The infrared band intensities of 1,4-dioxan and related compounds

Abstract

This thesis describes an investigation of the redistribution of electronic charge that occurs during the molecular vibrations of 1,4-dioxan and related compounds. The infrared band intensities of dioxan-dg, dioxan-dg, cyclohexane-d0, cyclohexane-d12 and tetrahydropyran were measured in both solution and gas phases. In the solution phase, overlapping band systems were split into their components by assuming Lorentzian band contours and producing the absorbance maximum, band half width and peak frequency from a least squares procedure. The individual gas phase intensities in such spectral regions were then derived by assuming that the distribution of intensity across the region was the same in both phases. Normal coordinate calculations were performed and extended to reduce the dioxan and cyclo-hexane experimental intensities to parameters known as atomic polar tensors (APTs). Each element of the APT for an atom represents the change in a component (x, y or z) of the molecular dipole moment on moving the atom along one of the molecule-fixed Cartesian axes. The sign ambiguity of the experimental dipole derivatives was satisfactorily resolved using constraints provided by the intensities of the deuterated compounds: APTs were calculated from each possible choice of signs and these were used in turn to predict the intensity of the deuterated molecule - the APT was rejected if the agreement was unsatisfactory (as measured by a "fit factor").The Gaussian 76 package for MO calculations was used to calculate ab initio APTs, and also, for dioxan, to derive a general valence force field. The latter, after least-squares refinement to the frequencies, proved more appropriate for the analysis of the dioxan intensities than a field derived from the Snyder-Zerbi generalized ether force field alone. A reasonable correspondence exists between the APTs derived from experiment and those from the Gaussian 76 package, and certain trends are apparent along the series of molecules that hopefully will aid future studies and will provide a route to the prediction of spectra of related molecules.