A PULSED ELECTRON-BEAM, VARIABLE TEMPERATURE, HIGH-PRESSURE MASS-SPECTROMETRIC REEVALUATION OF THE PROTON AFFINITY DIFFERENCE BETWEEN 2-METHYLPROPENE AND AMMONIA

A newly constructed pulsed electron beam high pressure mass spectrometer which incorporates a reverse geometry, BE, sector mass spectrometer has been used to examine proton transfer equilibria as a function of temperature for a series of bases intermediate in proton affinity between 2-methylpropene and ammonia. The data obtained support the recent conclusion of Meot-Ner and Sieck (J. Am. Chem. Soc., 113 (1991) 4448) that the proton affinity of ammonia should be revised upward by some 4 kcal mol-1. Examination of clustering equilibria at high temperatures for strongly bound proton bound dimer species establishes that temperatures are accurately measured in the new apparatus. Thermochemical data obtained for the association of NH4+ with 2-methylpropene suggest that the proton affinity of t-butylamine must also be revised upward by nearly 8 kcal mol-1. Use of the capability of the reverse geometry configuration of the mass spectrometer to examine collision-induced dissociation of mass-selected ions establishes that the cluster of NH4+ with 2-methylpropene has the same structure as protonated t-butylamine. Consideration of the kinetics of three-body association reactions reveals that temporal profiles in high pressure mass spectrometry experiments may have to be examined for up to 10 ms to ensure that true thermal equilibrium is established in clustering reactions.