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 Δν intervals in ²Π or ³Π 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_τ 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_τ selection rules are, however, abolished; (2) the uncoupled vectors l_τ are separately space-quantized with reference to the electric axis, giving component quantum numbers i_lτ; (3) the electron of the H atom (i_lτ=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_τ 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 Δν in ²Π and ³Π states of MH molecules in terms of Δν values of corresponding M atoms in states resulting from dissociation of MH. Usually ΔνMHΔνM is a little under 2/3; the factor 2/3 is that expected, according to theory, from the space-quantization of l_τ's to give i_l_τ's.