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 bee n used to investigate the unimolecular chemistry of gas-phase methyl 2-meth yl butanoate ions [CH3CH2CH(CH3)COOCH3.+]. This ester ion isomerizes to a l ower 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, a nd the analyzer function for threshold electrons, it was possible to extrac t 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 he at of formation, of methyl 2-methyl butanoate, we derive a 0 K heat of form ation of the distonic ion CH2CH2CH(CH3)COHOCH3.+ of 101.0 +/- 2.0 kcal/mol. The product ion is the enol of methyl prapionate, CH3CHCOHOCH3.+, which ha s a derived heat of formation at 0 K of 106.0 +/- 2.0 kcal/mol. (C) 1994 Am erican Society far Mass Spectrometry.