A mass spectrometric study of the effect of supersonic molecular beam sampling on the clustering of acetic, acid vapor

Abstract

A supersonic molecular beam, operated to preclude ion-molecule reactions, is used as a sampling system in a mass spectrometric study designed to test whether the molecular beam sampling process significantly disrupts the auto-association equilibrium between acetic acid monomers and dimers. From the temperature dependence of relative ion intensities, the heat of dimerization is determined to be −14.6 + 0.5 kcal mol −1. Agreement of this value with others in the literature indicates that this molecular beam technique can provide a specific approach to the study of cluster species at equilibrium in the nozzle. Analysis of the mass spectrum suggests a way to estimate the spectrum of the dimer and provides no evidence for any cluster larger than the dimer at pressures up to 40 torr. Photon ionization produces more extensive fragmentation than electron impact.