| Abstract
| - The gas-phase basicities (GBs) of nornicotine, nicotine, and model pyrrolidines have beenmeasured by FT-ICR. These experimental GBs are compared with those calculated (for the two sites ofprotonation in the case of nicotine and nornicotine) at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level,or those estimated from substituent effects on the GBs of 2-substituted pyrrolidines, 2-substitutedN-methylpyrrolidines, and 3-substituted pyridines. It is found that, in contrast to the Nsp3 protonation inwater, in the gas phase nornicotine is protonated on the pyridine nitrogen, because the effects of anintramolecular CH···Nsp3 hydrogen bond and of the polarizability of the 3-(pyrrolidin-2-yl) substituent addup on the Nsp2 basicity, while the polarizability effect of the 2-(3-pyridyl) substituent on the Nsp3 basicityis canceled by its field/inductive electron-withdrawing effect. The same structural effects operate on theNsp3 and Nsp2 basicities of nicotine, but here, the polarizability effect of the methyl group puts the pyrrolidinenitrogen basicity very close to that of pyridine. Consequently, protonated nicotine is a mixture of the Nsp3and Nsp2 protonated forms.
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