Chapter 3 The quest for the equilibrium structure of molecules

Abstract

This chapter explores that the determination of molecular structures has been one of the objectives of high resolution spectroscopy for a long time. This technique, together with gas phase electron diffraction, is the only practical method available to study structures of molecules in the gaseous state. In that state at sufficiently low pressures, molecules are free from interactions with other species and can therefore be studied isolated from the environment. It discusses that modern high resolution molecular spectroscopy has a tremendous resolving power and precision. Conventional Stark effect modulated microwave spectroscopy has a typical resolution of 1 in 10 5 and precision of 1 in 10 6 . A similar precision is achieved by high resolution Fourier transform infrared spectroscopy. Fourier transform microwave spectroscopy and laser based infrared spectroscopy have a precision close to 1 in 10 8 . If these spectroscopic methods are coupled with the supersonic molecular beam technique, the resolution approaches 1 in 10 7 . The chapter also discusses the theoretical models for fairly rigid molecules. With these models, the spectra observed for such molecules can be reproduced within the experimental precision. Many fitting parameters of these models—the spectroscopic constants—are obtained with high precision. Among them are the rotational constants, which sometimes have 8 or more significant digits. The chapter reviews the methods and techniques that are used to determine gas phase molecular structures from high resolution spectroscopic data. Principal techniques to obtain structures of molecules in the gas phase, electron diffraction are also discussed.