Vibrational Relaxation and Delocalization in Molecular Crystals and Biological Systems
Kosic, Thomas Joseph
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https://hdl.handle.net/2142/70312
Description
Title
Vibrational Relaxation and Delocalization in Molecular Crystals and Biological Systems
Author(s)
Kosic, Thomas Joseph
Issue Date
1985
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
Perturbations to the vibrational Hamiltonian of isolated gas molecules resulting from a crystalline environment are examined. FTIR is used to study the effects of two different crystal fields on the vibrational modes of perylene and Picosecond Coherent Raman Scattering (ps CARS) is applied to the investigation of vibrational dynamics of hydrogen bonded molecular crystals.
Perylene has two stable crystal forms at room temperature; (beta)-perylene having two molecules per unit cell and (alpha)-perylene consisting of four molecules per unit cell, arranged in face-to-face dimers. The IR and Raman spectra of (alpha) and (beta)-perylene crystal vibrations are presented. Frequencies and forms of the lattice vibrations are compared for the two crystal forms. An interesting case of vibron-phonon mixing in the region around 360 cm('-1) is discussed. Polarized ratios in (alpha) crystals are perturbed from the oriented gas molecule due to coupling of molecular vibrations to dimer vibrations. Lineshape analysis at low temperature yield lorentzian lines, suggesting delocalization for all the vibrations studied in both crystal forms. However, at 298 K the 1172 cm('-1) combination band is not lorentzian indicating vibron localization.
Vibrational relaxation in molecular crystals and systems of biological importance is examined. At low temperature ps CARS can be used to determine the vibrational lifetime T(,1) when the ps CARS decay is exponential, and a lower limit to T(,1) when it is not. Vibron ((OMEGA) = 150 - 1600 cm('-1)) lifetimes are less than 10 ps, whereas librons ((OMEGA) = 30 - 120 cm('-1)), which are torsional oscillations of the amino acid or peptide chains, have long lifetimes (10 ps < T(,1) < 5ns). The frequency dependence of T(,1) for low temperature librons is calculated using the theory of anharmonic processes.
An (OMEGA)('-4) dependence of T(,1) is observed for most crystals which is characteristic of spontaneous decay to two counterpropagating acoustic phonons at (OMEGA)/2. The crystal L-alanyl-L-tyrosine(.)3H(,2)O shows T(,1) (OMEGA)('-2), which is characteristic of decay to one libron and one acoustic phonon. The temperature dependent ps CARS data is discussed and is consistent with a mechanism involving absorption of acoustic phonons. Finally, the relationship between the crystal dynamics and protein dynamics, and results on crystalline lysozyme are discussed.
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