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The molecular dynamics (MD) simulations, which are based on the multistate empirical valence-bond methodology (MS-EVB) of Schmitt and Voth, are used to investigate the hydrogen-bond (HB) enthalpies and entropies for the hydronium solvation shells in liquid water. The systematic bond-energy differences have agreed with infrared (IR) study on proton hydration.

A combined effort of the experimental, theory, and the simulations revealed the elementary steps involved in the phenomenon of the photoacidity of ROH-type photoacids. The excitations triggered an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid.

The translational diffusion origin of preferential solvation is described. The main aspects of concentration and time dependence of spectral shifts are examined.

The solvatochromism of 2-naphthol (2OH) and 2-methoxynaphthalene (2OMe) have been studied by measuring the excitation and emission fluorescence spectra in a series of pure solvents. The empirical solvatochromic scale of Kamlet and Taft was used since it provides accurate values for dipolarity interactions as well as the HB accepting and donating power of the solvent. Results reveal that solvatochromism of 2OH is dominated by HBing rather than polarity effects. Both molecules have a small dipole moment in their ground electronic state which increases the excited (S1) state.

Research was conducted to examine the structure of concentrated HCl solutions. A pentagonal ring composed of four water molecules, one proton and one chloride anion, is proposed as the basic structural unit of HCl(H2O) and (HCl)2(H2O)6 in solution. It has bridged H5O2+ and Cl- ions, each of them having a coordination number around four. Results indicate that the ring structure is a likely candidate for the structure of protonated HCl clusters in the gas phase and for the contact ion-pair formed following HCl dissociation in solution.

The modern theories for many-body effects on diffusion-influenced reactions were used to derive approximate analytical expressions for the steady-state profile and the Laplace transform of the transient concentration profiles. The steady-state profiles were observed to have hyperbolic dependence on the distance of the substrate from the enzyme, with a prefactor containing the complexity of the manybody effects.

Different multistate generalizations of a two-state (EVB) model for proton hopping between water molecules are used for analyzing the proton transfer (PT) mechanism in liquid water. The transition in real time is monitored, which has shown how the average structure is converted to a distorted [H.sub.5][O.sub.2.sup.+] cation constituting the transitional complex for proton hopping between water molecules.

The direct proton transfer reaction between a photoacid, 2-naphthol-6-sulfonate, and a base, acetate anion, in water-glycerol mixtures is estimated by means of time-resolved fluorescence. The reaction dynamics illustrates a good approximation with transient regime of the Smoluchowski model, which is used to study the reaction kinetics of irreversible, diffusion-influenced reactions.

A diffusional model presented reveals the coupling of conformation change with enzyme action by simultaneous analysis of the on-time distribution and autocorrelation function of a single working cholesterol oxidase enzyme. Active-site oxidation induces a conformational change that opens the path for substrate entry.

Gas-phase H9O4+ has been considered an archetypal Eigen cation, H3O+(H2O)3. Yet ab initio molecular dynamics (AIMD) suggested that its infrared spectrum is explained by a linear-chain Zundel isomer, alone or in a mixture with the Eigen cation. Recently, hole-burning experiments suggested a single isomer, with a second-order vibrational perturbation theory (VPT2) spectrum agreeing with the Eigen cation. To resolve this discrepancy, we have extended both calculations to more advanced DFT functionals, better basis sets, and dispersion correction. For Zundel-isomers, we find VPT2 anharmonic frequencies for four low-frequency modes involving the excess proton unreliable, including the 1750 cm-1 band that is pivotal for differentiating between Zundel and Eigen isomers. Because the analogous bands of the H5O2+ cation show little effect of anharmonicity, we utilize the harmonic frequencies for these modes. With this caveat, both AIMD and VPT2 agree on the spectrum as originating from a Zundel isomer. VPT2 also shows that both isomers have the same spectrum in the high frequency region, so that the hole burning experiments should be extended to lower frequencies. [ABSTRACT FROM AUTHOR]

The infrared (IR) spectrum of the ammoniated ammonium dimer is more complex than those of the larger protonated ammonia clusters due to close-lying fundamental and combination bands and possible Fermi resonances (FR). To date, the only theoretical analysis involved partial dimensionality quantum nuclear dynamic simulations, assuming a symmetric structure (D3d) with the proton midway between the two nitrogen atoms. Here we report an extensive study of the less symmetric (C3v) dimer, utilizing both second order vibrational perturbation theory (VPT2) and ab initio molecular dynamics (AIMD), from which we calculated the Fourier transform (FT) of the dipole-moment autocorrelation function (DACF). The resultant IR spectrum was assigned using FTed velocity autocorrelation functions (VACFs) of several interatomic distances and angles. At 50 K, we have been able to assign all 21 AIMD fundamentals, in reasonable agreement with MP2-based VPT2, about 30 AIMD combination bands, and a difference band. The combinations involve a wag or the NN stretch as one of the components, and appear to follow symmetry selection rules. On this basis, we suggest possible assignments of the experimental spectrum. The VACF-analysis revealed two possible FR bands, one of which is the strongest peak in the computed spectrum. Raising the temperature to 180 K eliminated the "proton transfer mode" (PTM) fundamental, and reduced the number of observed combination bands and FRs. With increasing temperature, fundamentals red-shift, and the doubly degenerate wags exhibit larger anharmonic splittings in their VACF bending spectra. We have repeated the analysis for the H3ND+NH3 isotopologue, finding that it has a simplified spectrum, with all the strong peaks being fundamentals. Experimental study of this isotopologue may thus provide a good starting point for disentangling the N2H7+ spectrum. [ABSTRACT FROM AUTHOR]

Protonatedwater clusters that are hydrogen-bonded to a neutralbenzene molecule are a reductionist model for protons at hydrophobicsurfaces, which are of fundamental importance in biological energytransduction processes. Of particular interest is the protonated waterdimer (“Zundel ion”) on benzene, whose gas-phase messengerIR spectrum has been previously interpreted in terms of an asymmetricbinding of the protonated water dimer to the benzene ring througha single water molecule. This “standing Eigen” isomerhas a hydronium core. We have found an alternative “crouchingZundel” isomer, which attaches to the benzene ring symmetricallyvia both of its water molecules. When Ar-tagged, it has an IR spectrumin much better agreement with experiment than the standing Eigen isomer,particularly at the lower frequencies. These conclusions are basedon static harmonic (and anharmonic) normal-mode analysis using densityfunctional theory with various (dispersion corrected) functionalsand particularly on dynamic anharmonic spectra obtained from the dipoleautocorrelation functions from classical ab initio molecular dynamicswith the BLYP, PBE, and B3LYP functionals. Possible implications toprotons on water/organic-phase interfaces are discussed. [ABSTRACT FROM AUTHOR]

Research into excited-state photon transfer from an ultrafast photoacid, 5-cyano-2-naphthol, is presented. It was found that the reaction obeys the mechanism of geminate diffusion control over the entire composition range.

Super-photoacids, such as 5-cyano-2-naphthol, tend to undergo excited-sate proton transfer within their chromophore, when subjected to a series of time-resolved fluorescence. An empirical analysis involving the Kamlet-Taft approach indicate that majority of the shifting phenomenon in the excitation spectra is made possible through the action of hydrogen bonds, which have been donated from the OH group. The hydrogen bond contained by super-photoacids has also been shown to gather intensity during the excited state.