Stepwise hydration and multibody deprotonation with steep negative temperature dependence in the benzene.+ -water system

We studied the stepwise hydration and solvent-mediated deprotonation of the benzene*+ cation (Bz*+) and found several unusual features. The solvent binding energies DeltaH on-1,n for the reactions Bz*+(H2O)n-1 + H2O --> Bz*+(H2O)n are nearly constant at 9 +/- 1 kcal mol-1 for n = 1 to 8. We observed a remarkable sudden decrease in the entropy of association accompanying the formation of Bz*+(H2O)7 and Bz*+(H2O)8, indicating strong orientational restraint in the hydration shells of these clusters consistent with the formation of cagelike structures. We observed the size-dependent deprotonation of Bz*+ in a cooperative multibody process, where n H2O molecules (n >/= 4) can remove a proton from Bz*+ to form protonated water clusters. We measured, for the first time, the temperature dependence of such a process and found a negative temperature coefficient of a magnitude unprecedented in any chemical reaction, of the form k = AT-67+/- 4, or in an Arrhenius form having an activation energy of -34 +/- 1 kcal mol-1. The temperature effect may be explained by Bz*+ and four H2O molecules needing to be assembled from gas-phase components to form the reactive species. Such large temperature effects may be therefore general in solvent cluster-mediated reactions.     

Reactions of CH3OCH2+ with hydrocarbons and O,N, and S compounds: applications for chemical ionization in selected ion flow tube studies

We report the results of a flowing afterglow ion source-selected ion flow tube study (FA-SIFT) of the reactions of the methoxymethyl cation, CH3OCH2+. Rate coefficients and product branching ratios are reported for twenty nine reagent molecules including those that constitute the major ingredients of air, the hydrocarbons CH4, C2H6, C3H8, n-C4H10, C2H2, C2H4, C3H4 (allene and propyne), C6H6, and the S-containing molecules H2S, CH3SH, C2H5SH, (CH3)2SH, and (C2H5)2SH. In addition, we examined the reactions with the N-containing molecules NH3, CH3NH2, (CH3)2NH, (CH3)3N, pyrrole, pyridine as well as CH3COCH3. The results can be summarized under three general reaction types: Reaction at the CH3 carbon, reaction at the CH2 carbon, and association. The results also indicate that the methoxymethyl cation can be used as a chemical ionization source for the detection of trace levels of S-containing compounds in saturated hydrocarbons.     

Gas phase hydration and deprotonation of the cyclic C3H3+ cation. Solvation by acetonitrile, and comparison with the benzene radical cation

The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.     

Electron-impact spectra of ms-tetra-phenylchlorins and the stability of chlorins

The mass spectrum of ms-tetraphenylchlorin was observed to contain a significant porphyrin peak. Attempts to obtain metallochlorin spectra yeilded mainly metalloporphyrin spectra. Sublimation experiments established that the chlorin to porphyrin conversion resulted from electron-impact excitation and did not arise from thermolysis on the probe or from impurities. In conjunction with previous literature data, this is interpreted as showing that the apparent stability of chlorins with respect to porphyrins is mainly a matter of the activation energies required under specific constraints.     

The proton affinities of styrene,trsns-β-methylstyrene,and indene

We report experimental and theoretical AM1 proton affinities of styrene,-methylstyrenes, and indene. The computed AM1 proton affinities for the species of interest were in good agreement with the experimental values.trans--Methylstyrene was found to have a proton affinity slightly lower than that of styrene. This is an unusual result since methyl substitution in most classes of compounds increases the proton affinity by 2–4 kcal mol^–1. The lower basicity oftrans--methylstyrene compared to styrene is due to the greater stabilizing effect of the methyl group in the neutral species compared to the cation.     

Protonated p-Benzoquinone

The structure and energetics of protonated p-benzoquinone (pBQ) have been investigated using high-pressure mass spectrometry and ab initio calculations. The experimental proton affinity of pBQ is 801.4 +/- 8.9 kJ/mol (191.5 +/- 2.1 kcal/mol) (1sigma) from bracketing measurements and hydration thermochemistry. This value is supported by theory and by analogies with related compounds. In its protonation chemistry, pBQ behaves as an aliphatic ketone, both structurally and energetically. The dissociation of the hydrate (pBQH(+)).(H(2)O) is characterized by DeltaH degrees (D) = 90.0 +/- 2.3 kJ/mol and DeltaS degrees (D) = 123.4 +/- 4.9 J/mol.K (95% confidence).     

Charge localization and aromatic stabilization in large ring ions: Mass spectra of ms-porphyrins

The major mass spectrometric fragments of ms-tetraphenylporphin and ms-tetra(p-chloro)phenylporphin are [M ? H]⁺˙ and [M ? Cl]⁺˙, respectively. Metal derivatives of these compounds give a modified characteristic fragmentation pattern with peak groups ending in the ions [M ? 4H]⁺˙, [M ? ? ? 5H]⁺˙ and [M ? 2? ? 2H]⁺˙ for the metallo ms-tetraphenylporphins, and [M ? ?Cl ? 2Cl ? 3H]⁺˙ and [M ? 2?Cl ? Cl ? H]⁺˙ for Mgms-tetra(p-chloro)phenylporphin. Deuterated metal derivatives indicate random hydrogen loss from both phenyl and pyrrole carbons. However, metal substituents do not significantly modify the fragmentation pattern in the case of ms-tetra(p-methoxy)phenylporphin. These patterns can be explained in terms of aromatic stabilization of the fragmentation products, coupled with charge localization on the π system in the free base, on the metal atom in the metallo derivatives and on the methoxy function in the p-methoxyphenyl derivative.