Ultraviolet photoelectron, electron transmission and ab initio study of the factors determining the stability of imines

Abstract

The electronic structure of several alkyl-, N-bis(trimethylsilyl)methyl-(BTMSM), and C-phenyl-substituted imines 1–11 have been determined by ab initio 3-21G* calculations. The calculations reproduce with good accuracy the available geometry of methylenimine, 1, obtained by microwave spectroscopy and ab initio 6-31G** calculations as well as the outermost energy levels experimentally determined by ultraviolet photoelectron spectroscopy for several derivatives. The analysis of the results of the calculations indicate that alkyl substitution of 1 at the nitrogen atom reduces the charge separation at the double bond increasing stabilisation, while substitution at the carbon atom has the opposite effect. Steric hindrance of the substituent(s) and the energy gaps between filled (nitrogen lone pair, HOMO and π_CN) and virtual (π_CN*) orbitals play a secondary role on stability.

All the BTMSM and phenyl derivatives studied are stable in air at room temperature irrespective of other substituents present. Our analysis indicates that these substituents change the energy, ordering and localisation properties of the valence MOs. In particular, the nitrogen lone pair orbital is no longer the HOMO.