Structure and vibrational spectra of vinyl ether conformers. The comparison of B3LYP and MP2 predictions

Abstract

Two rotamers of vinyl ethers are distinctively characterized by experimental methods despite their small difference in energy, but their order of stability as well as its rationalization is still the problem of discussion. In this communication we report the application of popular B3LYP and MP2 methods with different basis sets (up to aug-cc-pVTZ) for solving this problem for H 3X–O–CH CH 2 and Me 3X–O–CH CH 2 (X = C, Si) molecules. Hyperconjugative effects play a dominant role in the preferred stability of synperiplanar conformations of H 3X–O–CH CH 2 molecules. However in the bulkier Me 3X–O–CH CH 2 molecules these effects are overridden by steric repulsion between Me 3X and CH CH 2 moieties and, due to these interactions, antiperiplanar conformations with X–O–C C torsional angle equal or close to 180° become more stable. The difference in the description of these interactions, which exists between B3LYP and MP2, results in the contrary predictions for the Me 3Si–O–CH CH 2 conformer stability: B3LYP in agreement with experimental data predicts the rotamer with the Si–O–C C torsional angle ( ϕ) close to 180° to be the most stable one, while in MP2 the 0° rotamer appears to be the main conformer. The first theoretical prediction of the vibrational spectra of trimethylsilyl vinyl isomers allowed us to assign the observed bands to a particular isomer and to confirm the previous conclusion that the main isomer of this molecule has the structure with ϕ close to 180°.