Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Vibronic interaction in the lower electronic states of benzene

“…The reduced intensities as given by Nieman for 0-2 per cent C~H6 in C6D6 at 4"2K are [8] 0-1 bands I1,0'= 1. with the x fine structure axis along a diagonal, arbitrarily labelled as C1-C4 [12]. Ponte Goncalves and Hutchison from their ENDOR studies further concluded that there are three distinct pairs of protons (in agreement with the inversion symmetry in the crystal), with the 1,4 pair having a substantially lower isotropic ENDOR shift than the two other, nearly equal pairs; also the molecule was found to be non-planar [13].…”

“…This then will lead to a favouring of molecular conformations in p, ~ space for which Vet is minimal over those where Ver is maximal and the molecule may look ' distorted ' to a suitably equipped observer, unless Ver were independent of ~. With the present expression (3.1) for Ver the quantity 4Aop simply represents the difference in interaction with the crystal field between the' quinoid ' and' antiquinoid ' conformations for a given degree of nonhexagonality p; see [12] figure 5. Or in loose language, one might picture the function A0p2 cos ~b as a furrow cut across the potential trough for the free molecule, towards which the molecular conformation is attracted.…”

“…The chief virtue of representing the effect of the crystal field by the potential (3.1) is the simplicity of the physical model behind it. This model has already been discussed in w 5 of [12] to which the reader is referred for an illustration. Briefly, the situation is as follows.…”

“…The two matrix elements (BI/~ I +> and (BI~ I -> in (4.12) can be calculated from the molecular orbitals of benzene via well-known methods; moreover, they are real and equal because of the circumstance that the two in-plane principal axes of the zero-field splitting tensor as determined in the crystal [12] coincide with symmetry axes of the free molecule. In terms of the MO's defined in figure 6 and table 1 of (4.14)…”

“…On the other hand, a weak crystal field which produces only minor changes in the vibronic energies, may still cause the marked departures from hexagonal symmetry of the electron spin distribution observed in magnetic resonance experiments [11][12][13] on the benzene crystal and may also explain the characteristic splitting observed for the strongest band of the To+-So absorption spectrum [10].…”