G. G. Meisels

On the dissociation dynamics of energy‐selected nitrobenzene ion

Fragmentation mechanisms of nitrobenzene ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown curves were obtained at internal energies of the molecular ion from 0 to 8 eV, and fragmentation pathways were elucidated. The precursor of C4H+3 has been identified clearly as the C6H+5 ion from the breakdown graph. Decay rates of metastable decompositions were measured from the asymmetric peak shapes of their coincidence time‐of‐flight (TOF) peaks for the C6H5O+, C6H+5, C4H+3, and NO+ ions. The activation energies for NO+ and C6H5O+ formation were estimated as 1.02 eV on the basis of RRKM/QET calculations. Average kinetic energy release for the formation of phenyl cation by direct C–N bond cleavage showed that about 4.2% of the internal energy is converted to translation; this is in good quantitative agreement with the calculations based on the statistical phase space theory. On the other hand, the dependence of the fragmentation rate...

Unimolecular dissociation of energy-selected methyl formate ion

Fragmentation mechanisms of the methyl formate and methyl formate‐d1 ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The breakdown diagrams and ion kinetic energies of fragment ions were measured at internal energies of the molecular ion from 0 to 7 eV, and fragmentation pathways were elucidated. The formation of HCO+ showed a large kinetic isotope effect. The formation of CH3O+ from methyl formate‐d1 is accompanied by a large kinetic energy release, about twice that calculated using quasiequilibrium theory (QET). Its relative abundance in the breakdown curve also appears to be inconsistent with statistical theory. Direct dissociation from an isolated state of the molecular ion was one possible explanation. Slow dissociations were observed for the formation of methanol ions resulting from a migration of the formyl hydrogen while QET predicts no metastable dissociation for this process. Two‐component reaction rates for methanol ion formation sugge...

Bimodal kinetic energy release in the unimolecular dissociation of energy‐selected formic acid ion

Dissociation of HCOOH+ and DCOOH+ have been studied using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown diagrams were obtained over internal energies of the molecular ion less than 8 eV. Kinetic energy release distributions (KERDs) for HCO+ and DCO+ were extracted from their flight time distributions by using a multiple linear regression method. The breakdown curve of HCO+ not only exhibited several maxima but also the surprisal analysis of its KERD demonstrated a considerable deviation from a single statistical distribution. Bimodal KERDs were observed for the formation of HCO+ and DCO+. The large KER component results from dissociation to the products in the ground state, HCO+ (1Σ+)+⋅OH (2Πi). Average kinetic energy releases observed for this process are, however, about twice higher than those calculated by QET/RRKM. The small KER component shows average KERs remarkably smaller than the statistical expectations and this value remains constant over the e...

Unimolecular dissociation of energy-selected dimethyl sulfoxide

Abstract The fragmentation mechanism of dimethyl sulfoxide ion was studied using threshold photoelectron photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown graph was obtained in the energy range 9.0–17.5 eV. The appearance potentials of major fragments determined from breakdown curves are lower than those reported from early electron impact studies. The breakdown graph calculated using RRKM-QET is in good agreement with experiment, suggesting that the dissociation of dimethyl sulfoxide ion is statistical.

Unimolecular decomposition of energy-selected anisole ions. Breakdown graph and metastable decay rates

Abstract The fragmentation behavior of the anisole ion (C6H5OCH+·3) was examined using threshold photoelectron-photoion coincident (TPEPICO) mass spectrometry. Four major fragments are observed in the ion time-of-flight spectra at photon energies less than 14 eV: C6H5O+ (m/z 93), C6H+7 (m/z 79), C6H+6 (m/z 78), and C5H+5 (m/z 65). Rate constants for the production of the ion m/z 78 near threshold were found to agree well with those calculated on the basis of RRKM theory. Most of the excess energy at threshold for this process is a result of the kinetic shift.

Fragmentation of acetic acid ions with selected internal energies

Abstract The unimolecular dissociation of acetic acid ion in the photon energy range 10.5–17.0 eV was studied using threshold photoelectron photoion coincidence mass spectrometry. The detailed breakdown graph was obtained and the fragmentation pathways were elucidated. The breakdown graph calculated using statistical theories was found to be consistent with the experimental data up to a photon energy of about 12.5 eV. The average kinetic energy release observed is higher than that calculated on the basis of quasi-equilibrium theory for the formation of COOH + while it seems to be statistical for the formation of CH 3 CO + . The origin of kinetic energy release accompanying the formation of these two ions is discussed. The structure of [COH 3 ] + ion ( m/z 31) is determined to be hydroxymethyl cation, CH 2 OH + , which could be formed by a two-step rearrangement prior to dissociation.

On the dissociation dynamics of energy-selected dimethylether ions

Abstract Dissociation of the dimethylether ion has been studied using threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry. Our TPEPICO breakdown curves showed a significant occurrence of electronic autoionization at around 12.2 eV and over 16 eV. For the formation of [CH 3 O] + , rectangular flight-time distributions owing to a large, sharp kinetic energy release were observed at high energies, and about 80% of internal excess energy of the molecular ion was found to be released as translational energy in this fragmentation. This remarkably high conversion was consistent with the spectator model of direct dissociation rather than with statistical theories (RRKM—QET). However, the relative abundance of [CH 3 O + ] remained less than 20% in the breakdown diagram although direct dissociation predicts its predoiminace. These results may be intepreted as a direct dissociation to CH 3 O + on a repulsive state reached by transition from an initial state or as a statistical fragmentation to CH 2 ue5fbO + H strongly controlled by a tight transition exit on a high reverse activation barrier.