The effect of substitution on the conformation in para-substituted ethylene glycol dibenzoate molecules

The series of ethylene glycol di-para-X-benzoates, where X = Me, OMe, CN, and NO 2 , has been studied by infrared spectroscopy and solid state CP/MAS 13 C nuclear magnetic resonance. The crystal structures were established for X = Me and OMe. Bond distances, bond angles, and torsion angles were compared to those of related molecules (X = H, Ph, Cl). It was found that the conformation of the COOCH 2 CH 2 OCO sequence is either trans-trans-trans or trans-gauche-trans. (trans = t, gauche = g). The CH 2 CH 2 bond distance ranging from 1.471(2) to 1.499(3) Å is always very short. The OCH 2 CH 2 bond angle is around 105° for the ttt conformation and 107° for the tgt conformation. It was confirmed that the infrared frequencies for the CH 2 bending, CH 2 wagging, C-C stretching, and CH 2 rocking modes are observed at 1470-1485, 1335-1340, 975-980, 845-855 cm −1 and at 1460, 1370-1375, 1040, 895-900 cm −1 for the ttt and tgt conformations respectively. The substitution effect on the nmr chemical shifts, both in solution and the solid state, can be rationalized in terms of induction and resonance contributions. On the basis of the ir and nmr spectra, it is proposed that for both the p-NO 2 and p-CN substituted molecules, for which no crystal structure has been established, the conformation of the methylenic sequence is trans-trans-trans.