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 arabinosides (substituents = H, OH, NH2, NAc, F) follow the Taft equation with sigma(F). The first-order rate constants for the pH-independent hydrolysis of these substrates follow rho(I), which is based on solution 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 evidence that the NAc substrate reacts through an ion-dipole complex. Energy profiles were calculated in AM1; while there are some apparent anomalies in the method that can be sorted out easily, the activation enthalpies and energies of the various structures are consistent with the proposed mechanism. A plot of the AM1-calculated values of Delta H double dagger for gasphase dissociation vs the log of the relative abundances for the respective species is linear, as is a plot of the solution Delta G double dagger and the gas-phase Delta H double dagger. Comparison of solution and gas-phase results suggests that an ion-dipole complex is an intermediate in both phases, but that the rate-limiting step is different.