Photoionization mass spectroscopy with synchrotron radiation of hydrogen-bonded alkylamine clusters produced in supersonic beams

Abstract

Hydrogen-bonded clusters of methyl, ethyl-, dimethyl-, and diethylamines are synthesized in a seeded supersonic expansion for a mass spectroscopic study. The mass spectra are compared by ionizing either with 21-eV electrons or with photons from dispersed synchrotron radiation. Photoionization efficiency curves are measured to determine threshold energies for the ionization and fragmentation of the clusters. The threshold values yield gas-phase proton affinities, association, and dissociation energies by thermochemical calculations. The derived thermochemical quantities of cluster ions depend on the amount of the association energies of the neutral clusters. The association energy data found in this study are compared with previously calculated values. Lower bounds to the bond dissociation energies of alkylamine cluster ions are presented. The absolute proton affinity values of CH/sub 3/NH/sub 2/ (930 +/- 15 kJ/mol), C/sub 2/H/sub 5/NH/sub 2/ (940 +/- 15 kJ/mol), (CH/sub 3/)/sub 2/NH (955 +/- 15 kJ/mol), and (C/sub 2/H/sub 5/)/sub 2/NH (965 +/- 15 kJ/mol) determined in this study are about 30 kJ/mol higher than the currently recommended reference data. The proton affinities of molecular aggregates are determined in order to quantify the first steps of gas-phase proton solvation energetics.