Geometry and electronic structure of model small nitrones and their tautomers

Ab initio MO studies have been performed on the parent nitrone [CH2 = N(H)O], its tautomers and methyl derivatives. The effect of non-dynamical electron correlation has been taken into account using the CASSCF wavefunction. Different kinds of excitation patterns have been applied in the (2,2) to (12,12) range in order to appreciate the effect of proper orbital selection. Not only the pi (a '') orbitals were found to be essential for a reliable description of nitrone molecular properties but also certain sigma (a') ones have also been shown to play an important role. Calculating the Mayer's bond indices and performing localization studies revealed four fully formed covalent bond and a fifth one which could be regarded either as a covalent bond polarised into the N --> O bond direction or as a partially back-donated lone pair into the O --> N direction. Comparing the calculated geometrical parameters of the parent nitrone to those of its oxime and nitroso tautomers (using several methods from HF/6-31G to CCSD(T)/TZP and MR-CISD/6-311G* levels of theory) also points to this direction. Since from CASSCF studies and natural orbital analyses the biradical contribution was found to be small, the electronic structure of the nitrone can be regarded as intermediate between the hypervalent and zwitterionic forms.