Methyl rotational barriers in the E-forms of methyl esters

Methyl nitrite has a normal 2.09 kcal/mol methyl rotation barrier in the Z-conformer, but only a small 0.03-0.06 kcal/mol barrier in the E-conformer as determined from microwave spectra (Turner et al., J. Phys. Chem., 83 (1979) 1473). Using dependable calculational methods, we show that this is a general result which carries over to another E-methyl ester, E-methyl formate, which does not have a sterically large substituent on the acid group, and is not specific to methyl nitrite alone. The small methyl barriers in E-methyl nitrite and E-methyl formate result from the absence of large steric interactions. Replacing H in the formyl group by sterically larger F, results in a normal O-methyl torsional barrier calculated to be 1.8 kcal/mol in the E-methyl fluoroformate.