CARBANIONS .2. INTRAMOLECULAR INTERACTIONS IN CARBANIONS STABILIZED BY CARBONYL, CYANO, ISOCYANO, AND NITRO GROUPS

摘要

The effect of a cyano group, an isocyano group, an aldehyde group, and a nitro group on the acidity of methane, propane, and cyclopropane has been studied via ab initio geometry optimizations at the MP2/6-31+G* theoretical level. Relative energies also were calculated using the CBS-4 and G2(MP2) theoretical models. The experimental gas phase acidities are well reproduced. The effect of substituents on the parent molecules and on the anions was examined by the use of group transfer reactions. Whereas cyclopropanecarboxaldehyde and nitrocyclopropane are about 10 kcal/mol less acidic than their isopropyl counterparts, there is little difference in acidity with the nitriles and isonitriles. The rotational barriers for the anions were calculated, allowing the total stabilization energies to be partitioned into sigma and pi components. Charge transfer from a carbanion to a substituent was examined using the Hirshfeld charges; for the substituents the charge transfer was NO2 similar to CHO > CN > NC, and for the carbanions it was CH3 similar to i-Pr > cyclopropyl. [References: 34]