Herbert C Georg

The electronic structure of phenol blue (PB) was investigated in several protic and aprotic solvents, in a wide range of dielectric constants, using atomistic simulations. We employed the sequential QM/MM and the free energy gradient methods to optimize the geometry of PB in each solvent at the MP2/aug-cc-pVTZ level. The ASEC mean field is used to include the ensemble average of the solute-solvent interaction into the molecular hamiltonian, both for the geometry optimization and for the calculations of the electronic properties. We found that the geometry of PB changes considerably, from a polyene-like structure in nonpolar solvents to a cyanine-like in water. Moreover, and quite interestingly, in protic solvents with higher dielectric constant than water, the structure of the molecule is less affected and lies in an intermediate state. The results illustrate the important role played by hydrogen bonds in the conformation of merocyanine dyes.

A combined and sequential use of Monte Carlo simulations and quantum mechanical calculations is made to analyze the spectral shift of the lowest pi-pi* transition of phenol in water. The solute polarization is included using electrostatic embedded calculations at the MP2/aug-cc-pVDZ level giving a dipole moment of 2.25 D, corresponding to an increase of 76% compared to the calculated gas-phase value. Using statistically uncorrelated configurations sampled from the MC simulation, first-principle size-extensive calculations are performed to obtain the solvatochromic shift. Analysis is then made of the origin of the blue shift. Results both at the optimized geometry and in room-temperature liquid water show that hydrogen bonds of water with phenol promote a red shift when phenol is the proton-donor and a blue shift when phenol is the proton-acceptor. In the case of the optimized clusters the calculated shifts are in very good agreement with results obtained from mass-selected free jet expansion experiments. In the liquid case the contribution of the solute-solvent hydrogen bonds partially cancels and the total shift obtained is dominated by the contribution of the outer solvent water molecules. Our best result, including both inner and outer water molecules, is 570 +/- 35 cm(-1), in very good agreement with the small experimental shift of 460 cm(-1) for the absorption maximum.

The dipole moment (μ), linear polarizability (α), and first hyperpolarizability (β(tot)) of the asymmetric unit of L-arginine phosphate (LAP) monohydrate crystal are investigated using the supermolecule approach in combination with an iterative electrostatic polarization scheme. Environment polarization effects are attained by assuring the convergence of the dipole moment of LAP embedded in the polarization field of the surrounding molecules whose atomic sites are treated as point charges. The results obtained show that in the presence of the embedding charges, the value of μ is increased by 9% but the static values of α and β(tot) are decreased, respectively, by 3% and 13%, as compared with the isolated situation. The MP2/6-311+G(d) model predicts for the in-crystal dipole moment the converged value of 33 D, in good concordance with the available experimental result of 32 D. Our estimates for the converged results of α and β(tot) are, respectively, 22.51×10(-24) and 5.01×10(-30) esu. Dispersion effects are found to have a small impact on the nonlinear optical responses of LAP in the visible region. In addition, MP2/6-311G results obtained for β(tot) by using isolated and embedded LAP dimers show that crystal packing effects have a significant contribution of the electrostatic interactions. Our results suggest that the role of the crystal environment is to minimize the effects of the intermolecular interactions in the electric properties. That is, μ and β(tot) gain a more additive character in the presence of the field of the embedding charges. This is specially marked for β(tot).

Using statistically uncorrelated solute-solvent configurations generated by Monte Carlo simulation a simpler and efficient implementation of the averaged solvent electrostatic potential is made. An average configuration alone is used such that one single quantum mechanical calculation reproduces the converged statistical average obtained from the entire simulation. Applications are presented for solvent effects in a variety of properties of acetone and aminopurine in water. In all cases, excellent agreement is obtained using the average configuration and the average from the full statistical distribution.

The electronic polarization of acetone in liquid water is obtained using an iterative procedure in the sequential Monte Carlo/quantum mechanics methodology. MP2/aug-cc-pVDZ calculations of the dipole moment of acetone in water are performed on statistically uncorrelated structures extracted from isothermic-isobaric MC simulations. After electrostatic equilibrium the calculated dipole moment of acetone is obtained as 4.80 ± 0.03 D, with an increase of 60% compared to the gas phase value. This solute polarization is used to obtain the shift of 1650 ± 42 cm -1 for the n-π ∗ transition of acetone in water, in agreement with the experimental result of 1500-1700 cm -1.

The sequential Monte Carlo (MC) quantum mechanics (QM) methodology, using time-dependent density-functional theory (TD-DFT), is used to study the solvatochromic shift of the n-pi* transition of trans-acrolein in water. Using structures obtained from the isothermal-isobaric Metropolis MC simulation TD-DFT calculations, within the B3LYP functional, are performed for the absorption spectrum of acrolein in water. In the average acrolein makes one hydrogen bond with water and the hydrogen-bond shell is responsible for 30% of the total solvatochromic shift, considerably less than the shift obtained for the minimum-energy configurations. MC configurations are sampled after analysis of the statistical correlation and 100 configurations are extracted for subsequent QM calculations. All-electron TD-DFT B3LYP calculations of the absorption transition including acrolein and all explicit solvent molecules within the first hydration shell, 26 water molecules, give a solvatochromic shift of 0.18 +...

... AND SYLVIO CANUTO1 1Instituto de Fısica, Universidade de S˜ao Paulo, CP 66318, 05315-970 S˜ao Paulo, SP, Brazil, e-mail: kaline@if.usp ... The calculated auto-correlation function (circles) is fitted to the exponential decay (line) and the correlation time τ is obtained using Eq. ...

A theoretical study of magnetic properties of hydrogen peroxide in water has been carried out by means of Monte Carlo simulation and quantum mechanics calculations. The solvent effects were evaluated in supermolecular structures generated by simulations in the NPT ensemble. The solute-solvent structure was analyzed in terms of radial distribution functions, and the solute-solvent hydrogen bonds were identified with geometric and energetic criteria. Approximately three water molecules are hydrogen bonded to H2O2 (0.6 and 0.8 in each hydrogen and oxygen atom, respectively, of the H2O2). Although, on average, both hydroxyls of the peroxide are equivalent, the distribution of hydrogen-bonded water molecules is highly asymmetric. Analyzing the statistics of the hydrogen bonds, we identify that only 34% of the configurations give symmetric distributions around the two hydroxyls of the H2O2 simultaneously. The magnetic shieldings and the indirect spin-spin coupling constants were calculate...

ABSTRACT The nuclear magnetic resonance chemical shielding of 17O is of great importance for biomolecular characterization in water environment. In these systems, oxygen atoms occupy important positions and are involved in hydrogen bonds with the water environment. In this work, different solvation models are used for the theoretical determination of the 17O chemical shielding of the nucleobase uracil and the substituted 5-fluorouracil in aqueous environment. Continuum, discrete and explicit solvent models are used, and an analysis is made of the role played by the solute polarization due the solvent. The best results are obtained using the sequential quantum mechanics/molecular mechanics methodology using an iterative procedure for the solute polarization, but a good compromise is obtained by using the electronic polarization provided by the polarizable continuum model. Quantum mechanical calculations of the chemical shieldings are made using density-functional theory in two different exchange–correlation approximations. Using an iterative procedure for the solute polarization and the mPW1PW91/aug-pcS-2 model in the electrostatic approximation, we obtained magnetic shielding constants for the two O atoms of uracil within 2 ppm of the experimental results. For 5-fluorouracil, the theoretical results, with the same model, are again in good agreement with the experimental values. An analysis of the influence of the solute–solvent hydrogen bonds in the chemical shielding of uracil case is also made, and it is concluded that the most important contribution to the calculated shielding derives from the electrostatic contribution to the solute–solvent interaction.

ABSTRACT Electronic polarization induced by the interaction of a reference molecule with a liquid environment is expected to affect the magnetic shielding constants. Understanding this effect using realistic theoretical models is important for proper use of nuclear magnetic resonance in molecular characterization. In this work, we consider the pyridine molecule in water as a model system to briefly investigate this aspect. Thus, Monte Carlo simulations and quantum mechanics calculations based on the B3LYP/6-311++G (d,p) are used to analyze different aspects of the solvent effects on the 15N magnetic shielding constant of pyridine in water. This includes in special the geometry relaxation and the electronic polarization of the solute by the solvent. The polarization effect is found to be very important, but, as expected for pyridine, the geometry relaxation contribution is essentially negligible. Using an average electrostatic model of the solvent, the magnetic shielding constant is calculated as −58.7 ppm, in good agreement with the experimental value of −56.3 ppm. The explicit inclusion of hydrogen-bonded water molecules embedded in the electrostatic field of the remaining solvent molecules gives the value of −61.8 ppm.

Monte Carlo simulation and quantum mechanics calculations based on the INDO/CIS and TD-DFT methods were utilized to study the solvatochromic shift of benzophenone when changing the environment from normal water to supercritical (P = 340.2 atm and T = 673 K) condition. Solute polarization increases the dipole moment of benzophenone, compared to gas phase, by 88 and 35% in normal and supercritical conditions, giving the in-solvent dipole value of 5.8 and 4.2 D, respectively. The average number of solute-solvent hydrogen bonds was analyzed, and a large decrease of 2.3 in normal water to only 0.8 in the supercritical environment was found. By using these polarized models of benzophenone in the two different conditions of water, we performed MC simulations to generate statistically uncorrelated configurations of the solute surrounded by the solvent molecules and subsequent quantum mechanics calculations on these configurations. When changing from normal to supercritical water environment, INDO/CIS calculations explicitly considering all valence electrons of the 235 solvent water molecules resulted in a solvatochromic shift of 1425 cm(-1) for the most intense pi-pi* transition of benzophenone, that is, slightly underestimated in comparison with the experimentally inferred result of 1700 cm(-1). TD-B3LYP/6-311+G(2d,p) calculations on the same configurations but with benzophenone electrostatically embedded in the 320 water molecules resulted in a solvatochromic shift of 1715 cm(-1) for this transition, in very good agreement with the experimental result. When using the unpolarized model of the benzophenone, this calculated solvatochromic shift was only 640 cm(-1). Additional calculations were also made by using BHandHLYP/6-311+G(2d,p) to analyze the effect of the asymptotic decay of the exchange functional. This study indicates that, contrary to the general expectation, there is a sizable solute polarization even in the low-density regime of supercritical condition and that the inclusion of this polarization is important for a reliable description of the spectral shifts considered here.

Ab initio calculations were employed to study the role of ipso carbon hybridization in halogenated compounds RX (R=methyl, phenyl, acetyl, H and X=F, Cl, Br and I) and its interaction with a phosphorus atom, as occurs in the halogen bonded complex type RX⋯PH3. The analysis was performed using ab initio MP2, MP4 and CCSD(T) methods. Systematic energy analysis found that the interaction energies are in the range -4.14 to -11.92 kJ mol(-1) (at MP2 level without ZPE correction). Effects of electronic correlation levels were evaluated at MP4 and CCSD(T) levels and a reduction of up to 27% in interaction energy obtained in MP2 was observed. Analysis of the electrostatic maps confirms that the PhCl⋯PH3 and all MeX⋯PH3 complexes are unstable. NBO analysis suggested that the charge transfer between the moieties is bigger when using iodine than bromine and chlorine. The electrical properties of these complexes (dipole and polarizability) were determined and the most important observed aspect was the systematic increase at the dipole polarizability, given by the interaction polarizability. This increase is in the range of 0.7-6.7 u.a. (about 3-7%).

ABSTRACT Using the gauge-including atomic orbitals approach with B3LYP exchange-correlation functional in combination with the 6-311++G(2d,2p) basis set, we have calculated the 13C chemical shifts [Δσ] and 13C13C indirect spin-spin coupling constants [J(C,C)] for the series of isolated planar polyacetylene chains C2nH2n+2, from n = 2 up to n = 11. For both cis and trans isomers, infinite polymer values can be estimated from converged chemical shifts in the central unit of each isomer. The theoretical model predicts the chemical shift difference between the cis and trans forms of 10.28 ppm, in very good concordance with experimental result. The 1J(CC) and 1J(CC) calculated values are more geometry-dependent but they exhibit notable regularity typical for such systems as the size of chain is increased. Variations between 1J(CC) and 1J(CC) in the central unit of the C22H24 chain are estimated around 11 Hz, independent of isomeric form. It is found for both 1J(CC) and 1J(CC) that variations between the cis and trans forms are in the range of 3–4 Hz, indicating also a distinction of the isomeric form. In addition, our results show that the presence of a structural change on these conjugated backbones has marked influence on the chemical shifts. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010