13C NMR studies of conformational dynamics in 2,2,5,7,8-pentamethylchroman-6-ol derivatives in solution and the solid state

Abstract

Conformational preferences of esters of 2,2,5,7,8-pentamethylchroman-6-ol were studied by means of dynamic 13C NMR in solution and the CP MAS technique in the solid phase. An increase in the rigidity of ester molecules in comparison with free chroman-6-ol was observed. The coalescence of the signals of the gem-dimethyl group was monitored and discussed in terms of hindered rotation around the C–Oester bond; the dynamic parameters (k, ΔG‡) were determined. The barrier is determined by the interplay of steric and electronic effects of substituents in the ester fragment. GIAO-CPHF MO calculations of shielding constants were performed for the conformations with the carbonyl group of the ester substituent located below and above the plane of the aromatic ring.