Solute-solvent clusters hydrogen bonded

Solvation, the interaction of a solute with the solvent, makes an important negative contribution to the entropy of a solution. Solvation can take the form of hydrogen bonding to donor or acceptor groups on the solute, or of a looser clustering of solvent molecules oriented around the solute (fig. 2.3). In general, the entropy of solvation by water be-... 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

The understanding of chemical reaction mechanisms in solution is often based on the nature of the interactions between reactants and solvent, which are governed by the physical properties of molecules, such as polarity, or by the possibility of bonds formation (e.g., hydrogen-bonding) and their dynamical evolution. The goal of the majority of works on molecular clusters is to try to fill the gap between the gas phase reaction and the condensed phase reaction by a step-by-step solvation of the reactive system. This approach will give useful... 

The term lyophobic interactions is intended to generalize the expres sion hydrophobic interactions to other solvents than water. Hydro-phobic interactions have been prominently implicated in determining the native configuration of proteins in aqueous solution. These interactions are actually not of a single relatively well-defined character, as are electrostatic or hydrogen bond interactions, but are rather a set of interactions responsible for the immiscibility of nonpolar substances and water. Proteins contain a substantial proportion of amino acids such as phenylalanine, valine, leucine, etc., with nonpolar side-chain residues. These nonpolar groups should tend, therefore, other factors permitting, to cluster on the... 

TD-DFT) they calculated the transition energies and dipole moments for NMA both in vacuum and in an aqueous solution. Moreover, in the treatment of the solvent they compared two different approaches, i.e., a polarizable-continuum method (COSMO) and a supermolecule approach. For the latter, the authors performed molecular-dynamics calculations using a force-field model and, subsequently, extracted a cluster containing the solute and 3 water molecules that form hydrogen bonds to the solute. Averages over 90 such configurations were ultimately determined. 

Neumark and co-workers [56] pointed out the similarity of the cluster results to the transient behavior in aqueous 1 solution, which has been studied via ultrafast pump-probe measurements [50]. Bradforth and co-workers [50] observed IR (800 nm) transient absorption after UV (255 nm) excitation with 50 fs time resolution. In 1 solution, a promptly arising transient disappears within 50 fs, and absorption due to solvated electron rises with a 200 fs time constant. For longer time-scales, the trapped electron shows a biexponential decay with time constants of 8 and 60 ps, which is due to recombination with the nearby iodine atom. The close resemblance of time-scales for the rise of the solvated electron and isomerization in I (water) ( = 5 and 6) implies that the electron trapping pathway in solution can be modeled as a rearrangement of the solvent hydrogen-bond network in gas-phase clusters. 

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