ESR investigation of the nitrobenzene anion radical in single crystals of benzoate salts

Abstract

ESR spectra have been obtained and interpreted for the nitrobenzene anion radical in single crystals of two benzoate salts. In trimethylphenylammonium benzoate, [(CH3)3C6H5N]+[C6H5CO2]−, there is a single magnetically unique site per unit cell, so reasonably simple spectra result. Principal values for the g tensor and the hyperfine tensors of N and the ortho and para protons are reported. Splitting by the meta protons was not resolved. The single crystal ESR data for the nitrobenzene anion radical doped into benzoate sites in this hosts, together with the unit cell dimensions and symmtery obtained from single crystal x-ray measurements on the host enabled a partial crystal structure determination. The crystals are monoclinic with four formula units per unit cell and are of C2/c symmetry with the four benzoate anions lying in special positions on a twofold axis such that the molecular twofold axis of each benzoate anion points along the b axis. Each of the four trimethylphenylammonium cations is likewise constrained under C2/c symmetry to possess either twofold or centrosymmetric symmetry, which necessitates an orientational disorder (not uncommon for tetraalkylammonium cations). In lithium benzoate–dimethylsulfloxide there are four magnetically inequivalent sites in the unit cell. The crystal structure of this host was determined in order to evaluate the principal values for the g tensor and the hyperfine tensors by spectral simulation. Eight formula units crystalize in a monoclinic unit cell P21/c. Two crystallographically distinct complexes were found. In one, the benzoate anion is essentially planar, whereas in the other complex, the CO2 group of the benzoate anion is twisted 8.2° with respect to the benzene ring. Although the chemical environment of the two crystallographically independent benzoate anions is similar, packing and Coulombic forces on the oxygen atom of one of the CO2 groups apparently twist this group out of the plane of the benzene ring. A most interesting feature of this structure is that, instead of the expected three-dimensional array of ions, there exist polymer chains of composition [Li+(C6H5CO2)−⋅ (CH3)2SO]∞ which are propagated by inversion symmetry in the direction of the b axis. The spacing between polymer chains is that characteristic of van der Waals forces, hence the packing of this benzoate salt behaves essentially as an ionic crystal in one dimension, but as a molecular crystal in the other two dimensions. Principal values and axes of the g tensor and the hyperfine tensors are also reported for the nitrobenzene anion radical in this host. In both hosts the para proton hyperfine tensor was found to be that expected for an isolated R2C–H fragment, whereas the ortho proton hyperfine tensor is twisted about 20° with respect to the C–H bond. The data are consistent with a preferred planar geometry for the nitrobenzene anion. A new method for incorporating electrochemically generated ion radicals, which are difficult to isolate, into single crystals of diamagnetic host suitable for ESR studies is also reported.