Abstract
The kinetic method is one of the most widely used experimental techniques for the measurement of thermochemical parameters by mass spectrometry. Recently it has been realized that it can also be used to determine reaction entropies, but the validity of this approach has not been established. This Perspective evaluates kinetic method plots in cases where there is a significant entropy difference between the competing fragmentation channels (i.e. between sample and reference compounds in the dissociating cluster ion). The concept underlying this study is to calculate mass spectra theoretically, based on known thermochemical parameters and as a function of experimental conditions. This can be done accurately using the RRKM‐based MassKinetics software. The resulting mass spectra are then interpreted by the kinetic method, yielding Δ__H__ and Δ__S__ values. These values are, in turn, compared with the true values used to generate the calculated mass spectra. The results show that the reaction entropy difference between sample and reference has a very large influence on kinetic method plots. This should always be considered when studying energy‐dependent mass spectra (using metastable ions or low‐ or high‐energy collision‐induced dissociation (CID)), even if only Δ__H__ is to be determined. Kinetic method plots are not strictly linear and this becomes a serious issue in the case of small molecules showing a large entropy effect. In such cases, results obtained at a low degree of excitation are more accurate. Energy and entropy effects can be evaluated in a relatively straightforward manner: first, the apparent Gibbs energy (Δ__G__~app~) and effective temperature (T~eff~) are determined from kinetic method plots (intercept and slope, respectively), obtained from experiments using various degrees of excitation. Second, the resulting Δ__G__~app~ is plotted against T~eff~, the slope yielding Δ__S__ while the intercept (extrapolation to zero temperature) yields Δ__H__. This data evaluation yields more accurate results than alternative methods used in the literature. The resulting Δ__H__ values are fairly accurate, with errors, in most cases, <4 kJ mol^−1^. On the other hand, Δ__S__ is systematically underestimated by 20–40%. Empirically scaling Δ__S__ values determined by the kinetic method by 1.35 results in a Δ__S__ value within 20% (or 10 J mol^−1^ K^−1^) of the theoretical value. Copyright © 2003 John Wiley & Sons, Ltd.