Estimation of the relative stabilities of the E,Z-isomers of alpha,beta-dialkylsubstituted methyl vinyl ethers by various computational methods

The relative thermodynamic stabilities (relative enthalpies) of the E,Z-iso mers of alpha,beta-dialkylsubstituted methyl vinyl ethers MeOC(R-1)=CHR2 ha ve been estimated by various computational methods including molecular mech anics, semiempirical, ab initio, and DFT calculations. The best performance , approaching the accuracy of the experimental method of chemical equilibra tion, is shown by the DFT calculations. Ab initio methods, provided that el ectron correlation is taken into account, an also satisfactory, but clearly less successful than the DPT calculations. The reliability of the semiempi rical methods AM1 and PM3 is considerably less good, and varies in an unpre dictable manner from case to case. The poorest general performance is shown by the MM2 and MM3 calculations, which may overestimate the relative stabi lity of the Z isomer by as much as 18 kJ mol(-1).