Ethene loss kinetics of methyl butanoate ions studied by threshold photoelectron–photoion coincidence:: The enol ion of methyl acetate heat of formation

Abstract

Threshold photoelectron–photoion coincidence spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl butanoate ions (CH 3CH 2CH 2COOCH 3 ·+). This ester ion isomerizes to a lower energy distonic ion (CH 2CH 2CH 2COHOCH 3 ·+) prior to dissociating by the loss of C 2H 4. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k( E) functions, the thermal energy distribution for room temperature and molecular beam samples, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization limit for methyl butanoate which at 0 K is 10.275 ± 0.010 eV as well as the dissociation barrier of the distonic ion of 0.735 ± 0.010 eV. By combining these with an estimated heat of formation of methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH 2CH 2CH 2COHOCH 3 ·+ of 119.0 ± 2.0 kcal/mol. The product ion is the enol of methyl acetate, CH 2COHOCH 3 ·+, which has a derived heat of formation at 0 K of 122.0 ± 2.0 kcal/mol. This energy agrees well with a derived energy obtained recently from the rate analysis of energy selected methyl acetate ions. Also measured was the adiabatic ionization energy of the methyl butanoate, 9.95 ± 0.05 eV.