Accurate Quantum Dynamics of Rare-Gas Heteroclusters and Hydrogen-Bonded Dimers

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This research is supported by the NSF theoretical and computational chemistry program. Rigorous quantum mechanical calculations will be carried out to study the internal dynamics of weakly bound systems, particularly, rare gas heteroclusters and hydrogen-bonded dimers. The project has three main components. (1) Quantum Monte Carlo methods will be applied to clusters consisting of a hydrogen halide molecule with several argon atoms to study the effects of solvent-solute interaction on energetics, structure, and the vibrational spectrum. (2) Improved computational methods will be developed for treating rovibrational states of four-atom clusters with one covalent bond. Large amplitude distortions of all five weak bonds will be treated explicitly. (3) Accurate quantum calculations will be carried out for mode-specific vibrational predissociation dynamics of hydrogen halide dimers to obtain theoretical predictions of predissociation lifetimes and rotational state distributions of the resulting monomer fragments. Advances during the past decade in molecular beam technology and laser spectroscopy have made it possible today to carry out detailed and highly precise measurements of the relative energies of vibrational and rotational states of very small molecular clusters in the gas phase. These experiments are potential sources of important, highly accurate, and detailed information about weak interatomic forces, but further theoretical work is required to fully extract this information. The present project is a theoretical study of three particular classes of clusters ranging in size from 4 to 17 atoms. The significant difference between these clusters and typical stable molecules with the same number of atoms are their extreme floppiness. Well developed mathematicals methods already exist for solving the quantum mechanical equations of motion for the small amplitude vibrations of typical molecules. The theory of the more compli cated, large amplitude, motion in weakly bound clusters is an active area of current research. This work is motivated, aside from inherent scientific curiosity, by the need for quantitative information about weak intermolecular forces to be used in subsequent computer simulations of molecular motion in large molecular systems, e.g. molecules of biological interest and molecules in liquid solution.