Theoretical studies on the iminium ion-forming reaction

Abstract

The iminium ion forming reactions of YC6H4NHCH2NH3+(I) and the corresponding carbon analog, YC6H4CH2CH2NH3+ (II) were studied theoretically at the MP2/6–31G*//RHF/6–31G* level. The iminium ion-forming reactions are favored mainly due to a large amount of electron donation (0.05–0.06e) from the nitrogen to the antibonding orbital of the cleaving bond, nN σ*(r3), by a first-neighbor vicinal charge-transfer interaction. At ca 50% progress of reaction, i.e. bond cleavage in the SN1 transition state (TS), bond contraction of the geminal bond (r2) is greater by ca 10% (60%), which is also greater than bond stretching of a vicinal bond (50%). The reaction through the imbalanced TS provides the minimum energy path in accordance with the basic laws of nature. The Hammett ρ values for the iminium ion-forming activation and equilibrium processes are ca −1.3 and −4.3, respectively. The negative activation energies for the SN2 processes with I in the gas phase are consistent with the experimental direct nucleophilic displacement by a strong nucleophile at the iminium ion-forming substrate in water. Copyright © 1999 John Wiley & Sons, Ltd.