14N Tensors Research Articles

ChemInform Abstract: Direct EPR Evidence for Si-N dπ-pπ Bonding in Silylamine Radical Cations

A series of amine radical cations with α-silyl substitutents has been studied by EPR spectroscopy, following γ-radiolysis of dilute solutions of the parent amines in CFCl3 at 77 K. It is clear that the nitrogen centred singly occupied molecular orbitals (SOMOs) are delocalised and extend appreciably onto the silyl groups, as shown by a fall in the magnitude of the anisotropic 14N couplings from those found in alkylamine radical cations, and thus the concept of Si–N dπ–pπ bonding in silylamines is strongly supported. The results contrast with prior conclusions for the isoelectronic alkyl radicals i.e. the extent of spin delocalisation onto the silyl group is negligible.Additionally, there is considerable variation in the effective McConnell QH-values throughout the series, and we propose that this is due to variations in the electron charge on the nitrogen atom centres.The radical cation of tetrakis(trimethylsilyl)hydrazine, (Me3Si)2N–N(SiMe3)2˙+, was found to adopt a twisted geometry with an angle between the two 14N tensors [and hence the N(2pz) orbitals] of <24°. This is essentially in accord with the conclusions drawn from a liquid phase study of this cation by Bock et al., but the twist angle is smaller than their estimate of 45°. This indicates that the N–N torsional potential may be quite shallow so that the cation is undergoing torsional averaging at the higher temperature used in the liquid state study.

An EPR and theoretical study of radicals formed from tetraalkylsulfamides [(R2N)2SO2

A series of sulfamides has been synthesised and their primary radical cations studied by EPR spectroscopy following γ-radiolysis in low-temperature freon matrices. These species are readily bleached by visible light (>570 nm), to give secondary radicals, the nature of which varies with the sulfamide used. The 14N hyperfine data show that the SOMO comprises both nitrogen centres and furthermore that the 14N tensors are parallel. This implies that a distortion of the sulfamide system occurs on ionisation, since the neutral species (at least for sulfamide and its tetramethyl derivative) adopt bisected structures with an appreciable angle between the nitrogen 2p orbitals. MO calculations indicate that the obvious possibility of a twisting of the N-centres through 90° to form regular ‘allylic ’structures is disfavoured energetically over the retention of the basic bisected geometry, and we suggest therefore that it is a ‘bent-bonding ’distortion that operates to render the N-orbital axes parallel, in accord with the EPR data. Additionally, we have studied radicals formed by radiolysis of the pure sulfamides at 77 K, and conclude that much of the radiation chemistry in these pure systems is driven by electron capture events: these are not possible in the freon media which are themselves efficient electron capture materials, and thus remove them from the reaction system.