Proton hyperfine structure in the electron spin resonance spectrum of the acetonitrile dimer radical anion

Abstract

The e.s.r. spectrum of (CH₃CN)^–₂· has been observed in a γ-irradiated single crystal of acetonitrile at 77 K. At a crystal orientation for which the spectra from magnetically distinguishable sites coalesce, the spectrum consists of a well-resolved hyperfine structure arising from a ¹⁴N quintet of ¹H septets, typical coupling constants being A(¹⁴N)= 0.85 mT and A(¹H)= 0.21 mT. This spectrum saturates readily with increasing microwave power and is photobleachable by red light, these effects paralleling those observed for the ¹⁴N quintet spectrum of (CD₃CN)^–₂·. The lack of spectral resolution at other orientations is attributable to interference by site splitting and to a decrease in the proton coupling as the nitrogen coupling increases to its maximum value in the rotation plane. Confirmation of the latter observation has been obtained by similar studies on CH₂DCN and CHD₂CN. Both the magnitude of the proton coupling and its relationship to the nitrogen coupling are shown to be consistent with information on the structure and spin density distribution derived from studies of the dimer radical anion in deuterated acetonitriles.