Equilibrium studies by pulsed high pressure mass spectrometry: a calibration, and some pitfalls and solutions

Abstract

The accuracy of pulsed high pressure mass spectrometry (PHPMS) thermochemical measurements was assessed using a ladder of charge transfer equilibria involving aromatic molecules. The ladder was constructed using Δ H° values from variable temperature measurements. For the 14 compounds in the ladder, the average agreement of the equilibrium results with spectroscopic ionisation potentials (IPs) is ± 0.6 kcal mol −1. Over the span of the ladder, from 1,2,3,5-tetramethylbenzene to C 6F 6, the thermochemical ladder yields a ΔIP of 40.7 kcal mol −1, compared with the spectroscopic value of 42.3 kcal mol −1 i.e., the equilibrium values cover the span of 40 kcal mol −1 with an accuracy of ±5%. The sources of error in these studies are evaluated, and some common pitfalls in PHPMS studies are discussed. These include: steady states that appear as equilibria; isomerisation; pyrolysis of ions and neutrals; slow reactions and non-reactive systems; reaction outside the ion source; vapor pressure problems; and mass coincidences. Tests and solutions for these complications are also discussed.