Investigations in the infra-red region of the spectrum. Part IX.—The absorption spectrum of chlorine monoxide (Cl 2 O)

Recent applications of quantum mechanics to polyatomic molecules have given new theoretical rules governing molecular structure, and seem likely to provide much information regarding the nature of the forces acting between the constituent atoms. In many cases we find that the bonds between neighbouring atoms may be classified according to the resultant component of angular momentum of the bonding electrons in the direction of the line joining the atoms. Thus bonds may be of the σ-, π-, δ- and similar types, together with the non-localized bond obtained when the electrons available for bond formation are present in excess of the number usually required. Theory has almost outrun experiment in that data, especially of a spectroscopic nature, are at present confined to the simpler triatomic molecules in which no great constitutional difficulties are to be expected, and apart from the somewhat indefinite and doubtful hypothesis of semipolar and one-electron bonds, there is no experimental result which contradicts classical theory yet finds a natural explanation in quantum mechanics. The structure of Cl2 O provides such a crucial test. The molecule consists of two atoms of chlorine each of large polarizability compared with the central oxygen atom to which they are joined; if the stability of a molecule be determined by the polarizability of the constituent atoms, the substance should have a linear structure, apart from anyad hoc evidence adduced from other sources as to the angle between two oxygen bonds. According to the quantum mechanics, however, the stable structure of a molecule is determined not by the polarizability alone, but rather by a combination of the polarizability of the atoms, and by the interactions of the valency electrons, the second factor playing by far the larger part in determining the structure. We shall see that the triangular molecule with a vertical angle not far removed from 90° is demanded by the quantum theory and supported by the experimental evidence. The gas absorbs continuously in the ultra-violet, and evidence as to the fundamental frequencies and force constants must consequently be sought in the infra-red.