Conformational analysis of small ring molecules using molecular mechanics, band contour simulation and vibrational spectroscopy

Abstract

Semi empirical computational methods can be used in conjunction with low frequency infrared and Raman spectra of gas -phase molecules to obtain information about large amplitude vibrations which are important for understanding molecular conformations. These methods are combined to create a self-consistent description of the molecular dynamics of a molecule. In this work, the most stable conformations and relative energies as well as potential energy functions governing the dynamics of selected small molecules have been elucidated. Molecular mechanics methods were applied for calculating the energies for the conformations of four-membered and five-membered rings. These methods provided data for the estimation of the barriers to planarity and pseudorotation. Potential energy functions were constructed for the investigation of the energy minima in the bicyclo-(3.1.0)hexanes and the bicyclo(3.2.0)- hept-6-enes. Mulitphase infrared and Raman spectroscopy were combined with band contour simulation and molecular mechanics methods to investigate the conformational dynamics of isopropyl formate and four of its deuterated isotopes. Band contour simulation of the carbonyl stretch of the do isotope and analysis of the spectra of the other isotopes confirmed that the s-cis, gauche was the most stable conformer present in the gas phase. A 4.3 m heatable long-path cell using silicon windows was constructed for the purpose of vapor-phase far-infrared spectroscopy of samples with low volatility. Nichrome resistance wire was used to heat the 12' pipe and heat tape was used to heat the six-inch sections extending beyond each of the windows. Twelve-inch sections of copper pipe, water-cooled with 1/4" copper coils were attached to either end of the cell to prevent the heating of the bolometer or the spectrometer.