The molecular structures of styrene and 4-bromostyrene and the deuterium quadrupole coupling constants of styrene-d₈ determined from the proton and deuterium magnetic resonance spectra of nematic solutions

Abstract

The proton spectra of styrene and 4-bromostyrene as solutes in liquid crystalline solvents have been analysed to yield sets of dipolar coupling constants. It is found that for both molecules the dipolar couplings are consistent with planar ground state structures only if the torsional motion of lowest frequency occurs at about 80 cm^-1, in contrast to values of around 40 cm^-1 assigned to this mode by other methods. The structure of the olefin fragment, assuming rr _CH bond lengths of 1.08±0.01 Å, has r(CC) = 1.31 ± 0.06 Å (styrene) and 1.32 ± 0.06 Å (bromostyrene) and CCC bond angles of 136 ± 6° (styrene) and 134 ± 6° (bromostyrene). The deuterium spectrum of styrene-d ₈ has been analysed under the same conditions of concentration and temperature as the proton spectrum and the results used to derive the quadrupole coupling constant for the aromatic CD bonds as 186 ± 2 kHz with an asymmetry parameter of 0.03 ± 0.03. These same properties for the olefinic CD bonds are derived to be 181 ± 9 kHz and -0.01±0.03 respectively.