The carbonyl group frequency. Part VI. Alkyl carbonates

Abstract

Calculations for a simplified model suggest that the similarity in carbonyl frequencies for carbonate and carboxylate esters arises from compensating changes in k_c0 and other vibration parameters. A revised assignment for the in-plane skeletal bands of dimethyl carbonate is proposed. The carbonyl frequencies of di-methyl, -ethyl, and -isopropyl carbonate show regular substituent and solvent effects; variation in the values for di-t-butyl carbonate accords with the s-cis–s-trans-form (3) being more stable in polar solvents. The high group frequency and solvent sensitivity shown by propylene carbonate is attributed to the effects of electronic repulsions in the s-trans–s-trans-structure.