E.P.R. and zero-field odmr studies of the T₁ states of halogen-containing benzonitriles, anisoles and anilines

Abstract

The magnetic and dynamic properties of bromo and chloro substituted benzonitriles, anisoles and anilines were studied by means of E.P.R. and zero-field odmr at 1·2–4·2 K.

Single-crystal E.P.R. studies showed that the zero-field splitting parameter D of bromo substituted molecules are positive as in other aromatic molecules. It was also shown that para bromo substitution depresses the energies of the T_x and T_y sublevels preferentially. This indicates that the second-order effect of the spin-orbit mixing of the T ₁ state with the ³ nπ* state is the most important in affecting the zero-field splitting.

Single-crystal E.P.R. spectra of 4-bromobenzonitrile and 4-bromoanisole gave resolved hyperfine splittings at certain orientations. The analysis of the in-plane hyperfine structure of 4-bromobenzonitrile gives A_zz = 16 G and A_yy = 13 G, which are considerably different from the values expected from the spin densities in benzonitrile and the ⁸¹Br hyperfine coupling of α-bromomaleic acid. The large value of A_xx indicates considerable delocalization of the π electron onto the bromine atom. Hyperfine data were also used to ascertain the zero-field sublevel schemes and the directions of the principal axes of the zero-field tensor.

It was found that the T_z sublevel is radiatively active at the electronic origin in most of the systems studied here. Possible causes of this activity are considered. In particular, mixing of the ³nπ* and ¹nπ* states with the T ₁ state is suggested as the possible cause of the radiative activity of the T_z sublevel.