ESR, 1H, and 2D Studies of the Radical Anion of 1,3,5-Triphenylbenzene

Abstract

MO calculations predict that the monoanion of 1,3,5-triphmylbenzene (Tpb) is in an orbitally degenerate ground state. Nevertheless, the ESR spectrum in liquid NHj is well resolved; the derivative linewidth is equal to 0.06 Oe only. The g value and the electronic relaxation times Ti and T-t were comparable in magnitude to those of orbitally nondegenerate radicals. It is suggested that twisting of the phenyl rings out of the plane of the molecule, which may lift the orbital degeneracy, is the principal reason of this behavior. 'H and ZD N M R experiments were performed on a reduced sample of Tpb, containing both normal and perdeutero-Tpb. From the Fermi contact shifts signs and magnitudes of all hyperfine splitting constants have been determined, and they agreed very well with the ESR splitting constants. From the proton linewidths the electronic and dipolar correlation times were inferred. Quadrupole coupling constants for all deuterium nuclei were calculated from the 2D linewidths.

Electron spin distribution and molecular dynamics of aromatic hydrocarbon radical anions studied by1H and2D N.M.R.

“…For our experiments the correlation times are of the order of 10-11-10-1~ s [2,3,5], whereas w e is approximately 3 • 1011 rad/s. Hence the various relaxation times will be proportional to the corresponding correlation times.…”

“…If the molecular orbitals (MO) of the radical anion are expressed as a linear combination of the set of carbon 2p.-AO's{Xm}, then the following relation holds [3] :…”

“…T2-a(intramolecular) = T2 ' ~0-1 + T2 ' Dip--1 + T2 ' Q-1 (6) For a nucleus in a radical subjected to rapid spin exchange and rapid isotropic rotational diffusion the following relations can be derived [3,20] :…”

“…spectra of these radicals [1][2][3]. The fast relaxation of the electron spin produces simple spectra, which are comparable to the high power ENDOR spectra.…”

“…and ENDOR measurements [4] usually yield only the absolute value of the hfsc. In previous papers [2,3,5] we pointed out that the aH and 2D linewidths in the N.M.R. spectra of alkali radical ion pairs contain valuable information about the electronic structure of these ion pairs and about the dynamics of these species in ethereal solution.…”