Accuracy of enthalpy and entropy determination using the kinetic method: are we approaching a consensus?

✍ Scribed by László Drahos; Csaba Peltz; Károly Vékey

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 354 KB
• Volume: 39
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.683

✦ Synopsis

Abstract

There is an emerging consensus regarding the applicability of the kinetic method. All parties acknowledge that it is an approximate quantitative technique, capable of yielding not only enthalpy, but also entropy values. Opinions differ mainly on the accuracy of the results but it is agreed that the energy (effective temperature) dependence of kinetic method plots needs to be checked in all but the simplest of cases. When the ‘apparent basicity’ is found to depend on collision energy (and hence effective temperature), the extended kinetic method must be used. We have performed a large‐scale modeling study, involving thousands of randomly selected molecular systems and a variety of experimental conditions, using exact calculations and realistic data sets. The results show that when the measured entropy difference between the two competing reaction channels is less than ∼35 J mol^−1^ K^−1^, overall errors (standard deviations) of Δ__H__~298~ determined by the kinetic method are ±5 kJ mol^−1^; those of Δ__S__~298~ are ±10 J mol^−1^ K^−1^. These include not only inherent errors of the kinetic method, but also errors in ion abundance measurement (5%) and inaccurate knowledge of reference compound thermochemistry (±2 kJ mol^−1^, on average). We recommend, in general, that these errors be reported in kinetic method studies. When the measured entropy difference between the two competing fragmentation channels is large (>35 J mol^−1^ K^−1^), it is likely to be significantly underestimated and errors of the kinetic method increase significantly. Copyright © 2004 John Wiley & Sons, Ltd.