Intracluster Proton‐Transfer between Methylsubstituted Benzene Cations and Polar Solvent Molecules Studied by Resonant Two‐Photon Ionization

Abstract

Intraclustcr proton-transfc~was~bserved for mixed clusters of toluene and p-xylene with polar solvent molecules (NH 3 , CH 30H, 0 20) after~esonant two-photon IOmza~lOn.of the aromatic moiety. The transfer is deduced from the appearance of protonated solvent clusters, showing the R2PI spectra of their mixed cluster precursors. This process, observable only in combination with subsequent dissociation is cluster size dependent. The minimum number of solvent molecules necessary for a transfer to take place depends on the solvent's proton solvation energy.

The Structure of Microsolvated Benzene Derivatives and the Role of Aromatic Substituents

“…These solvation- and excitation/ionization-dependent acid−base reactions represent under conditions of discrete microsolvation the cluster analogy of condensed-phase acid−base equilibria in solvents of different basicity. Sato and Mikami 167 observed a size-dependent intracluster dissociative proton transfer (dPT) in PH + ·W n similar to that reported first for TO + ·W n . , While dPT from TO + to the water subclusters necessitates three water molecules, with PH + it sets in with four water molecules. The threshold size was latter confirmed …”

“…Due to the obstacles mentioned above, the R2PI spectra of product ions of quantitatively reacting or fragmenting clusters are in general not assignable. Exceptions are the fingerprints of clusters, which react in a particular manner, such as the cations of microsolvated toluene . Here the ionized chromophore may stabilize to the benzyl radical by quantitatively transferring a proton to the solvent moiety of a certain minimum size.…”

“…For toluene + (TO + ) and p -xylene + (PX + ) one observes a dissociative proton transfer (dPT) to the solvent moiety. The only precondition for this reaction to occur is that the proton affinity of the latter is larger than that of the conjugated bases of the cations, i.e., the benzyl or the methylbenzyl radicals. ,, …”

“…Exceptions are the fingerprints of clusters, which react in a particular manner, such as the cations of microsolvated toluene. 71 Here the ionized chromophore may stabilize to the benzyl radical by quantitatively transferring a proton to the solvent moiety of a certain minimum size. If this intracluster proton transfer is exothermic and induces the dissociation of the solvent moiety from the protondonating cation, without fragmenting the protonated species itself, the latter is detected intact in the mass spectrometer.…”

“…If this intracluster proton transfer is exothermic and induces the dissociation of the solvent moiety from the protondonating cation, without fragmenting the protonated species itself, the latter is detected intact in the mass spectrometer. Early studies of such dissociative protontransfer reactions (dPT) 71,72 substantiated that protic solvent molecules form subclusters attached to the aromatic chromophore. The predominance of subcluster formation for protic solvent molecules is a consequence of the different strength and nature of the intermolecular bonds.…”