Termolecular proton transfer reactions assisted by ionic hydrogen bond formation: Reactions of aromatic cations with polar molecules

Abstract

We present a new method that applies resonant-two-photon ionization to generate reactant ions selectively in the source of a high-pressure mass spectrometer (R2PI-HPMS) for kinetic and equilibrium studies. Applications to reactions that would be obscured otherwise in a complex system are illustrated in mixtures of benzene with polar solvent molecules (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 protonated solvent S2H+ dimers. The reactions are driven by the strong hydrogen bonds in the S2H+ dimers and therefore require the formation of the hydrogen bond concertedly with proton transfer, to make the process energetically feasible. The adducts (C6H6+•)S are observed with blocked solvent molecules where the subsequent switching reaction to yield S2H+ is slow, but not with alcohol reactants that can form hydrogen-bonded chains that facilitate fast subsequent proton extraction. Correspondingly, kinetic simulations suggest that the mechanism proceeds through (C6H6+•)S+S→S2H++C6H5• and C6H6+•+2S→S2H++C6H5• reactions, respectively. The rate coefficients of these reactions are in the range 10−13−10−12 cm3 s−1 for the reaction through a bimolecular switching channel and in the range 10−26−10−28 cm6 s−1 for reaction through a direct termolecular proton extraction mechanism. The relation to energetics and reactant structure is examined.

Protonation of Naphthalene–(Water)n Nanoclusters: Intracluster Proton Transfer to Hydration Shell Revealed by Infrared Photodissociation Spectroscopy

“…Protonated aromatic molecules are ubiquitous in (bio)­chemical environments, ranging from biology, organic chemistry, and combustion to astrochemistry. − They often appear as reactive intermediates in chemical reactions . At physiological pH, many proteins and enzymes exist in their protonated form to ensure their function and catalytic activity. − Because hydration is crucial for their stability and reactivity, “interfacial” or “biological” water is nowadays considered as an integral part of these bioactive molecules. − The hydration shell surrounding these molecules often acts as a proton transfer or proton pump medium through the formation of an extended hydrogen-bonded (H-bonded) network. ,− Such intermolecular proton transfer (PT) reactions often control various biological phenomena, and examples include respiration, photosynthesis, the photostability of DNA bases, ion transport, and hole hopping processes through DNA. − Electronic excitation or ionization of the donor–acceptor partners frequently fosters the PT reaction. , Detailed insight into the structure and energetics of such intermolecular PT at the molecular level has been provided by spectroscopy of size-selected microhydrated aromatic clusters in the gas phase. Depending on the proton affinity (PA) and the solvation energy (or complexation energy) of the aromatic compound and the water cluster, the excess proton is attached either to the arene or to the solvent.…”

The Deprotonation of Benzyl Alcohol Radical Cations: A Mechanistic Dichotomy in the Gas Phase as in Solution

“…[5,10] A notable exception to the expectedly higher kinetic acidity of the toluene radical cation with respect to the benzene radical cation has been reported recently for the termolecular proton transfer to two polar molecules; this reaction has been ascribed, however, to the association of the polar molecules with the aromatic ring within the reaction complexes. [11] The question is then left as to which sites are involved in the deprotonation process; among these sites are the conceivable a-Cand O-hydrogens in the parent benzyl alcohol radical cation, but this leads to the problem of ascertaining the kinetic and thermodynamic features of the processes involving the two individual sites. The problem can be approached in a stepwise fashion, starting from species where only one acidic site may come into play.…”

Kinetics of ion-induced nucleation in a vapor-gas mixture

“…Note that for homogeneous nucleation ͑i.e., n i =0͒ the above derivation gives C Ϸ n, as in the classical theory for homogeneous nucleation. Mass spectrometric investigation of the initial ionmolecule clustering steps 44 shows that for reaction of benzene ions ͑C 6 H 6 + ͒ with methanol molecules at room temperature, ␤⌬⌽͑1,q͒ϳ8. 45 This suggests that the approximation of the normalization coefficient used in Eq.…”