Elucidating the conformational change and electronic absorption spectrum of p-dimethylamino-cinnamaldehyde merocyanine across different solvent polarities.

We present a theoretical study on the structural and electronic properties of the p-dimethylamino-cinnamaldehyde (DMACA) merocyanine molecule in solvents of different polarities by combining the free energy gradient and the average solvent electrostatic configuration methods via an iterative procedure based on the sequential quantum mechanics/molecular mechanics hybrid methodology. Studying such a system in solution is a crucial step for understanding the solvent effects on its properties, which can have implications in fields such as optoelectronics and biophysics. We found that the DMACA molecule presents different geometries in nonpolar and polar solvents, changing from a polyene-like structure with a pyramidal dimethylamino group (in gas phase or nonpolar solvents) to a cyanine-like structure with a planar dimethylamino group in water due to the stabilizing effect of hydrogen bonds between DMACA and water. The molecular absorption spectrum showed a significant change, increasing solvent polarity with a large shift of the lower energy band, while the other two low lying bands did not shift significantly. The study accurately described the solvatochromic shift of the lowest-energy band and analyzed the structure of the excited states in terms of the one-electron transition density matrix, which showed that the dominant excited state (associated with the first lower energy band) is characterized by a local excitation on the benzene ring with charge transfer character to the carbon conjugated segment.

Theoretical investigation on the linear and nonlinear optical properties of DAPSH crystal.

The linear polarizability, first and second hyperpolarizabilities of the asymmetric unit of DAPSH crystal are studied and compared with available experimental results. The polarization effects are included using an iterative polarization procedure, which ensures the convergence of the dipole moment of DAPSH embedded within a polarization field generated by the surrounding asymmetric units whose atomic sites are considered as point charges. We estimate macroscopic susceptibilities from the results of the polarized asymmetric units in the unit cell, considering the significant contribution of the electrostatic interactions in crystal packing. The results show that the influence of the polarization effects leads to a marked decrease of the first hyperpolarizability, compared with the respective isolated counterpart, which improves the concordance with the experiment. There is a minor influence of polarization effects on the second hyperpolarizability but our estimated result for the third-order susceptibility, related to the NLO process of the intensity dependent refractive index, is significant as compared with the results for other organic crystals, such as chalcone-derivatives. In addition, supermolecule calculations are conducted for explicit dimers in presence of the electrostatic embedding to illustrate the role played by the electrostatic interactions in the hyperpolarizabilities of the DAPSH crystal.

Preferential solvation and optical properties of eumelanin building blocks in binary mixture of methanol and water.

Employing a sequential quantum mechanical/molecular mechanical approach for polar protic solvents, we study the absorption spectrum of eumelanin building blocks including monomers, dimers, and tetramers in pure water and methanol and three water-methanol binary mixtures having water molar fractions (Xw = 0.25, 0.50, and 0.75). The binary mixture of solvents is a common situation in experiments, but theoretical studies are limited to the use of continuum models. Here, we use explicit solvent molecules, and specific solute-solvent interaction is analyzed and seen to play an important role. Effects of the electronic polarization of solute by the environment were included using a reliable iterative scheme. The results illustrate that the monomers, dimers, and tetramers are preferably solvated by methanol, but the composition of the mixture in the vicinity of the solute molecules is different from the bulk composition with a preferential microsolvation (hydrogen bonds) in water for most species considered. It is observed that the short-range electrostatic polarization effects of the hydrogen bonds lead to a slight blue shift of the excitation energies when the concentration of water in the mixture is enhanced. For the same species, there is an enhancement of the higher-energy absorption intensity caused by long-range electrostatic interactions with the environment and that the behavior of the experimental spectrum, which is characterized by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced by the superposition of the absorption spectra of monomers, dimers, and tetramers in the liquid phase.

Applicability of DFT functionals for evaluating the first hyperpolarizability of phenol blue in solution.

The first electronic hyperpolarizability (ß) of phenol blue (PB) in several solvents in a wide range of dielectric constants is investigated using the density functional theory (DFT). The reliability of various exchange-correlation functionals is assessed by a comparison to reference Møller-Plesset second-order perturbation theory (MP2) calculations. The equilibrium geometry of PB in each solvent is obtained by using the average solvent electrostatic configuration/free energy gradient method, which performs optimizations on the free energy hyper-surface by employing iteratively the sequential quantum mechanics/molecular mechanics methodology. The dependence of ß on the bond length alternation (BLA) coordinate is rationalized by means of the two-level model. Within the employed exchange-correlation functionals, the LC-BLYP functional shows the best performance for describing the static and dynamic MP2 results of ß, which increases as the BLA diminishes, reaching a maximum in an intermediate value of BLA. The results also illustrate the role played by the difference between the ground- and excited-state dipole moments (Δµ) in determining the hyperpolarizability behavior in solution. Particularly, in the aqueous solution case, Δµ goes to around zero when BLA is near zero, leading to an abrupt decline in the ß value. The DFT results of this study, therefore, indicate a clear relationship between the first hyperpolarizability and the BLA coordinate for the PB in solution, in agreement with experiment.

15N NMR Shifts of Eumelanin Building Blocks in Water: A Combined Quantum Mechanics/Statistical Mechanics Approach.

Theoretical results for the magnetic shielding of protonated and unprotonated nitrogens of eumelanin building blocks including monomers, dimers, and tetramers in gas phase and water are presented. The magnetic property in water was determined by carrying out Monte Carlo statistical mechanics sampling combined with quantum mechanics calculations based on the gauge-including atomic orbitals approach. The results show that the environment polarization can have a marked effect on nitrogen magnetic shieldings, especially for the unprotonated nitrogens. Large contrasts of the oligomerization effect on magnetic shielding show a clear distinction between eumelanin building blocks in solution, which could be detected in nuclear magnetic resonance experiments. Calculations for a π-stacked structure defined by the dimer of a tetrameric building block indicate that unprotonated N atoms are significantly deshielded upon π stacking, whereas protonated N atoms are slightly shielded. The results stress the interest of NMR experiments for a better understanding of the eumelanin complex structure.

Assessing the structure and first hyperpolarizability of Li@B10H14 in solution: a sequential QM/MM study using the ASEC-FEG method.

The structure and electronic properties of the lithium decahydroborate (Li@B10H14) complex in chloroform and water in normal thermodynamic conditions have been investigated using sequential QM/MM calculations by means of the average solvent electrostatic configuration (ASEC) and the Free Energy Gradient (FEG) methods. To obtain the structure of the Li@B10H14 complex in each of the solvents considered, we have performed geometry optimizations in solution using the ASEC-FEG method. The results show, for the first time with a realistic model of the molecular environment, that this alkali-metal-borane cluster is stable in chloroform but unstable in water. We have also explored the role of the electronic polarization of the solute due to solvent in the static first hyperpolarizability. The results show that, despite the reduction due to the effect of electrostatic polarization in chloroform, the Li@B10H14 complex still exhibits a large electronic first hyperpolarizability, with potential for application as a second-order nonlinear optical (NLO) material. In water, in contrast, the contribution of the excess electron for NLO responses is significantly affected by the electrostatic polarization effects. Therefore our results reveal that the influence of the environment must be considered in the design of new stable NLO materials.

Electronic and Vibrational Hyperpolarizabilities of Lithium Substituted (Aza)benzenes and (Aza)naphthalenes.

In this article we report results for the electronic and vibrational hyperpolarizabilities of ten molecules: Li@benzene, Li@pyridine, Li@pyrimidine, and Li@pyrazine; Li@naphthalene, Li@quinoline, Li@isoquinoline, Li@cinnolin, Li@quinazoline, and Li@quinoxaline. An electron correlation study shows that second-order many-body perturbation theory and density functional theory with CAM-B3LYP and M05-2X functionals give results for the electronic hyperpolarizabilities in good agreement with coupled cluster with single and doubles reference values. Static and dynamic vibrational corrections were computed through the perturbation theoretical method of Bishop and Kirtman and using a variational approach. In general, we obtained notable discrepancies between the results obtained by the two methods for the pure vibrational corrections because of the deficiency of the perturbation method to properly treat low-frequency normal modes present in the investigated systems. However, both methods give results similar to the zero-point vibrational average corrections.

Confirming the relationship between first hyperpolarizability and the bond length alternation coordinate for merocyanine dyes.

We investigated the first electronic hyperpolarizability of a typical merocyanine dye in several solvents in a wide range of dielectric constants. The equilibrium geometry of the molecule was obtained in each solvent by employing an optimization technique allied to atomistic simulations. The results confirm, for the first time with a realistic model of the molecular environment, the relationship between the first electronic hyperpolarizability (ß) and the bond length alternation (BLA) coordinate, with a maximum value of ß for intermediate positive BLA and a vanishing ß when the BLA goes to zero.

Electronic and vibrational second hyperpolarizabilities of (MgO)n clusters.

In this work, we report results for the static second hyperpolarizability of magnesium oxide clusters including electronic and vibrational contributions. The comparison between second-order Møller-Plesset (MP2) perturbation theory and coupled cluster results to the electronic contribution points out that MP2 is a suitable method to compute this property. When computed at the MP2 level, the electronic contribution per atom converges to approximately 5000 a.u. Vibrational corrections were computed at the MP2 level through the perturbation theoretical method of Bishop and Kirtman. Results obtained showed that the term [α2]0,0 represents around 20% of the electronic counterpart while the term [µß]0,0 is comparable to it. Modes that contribute significantly to [α2]0,0 are those in which all or part of the bond lengths simultaneously increase and decrease, leading to large polarizability derivatives. In turn, modes that provide relevant contributions to [µß]0,0 are those in which oxygen anions move in opposite directions to the magnesium cations yielding large derivatives of the dipole moment and first hyperpolarizability.

Elucidating the structure of merocyanine dyes with the ASEC-FEG method. Phenol blue in solution.

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.

Vibrational corrections to the second hyperpolarizabilities of Al(n)P(n) clusters.

In this work, we report results of vibrational corrections to the second hyperpolarizabilities of Al2P2, Al3P3, Al4P4, Al6P6, and Al9P9 clusters. The vibrational corrections were calculated through the perturbation theoretic method of Bishop and Kirtman and also using a variational methodology at the second order Møller-Plesset perturbation theory level with the aug-cc-pVDZ basis set. Results show that the vibrational corrections are important, accounting for more than half of the corresponding electronic second hyperpolarizabilities at the static limit. Comparisons between results obtained through both methods show very good agreements for the terms [α(2)] and [µß] but significant differences for the term [µ(2)α]. Dynamic vibrational corrections to the second hyperpolarizabilities related to the dc-second harmonic generation, intensity dependent refractive index, and dc-Kerr nonlinear optical processes are also reported.

Assessing the hydration free energy of a homologous series of polyols with classical and quantum mechanical solvation models.

Molecular dynamics (MD) simulations associated with the thermodynamic integration (TI) scheme and the polarizable continuum model (PCM) in combination with the SMD solvation model were used to study the hydration free energy of the homologous series of polyols, C(n)H(n+2)(OH)n (1 ≤ n ≤ 7). Both solvation models predict a nonlinear behavior for the hydration free energy with the increase of the number of hydroxyl groups. This study also indicates that there is a sizable solute polarization in aqueous solution and that the inclusion of the polarization effect is important for a reliable description of the free energy differences considered here.

Vibrational corrections to the first hyperpolarizability of the lithium salt of pyridazine Li-H3C4N2.

In this work we report results of vibrational corrections to the polarizability and first hyperpolarizability of the lithium salt of pyridazine Li-H3C4N2 obtained at the second-order Mo̸ller-Plesset theory level with the aug-cc-pVDZ basis set. The calculations were carried out by means of the perturbation theoretical method of Bishop and Kirtman and also using a variational approach proposed here. The results obtained show that at the static limit, the pure vibrational corrections for the polarizability and first hyperpolarizability have the same order of magnitude of the corresponding electronic contributions. Comparisons between the results obtained through the two methods show that the perturbation theoretical method is not suitable to treat the system studied, while the variational methodology presented seems to be an alternative approach to treat anharmonic systems.

Dynamic (hyper)polarizabilities of the sulphur dioxide molecule: coupled cluster calculations including vibrational corrections.

In this work we report results for dynamical (hyper)polarizabilities of the sulphur dioxide molecule with inclusion of vibrational corrections. The electronic contributions were computed analytically at the single and double coupled cluster level through response theories for the frequencies 0, 0.0239, 0.0428, 0.0656, 0.0720, and 0.0886 hartree. Contributions of the connected triple excitations to the dynamic electronic properties were also estimated through the multiplicative correction scheme. Vibrational corrections were calculated by means of the perturbation theoretical method. The results obtained show that the zero point vibrational correction is very small for all properties studied while the pure vibrational correction is relevant for the dc-Pockels effect, intensity dependent refractive index, and dc-Kerr effect. For these nonlinear optical processes, the pure vibrational corrections represent approximately 75%, 13%, and 6% of the corresponding electronic contributions for the higher frequencies quoted. The results presented for the polarizability are in good agreement with experimental values available in the literature. For the hyperpolarizabilities we have not obtained experimental results with precision sufficient for comparison.

Dynamic (hyper)polarizabilities of the ozone molecule: coupled cluster calculations including vibrational corrections.

In this work, we present results for dynamical (hyper)polarizabilities of the ozone molecule with inclusion of vibrational corrections. Electronic contributions for dynamic properties were computed analytically at the single and double coupled cluster level through response theories for the frequencies 0, 0.0239, 0.0428, and 0.0656 hartree. In the static limit, the electronic contributions were also computed at the single and double coupled cluster with perturbative correction of connected triple excitations level by means of the finite-field method. It was found that the inclusion of connected triple excitations is important, especially for a reliable description of the hyperpolarizabilities. Vibrational corrections were calculated by means of the perturbation theoretical method. The zero-point vibrational average correction was found to be relevant only for the linear polarizability, representing approximately 8% of the corresponding electronic contribution. Results also showed that the pure vibrational correction is relevant for the dc-Pockels effect, dc-second harmonic generation, intensity dependent refractive index, and dc-Kerr effect nonlinear optical processes. The double-harmonic approximation is in general suitable to compute this correction, the anharmonicity being small for the dc-Kerr effect and negligible for the other processes.

Hydrogen bond interactions between acetone and supercritical water.

Hydrogen bond interactions between acetone and supercritical water are investigated using a combined and sequential Monte Carlo/quantum mechanics (S-MC/QM) approach. Simulation results show a dominant presence of configurations with one hydrogen bond for different supercritical states, indicating that this specific interaction plays an important role on the solvation properties of acetone in supercritical water. Using QM MP2/aug-cc-pVDZ the calculated average interaction energy reveals that the hydrogen-bonded acetone-water complex is energetically more stable under supercritical conditions than ambient conditions and its stability is little affected by variations of temperature and/or pressure. All average results reported here are statistically converged.

Hyperpolarizabilities of the methanol molecule: A CCSD calculation including vibrational corrections.

In this work we present the results for hyperpolarizabilities of the methanol molecule including vibrational corrections and electron correlation effects at the CCSD level. Comparisons to random phase approximation results previously reported show that the electron correlation is in general important for both electronic contribution and vibrational corrections. The role played by the anharmonicities on the calculations of the vibrational corrections has also been analyzed and the obtained results indicate that the anharmonic terms are important for the dc-Pockels and dc-Kerr effects. For the other nonlinear optical properties studied the double-harmonic approximation is found to be suitable. Comparison to available experimental result in gas phase for the dc-second harmonic generation second hyperpolarizability shows a very good agreement with the electronic contribution calculated here while our total value is 14% larger than the experimental value.

The isotropic nuclear magnetic shielding constants of acetone in supercritical water: a sequential Monte Carlo/quantum mechanics study including solute polarization.

The nuclear isotropic shielding constants sigma((17)O) and sigma((13)C) of the carbonyl bond of acetone in water at supercritical (P=340.2 atm and T=673 K) and normal water conditions have been studied theoretically using Monte Carlo simulation and quantum mechanics calculations based on the B3LYP6-311++G(2d,2p) method. Statistically uncorrelated configurations have been obtained from Monte Carlo simulations with unpolarized and in-solution polarized solute. The results show that solvent effects on the shielding constants have a significant contribution of the electrostatic interactions and that quantitative estimates for solvent shifts of shielding constants can be obtained modeling the water molecules by point charges (electrostatic embedding). In supercritical water, there is a decrease in the magnitude of sigma((13)C) but a sizable increase in the magnitude of sigma((17)O) when compared with the results obtained in normal water. It is found that the influence of the solute polarization is mild in the supercritical regime but it is particularly important for sigma((17)O) in normal water and its shielding effect reflects the increase in the average number of hydrogen bonds between acetone and water. Changing the solvent environment from normal to supercritical water condition, the B3LYP6-311++G(2d,2p) calculations on the statistically uncorrelated configurations sampled from the Monte Carlo simulation give a (13)C chemical shift of 11.7+/-0.6 ppm for polarized acetone in good agreement with the experimentally inferred result of 9-11 ppm.

Vibrational effects on the dynamic electric properties of hydrogen peroxide.

In this work we present a method based on the perturbation theoretic approach of Bishop and co-workers [J. Chem. Phys. 95, 2646 (1991); 97, 5255 (1992); 108, 10013 (1998)] to calculate the effect of torsional motion on the polarizability and hyperpolarizabilities of hydrogen peroxide. The frequency dependence has been evaluated using the time-dependent Hartree-Fock method. The results obtained show that the zero-point vibrational averaging contributions are small compared to the corresponding electronic contributions. In the static limit the pure vibrational contributions are very large, specially for beta and gamma. These contributions are significant for the hyperpolarizabilities even in the visible region, except for the second harmonic generation and third harmonic generation processes.

Probing supercritical water with the n-pi* transition of acetone: a Monte Carlo/quantum mechanics study.

The n-pi(*) electronic transition of acetone is a convenient and important probe to study supercritical water. The solvatochromic shift of this transition in supercritical water (adopting the experimental condition of P=340.2 atm and T=673 K) has been studied theoretically using Metropolis NPT Monte Carlo (MC) simulation and quantum mechanics (QM) calculations based on INDO/CIS and TDDFT-B3LYP6-31+G(d) methods. MC simulations are used to analyze hydration shells, solute-solvent interaction, and for generating statistically relevant configurations for subsequent QM calculations of the n-pi(*) transition of acetone. The results show that the average number of hydrogen bonds between acetone and water is essentially 13 of that in normal water condition of temperature and pressure. But these hydrogen bonds have an important contribution in the solute stabilization and in the solute-solvent interaction. In addition, they respond for nearly half of the solvatochromic shift. The INDO/CIS calculations explicitly considering all valence electrons of the water molecules, using different solvation shells, up to the third shell (170 water molecules), give a solvatochromic shift of 670+/-36 cm(-1) in very good agreement with the experimentally inferred result of 500-700 cm(-1). It is found that the solvatochromic effect on n-pi(*) transition of acetone in the supercritical condition is essentially given by the first solvation shell. The time-dependent density-functional theory (TDDFT) calculations are also performed including all solvent molecules up to the third shell, now represented by point charges. This TDDFT-B3LYP6-31+G(d) also gives a good but slightly overestimated result of 825+/-65 cm(-1). For comparison the same study is also made for acetone in water at normal condition. Finally, all average results reported here are statistically converged.