TERMOLECULAR PROTON-TRANSFER REACTIONS ASSISTED BY IONIC HYDROGEN-BOND FORMATION - REACTIONS OF AROMATIC CATIONS WITH POLAR-MOLECULES

We present a new method that applies resonant-two-photon ionization to generate reactant ions selectively in the source of a high-pressure m ass spectrometer (R2PI-HPMS) for kinetic and equilibrium studies. Appl ications to reactions that would be obscured otherwise in a complex sy stem are illustrated in mixtures of benzene with polar solvent molecul es (S). We observe a novel type of proton transfer reactions from C6H6 +. to two S molecules where S=CH3CN, CH3OH, C2H5OH and CH3COOC2H5, and from C6H5CH3+. to two S molecules where S=CH3OH and C2H5OH to form pr otonated solvent S2H+ dimers. The reactions are driven by the strong h ydrogen bonds in the S2H+ dimers and therefore require the formation o f the hydrogen bond concertedly with proton transfer, to make the proc ess energetically feasible. The adducts (C6H6+.)S are observed with bl ocked solvent molecules where the subsequent switching reaction to yie ld S2H+ is slow, but not with alcohol reactants that can form hydrogen -bonded chains that facilitate fast subsequent proton extraction. Corr espondingly, kinetic simulations suggest that the mechanism proceeds t hrough ((C6H6S)-S-+.+S-->S2H++C6H5. and C6H6+.+2S-->S2H++C6H5. reactio ns, respectively. The rate coefficients of these reactions are in the range 10(-13)-10(-12) cm(3) s(-1) for the reaction through a bimolecul ar switching channel and in the range 10(-26)-10(-28) cm(6) s(-1) for reaction through a direct termolecular proton extraction mechanism. Th e relation to energetics and reactant structure is examined. (C) 1996 American Institute of Physics.