The heat of formation of 2-C3H7+ and proton affinity of C3H6 determined by pulsed field ionization-photoelectron photoion coincidence spectroscopy

Pulsed field ionization combined with ion coincidence has been used with synchrotron radiation at the Advance Light Source (ALS) operated in the multibunch mode to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar on this instrument is 0.6 meV. The production of the 2-C3H7+ ion was investigated by measuring the dissociative ionization onset from the 2-propyl chloride, bromide, and iodide. Because the dissociation limits for halogen atom loss lie in the Franck-Condon gap, the yield of threshold electrons generated by pulsed field ionization was found to be extremely low. Nevertheless, the heat of formation of 2-C3H7+ determined from the three molecules agreed to within 2 kJ/mol. The average and now recommended values for this heat of formation are 825.0 +/- 1.5 kJ/mol (0 K) and 807.5 +/- 1.5 kJ/mol (298 K). This can be combined with the known heats of formation of C3H6 and H+ to yield a proton affinity of C3H6 of 742.3 +/- 1.5 kJ/mol (298 K). This new value, which is 9.3 kJ/mol lower than the recommended value of Hunter and Lias, is in much better agreement with two theoretical values (744 kJ/mol) as well as the determination of Szulejko and McMahon (746.4 kJ/mol).