Electron paramagnetic resonance study of imine radical cations in low-temperature solid matrices

Abstract

A series of imine radical cations has been studied by EPR spectroscopy, generated in low-temperature halogenocarbon matrices by γ-radiolysis. Radicals of the type ArCHNR^˙+[Ar = Ph or p-Me-C₆H^–₄] tend to be formed in the ²A′ state, corresponding to ionisation of the in-plane (mainly the nitrogen lone-pair) a′ orbital (I). These are isostructural with the corresponding vinyl radicals and show angular geometries at the radical centres and large couplings (ca. 85 G) to the proton trans to the nominally nitrogen-localised SOMO (I). When R = Bu^t, a 12 G long-range coupling is observed to a single proton from the Bu^t group, arising via the W-configuration (II). This coupling was found to persist in the CFCl₃ matrix up to its melting point (ca. 160 K), but was lost reversibly in the higher melting CCl₄ matrix at ca. 200 K.

When R = phenyl, the radical centre is rendered linear, because of the partial π-bonding between the aromatic ring and the nitrogen atom due to delocalisation of the unpaired electron, as shown by a fall in the parallel (¹⁴N) coupling with a concomitant increase in the nitrogen 2p/2s ratio, and by the observation of couplings to the ortho and para phenyl protons.

For imines derived from benzophenone (Ph₂CNR) the tendency is for ionisation to occur from the π-orbital (III) which has large spin densities on the para carbon atoms.

Benzophenone imine (Ph₂CNH), in addition to forming the π-state (III), was found to deprotonate, giving rise to the Ph₂CN˙ radical. Similar behaviour was shown by benzophenone oxime, which gave only the corresponding iminoxyl radical (Ph₂CN—O˙), and we propose that this is due to the dissolution of the latter two materials in the form of hydrogen-bonded molecular clusters in the non-polar freon solvent, thus facilitating deprotonation of the primary cations.