Matrix isolation FTIR and molecular orbital study of E and Z acetaldoxime monomers

The molecular structures and vibrational spectra of the monomers of the Z and E acetaldoxime isomers were investigated by ab initio MP2/6-311++G** and DFT (B3LYP) /6-311++G** calculations. Experimentally, acetaldoxime was studied using FTIR spectroscopy combined with the technique of isolation in low temperature argon matrixes. Significant dependence of the ratio of E and Z isomers in the matrix on the method of sample preparation was observed. Matrixes with high predominance of the Z form were deposited, as well as such with dominating E form or with both forms in nearly the same amount. That allowed separation of the infrared spectra of both isomers. An assignment of the vibrational spectra of the two isomeric forms was based on comparison with the spectra theoretically calculated at the DFT and MP2 levels. The vibrational data were correlated with some important structural parameters, in particular the different methyl torsional barriers present in the two isomers.