7: Ring Current in the Benzene Negative Ion
Studies of Weissman and collaborators, and others, have shown that the paramagnetic resonance line widths are unusually broad in rotationally symmetric aromatic ions compared to non symmetrical aromatic ions. See (70) and (71) for leading references. As will be evident from the following notes this issue is related to the rates of intramolecular transfer discussed above.
McLachlan and I made a study of the dynamic Jahn-Teller effect in the benzene negative ion (70). The main conclusion was that there is a dynamic JT effect, the ground state of the molecule remains degenerate in the absence of external forces. Of course this degeneracy is lifted in solution, but nonetheless there should be fluctuations in the spin distribution due to solvent fluctuations. We erroneously concluded that these fluctuations might account for the line proton hyperfine broadening in the benzene negative ion. We failed to look at the original data carefully enough, and note that the proton nuclear spin state Iz = 3 was just as broad as the other nuclear spin states, thus ruling out this contribution to the enhanced line widths. Fortunately our effort was not in vain, as it provided a theoretical basis for the contribution of spin orbit effects to the enhanced line widths.
In (71) I started with this conclusion of a degenerate JT ground state for the benzene negative ion and made a rough calculation of the spin orbit interaction that would ensue if there were transient fluctuations in solvent configuration that left the JT degeneracy intact. The electron spin relaxation effect was indeed large and left no doubt in my mind that a combination of spin orbit interactions and transient JT degeneracy were responsible for enhanced line broadening in most if not all of the symmetrical ion radicals. This line broadening does not involve the proton hyperfine interaction, as should be demonstrable using per deuterobenzene negative ion. The reader will note the parallel between these systems and those above involving intramolecular electron transfer.
The CIT to Stanford Transition, 1964
I moved from CIT to Stanford University with great reluctance; both are great institutions with strong chemistry departments and great faculty. I moved because I had a strong distaste for the LA smog prevalent at that time. Had it not been for the smog, I would have certainly stayed at CIT. (Also I was depressed by a conversation I had with a CIT trustee who said that CIT would not take an opportunity to move to Santa Barbara because “the engineering faculty would lose their consulting jobs.”) The smog situation has greatly improved in LA in recent years.
Although it was only a coincidence, my departure from CIT did coincide with the completion my main ambition. I wanted to see a simple, reliable test of the most elementary aspects of molecular electronic structure theory. Spin density distributions as measured by aromatic proton hyperfine interactions and calculated by molecular orbital and valence bond theory provided just such a test, and the results gave me much pleasure and satisfaction.
Chapter 8: Spin Labels (Stanford)
