Structure and Dynamics of Van Der Waals Molecules From Microwave Spectroscopy
Keenan, Michael Robert
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https://hdl.handle.net/2142/70160
Description
Title
Structure and Dynamics of Van Der Waals Molecules From Microwave Spectroscopy
Author(s)
Keenan, Michael Robert
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
The microwave rotational spectra of several weakly bound molecular complexes, or van der Waals molecules, involving the binding between a rare gas atom and a linear molecule are examined here. The gas phase rotational spectra were obtained by using the recently developed technique of combining pulsed Fourier transform microwave spectroscopy in a Fabry-Perot cavity with a pulsed supersonic nozzle as the molecular source. Analyses of the spectra give information concerning the structures and intramolecular potential energy surfaces of these complexes.
The first chapter briefly reviews the literature of van der Waals molecules from the standpoint of intermolecular forces. The importance of studying van der Waals molecules for elucidating intermolecular potential energy surfaces is indicated. In addition, the type of information derivable from the rotational spectra and the experimental method are described.
In the second chapter, rotational spectra of KrHCl, XeHCl, ArHBr, KrHBr and ArHF are presented and the general methods used to treat the rare gas-hydrogen halides are described. Several spectroscopic features are discussed in terms of the molecular structure and dynamics. Finally, the properties of all studied rare gas-hydrogen halide complexes are compared and models are proposed to account for these properties in a self-consistent manner.
Chapter III describes the bending motion of the rare gas-hydrogen halide molecular complex in terms of a hindered internal rotation of the hydrogen halide subunit. The barrier to rotation is first chosen to reproduce the experimental structures and, secondly, described by a multipole series without adjustable parameters. For this second choice of the barrier, excellent agreement with experiment is found.
In Chapter IV, the Xe nulcear quadrupole coupling in ('131)XeHCl is examined. In a free, ground state rare gas atom, the coupling constant is identically zero. The measurement of a non-zero nuclear quadrupole coupling constant for Xe in ('131)XeHCl indicates that the Xe electronic distribution is significantly perturbed in the complex. The observed coupling constant is interpreted in terms of Sternheimer antishielding and formation of the weak Xe-HCl van der Waals bond.
Finally, Chapter V presents the rotational spectra of ArClCN. ArClCN is a T-shaped asymmetric rotor and the data were fit to the Watson rotational parameters and an exact expression for the Cl and N nuclear quadrupole coupling. The centrifugal distortion constants are used to derive the intramolecular force field and a normal coordinate analysis is performed. The Cl nuclear quadrupole coupling tensor indicates that the electric field gradients in ClCN are slightly perturbed upon complex formation but not by enough to proscribe their use in structural determinations of weakly bound complexes.
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