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–O_ester 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.