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Cooperative intramolecular hydrogen bonding effect and basicity - An ab initio and DFT study of the superbasic properties of N-dimethylaminoalkyl-2, 3-diamino-cyclopropeneimines

The MP2(fc)/6-311+G**//HF/6-31G* and B3LYP/6-311+G**//HF/6-31G* calculations show that N-dimethylaminopropyl and N-dimethylaminobutyl derivatives of 2, 3-diamino-cyclopropeneimine (CPI) possess high gas phase proton affinities and pronounced basicities in acetonitrile (MeCN). Tris-(N-dimethylaminopropyl(butyl))-CPI derivatives achieve superbasic PA and pKa values. The reason behind these remarkable features is stability of the resulting conjugate acids. They are stabilized by aromatization of the three-membered ring accompanied by the cationic resonance between cyclopropyl ring and amino nitrogens of the side chains, and, last but not least, by the cooperative multiple intramolecular hydrogen bonding effect. The latter is particularly enhanced in tris-substituted CPIs. It is found that N-dimathylaminobutyl side chains contribute more to the stability of conjugate acids then the corresponding N-dimethylaminopropyl ones, because they undergo a somewhat larger relaxation upon protonation. The studied compounds might be helpful in building up a dense ladder of strong bases and superbases, which could contribute to a development of the chemistry of superacids and superbases.

neutral organic superbases; superbasicity; proton affinities; hydrogen bonding; aromaticity

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