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SAA 13648
No. of Pages 12, Model 5G
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2015) xxx–xxx
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
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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
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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.
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a r t i c l e
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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.
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Introduction
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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
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⇑ 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
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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.
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Experimental
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(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]:
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f ðm0 mi Þ4 Si
Ii ¼
mi ½1 expðhcmi =kTÞ
ð1Þ
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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.
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Infrared and Raman spectroscopy
Results and discussion
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Quantum chemical calculations
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Computational methods
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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
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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.
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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
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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
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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.
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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
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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
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Fig. 2. Molecular structure for the conformers of CH3CH2CH2C(O)SCH3. The complete description is presented in the body of the text.
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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
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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/
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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:
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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/
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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
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415
416
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422
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433
434
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436
437
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439
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443
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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/
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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/
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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
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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/
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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/
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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.
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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
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[27].
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Appendix A. Supplementary data
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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.
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References
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10.1016/j.saa.2015.04.097