Distonic radical cations : Guidelines for the assessment of their stability

Abstract

Ab initio molecular orbital calculations on the distonic radical cations CH 2(CH 2) nN +H 3 and their conventional isomers CH 3(CH 2) nNH 2+ ( n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH 3·H 2+CH 3 +NH 3 (representing the distonic isomer) and CH 3CH 3+CH 3NH 2+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH 3·Y+CH 3X +H and CH 3YH+CH 3X +. (or CH 3YH +. + CH 3X) to predict the relative energies of the distonic ions ·Y(CH 2) nX +H and their conventional isomers HY(CH 2) nX +. (X = NH 2, OH, F, PH 2, SH, Cl; Y = CH 2, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.