Cryogenic Ion Kinetics and Thermodynamics in a Temperature-Controlled, Radiofrequency Ion Trap: A Case Study of the “Tag Isomers” Adopted by the Py-CH2-COOH+·D2 Cation

Two spectroscopically distinct isomers of the Py-CH2-COOH+ cation are formed upon D2 condensation in a cryogenic radiofrequency ion trap. This provides a useful platform upon which to elucidate the bimolecular kinetics that underlie the temperature-dependent distribution of the isomers. This equilibrium is maintained by isomer interconversion reactions involving collisions with D2 molecules present at low pressure in the trap. The rates are determined by measuring the transient population recoveries after one of the isomers is selectively removed by resonant vibrational excitation with a 5 ns infrared laser. The correlated spectral response of the two isomers yields a direct experimental measurement of the relative optical cross sections of the D2-bound and free fundamental transitions of the OH group on the acid. The reaction thermochemistry is extracted using van 't Hoff analysis of the temperature-dependent isomer populations.