Methyl Loss Kinetics of Energy-Selected 1,3-Butadiene and Methylcyclopropene Cations

Abstract

The kinetics of the metastable dissociation of energy-selected 1,3-butadiene and 3-methylcyclopropene cations to form C 3 H 3 + (cyclopropenyl cation) and CH 3 have been investigated by threshold photoelectron photoion coincidence (TPEPICO) time-of-flight mass spectrometry, ab initio molecular orbital calculations, and RRKM statistical theory. Both the experimental results and the molecular orbital calculations indicate that 1,3-butadiene ions lose CH 3 by first isomerizing to a higher energy structure (3-methylcyclopropene cation) which can rapidly lose CH 3 or isomerize back to the 1,3-butadiene cation. A complete kinetic model of the twowell potential and its three rate constants is necessary to account for the measured dissociation rates as a function of the ion internal energy. At low energies, the dissociation rate is limited by the bond cleavage step, while at higher energies, the bottleneck shifts to the lower energy isomerization step. A fit of calculated RRKM rate constants to the experimental data yields a 0 K isomerization barrier (relative to the 1,3-butadiene ion) of 2.02 ( 0.03 eV and an activation entropy at 600 K of -4 cal K -1 mol -1 . The entropy of activation for the dissociation step from 3-methylcyclopropene was found to be +7 cal K -1 mol -1 . 3-Methylcyclopropene was found to have an adiabatic ionization energy of 9.28 ( 0.05 eV and a neutral heat of formation of 273( 2 kJ mol -1 at 0 K (257 ( 2 kJ mol -1 at 298 K). This is the first experimental determination of this value.

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

“…Afterward, INT27 undergoes ring closure via transition state TS30 with an energy barrier of 0.29 eV to give the same intermediate INT26 as described above. This pathway has also been investigated in the study of dissociation of 1,3‐butadiene by Keister et al and Fang et al at the MP2 and G3B3 level of theory, respectively.…”

Proton Tunneling in the Loss of Hydrogen Bromide from Energy-Selected Gas-Phase 2-Bromobutane Cations

“…The solid line through the data points thus corresponds to a calculated TOF distribution with an "effective" rate constant obtained by accounting for the thermal energy distribution. This method has recently been applied to the dissociation of butadiene at 298 and 220 K. 24 The LURE data in Figure 3 are a composite of three TOF distributions obtained by varying the delay in extracting the ions out of the acceleration region. As in the UNC data, the C 4 H 9 + (m/z 57) peak is symmetric because it is the result of a fast dissociation.…”

“…The solid line through the data points thus corresponds to a calculated TOF distribution with an “effective” rate constant obtained by accounting for the thermal energy distribution. This method has recently been applied to the dissociation of butadiene at 298 and 220 K 2 Sample TOF data obtained at UNC.…”

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

“…The concepts for how to use synchrotron radiation for chemical dynamics investigations, embodied in these two platforms, have not changed significantly over the years. In one case, high spectral resolution provides narrow band tunable synchrotron radiation to measure spectroscopic features of molecular ionization, the results of which also provide important energetic and thermodynamic information; [23][24][25] these spectral measurements thus reveal structural information about novel molecules, clusters, and isomers (molecules with the same chemical formula but different structural configurations). Coincidence methods are sometimes incorporated to enhance the power of such spectroscopic investigations, 26 and threshold ionization coincidence techniques also provide state-selected preparation of ions for novel state-resolved reaction dynamics studies.…”