 285 286 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 for the title compound using the B3LYP/6-311++G(d,p) basis sets are shown in Table S2. Fig. 3 shows the Fourier decomposition of the total energy function calculated at the same level of theory. With their large values, V1 and V2 are the main contributions to the rotational barrier, with V2 > V1 > V3. The terms V4–6 are less significant when deconvoluting the potential-energy curve. The magnitudes and signs of the two main terms are similar regardless of the level of theory used to calculate them. V2 is usually associated with conjugative and hyper-conjugative effects that have a periodicity of 180°. As for V1, it usually accounts for interactions between local dipoles and for steric interactions. The V3 term is associated with unfavorable bond-bond eclipsing interactions, exhibiting a three-fold periodicity for a torsion involving sp3hybridized sulfur atoms. Fig. 3(a) shows that the barrier between the syn and anti forms is approximately 24 kJ mol1. It can be seen that the highest value corresponds to the V2 parameter, but this contributes equally to the two forms. The values of both V1 and V3 are the determinants for the stability of the syn form. Both parameters are dependent by electrostatic interactions. This is further supported by the value of the dipole moment for the syn form (1.15 D) that is much smaller than the anti form (4.21 D). Fig. 3(b) shows that the barrier between the pseudo anti–syn and gauche– syn forms is approximately 10 kJ/mol. The large V3 and V1 values are the main contributions to the rotational barrier, while V2 > V5 > V4 > V6 are less significant when deconvoluting the potential energy curve. The V3 term is large and negative, showing that there is a strong preference for the pseudo anti–syn and gauche– syn form over the anti–syn form. The V3 term is associated with unfavorable bond-bond eclipsing interactions between the CH2 groups, exhibiting a 3-fold periodicity for a torsion involving sp3-hybridized carbon atoms while the behavior of the V1 term is less favorable for both transition states. The balance between the V3 and V1 terms contributed to the stabilization of the pseudo anti–syn form. The absolute values of V3 and V1 gave the barrier energy and form respectively. NBO analysis Natural bond orbital (NBO) analysis is a useful tool for understanding delocalization of electron density from occupied Lewis-type (donor) NBOs to properly unoccupied non-Lewis type (acceptor) NBOs within the molecule. The stabilization of orbital interaction is proportional to the energy difference between the interacting orbitals. Therefore, the interaction having strongest stabilization takes place between effective donors and effective acceptors. The interaction between bonding and anti-bonding molecular orbitals can be quantitatively described in terms of the NBO approach that is expressed by means of second-order perturbation interaction energy E(2). This energy represents the estimate of the off-diagonal NBO Fock matrix element. The stabilization energy E(2) associated with i (donor) ? j (acceptor) delocalization is estimated from the second-order perturbation approach as given below: 338 340 341 342 343 344 345 346 347 348 349 350 Eð2Þ ¼ Eij ¼ qi F 2 ði; jÞ ej  ei ð3Þ where qi is the donor orbital occupancy, ei and ej are diagonal elements (orbital energies) and F(i,j) is the off-diagonal Fock matrix element. The role of hyper-conjugative interactions in the stabilization of the conformations observed for the title compound has been studied using NBO analysis, where the hyper-conjugation represents the transfer of an electron between a lone pair or bonding orbital and an anti-bonding orbital. Table 3 shows the main hyperconjugative interactions for the anti–syn (conformer I), pseudo anti–syn (conformer II) and gauche–syn (conformer III) of S-methyl 5 thiobutanoate. In terms of the NBO analysis, the hyper-conjugative interactions are more favored in the pseudo anti–syn conformation than the others. Thus, lone pairs of the oxygen and sulfur atoms transfer electronic charge to the anti-bonding r⁄ orbital of the C–S and C@O bonds and these stabilizing interactions are stronger for the pseudo anti–syn form. For a larger anomeric effect lpr S(3) ? r⁄ C(1)–O(2) a C–S bond strengthening is expected. As seen in Table 3, this interaction for the pseudo anti–syn conformer is higher than for the gauche–syn form. This is reflected in the higher value of the stretching frequency of the C(O)–S bond for the pseudo anti–syn conformer (703 cm1) with respect to the gauche–syn (697 cm1). A comparative study of the skeleton internal barrier and the corresponding NBO analysis for this family of compounds was performed by the authors and will be the subject of another paper to be published. The values of occupation and energy of the different natural bond orbitals are shown in Table S3. 351 AIM analysis The quantum theory of atoms in molecules has been useful in the characterization of bonds through a topological analysis of the electronic charge density and their Laplacian at the bond critical point (BCP) [22]. In the AIM theory the nature of the bonding interaction can be determined through an analysis of the proper- 367 ties of the charge density, q, and its Laplacian r2 ðqÞ at the BCP, and through the properties of the atoms, which are obtained by integrating the charge density over the atom orbitals [22]. The molecular graph for the pseudo anti–syn and gauche–syn conformers of the title compound using the AIM program calculated at B3LYP/6-311++G(d,p) level is presented in Fig. S2. Table S4 shows the bond critical point data for S-methyl thiobutanoate molecule. As seen in Table S4, the values of charge density for the C(1)–O(2), C(1)–S(3), S(3)–C(4) and C(1)–C(8) bond critical points 373 of both conformations are relatively high and the r2 ðqÞ is negative. These results indicate that the charge density has been concentrated in the inter-nuclear region. The AIM methodology self-consistently partitioned any system and its properties into its atomic fragments, considering the gradient vector field of its electron density distribution. Koch et al. have proposed criteria based on the AIM theory to establish hydrogen bonding; the electron density at the BCP and its Laplacian are the most representative for this kind of interaction [28]. However, the energy density at the bond critical point (HBCP) has proved to 382 be a more sensible and appropriate index than r2 ðqÞ to characterize the nature of hydrogen bonds [29]. The results obtained for electron density (qBCP), its Laplacian 392 (r2 ðqÞBCP ), electron kinetic energy density (GBCP), electron potential energy density (VBCP) and total electron energy density (HBCP) at the bond critical points (BCPs) for pseudo anti–syn and gauche– syn conformers of the title compound evaluated by means of the AIM approach at the B3LYP/6-311++G(d,p) level are presented in Table S5. The values of electron density at the BCP are in agreement with the values range reported by Koch and Popelier (0.002– 0.004). Rozas et al. have suggested criteria that can be used to characterize hydrogen bonds (HB) [29]. Weak HB interactions 395 show both r2 ðqÞBCP and HBCP > 0, and medium HB interactions 404 show r2 ðqÞBCP > 0 and HBCP < 0, while strong HB interactions show 405 both r2 ðqÞBCP and HBCP < 0. According to the values reported in 406 Table S5, all r2 ðqÞBCP and HBCP parameters for both conformers were greater than zero indicating that O(2)  H(7) hydrogen bonds are weak interactions. The ellipticity (e) at the BCP is a sensitive index to monitor the p-character of bond. The e is related to k1 and k2, which correspond to the eigen values of the Hessian and is defined by the relationship: e = (k1/k2)  1. The ellipticity values for bonds C(1)–O(2) 407 Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 368 369 370 371 372 374 375 376 377 378 379 380 381 383 384 385 386 387 388 389 390 391 393 394 396 397 398 399 400 401 402 403 408 409 410 411 412 413 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 6 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Fig. 3. Fourier decomposition of the potential function V(h) for CH3CH2CH2C(O)SCH3 for the CSCC and CCCC dihedral angles calculated at B3LYP/6-311++G(d,p) level. Table 3 Relevant hyper-conjugative interactions in kJ mol1 for CH3CH2CH2C(O)SCH3 calculated at the B3LYP/6-311++G(d,p) level. a 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 Interactions (donor ? acceptor)a Anti–syn (I) Pseudo anti – syn (II) Gauche–syn (III) lpr O(2) ? r⁄ C(1)–S(3) lpr O(2) ? r⁄ C(1)–C(8) lpp O(2) ? r⁄ C(1)–S(3) lpp O(2) ? r⁄ C(1)–C(8) lpr O(2) ? r⁄ C(4)–H(7) lpr S(3) ? r⁄ C(1)–O(2) lpr S(3) ? r⁄ C(1)–O(2) Total 5.98 10.62 147.34 72.44 2.26 20.02 125.27 383.93 7.11 10.12 146.72 69.89 3.89 22.32 128.87 388.92 6.94 10.37 145.84 71.48 3.55 22.49 126.15 386.82 lp denotes a lone pair of r or p orbital on the specified atom. and C(1)–S(3) were 0.0329 and 0.2350, respectively for the pseudo anti–syn conformer. The e values for the gauche–syn conformer corresponding to these bonds were 0.0313 and 0.2329. The lower values of the ellipticity index confirm that there is electron delocalization through the corresponding atoms. However, the higher ellipticity values for C(1)–S(3) indicate that the electrons of these bonds are not delocalized [22]. Molecular electrostatic potential (MEP) The molecular electrostatic potential surface (MEP) was determined by B3LYP/6-311++G (d,p) level in order to understand the relative polarity of the molecule. The MEP (electrostatic potential mapped onto an electron iso-density surface) may be used to predict reactive sites for electrophilic attack (electron rich region) and nucleophilic attack (electron poor region). Even when the two molecules are structurally very similar, MEPs make clear that this similarity is not carried over into their electrophilic/nucleophilic reactivates. The MEP surface simultaneously displays molecular size, shape and electrostatic potential in terms of color grading and is a very useful tool in the investigation of correlation between molecular structure and the physicochemical property relationship of molecules including biomolecules and drugs [30–32]. The red and blue region refers to the electron rich and electron poor region while the slightly electron rich region is indicated by yellow and the green region in MEPs suggests an almost neutral region. The variation in electrostatic potential produced by a molecule is largely responsible for the binding of a drug to its receptor binding sites, as the binding site in general is expected to have opposite areas of electrostatic potential. The MEPs map and contour plot of the pseudo anti–syn conformer of S-methyl thiobutanoate generated at optimized geometry of the title molecule using the GaussView 05 software is shown in Fig. 4. It is evident from the MEPs map that the region around the hydrogen atoms of the carbon atoms is electron deficient (light blue color), therefore binding site for electrophiles. The region around the oxygen atom corresponding to the C@O group represents the most electron rich region and it is the binding site for nucleophiles. As seen in Fig. 4, the region around the sulfur atom is slightly electron rich. 446 HOMO–LUMO analysis The energy gap between the highest occupied and the lowest unoccupied molecular orbital is an important quantum chemical parameter that determines molecular electrical transport properties and is a measure of electron conductivity. The HOMO energy characterizes electron ability to give while the LUMO energy characterizes electron ability to accept, and the gap between the HOMO and LUMO molecular orbital characterizes the chemical reactivity and kinetic stability of the molecule. A molecule with a small energy gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as a soft molecule [33]. The HOMO and LUMO plot for the title compound with the corresponding energies and energy gap is presented in Fig. 5. The HOMO of the title molecule (7.0736 eV) is located in the sulfur atom, the oxygen of the C@O group and in the methyl group bound to the S atom. The LUMO (0.8416 eV) is spread over the entire molecule. The high value of the energy gap between HOMO–LUMO indicates that the molecule shows high chemical stability and low reactivity. Table S6 shows the band gap of the anti and gauche conformers of some thioesters such as CF3C(O)SCH2CH3, CH3C(O)SCH2CH3 and for the most stable conformers of the title compound. As can be seen in Table S6, the gap band between the HOMO and LUMO frontier molecular orbitals is higher in the thioesters where the carbonyl group is bound to an alkyl group (methyl or propyl), but the lowest gap band value is observed in the compound bound to the CF3 group. It indicates that the CF3C(O)SCH2CH3 in both conformations is more reactive and more stable than CH3C(O)SCH2CH3 and CH3CH2CH2C(O)SCH3. 451 Global and local reactivity descriptors The global reactivity descriptors like chemical potential, electronegativity, hardness, softness and global electrophilicity index can be calculated using DFT methods. Following Parr and Pearson [34], the electronic chemical potential describing the escaping tendency of the electron from a stable system can be calculated as: 479 ðI þ AÞ l¼ 2 ð4Þ where I = ionization potential and A = electron affinity. Electronegativity (v) is described as the negative of the electronic chemical potential. Chemical hardness (g) demonstrates the resistance to alteration in electron distribution and is well correlated Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 447 448 449 450 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 480 481 482 483 484 485 487 488 489 490 491 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 7 Fig. 4. MEP surface and contour plots for CH3CH2CH2C(O)SCH3 calculated at B3LYP/6-311++G(d,p) level. Fig. 5. HOMO and LUMO plots for CH3CH2CH2C(O)SCH3 calculated at B3LYP/6-311++G(d,p) level with the corresponding energies and band gap. 503 492 493 with the stability and reactivity of the chemical system. Hardness is expressed by the following equation: 494 496 497 g¼ IA 2 ð5Þ The inverse of hardness is expressed as global softness: 498 500 501 502 1 s¼ 2g ð6Þ The global electrophilicity index (x), introduced by Parr et al. [34] is calculated in terms of chemical potential and hardness: l2 x¼ 2g ð7Þ This value assesses energy decreasing due to maximal electron flow between donor and acceptor. The calculated values of the global reactivity descriptors for the title molecule are collected in Table 4. The value of chemical hardness is 3.1160 eV. In terms of chemical hardness, if a molecule has a large HOMO–LUMO gap, it is hard. Conversely, if the HOMO–LUMO gap is small, it is soft. One can also relate molecular stability to hardness, which means that the molecule with smaller HOMO–LUMO gap is more reactive. Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 505 506 507 508 509 510 511 512 513 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 8 514 515 516 517 518 519 520 521 523 524 526 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx The local reactivity descriptor like the Fukui function indicates the preferred regions where a chemical species (molecule) will amend its density when the electron number is modified or indicates the tendency of the electron density to deform at a given position upon accepting or donating electrons [35]. The condensed or atomic Fukui functions on the kth atom site, for electrophilic + 0 (f k ), nucleophilic (fk ) and free radical (fk ) attacks are defined as: þ ð8Þ  ð9Þ f k ¼ ½qðN þ 1Þ  qðNÞ for nucleophilic attack f k ¼ ½qðNÞ  qðN  1Þ for electrophilic attack 527 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 549 0 fk 1 ¼ ½qðN þ 1Þ  qðN  1Þ forradicalattack 2 ð10Þ where qk is the atomic charge (Mulliken, NBO, etc.) at the kth atomic site in the anionic (N + 1), cationic (N  1) or neutral molecule. According to Parr and Yang [35], the sites in chemical species with the largest of Fukui function (fk) values show high reactivity for corresponding attacks. The Fukui functions (FF) were calculated using the program proposed by Chamarro et al. [36] The FF values for the title compound calculated at B3LYP/6-311++G(d,p) level 0 are shown in Table 5 and Fig. S3. The FF (f+k, f k , fk), local softness  0 + 0 (S+k, S k , Sk) and electrophilic indices (xk, xk , xk) of the individual atoms of CH3CH2CH2C(O)SCH3 are presented in Table S7. The highest f+k value was predicted for the C(1) atom indicating that it is the preferred site for nucleophilic attack. According to the values reported in Table 4, the reactivity order for nucleophilic attack is C(1) > O(2) > S(3). The electrophilic reactivity order is: S(3) > O(2) while the order of sites for free radical attack is: 0 S(3) > C(1) > O(2). Local softness (S+k, S k , Sk) and electrophilic indices  + 0 (xk, xk , xk) are calculated using the following equations [37]: þ Sþk ¼ Sf k ;  Sk ¼ Sf k ; 0 S0k ¼ Sf k Table 4 Global reactivity descriptors data for CH3CH2CH2C(O)SCH3 calculated at B3LYP/6311++G(d,p) level. Ionization potential, I (eV) Electron affinity, A (eV) Electronegativity, v (eV) Chemical potential, l (eV) Chemical hardness, g (eV) Global softness, s (eV1) Global electrophilicity index, x (eV) 7.0736 0.8416 3.9576 3.9576 3.1160 0.1605 2.5132 Table 5 Values of Fukui functions for S-methyl thiobutanoate.a a b Atomsb f() f(+) f(0) C(1) O(2) S(3) C(4) H(5) H(6) H(7) C(8) H(9) H(10) C(11) H(12) H(13) C(14) H(15) H(16) H(17) 0.0136 0.1151 0.7961 0.0212 0.0219 0.0240 0.0001 0.0028 0.0024 0.0002 0.0016 0.0008 0.0000 0.0000 0.0000 0.0000 0.0001 0.5421 0.2664 0.0900 0.0020 0.0020 0.0022 0.0001 0.0199 0.0515 0.0049 0.0088 0.0055 0.0001 0.0028 0.0002 0.0002 0.0013 0.2778 0.1908 0.4430 0.0116 0.0120 0.0131 0.0001 0.0113 0.0269 0.0026 0.0052 0.0032 0.0000 0.0014 0.0001 0.0001 0.0007 Calculated at B3LYP/6-311++G(d,p) level. See Fig. 4 for atoms numbering. ð11Þ 550 552 xþk ¼ xf þk ; xk ¼ xf k ; x0k ¼ xf 0k 553 559 where +,  and 0 signs show nucleophilic, electrophilic and radical attacks, respectively. These equations predict the most electrophilic site in a system with the maximum value of S+k and x+k while max imum value of S k and xk corresponds to the nucleophilic site in the molecule. According to the values of local softness and local electrophilic indices reported in Table S7, the reactivity order for nucle0 ophilic attack is identical to f+k, f k and fk. 560 Vibrational analysis 561 The assignment of the experimental IR and Raman bands to the normal modes of vibration of CH3CH2CH2C(O)SCH3 was based on the comparison of related molecules [11,38,39] and assisted by the theoretical calculations performed in this work with different levels of theory. DFT calculations reproduced the normal wavenumbers of vibrations with the following root-mean-square deviations (RMSD) for each basis set: 65 cm1 for 6-31G(d), 56 cm1 for 6-311G(d,p) and 53 cm1 for 6-311++G(d,p). The results with the combination B3LYP/6-311++G(d,p) were used for the vibrational analysis to facilitate the comparison of the present results with those obtained previously for related molecules [11,38,39]. At room temperature, most bands are attributable to the same fundamental for pseudo anti–syn and gauche–syn conformations. The IR and Raman spectra of the liquid substance demonstrate the presence of both conformers of the title compound by resolution of several fundamental modes of vibration. The FTIR and Raman spectra of the liquid substance are shown in Fig. 6. The wavenumbers of the observed spectra and the approximate description of modes of both conformers of the title compound are given in Table 6. 554 555 556 557 558 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 ð12Þ Fig. 6. Experimental IR and Raman spectra of CH3CH2CH2C(O)SCH3 in liquid phase. Assignment of bands Methyl and methylene group modes. The shoulder observed at 2984 cm1 in the Raman spectrum is assigned to the anti-symmetric stretching mode of the CH3 group bound to the S atom. The band corresponding to the CH3 symmetric stretching mode appears as a shoulder at 2948 cm1 in the Raman spectrum. The band located at 2840 cm1 in the IR spectrum (2915 cm1 in Raman) is assigned to the symmetric stretching mode of the methyl bound to the CH2 group. The bands located at 1463 and 1458 cm1 in the IR spectrum are assigned to the CH3(CH2) anti-symmetric bending mode. The weak band located at 1425 cm1 in the IR spectrum (1430 and 1422 cm1 in Raman) is assigned to the CH3(S) anti-symmetric Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 581 582 583 584 585 586 587 588 589 590 591 592 593 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 9 Table 6 Observed bands in the IR and Raman spectra of CH3CH2CH2C(O)SCH3. Calculated wavenumbers, IR and Raman intensities for gauche–syn and pseudo anti–syn conformers of the title molecule and their tentative assignments. Calculatedc Experimental Infrared a Raman b Approximate description of moded Syn, pseudo anti IR int. Raman int. Syn, gauche IR int. Raman int. 3143 3139 3093 3089 3075 3061 3048 3033 3023 3021 1760 1509 1499 1495 1481 1479 1473 1414 1379 1346 6 1 52 36 1 2 29 28 0.50 29 258 10 8 1 10 6 6 2 13 8 82 46 22 104 3 123 175 13 225 148 9 1 8 12 11 9 11 0.2 3 1 3144 3140 3094 3091 3073 3063 3049 3036 3024 3022 1761 1506 1502 1487 1484 1475 1463 1417 1379 1369 1346 4 3 42 43 4 3 28 30 14 20 244 9 8 6 8 8 11 6 10 1 10 71 54 17 78 34 110 170 220 194 27 8 4 4 11 9 10 9 .01 4 1 1 1332 1304 1251 1139 1104 4 10 2 56 4 7 2 1 4 16 1290 1240 1134 8 0.5 38 3 6 4 1085 1058 9 23 1 5 1003 986 978 893 123 7 22 7 3 3 4 4 q SCH3 q SCH3 q CH2 864 807 15 32 5 12 ms C–C–CH3 q CH2 710 697 6 15 7 14 544 14 4 490 391 330 292 235 179 130 81 37 24 1 6 3 0.5 2.2 0.6 0.26 0.3 0.2 0.3 4 1 7 0.7 2.6 0.78 0.02 1.0 0.3 0.5 Liquid (R.T.) – – 2966 s – – – – 2932 s 2932 s 2840 sh 1693 s, br 1463 sh 1458 w 1451 sh 1425 w – 1417 w 1381 w 1346 w 1311 w 1278 w 1259 w 1221 w 1221 w 1138 s 1116 s 1093 sh 1050 w 1040 w 1014 vs 999 vs – 964 sh 887 m 887 m 861 m 801 m 763 m 740 sh – – 600 m 580 sh 503 vw – – – – – – – – – – a b c d 594 595 596 597 598 599 600 601 602 2984 sh – 2968 sh – – – 2948 sh 2934 (100) 2934 (100) 2915 (43) 1693 (14) – – 1452 (16) 1430 (12) 1422 sh – – – – – – – – – 1116 (15) 1093(10) 1053 (12) 1043 (15) – – – – 888 (12) 888 (12) 865 (10) 803 (6) 766 (6) 725 699 604 578 503 494 339 309 255 – – – – – – (34) (20) (15) sh (15) sh (11) (21) (15) 1043 1019 988 982 896 884 758 736 20 146 1 0.2 9 3 26 32 5 3 2 5 4 6 4 24 14 8 502 3 5 377 291 250 240 195 102 80 65 22 3 2 11 1 0.5 0.3 0.05 0.01 0.02 1 5 1 1 1.0 0.01 0.1 0.5 0.1 da CH3(CH2) da CH3(CH2) d CH2 da SCH3 d CH2(CO) ds CH3(CH2) x CH2 ds SCH3 x CH2 tx CH2 q CH3(CH2) q CH3(CH2) ma C–C–CH3 m C–C(O) 1 2 703 575 ma SCH3 ma SCH3 ma CH2 ma CH2 ma CH3(CH2) ma CH3(CH2) ms SCH3 ms CH2 ms CH2 ms CH3(CH2) m C@O m S–CH2 m C(O)–S qout of plane C@O qin plane C@O d C–C–CH3 d C–C(O)–C s CH3 d C–C–C(O) s C–C(CH3) s C–C(O) s SCH3 s C–S s structural sh, shoulder; s, strong; w, weak; m, medium; v, very. Relative band heights in parentheses. Calculated at B3LYP/6-311++G(d,p) level. Frequencies in cm1 and IR intensity in km mol1. m: stretching, d: in-plane deformation, c: out-of-plane deformation, q: rocking, x: wagging, sx: twisting, s: torsion modes. bending mode. The bands located at 1381 and 1311 cm1 in the IR spectrum are assigned to the symmetric bending mode of the CH3(CH2) and CH3(S), respectively. The bands corresponding to the CH3(CH2) rocking mode appear split in the IR and Raman spectra (See Table 6) indicating the presence of the two conformations mentioned above. The strong band located at 2966 cm1 in the IR spectrum (2968 cm1 in Raman) is assigned to the CH2 anti-symmetric stretching mode. The most intense band located at 2934 cm1 in the Raman spectrum (2932 cm1 in IR) is assigned to the CH2 symmetric stretching mode. The shoulder located at 1451 cm1 in the IR spectrum (1452 cm1 in Raman) is assigned to the CH2 bending mode. The bands corresponding to the CH2 wagging mode appear at 1346, 1278 and 1259 cm1 in the IR spectrum. These bands appear split indicating the presence of the two most stable conformers of the title compound. The experimental IR spectrum between 1550 and 500 cm1 and the corresponding simulated IR of the gauche– Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 603 604 605 606 607 608 609 610 611 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 10 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx Fig. 7. Experimental and calculated IR spectra (between 1550 and 500 cm1) for the gauche–syn and pseudo anti–syn conformers of CH3CH2CH2C(O)SCH3. 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 syn and pseudo anti–syn conformers of the title compound are shown in Fig. 7. The weak band observed at 1221 cm1 in the IR spectrum is assigned to the CH2 twisting mode and the bands at 887, 801 and 763 cm1 are assigned to the CH2 rocking mode. The bands mentioned above appear split in the IR spectrum indicating that these bands are important to identify the conformations present in the liquid substance. Carbonyl group modes. The very strong band located at 1693 cm1 in the IR spectrum is assigned to the C@O stretching mode. The band corresponding to the same mode of vibration appears in the Raman spectrum as a weak band located at 1693 cm1. These bands are in agreement with the values predicted by calculations performed at B3LYP/6-311++G(d,p) level. The wavenumbers predicted by calculations were 1760 and 1761 cm1 for the pseudo anti–syn and gauche–syn conformers, respectively. The bands observed at 600 and 580 cm1 in the IR spectrum (604 and 578 cm1 in Raman) are assigned to the C@O out-ofplane bending mode. The band corresponding to the mode mentioned above appears split showing the presence of the two conformations (See Fig. 7 and Table 6). The very weak band located at 503 cm1 in the IR spectrum is assigned to the C@O in-plane bending mode. The Raman spectrum shows two bands at 503 and 494 cm1 corresponding to the mode mentioned above. Skeletal modes. The band corresponding to the C–C–CH3 anti-symmetric stretching mode appears split into two components at 1050 and 1040 cm1 in the IR spectrum (1053 and 1043 cm1 in Raman) indicating the presence of the two conformers. This assignment is in agreement with the calculated values 1058 and 1043 cm1 for gauche–syn and pseudo anti–syn conformers, respectively. The bands located at 887 and 861 cm1 in the IR spectrum (888 and 865 cm1 in Raman) are assigned to the C–C–CH3 symmetric stretching mode of pseudo anti–syn and gauche–syn conformers, respectively. This assignment is in agreement with the values calculated for both conformers (884 and 864 cm1). The IR spectrum shows a shoulder located at 740 cm1 and the Raman spectrum shows a band located at 725 cm1. These bands are assigned to the S-CH2 stretching mode for pseudo anti–syn and gauche–syn conformers, respectively. The band located at 699 cm1 in the Raman spectrum is assigned to the C(O)–S stretching mode. The bands located at 339 and 309 cm1 in the Raman spectrum are assigned to the C–C–CH3 and C–C(O)–C bending modes, respectively. 651 Torsional modes. The band located at 255 cm1 in the Raman spectrum is assigned to the CH3 torsional mode. The bands corresponding to the other torsional modes have not been observed in the Raman spectrum of the liquid substance. 654 Conclusions 658 The optimized molecular geometries and the conformational analysis for S-methyl thiobutanoate compound, CH3CH2CH2C(O)SCH3 have been calculated using MP2 and DFT methods (B3LYP and mPW1PW91) and different basis sets. The structural results indicate that the pseudo anti–syn conformation is the most stable form. The geometrical parameters of the title compound calculated are in good agreement with the values reported for related molecules by means of GED measurements. The NBO analysis was performed to justify the preferred conformation of the title compound. The hyper-conjugative interactions are more favored in the pseudo anti–syn conformer than in the others. The analysis performed by means of the AIM indicates that the intramolecular hydrogen interaction O(2)  H(7) is very weak and the C(1)–S(3) ellipticity value shows that the electrons of this bond are not delocalized. HOMO–LUMO calculations have been performed for the title compound. The energy gap between the HOMO and LUMO molecular orbital is predicted to be 6.232 eV, a relatively high value that shows high molecular chemical stability and low reactivity. Fukui functions, local softness and electrophilicity indices were calculated in order to determine local reactive sites for the molecular system during electrophilic, nucleophilic and radical attacks. According to f±, S± and x± values, the C(1) site is the most favorable for nucleophilic attack. The most favorable site for electrophilic and radical attacks is S(3). The analysis of the IR and Raman spectra of the title compound in liquid phase agrees with the presence of pseudo anti–syn and gauche–syn conformers by resolution of some normal modes of vibration. 659 Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097 652 653 655 656 657 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 SAA 13648 No. of Pages 12, Model 5G 2 May 2015 D.M. Gil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx 688 689 Q3 Uncited reference [27]. 690 Appendix A. Supplementary data 691 692 Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.04.097. 693 References 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 [1] C. Deduve, Am. Sci. 83 (1995) 428. [2] A. Lehninger, D.L. Nelson, M.M. Cox, Princ. Biochem., Fourth ed., Omega, Spain, 2005. [3] A.M. Sourabié, H.E. Spinnler, Bourdat-Dechamps, M.R. Tallon, S. Landaud, P. Bonnarme, Appl. Microbiol. Biotechnol. 93 (2012) 1673. [4] C.O. Della Védova, R.M. Romano, H. Oberhammer, in: J. Org. Chem. 69 (2004) 5395. [5] A.M.M. El-Aasar, C.P. Nash, L.L. Ingraham, Biochemistry 21 (1982) 1972. [6] G.L. Jones, D.G. Lister, N.L. Owen, M.C.L. Gerry, P.J. Palmieri, Mol. Spectrosc. 60 (1976) 34. [7] M.F. Erben, R. Boese, Védova C. Della, O. Oberhammer, H. Willner, J. Org. Chem. 71 (2006) 616. [8] M.E. Defonsi Lestard, M.E. Tuttolomondo, E.L. Varetti, D.A. Wann, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, in: J. Raman Spectrosc. 40 (2009) 2053. [9] M.E. Defonsi Lestard, M.E. Tuttolomondo, D.A. Wann, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, J. Raman Spectrosc. 41 (2010) 1357. [10] M.E. Defonsi Lestard, M.E. Tuttolomondo, E.L. Varetti, D.A. Wann, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, J. Mol. Struct. 917 (2009) 183. [11] M.E. Defonsi Lestard, M.E. Tuttolomondo, D.A. Wann, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, J. Chem. Phys. 131 (2009) 214303. [12] Frisch M.J., Pople J.A., Binkley J.S., J. Chem. Phys. 80 (1984) 3265. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Montgomery, Jr., J.A.; Vreven, T.; Kudin, K.N.; Burant, J.C.; Millam, J.M.; Iyengar, S.S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J.E.; Hratchian, H.P.; Cross, J.B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.E.; Yazyev, O.; Austin, A.J.; Cammi, R.; Pomelli, C.; Ochterski, J.W.; Ayala, P.Y.; Morokuma, K.; Voth, G.A.; Salvador, P.; Dannenberg, J.J.; Zakrzewski, V.G.; Dapprich, S.; Daniels, A.D.; Strain, M.C.; Farkas, O.; Malick, D.K.; Rabuck, A.D.; Raghavachari, K.; Foresman, J.B.; Ortiz, [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] 11 J.V.; Cui, Q.; Baboul, A.G.; Clifford, S.; Cioslowski, J.; Stefanov, B.B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R.L.; Fox, D.J.; Keith, T.; AlLaham, M.A.; Peng, C.Y.; Nanayakkara, A.; Challacombe, M.; Gill, P.M.W.; Johnson, B.; Chen, W.; Wong, M.W.; Gonzalez, C.; Pople, J.A.; Gaussian 03, Revision C.01. Gaussian Inc, Wallingford CT (2004). C. Møller, M.S. Plesset, Phys. Rev. 46 (1934) 618. R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650. A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639. M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265. W.J. Hehre, P.V.R. Schleyer, J.A. Pople, Ab initio Molecular Orbital Theory, Wiley, New York, 1986. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. C. Alamo, B. Barone, J. Chem. Phys. 108 (1998) 664. E.D. Glendening, J.K. Badenhoop, A.D. Reed, J.E. Carpenter, F.F. Weinhold, Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 1996. R.F.W. Bader, Atoms in Molecules, A quantum Theory, Clarendon Press, Oxford, 1990. F. Biegler-Köning, J. Schönnbohn, D.J. Bayles, Comput. Chem. 22 (2001) 545. V. Krishnakumar, G. Keresztury, T. Sundius, R.J. Ramanamy, Mol. Struct. 702 (2004) 9. M.E. Tuttolomondo, A. Navarro, T.P. Ruiz, E.L. Varetti, S.A. Hayes, D.A. Wann, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, J. Phys. Chem. A 111 (2007) 9952. D.M. Gil, M.E. Tuttolomondo, A. Ben Altabef, J. Spectrochim. Acta A 123 (2014) 290. U. Koch, P.L.A. Popelier, J. Phys. Chem. 99 (1995) 9747. W. Koch, G. Frenking, J. Gauss, D. Cremer, J.R. Collins, J. Am. Chem. Soc. 109 (1987) 5917. I. Rozas, I. Alkorta, J. Elguero, J. Am. Chem. Soc. 122 (2000) 11154. V. Arjuan, M. Kalaivani, S. Senthikumari, S. Mohan, J. Spectrochim. Acta A 115 (2013) 154. J. Sponer, P. Hobza, Int. J. Quantum Chem. 57 (1996) 959. M. Karabacak, L. Sinha, O. Prasad, Z. Cinar, M.J. Cinar, Spectrochim. Acta A 93 (2012) 33. S.K. Pathak, R. Srivastava, K.A. Sachan, O. Prasad, L. Sinha, A.M. Asiri, M. Karabacak, J. Spectrochim. Acta A 135 (2015) 283. R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512. R.G. Parr, W.J. Yang, J. Am. Chem. Soc. 106 (1984) 4048. E. Chamorro, P. Pérez, J. Chem. Phys. 123 (2005) 14107. J. Padmanabhan, R. Parthasarathi, V. Subramanian, P. Chattaraj, J. Phys. Chem. A 111 (2007) 1358. S.E. Ulic, E.M. Coyanis, R.M. Romano, C.O. Della Védova, J. Spectrochim. Acta A 54 (1998) 695. M.E. Defonsi Lestard, M.E. Tuttolomondo, A. Ben Altabef, in: J. Spectrochim. Acta A 135 (2015) 907. 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 Please cite this article in press as: D.M. Gil et al., Vibrational studies (FTIR and Raman), conformational analysis, NBO, HOMO–LUMO and reactivity descriptors of S-methyl thiobutanoate, CH3CH2CH2C(O)SCH3, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), http://dx.doi.org/ 10.1016/j.saa.2015.04.097