Towards a More Quantitative Understanding of Intermolecular Interactions: Biologically Significant Intermolecular Clusters

Author: Suzuki, Sakae

Year: 1996

Degree: Dissertation (Ph.D.)

Advisor: Blake, Geoffrey A.

Committee Members: Okumura, Mitchio; McKoy, Basil Vincent; Gray, Harry B.; Blake, Geoffrey A.

Option: Chemistry

DOI: 10.7907/z68g-sj24

Abstract

Intermolecular potential energy surfaces (IPS's) of weakly bound clusters with potential applications to the interpretation of biological and other natural phenomena, namely Ar-D2O, Ar-DOH, C6H6-H2O, C6H6-D2O, C6H6-DOH, and (D2O)3, were studied with fully rotationally resolved spectroscopy, ab intio calculations and diffision Monte Carlo simulations. These results contributed to the characterization of the Ar-water, benzene-water and water trimer IPS's and reinforced the need to treat complete the complex dynamics of intermolecular forces.

The Caltech tunable far-infrared spectrometer (TuFIR) was used to observe the Σ+000 → Π111 transition at 19.32 cm-1 and the Σ+000 → Σ+111 transition at 20.65 cm-1 for Ar-D2O, and the Σ+000 → Π101 transition at 19.9 cm-1 for Ar-HDO. These transitions probed previously unexplored regions of the Ar-water IPS and enabled the generation of an improved multi-dimensional IPS for this simplest model system of hydrophobic-polar interactions.

The Caltech direct absorption microwave spectrometer was used to record the symmetric top rotational spectrum for the J=4 → 5 to 9 → 10 transitions in the m = 0 and m = ±1 manifold of C6H6-H2O, C6H6-D2O, and C6H6-DOH, to examine the nature of "aromatic-polar" interactions. The rotational constants from the three isotopomers demonstrated unambiguously that water is positioned above the benzene plane in nearly free internal rotation with the hydrogen atoms pointing toward the benzene π-electron cloud. A D0 of 1.9 kcal/mol was estimated based on the spectra. Ab initio calculations supported the aromatic-π-H bond geometry and predicted a binding energy of De less than or equal to 1.78 kcal/mol. Diffusion Monte Carlo simulations on the ab initio surface were performed to visualize the large amplitude motions in this dimer. The TuFIR was also employed to measure a C6H6-D2O intermolecular vibrational band at 19.5 cm-1. The symmetric top pattern and l-type doubling in the excited state revealed a strong coupling between the internal rotation and bending

Finally, a previously unpredicted c-type FIR band of (D2O)3 centered at 41.1 cm-1 was observed, which prompted the replacement of a simple one-dimensional model with a more realistic analysis of the (D2O)3 dynamics.

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