STABILIZATION OF NITROGEN-CONTAINING 3-MEMBERED RINGS BY H+ AND LI+ ASSOCIATION IN THE GAS-PHASE

Ab initio calculations have been performed for three-membered nitrogen-containing rings, including all saturated and unsaturated compounds and their protonated and lithiated species. Geometry optimizations were carried out at the HF/6-31G* level and, for some suitable examples, at the MP2/6-31G* level. Protonation energies and Li+ binding energies were evaluated at the MP4/6-31+G(d,p) level including ZPVE corrections. We have found that the intrinsic basicities of saturated nitrogen-containing three-membered rings decrease as the number of basic centers increases. However, the same trend is not observed in the series of unsaturated analogues, where triazirine is predicted to be much more basic than diazirine. This finding is related to the considerable alleviation of the antiaromatic character of the former when it becomes protonated. This enhanced gas-phase basicity is observed for all of the antiaromatic compounds studied, explaining why some antiaromatic species become more stable than the nonaromatic counterparts when protonated in the gas phase. When the reference acid is Li+ this enhancement is considerably smaller, but sizable. Saturated systems with two or more basic centers also present enhanced Li+ binding energies due to the ability of Li+ to bridge between two neighbor centers bearing pairs of electrons. No stable bridged structures were found for unsaturated compounds. The Li+ derivative of triaziridine in which the alkali cation is tricoordinated is a minimum of the potential energy surface which lies about 3 kcal/mol above the global one, in which Li+ appears dicoordinated.