Sensitivity of Calculated Wave Numbers of a Normal Coordinate Treatment to Assumed Molecular Geometry

Abstract

Normal coordinate treatments rarely include a consideration of the effect on the treatment of experimental uncertainties in bond lengths and interbond angles. At the same time, the agreement between calculated and observed wave numbers is taken as the major criterion of the validities of the potential-energy constants obtained, of the form of internuclear potential-energy function assumed, and of such band assignments as may have been based on the treatment. To help understand this effect, numerical results were obtained for the changes of calculated wave numbers resulting from small alterations in bond lengths and interbond angles. The molecules used were thirteen types of substituted methanes, for which potential-energy constants had previously been obtained in this laboratory. The use of such molecules also helps in understanding how seriously the common assumption of tetrahedral structure for substituted methanes affects the many normal coordinate treatments of them that have appeared in the literature. The significance of the unexpectedly large changes obtained for normal coordinate treatments of more complex and structurally less well-determined molecules is discussed.

One of the investigations carried out in partial fulfillment of requirements for the doctoral degree.