General Physics Department

Monte Carlo simulations of one and two benzene molecules in water have been performed to analyze the hydrophobic hydration and hydrophobic interaction effects. Also, Monte Carlo structures have been used in the quantum mechanical calculations of the red shift of the B2u(Tr-7r *) band of benzene in water and the role of hydrophobicity analyzed. It has been found, on average, that the water molecule closest to benzene has one of the O-H bonds tangential to the benzene plane; the water-induced effect essentially doubles the benzene-benzene interaction and hydrophobicity has a small, but non-negligible, effect on the red shift of the first absorption band of benzene in water.

We address the effect of solvation on the lowest electronic excitation energy of camphor. The solvents considered represent a large variation in-solvent polarity. We consider three conceptually different ways of accounting for the solvent using either an implicit, a discrete or an explicit solvation model. The solvatochromic shifts in polar solvents are found to be in good agreement with the experimental data for all three solvent models. However, both the implicit and discrete solvation models are less successful in predicting solvatochromic shifts for solvents of low polarity. The results presented suggest the importance of using explicit solvent molecules in the case of nonpolar solvents.

Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction ͑INDO/CIS͒ are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 Å from the center of mass of formaldehyde, showing the very long-range effects on the solvation shift of this polar molecule. The largest cluster includes one formaldehyde and 142 water molecules. INDO/CIS calculations are performed on this cluster with a properly antisymmetric reference ground state wave function involving all valence electrons. The estimated limit value for the solvatochromic shift of the n-* emission transition of fully relaxed formaldehyde in water, compared to the gas phase, is Ϸ1650 cm Ϫ1 . The total Stokes shift of formaldehyde in water is calculated as Ϸ550 cm Ϫ1 .

The sequential Monte Carlo ͑MC͒ quantum mechanics ͑QM͒ methodology, using time-dependent density-functional theory ͑TD-DFT͒, is used to study the solvatochromic shift of the n -* 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± 0.11 eV. Using simple point charges to represent the solvent the shifts are calculated for the first, second, and third solvation shells. The results converge for the calculated shift of 0.20± 0.10 eV in very good agreement with the experimentally inferred result of 0.20± 0.05 eV. All average results presented are statistically converged.

A sequential combination of Monte Carlo simulation and quantum mechanics calculation is used to study the solvatochromic shift of the n ] p* absorption transition of pyrimidine in water and in carbon tetrachloride. Super-molecular conÐgurations are generated from NVT Monte Carlo simulations and are used for subsequent extensive quantum mechanical calculations. The auto-correlation function of the energy is used to analyze the statistical correlation between the conÐgurations used in the quantum mechanical calculations. The total number of molecules used in the super-molecules is obtained after analysis of the radial distribution function that deÐnes the solvation shells. For the case of pyrimidine in water, full quantum mechanical INDO/CIS calculations are performed in the super-molecular clusters corresponding to the Ðrst, second and third solvation shells, extending up to nearly 11.5 away from the center of mass of pyrimidine. For the A largest calculation, made for the third solvation shell, it includes 1 pyrimidine and 213 water molecules, with a total of 1734 valence electrons explicitly included. Using the results obtained for the di †erent solvation shells the solvatochromic shift is extrapolated to the bulk limit. This gives our best result of 2223^60 cm~1, in good agreement with the experimental value of 2700^300 cm~1 and explicitly conÐrming that the polarization e †ects of pyrimidine in protic solvents extend to a very long distance from the solute. For pyrimidine in carbon tetrachloride, a non-polar and aprotic solvent, the use of only the Ðrst solvation shell gives a stable result of D100 cm~1 for the n ] p* blue shift. INDO/CIS calculations, showed that the n ] p* blue shift of pyrimidine in water, modeled using a continuum solvation model, can only be adequately predicted with the inclusion of

Monte Carlo simulations and thermodynamic perturbation theory calculations have been carried out to analyze the differential hydration of phenol (PhOH) and phenoxy radical (PhO • ). The hydration enthalpy of phenol predicted by different phenol-water interaction models is in good agreement with experimental data. On the basis of the difference in the hydration enthalpy of phenol and phenoxy radical, we find that the O-H bond dissociation enthalpy in water is above the recommended experimental value for the gas phase by ca. 7 kcal/mol. This result is in agreement with photoacoustic calorimetry measurements for phenol in other polar solvents. Thermodynamic perturbation theory results for the relative hydration Gibbs energy of phenol and phenoxy radical are also reported. The structure of the solutions suggests that the differential solvation of phenol and phenoxy radical can be related to the strong character of phenol as a hydrogen bond donor in comparison with the role played by phenoxy radical as a hydrogen bond acceptor.

The sequential Monte Carlo quantum mechanics methodology is used to obtain the solvent effects on the Stokes shift of acetone in water. One of the great advantages of this methodology is that all the important statistical information is known before running into the costly quantum mechanical calculations. This advantage is discussed not only with respect to the statistical correlation between the different structures generated by the simulation but also in the proper identification of hydrogen bonds in liquids. To obtain the solvent effects in the Stokes shift of the n -p p absorption transition of acetone in water, quantum-mechanical calculations are performed in super-molecular structures generated by NVT Monte Carlo simulation. The statistical correlation between configurations is analyzed using the auto-correlation function of the energy. The largest calculations include one acetone and 170 water molecules. One-hundred INDO/CIS super-molecular calculations are performed for each solvation shell to obtain the statistical average value. The calculated solvatochromic shift of the n -p p absorption transition of acetone in water, compared to gas phase, is , 1310 cm 21 in good agreement with the experimental blue shift of 1500^200 cm 21 . For the emission of the relaxed excited state, the calculated shift is , 1850 cm 21 . The total calculated solvent effect on the Stokes shift of acetone in aqueous solution is thus 540 cm 21 . A detailed analysis of the sampling of the configurations obtained in the Monte Carlo simulation is made and it is shown that all results represent statistically converged values. q