33S nuclear quadrupole resonance study of dibenzyl disulfide toward understanding of cross-linked structures in rubber

Experimental and theoretical investigations of a sulfur-33 electric-field-gradient (EFG) tensor of disulfide bonds in 33S-labled dibenzyl disulfide have been presented. Temperature dependence of quadrupolar frequencies, νQ, is observed in the temperature range between 80 and 280 K, in which single peaks appear in all the 33S nuclear quadrupole resonance (NQR) spectra. Analysis of nutation echo 33S NQR spectra at 200 K yields the quadrupolar coupling constant, CQ value, of 46.8(6) MHz and the asymmetry parameter, ηQ, of 0.48(7). The orientation of the 33S EFG tensor of the disulfide is obtained by quantum chemical calculations: the largest EFG tensor component, VZZ, is approximately perpendicular to the molecular plane (C-S-S), and the smallest component, VXX, is approximately 41° off the C-S bond. Extensive quantum chemical calculations are systematically performed to investigate dependences of 33S EFG tensors on changes of torsion angles in disulfide and trisulfide bonds, indicating that analysis of νQ and CQ values potentially makes it possible to assign the secondary structures of cross-linking in a rubber.