Chapter 4 The accuracy of molecular geometry predictions by quantum chemical methods

Abstract

Publisher Summary This chapter explores that there are relatively few experimental techniques for the determination of accurate molecular structures of free molecules in their ground states in the gas phase. The chapter focuses on the subdivisions of spectroscopy: microwave, infrared, and Raman. The common techniques come under either spectroscopic or electron diffraction methods. There are several quantum chemical (QC) methods and many basis sets, and different combinations of these. In QC methods, energy minimization is used usually without constraints and hence the structures obtained are predictions of r e structures in the spectroscopic sense. The r e structures obtained theoretically is corrected for zero-point and vibrational averaging. In spectroscopy studies, r g and r z are not of much value in analysis of spectra. The differences between r e , r s , and r 0 structures are fairly large for X–H bonds but are sometimes within experimental error for bonds between first-row atoms and certainly within the differences obtained by different methods/basis sets. The geometrical structures of hydrocarbons are also easy to predict if no multiple bonds are present apart from some effects on the C–H bond lengths. This means small calculations can be done using small basis sets and simple methods so that no great expenditure of computer time is required. The greatest difficulties arise in predicting the structures of molecules with electronegative atoms and/or multiple bonds.