REACTIONS OF CHARGED SUBSTRATES .2. GAS-PHASE DISSOCIATION OF 2'-SUBSTITUTED NICOTINAMIDE ARABINOSIDES

The relative abundances of ribosyl oxocarbenium ion-related cations in the gas-phase dissociation of five 2'-substituted beta-nicotinamide a rabinosides (substituents = H, OH, NH2, NAc, F) follow the Taft equati on with sigma(F). The first-order rate constants for the pH-independen t hydrolysis of these substrates follow rho(I), which is based on solu tion acidities of the same series of compounds used to define sigma(F) in the gas phase. The value of rho is much greater in solution (rho(I ) = -6.7) than in the gas phase (rho(F) = -0.75). There is direct evid ence that the NAc substrate reacts through an ion-dipole complex. Ener gy profiles were calculated in AM1; while there are some apparent anom alies in the method that can be sorted out easily, the activation enth alpies and energies of the various structures are consistent with the proposed mechanism. A plot of the AM1-calculated values of Delta H dou ble dagger for gasphase dissociation vs the log of the relative abunda nces for the respective species is linear, as is a plot of the solutio n Delta G double dagger and the gas-phase Delta H double dagger. Compa rison of solution and gas-phase results suggests that an ion-dipole co mplex is an intermediate in both phases, but that the rate-limiting st ep is different.