The proton affinity for isobutene is an important thermochemical quantity because it is widely used as a primary standard to assign absolute values for relative gas basicity and proton affinity scales. The widely used recommended value of 820 kJ/mol is based on a 298 K enthalpy of formation of 693.7 kJ/mol for the t-butyl cation, which was derived from measurements of the equilibrium constant for the reaction between t-butyl chloride and the benzyl cation [1,2]. However, several theoretical [3,4] and experimental [5,6] studies have challenged this, proposing a downward revision to 802 kJ/mol. Given the importance of the isobutene reference it is vital that such a large discrepancy be resolved.

Threshold photoionization mass spectrometry has been used to measure the appearance energy for the formation of C₄H₉⁺ from isobutane. From the value of 10.82 ± 0.01 eV, a 298 K enthalpy of formation for the t-butyl cation of 711.4 ± 1. l kJ/mol is derived, which leads to an absolute proton affinity for isobutene of 801.7 ± 1.4 kJ/mol. This is in excellent agreement with the more recent experiments and is supported by similar appearance energy data for neopentane.

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2. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1.
3. Smith, B. J.; Radom, L. J. Am. Chem. Soc. 1993,115, 4885.
4. Smith, B. J.; Radom, L. J. Phys. Chem. 1995, 99, 6468.
5. Szulejko, J. E.; McMahon, T. B. J. Am. Chem. Soc. 1993,115, 7839.
6. Keister, J. W.; Riley, J. S.; Baer, T. J. Am. Chem. Soc. 1993, 115, 12613.