Molecular energy levels of azines; ab initio calculations and correlation with photoelectron spectroscopy

Abstract

Non-empirical calculations using a standard gaussian atomic orbital basis (LCGO) have been carried out for known and unknown azines. The wavefunctions show pseudo D_6h symmetry and have been correlated with those of benzene; this correlation has been used to interpret the combinations of ‘lone pair’ orbitals that occur. In the case of the tetrazines the splitting of the lone pair levels leads to extensive overlap with the CH region of energy so that mixing occurs and 1,2,4,5-tetrazine shows five orbitals which can be described as ‘lone pair orbitals’. The orbital energy ordering is effectively identical with that obtained from recent double ζ calculations on a few of the molecules; these energies correlate well with photoelectron spectra and obey the relationship IP_obs= 0·790IP_calc. Correlations using CNDO/2, INDO, and the Extended Huckel Method (EHM) are less satisfactory both in grouping of the orbitals and absolute energies. The LCGO band assignments are generally in good agreement with those made by Lindholm et al. based upon the EHM. The thermodynamic stability of the azines with respect to fragmentation is discussed; the difference in energies between the classical alternating bond lengths and the observed ones for selected examples are in the region 6–10 kcal mol^–1.