Ab initio study of conformations of biological molecules and carbon dioxide-laser/microwave-sideband spectroscopy

In the first part, we have conducted extensive ab initio calculations for a series of large molecules of biological interest, including the molecules 2-(ethylthio)ethanol, thiodiglycol, ethyldiethanolamine and methyldiethanolamine. One of the perplexing issues that remain is the failure to observe some of the ab initio calculated low energy conformers for 2-(ethylthio)ethanol and methyldiethanolamine, leaving an unsolved question in this study. In addition to the detailed conformational studies, in this thesis we also report some preliminary studies of a "diatomic"-twist approach for anharmonic potential corrections to ab initio harmonic frequencies for large amplitude torsions.; The second part of this thesis reports the dual-mode operation of a widely tunable CO2-laser/microwave-sideband spectrometer employing a Cheo waveguide modulator and a CO2 laser that can operate both on 9 mum hot and sequence band lines as well as low-J and high- J regular band lines. The spectrometer has a wide-scan mode at Doppler-limited resolution for spectral searching and assignment, and a narrow-band sub-Doppler mode for precision measurements. The instrumental performance is demonstrated via spectra of the C-O stretching fundamental vibrational band of methanol (CH3OH). The measurement precision is confirmed for several CH 3OH transition systems through closure of combination loops. Our results illustrate the wide spectral coverage and excellent resolution obtainable with CO2-sideband infrared laser radiation in a broadly tunable and convenient infrared source with high potential for spectroscopic applications. (Abstract shortened by UMI.)...