Joong Chul Choe

Reinvestigation of the photodissociation kinetics of m-iodotoluene molecular ion

Abstract Photodissociation kinetics of the m -iodotoluene molecular ion has been investigated on a nanosecond time scale by mass-analyzed ion kinetic energy spectrometry. Photodissociation rate constants have been determined at molecular ion internal energies of 3.7–4.3 eV. The influence of the collisional relaxation of the molecular ion occurring in the ion source on the rate constant has been corrected, which was not considered in the previous photodissociation experiment. The kinetic energy release distributions (KERDs) determined from the ion kinetic energy profiles are composite due to the participation of two competing reaction channels, one leading to the formation of m -tolyl ion and the other to tropylium ion. By separating these bimodal KERDs, the branching ratios for the two channels have been obtained. From the overall rate constants and the branching ratios, rate constants for each channel have been determined on a nanosecond time scale. It has been found that the rate-energy data for each channel can be explained adequately by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The production of m -tolyl ion proceeds via a loose transition state while that of tropylium ion proceeds via a tight transition state. Refined experimental data together with the microsecond data reported by Lin and Dunbar have been found to be essential to arrive at the correct mechanistic pictures for these reactions

Discovery of benzene cation in a very long-lived excited electronic state

Presence of benzene cation in a long-lived excited electronic state at ∼2.3 eV above the ground state was found through photodissociation kinetics and charge exchange ionization mass spectrometry. The lifetime of this state seems to be longer than 10 μs, maybe much longer. The experimental energy level of this state suggests that B 2E2g is the best candidate. The radiative transition from the latter state to the ground state, B 2E2g→X 2E1g, is electric dipole forbidden. Then, a very long lifetime requires an inefficient radiationless decay of the state. This is in contrast with the very fast decay proposed previously to account for the lack of fluorescence from the B 2E2g state. The present finding opens the possibility of studying a new excited state chemistry.

Internal Energy Content of n-Butylbenzene, Bromobenzene, Iodobenzene and Aniline Molecular Ions Generated by Two-photon Ionization at 266 nm. A Photodissociation Study

A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the n-butylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The ion internal energies have been estimated by comparing the measured kinetic data with the previous energy dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization contribute to the photodissociation signals. Around half of the available energy has been found to remain as molecular ion internal energy in the two-photon ionization process. Copyright 1999 John Wiley & Sons, Ltd.

Charge Exchange Ionization in Collision Cells as a Method to Detect the Presence of Long-lived Excited Electronic States of Polyatomic Ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C4H6, CS2, CH3Cl) were lower than the recombination energy (∼11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH3F, CH4) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.

Anisotropic photodissociation of CH3Cl

Photodissociation of the methyl chloride ion has been investigated using mass-analyzed ion kinetic spectrometry (MIKES). The MIKE spectrum for the chlorine atom loss from the methyl chloride ion has been measured as a function of the laser polarization angle at 357, 488.0, and 514.5 nm. The anisotropy parameters and kinetic energy release distributions have been determined. At all the wavelengths used, an anisotropic dissociation (β=1.2) in the repulsive excited electronic state has been observed. Results from quantum chemical calculations carried out at the various levels suggest that the methyl chloride ion is excited to the first excited electronic state A, and dissociates repulsively in this state.

Dissociation dynamics of propargyl chloride molecular ion near the reaction threshold: manifestation of quantum mechanical tunneling

The Cl loss from the propargyl chloride molecular ion has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES). The kinetic energy release distribution in the unimolecular dissociation has been determined. The potential energy surface for the mechanistic pathway has been calculated at the B3LYP/6-311G** density functional theory level. The calculated potential energy surface suggested that the threshold dissociation of the propargyl chloride molecular ion produces the C(3)H(3)(+) ion, only with the cyclopropenium structure, and with the release of a large amount of kinetic energy. This is in agreement with experimental results. Also, calculation of the rate constants with statistical rate models predicted that the reaction observed on a microsecond time scale occurs via tunneling through the rate-determining isomerization barrier for H-atom transfer. It has been found that a broad lifetime distribution is a manifestation of quantum mechanical tunneling of a precursor prepared under thermal conditions. Reinterpretation of previous photoelectron-photoion coincidence results taking into account the tunneling effect necessitated raising the critical energy to 0.64 eV from the energy of 0.34 eV reported previously. Copyright 2000 John Wiley & Sons, Ltd.