Internal and External Vibrations of Weakly Bound Molecular Complexes: Infrared Spectroscopy and Vibrational Analysis
Kolenbrander, Kirk David
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https://hdl.handle.net/2142/70412
Description
Title
Internal and External Vibrations of Weakly Bound Molecular Complexes: Infrared Spectroscopy and Vibrational Analysis
Author(s)
Kolenbrander, Kirk David
Issue Date
1988
Doctoral Committee Chair(s)
Lisy, James M.
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
Infrared vibrational predissociation spectroscopy has been used to study the internal (intramolecular) and external (intermolecular) vibrations of weakly bound molecular complexes in a molecular beam apparatus. The apparatus features a continuous supersonic expansion source and mass selective detection which employs electron impact ionization and quadrupole mass filtering. A Nd:YAG-pumped optical parametric oscillator (OPO) provided tunable infrared radiation in the 2.5 to 3.5 $\mu$ region.
Studies of internal vibrations of weakly bound molecular complexes include the investigation of the $\nu\sb9$ and $\nu\sb $ C-H stretching vibrations of (C$\sb2$H$\sb4)\sb2$ and Ar-C$\sb2$H$\sb4$, and the $\nu\sb{s}$ H-F stretching vibration of a series of binary complexes of HF with N$\sb2$, acetylene, ethylene, and cyclopropane. In addition, a C-H stretching vibration of benzene dimer was examined in an application of a new technique which features isotopic substitution in the unambiguous assignment of observed absorptions to a specific complex. No evidence of homogeneous broadening was observed in any of the studies of the internal vibrations.
The in-plane and out-of-plane external torsional vibrations of cyclic hydrogen fluoride trimer have been characterized and assigned using a concerted theoretical and experimental effort. The cyclic (HF)$\sb3$ complex was modeled as a three-dimensional system of fully coupled hindered rotors. Vibrational energy levels, wavefunctions, and oscillator strengths were calculated for each of the two vibrational systems, and complete vibrational manifolds are presented. Two vibrational bands, an in-plane overtone and an out-of-plane combination, were calculated to exist within the frequency range of the CO$\sb2$ laser. Using a two-laser, IR-IR double resonance technique, two rovibrational transitions were observed, corresponding to these predicted bands. Theoretical determinations of oscillator strengths and the effects of deuterium isotopic substitution on the vibrational energy levels were experimentally verified to establish the assignment of the observed transitions to their specific vibrational modes. The theoretical analysis was continued to examine the anharmonicity, amplitude of vibrational motion, and oscillator strengths of the vibrational modes of the complex in detail. The experimentally determined linewidths of the observed transitions are compared to the previously reported linewidth of the (HF)$\sb3$ predissociative H-F stretching vibration.
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