Non-van der Waals treatment of the hydrophobic solubilities of CF4.

A quasi-chemical theory implemented on the basis of molecular simulation is derived and tested for the hydrophobic hydration of CF4(aq). The theory formulated here subsumes a van der Waals treatment of solvation and identifies contributions to the hydration free energy of CF4(aq) that naturally arise from chemical contributions defined by quasi-chemical theory and fluctuation contributions analogous to Debye-Hückel or random phase approximations. The resulting Gaussian statistical thermodynamic model avoids consideration of hypothetical drying-then-rewetting problems and is physically reliable in these applications as judged by the size of the fluctuation contribution. The specific results here confirm that unfavorable tails of binding energy distributions reflect few-body close solute-solvent encounters. The solvent near-neighbors are pushed by the medium into unfavorable interactions with the solute, in contrast to the alternative view that a preformed interface is pulled by the solute-solvent attractive interactions into contact with the solute. The polyatomic model of CF4(aq) studied gives a satisfactory description of the experimental solubilities including the temperature dependence. The proximal distributions evaluated here for polyatomic solutes accurately reconstruct the observed distributions of water near these molecules which are nonspherical. These results suggest that drying is not an essential consideration for the hydrophobic solubilities of CF4, or of C(CH3)4 which is more soluble.

Vibrational spectral diffusion of azide in water.

Vibrational spectral diffusion denotes the time-dependent fluctuations of a solute's vibrational frequencies due to local environmental dynamics. Vibrational line shapes are weakly sensitive to spectral diffusion, whereas three-pulse vibrational echoes are much more sensitive. We report here on theoretical studies of spectral diffusion of the asymmetric stretch of the azide anion in heavy water. We run a classical molecular dynamics simulation of rigid azide in rigid water, and at every time step we calculate the azide's anharmonic asymmetric stretch frequency using an optimized quantum mechanics/molecular mechanics method developed earlier. This generates a frequency trajectory, which we use to calculate the absorption line shape and integrated three-pulse echo intensity. Our results for both the line width and the integrated echo intensity are in excellent agreement with experiment. Our calculated frequency time-correlation function is in excellent agreement with experiment for long times (greater than 250 fs) but differs considerably from experiment at short times; our theoretical correlation function has a very pronounced oscillation, presumably due to intermolecular azide-water hydrogen-bond stretching dynamics.

Signatures of beta-peptide unfolding in two-dimensional vibrational echo spectroscopy: a simulation study.

An ensemble of exciton Hamiltonians for the amide-I band of the folded and unfolded states of a helical beta-heptapeptide is generated using a molecular dynamics (MD) simulation. The correlated fluctuations of its parameters and their signatures in two-dimensional (2D) vibrational echo spectroscopy are computed. This technique uses infrared pulse sequences to provide ultrafast snapshots of molecular structural fluctuations, in analogy with multidimensional NMR. The present study demonstrates that, by combining a method of calculating the vibrational Hamiltonian from MD snapshots and the nonlinear exciton equations (NEE), it may be possible to simulate realistic multidimensional IR spectra of chemically and biologically interesting systems.