Quadrupole relaxation and asymmetric linewidth effects in the e.s.r. spectra of a bromoacyl nitroxide

Abstract

The room temperature e.s.r. spectrum of the bromoacyl nitroxide BrC(O)N(O)Bu^t shows hyperfine coupling from both nitrogen and bromine. However, as the temperature is lowered, the bromine quartet collapses and at –60°C a three line spectrum is observed. At still lower temperatures there is pronounced asymmetry in the widths of the ¹⁴N triplet. These temperature dependent effects have been analysed in terms of appropriate theories of spin relaxation and by computer simulation. The ⁷⁹Br quadrupole coupling constant is estimated to be 268 MHz. INDO calculations have been performed on the model halogenoacyl nitroxide ClC(O)N(O)CH₃ which is predicted to prefer a planar trans-conformation.