AM1 AND PM3 CALCULATIONS ON THE EFFECT OF SUBSTITUENTS ON THE STABILITIES OF CARBOCATIONS IN THE GAS-PHASE AND IN SOLUTION

The effect of substituents on the stability of Ph(n)C(+) H(2-n)X (n = 0-2) carbocations, X = F, OH, OCH3, NH2, CH3, C6H5, CH=CH2, C=CK, CN, CHO, COOK, and NO2, has been studied with AM1 and PM3 semiempirical molecular orbital theories both in the gas phase and in solution. The results indicate that the amino group gives the most stabilization by pi-electron donation, whereas the nitro group is most destabilizing because of its strong sigma- and pi-accepting nature. Carbocation substituent stabilization energies decrease with increasing number of phenyl rings directly attached to the cation center. Electron-withdrawing substituents, (CN, CKO, and COOH) destabilize the cation center to a lesser extent than might be expected because they act as pi-electron donors when directly attached to a cation center. On going from the gas phase to solution, the stabilization energies of primary carbocations decrease, whereas for tertiary cations they increase. The secondary carbocations are of intermediate behavior.