Some aspects of molecular symmetry

Abstract

Any object with a characteristic shape possesses symmetry. Such an object could be a house, a tennis ball or a molecule. This discussion is concerned with the symmetry of molecules. (The aesthetic beauty of molecular architecture has been the subject of a book written by Pauling and Hayward some time ago). One can instinctively tell that a molecule like methane is more 'symmetrical' than a molecule like chlorofluoromethane, which in tum, is more symmetrical than a molecule of the species FClSO. However, in order to be able to determine the relative degrees of symmetry of different molecular species, a set of basic rules must be established and used as exact criteria of symmetry. Indeed, molecular symmetry has become a quantitative science based on the principles of that branch of mathematics called Group Theory. In this article, the basic ideas of molecular symmetry are introduced and the symbolism employed in the symmetry analysis of molecules is explained. With this information one is then able to precisely define the shape of a molecule or ion by its symmetry symbol without having to give long verbal descriptions. Problems connected with optical isomerism are then tackled from the point of view of symmetry.