Conformational and Structural Investigation of Substituted Hydrocarbons and Ring Compounds by Vibrational and Microwave Spectroscopy

Abstract

The physiochemical properties of a molecule of interest are usually obtained from its molecular structure, and it involves the orientation of one part of the molecule about a particular bond with respect to the rest of the molecule. These properties depend mainly upon the three-dimensional arrangement of one part of the molecule in space with respect to the other and, additionally, the way the molecule will react chemically and its available reaction pathways are often critically dependent upon the orientation of the reactant molecule. The energy differences of various conformations generally strongly couple or interact by way of nonbonded interactions which even though are individually too weak to determine, any single geometric feature may nevertheless act together to uniquely determine the spatial structures of large and complicated molecules such as proteins and DNA. Therefore, conformational analysis can lead to significant improvement in the understanding of more complex system. The infrared (3100-40 cm⁻¹) and Raman spectra (3200-20 cm⁻¹) of a number of substituted (thiol, isocyanide, cyanide, acetylchloride, silane, amine, and phosphine) ring and straight chain molecules were recorded in the gaseous, liquid and solid phases. Additionally, variable temperature studies of the infrared spectra of the sample dissolved in xenon has been carried out. From these spectral data, the possible stable conformers have been identified and the enthalpy differences are given among the various forms for each molecule. By utilizing microwave determined rotational constants for the isotopomer(s) combined with the structural parameters predicted from the MP2(full)/6 311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the stable forms of some molecules. Complete vibrational assignments are proposed for the stable conformers of each molecule. To support the vibrational assignments, normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results will be discussed and compared to the corresponding properties of some analogous molecules wherever possible.