Ethene loss kinetics of methyl 2-methyl butanoate ions studied by threshold photoelectron-photoion coincidence: The enol ion of methyl propionate heat of formation

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl 2-methyl butanoate ions [CH₃CH₂CH(CH₃)COOCH₃^·+]. This ester ion isomerizes to a lower energy distonic ion [CH₂CH₂CH(CH₃)COHOCH₃^·+] prior to dissociating by the loss of C₂H₄. 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 sample, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization threshold for methyl 2-methyl butanoate which at 0 K is 9.80 ± 0.01 eV as well as the dissociation barrier of the distonic ion of 0.86 ± 0.01 eV. By combining these with an estimated heat of formation of methyl 2-methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH₂CH₂CH(CH₃)COHOCH₃^·+ of 101.0 ± 2.0 kcal/mol. The product ion is the enol of methyl propionate, CH₃CHCOHOCH₃^·+, which has a derived heat of formation at 0 K of 106.0 ± 2.0 kcal/mol.