Nuclear Magnetic Resonance Study of Exchanging Systems. X. Rotational Barriers to trans-cis Isomerization in Esters

The 13C NMR spectra of t-butyl formate (1), 1,1-diethylpropyl formate (2), and 2,6-dimethylphenyl formate (3) were measured at various temperatures. It was found that the signals of the trans and cis isomers in 1 and 2 are separated at very low temperatures, and that the signal of 3 only shows a slight line broadening. The complete line shape analyses of these temperature-dependent spectra were performed. The activation energies Ea(t-c) of the isomerization from the trans isomer to the cis were determined to be 10.9 kcal/mol for 1 and 7.2 kcal/mol for 2.