tokens: can form a favourable overlap elsewhere on the backbone than at C2. Some electronic redistribution is required so it may not be as favourable for H to attach to C3 or N as it is at C2. This can account for the lower stability of the ketenimine product and the observed ∼9:1 ratio of the two products shown in Table 1 . We explored these two pathways in the related but simpler system H 2 CCN + H. Replacing the methyl group of CH 3 CHCN with H should not greatly affect the energetics of H-addition. Like CH 3 CHCN, the H 2 CCN radical shows resonance behaviour between two forms, as indicated by radical character on all three heavy atoms. For H 2 C1-C2-N, the Mulliken spin populations at the UMP2/aug-cc-pVTZ level on C1, C2, and N are 1.09, −0.64. and 0.65, respectively, which are quite similar to the spin densities on the analogous atoms in CH 3 -CH-CN above (there are also minor amounts of spin density on the two H atoms). We might therefore expect to form both CH 3 CN (methyl cyanide) and H 2 CCNH (ketenimine) by adding H to C1 and N of H 2 CCN, respectively. To test the likelihood of forming both products, we ran additional calculations. These calculations required a multireference treatment since closed-shell singlet molecules are being formed from open-shell doublet reactants. At the MRCI + Q/aug-cc-pVTZ level, we found that neither addition involves a reaction barrier. The issue thus becomes trying to understand why there is a preference for the cyanide product over the ketenimine product. With no barriers for either addition, the favourability of one product over the other must be due to the specific character of the full reaction surface. In addition of H to CH 3 CHCN, ethyl cyanide is more stable than the ketenimine product, and the same is true when adding H to H 2 CCN: methyl cyanide is more stable than ketenimine. The first channel has a ‘wider’ entrance on the potential energy surface; it is deeper and extends further out from C1. This is likely what favours cyanide formation over ketenimine formation in these reaction systems. To gain further insight into the ratio of the two products, we note that the study of delocalized radicals containing nitrogen atoms has been theoretically investigated in the case of the simplest of the cyanoalkyl radicals, CH 2 CN. Delbecq ( 1983 ) has shown using valence-bond analysis that cyanomethyl radical CH 2 CN has two different limit structures, C°H 2 CN and CH 2 = C = N° with relative weights of 83 per cent and 14 per cent, respectively. The H 2 CN π-delocalized structure was found with a low weight of the order of 3 per cent. Similarly, under our experimental conditions, the formation of CH 3 − C°H − C≡N through CH 2 = CH − C≡N + H reaction, would lead as shown in Fig. 8 , to the formation of a second localized form with ketenimine character CH 3 − CH = C = N° that explains the possibility of reaction at the nitrogen end to form CH 3 CH = C = NH. Figure 8. Two different limit structures and one
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