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

Conformational preferences of esters of 2,2,5,7,8-pentamethylchroman-6-ol w ere studied by means of dynamic C-13 NMR in solution and the CP MAS techniq ue in the solid phase. An increase in the rigidity of ester molecules in co mparison with free chroman-6-ol was observed. The coalescence of the signal s of the gem-dimethyl group was monitored and discussed in terms of hindere d rotation around the C-O-ester bond; the dynamic parameters (k, Delta G do uble dagger) 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 conforma tions with the carbonyl group of the ester substituent located below and ab ove the plane of the aromatic ring.