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

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 H3X-O-CHCH2 and Me3X-O-CHCH2 (X = C, Si) molecules. Hyperconjugative effects play a dominant role in the preferred stability of synperiplanar conformations of H3X-O-CHCH2 molecules. However in the bulkier Me3X-O-CHCH2 molecules these effects are overridden by steric repulsion between Me3X and CHCH2 moieties and, due to these interactions, antiperiplanar conformations with X-O-CC 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 Me3Si-O-CHCH2 conformer stability: B3LYP in agreement with experimental data predicts the rotamer with the Si-O-CC 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°.