New Methods for the Study of Intramolecular and Solvent Dynamics

Citation

Mehta, Aseem (1997) New Methods for the Study of Intramolecular and Solvent Dynamics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/75xn-rr09. https://resolver.caltech.edu/CaltechTHESIS:02172022-233103988

Abstract

Some of the fastest processes of relevance to chemical physicists occur on pico to femtosecond timescales. In the following chapters two of such fast processes axe investigated with novel theoretical methods to obtain insight into experimental observations at the molecular level.

One of the major topics of interest in chemical physics has been about energy localization in polyatomic molecules. The "golden rule" formula states that the rate for the intramolecular relaxation of energy (IVR) that is initially localized in one part of the molecule is proportional to the density of states at that energy. Here, a general mechanism of the energy redistribution out of an initially populated "light" or "bright" state is elucidated. It is shown that, for a family of acetylenic molecules, the relaxation is due to a sequence of weak off-resonant directly coupled states rather than all the available states. This mechanism shows how the rates of IVR can be significantly slower than those predicted by a naive application of the "golden rule," since mainly only the initial weak off-resonant couplings govern the rate of IVR.

Another topic that has attracted substantial interest in the chemical physics community is that of solvation. Various heavily applied theories of reaction rates, such as the electron transfer theory, have viewed the solvent as a dielectric continuum. Recent experiments and simulations have shown that the very fast solvation response provides interesting information on the molecular nature of the solvent. Here, a new method for doing molecular dynamics (MD) calculations for solvation is developed. This method uses the reaction field method to obtain the long range potential for a small cluster of molecules rather than using the usual Ewald sum technique with periodic boundary conditions (PBC). It is shown, here, that this method may be used successfully for solvent dynamics simulations. This method may prove superior for such calculations as compared to the PBC approach, since it does not impose an artificial isotropy on the problem as is the case with the PBC calculations.

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