Vanishing First- and Second-Order Intramolecular Heavy-Atom Effects on the (π*→n) Phosphorescence in Carbonyls

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Abstract
The coupling schemes and matrix elements resulting from the first‐order spin—orbit perturbation of the lowest n,π* triplet state in halogenated carbonyl compounds are examined. Because of the one‐electron nature of H s·o·, only seven of the 24 states resulting from one‐electron transitions in X2CO, are found to perturb the 3 A 2(n,π*) state. Of these seven, only three states give rise to interactions involving the halogen atom: 1 B 2(n,σ2 *) through two‐center terms and small contribution of one‐center term, 1 A 1(n,σ1 *) through a small contribution of two‐center term, and 1 B 1(σ1,π*) through three‐center terms. Qualitative calculation indicates that of these three only the first interaction, when X=iodine, can be an order of magnitude larger than the one‐center term on oxygen in H2CO. The absence of one‐ and two‐center terms in the moment of the 1 B 2(n,σ2 *)↔1 A 1 transition, however, reduces the importance of this state in inducing intramolecular heavy‐atom effects. Because of the one‐electron property of the spin—orbit and vibronic operators, only two states, the 1 B 1(σ1,π*) and 1 B 2(n,σ2 *), can be used for second‐order (s.o. and vibronic) intramolecular heavy‐atom perturbations. It is found that there are no intermediate states through which these states can couple with the emitting triplet state. These conclusions indicate that intramolecular heavy‐atom effects on the π*→nphosphorescence radiative lifetime are expected to be small in X2CO or in any other carbonyl. It is pointed out that the large intramolecular heavy‐atom effects observed for the π*→π phosphorescence is a result of the relatively small spin—orbit perturbation present in the parent heavy‐atom‐free molecule.