Relationship between effective temperature of thermalized ions and ion source temperature

The spontaneous dissociation of mixed proton bound dimers of nitrile bases has been studied using mass-analyzed ion kinetic energy spectra in a reverse geometry double focusing instrument coupled to a high pressure ion source. A linear correlation was found between the relative ion abundance ratios of the fragments and the ion source temperature. Proton exchange equilibria between the bases were studied using the time resolved capability of the high pressure mass spectrometry system, and relative ΔG^o, ΔH^o, and ΔSo values were extracted from the van't Hoff plots. The proton affinities (PA) of the nitriles studied were found to be PA(MeCN) = 186.8 kcal/mol, PA(EtCN) = 189.9 kcal/mol, PA (n-PrCN) = 191.0 kcal/mol, and PA(i-PrCN) = 192.2 kcal/mol. The effective temperatures of the metastable protonated nitrile dimers at different ion source temperatures were obtained from a plot of 1n(I_RCN/I_ref.) versus ΔGB (relative gas phase basicity). From a plot of the effective temperature of the protonated metastable nitrile dimers reacting in the second field free region of the mass spectrometer versus the ion source temperature, it was found that the effective temperature decreases with increasing ion source temperature, which can be explained from qualitative considerations of the relevant thermal Boltzmann distributions. (Int J Mass Spectrom 176 (1998) 87-97)