Electronic Orbitals, Shapes and Spectra of Polyatomic Molecules

Abstract

OUR ideas and knowledge of the shapes of polyatomic molecules are largely confined to the ground electronic states. Comparatively little progress has been made in knowledge of the symmetries of electronically excited states. Not infrequently it has been tacitly, but unjustifiably, assumed that the excited state of a molecule has the same symmetry as the ground-state. Bound up with this, spectroscopists, having made a vibrational analysis of a spectrum, have often been content to leave it at that; they have not tried to answer the question as to why particular vibrations occur. There have been exceptions, of course—the occurrence of the breathing vibration of benzene on electronic excitation near 2600 A. has been, for example, very reasonably attributed to an expansion of the benzene ring in the upper state—but, especially with non-totally symmetrical vibrations, it is usually true that no explanation has been offered for the occurrence of particular modes of vibration. Why, for example, do e_g⁺ vibrations occur in the near ultra-violet spectrum of benzene? That these enable an otherwise forbidden transition to become allowed is, of course, no explanation of why they occur.