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Engineering Neutral Organic Bases and Superbases by Computational DFT Methods - Carbonyl Polyenes

The gas-phase proton affinities and basicities of a large number of the open-chain zig-zag polyenes involving a carbonyl-functional group are examined by using B3LYP/6-311+G(d, p)//B3LYP/6-31+G(d) computational scheme. It turned out that absolute proton affinities (APAs) increased with the number of polyene subunits. The most basic site in all cases was carbonyl-oxygen. It was conclusively shown that basicity of polyenes was dramatically amplified by substitution of NMe2 and Me groups at the strategic positions on the molecular backbone. Triadic analysis revealed that the dominating effect was relaxation energy released upon formation of the molecular radical cations. Relation between the relaxation and cationic resonance effect was briefly discussed. Moreover, it was shown that the carbonyl-oxygen was more basic than the amino nitrogen of the NMe2 groups because of the larger bond association energy of the hydrogen atom attached to O compared to N atoms, in the protonated conjugate acid in question.

proton affinity; basicity; bond energy; relaxation effect; homodesmotic reactions

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