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Characterizing Nitrilimines with Nuclear Magnetic Resonance Spectroscopy. A Theoretical Study†
journal contribution
posted on 2008-01-17, 00:00 authored by Robert C. Mawhinney, Gilles H. Peslherbe, Heidi M. MuchallThe 13C chemical shifts in selected nitrilimines, nitriles, acetylenes, allenes, and singlet carbenes have been
calculated using density-functional theory [PBE0/6-311++G(2df,pd)] and the gauge including atomic orbital
(GIAO) method. The effects of substitution on the 13C chemical shifts in nitrilimines, R−CNN−R, have
been examined. The carbon nucleus is generally found to be deshielded by substituents in the order CH3 <
NH2 < OH < F. Comparison with nitriles, acetylenes, and allenes shows that this effect is related to the
presence of the cumulated functionality, CNN. Terminal N-substitution is found to have a larger effect
than C-substitution due to a large increase in chemical shielding anisotropy. The electronic structure of
nitrilimines has recently been shown to possess a carbene component whose resonance contribution varies
widely with substitution, and, as previously reported, insight into the electronic structure can be gained by an
analysis of the shielding tensor, especially for carbenes. Accordingly, the components of the diagonalized
13C shielding tensor for nitrilimines and stable singlet carbenes have been examined. This analysis suggests
that diaminonitrilimine, H2N−CNN−NH2, may be a stable carbene, and, to the best of our knowledge, it
would be the first acyclic, unsaturated stable carbene ever reported. Finally, a detailed analysis of the 13C
chemical shifts shows that an increase in the dipolar character of nitrilimines induces a shielding at the carbon
nucleus, while an increase in allenic or carbenic character tends to cause a deshielding.