Isomerization and Dissociation in Competition. The Two-Component Decay Rates of Energy-Selected Trimethyl Borate Ions

Abstract

Threshold photoelectron−photoion coincidence spectroscopy has been used to investigate the unimolecular chemistry of metastable trimethyl borate ions, B(OCH3)3•+. The ions were found to fragment by two-component decay rates via four distinct channels with the loss of CH3O•, •CH2OH, •CH3, and H•. The rate of molecular ion fragmentation as a function of ion internal energy was obtained from the coincidence data and used in conjunction with RRKM and ab initio molecular orbital calculations to model the fragmentation mechanism. The data were found to be consistent with a three-well potential surface including trimethyl borate molecular ions (I) and two isomers formed by consecutive 1,2- and 1,5-hydrogen shifts. The molecular ion undergoes a fast, but metastable, loss of CH3O• and a competative 1,2-H shift to isomer II, (CH3O)2BO(H)CH2•+. This isomer is responsible for the metastable loss of •CH2OH and isomerization to the second isomer, III, which loses a methyl group. The 0 K heat of formation of CH3OBOCH3+ was found to be 196 ± 5 kJ mol-1 from the RRKM fit to the experimental rate constants for the simple bond cleavage reaction forming CH3O• radicals from I. An upper limit to ΔfH°0 of (CH3O)2BOCH2+, −12 ± 10 kJ mol-1, was obtained in a similar manner. Also calculated from RRKM theory and experimental rate constants were ΔfH°0(II), −12 ± 5 kJ mol-1, and the barrier heights for the isomerization reactions, +105 kJ mol-1 for I−II and +135 kJ mol-1 for II−III.