The Assignment of Quantum Numbers for Electrons in Molecules. III. Diatomic Hydrides

The known electronic states of diatomic hydride molecules (MH) are derivable from unexcited H plus familiar low-energy states of M atoms (Hund, Hulth\'en, Mecke, Mulliken: cf. Table I). Observed states, and \`especially observed intervals in ^2 or ^3 states of such MH molecules (cf. Table III), indicate that the effects of the H on the M atom are confined essentially to the following: (1) the couplings, when present, between l_ vectors of M atom outer electrons to give a resultant l are completely broken down by the field of the H nucleus; the M atom orbits are otherwise scarcely changed, except for slight shielding or similar effects produced by the H electron and nucleus; the usual l_ selection rules are, however, abolished; (2) the uncoupled vectors l_ are separately space-quantized with reference to the electric axis, giving component quantum numbers i₋; (3) the electron of the H atom (i₋=0) is promoted and takes its place with the M electrons, sometimes becoming equivalent to one of them giving a new closed shell (of two electrons) ; the H nucleus, however, stays on the outside edge of the M electron cloud, so that the hydrides should in general be strongly polar, in agreement with Mecke's conclusions: (4) the original couplings of s_ vectors are often broken down by the advent of the H electron spin; always, the latter alters the original multiplicity by one unit. In Table II and the related discussion, data are presented as evidence that molecular stability is primarily a matter of promotion energy, rather than of valence bonds in the sense of Lewis or London. In connection with Table III, a simple explanation is given of observed multiplet widths in ^2 and ^3 states of MH molecules in terms of values of corresponding M atoms in states resulting from dissociation of MH. Usually {₌₇}{₌} is a little under 23; the factor 23 is that expected, according to theory, from the space-quantization of l_'s to give i₋_{}'s.