The benzoyl ion. Thermochemistry and kinetic energy release

Abstract

The thermochemistry of benzoyl ion formation from a variety of sources has been examined by using the measured kinetic energy release for metastable ions to estimate the excess energy of the activated complex. A correlation is observed between this estimated excess energy and literature values of the heat of formation of the benzoyl ion. From this relationship and the observed correlation between the uncorrected heat of formation and the difference between the appearance potential of [C,H,CO]c and the ionization potential of the parent compound, the large range of reported values for AH,[C,H,CO]+ is seen to be due, at least in part, to variation in the kinetic shift with the critical energy of the reaction. With the exception of the ion generated from trifluoroacetophenone and possibly that from benzaldehyde, the fragmenting [C,H,O]+ ions are shown, from kinetic energy release data, to be structurally identical. The approach adopted here may have general merit in improving or testing the accuracy of thermochemical data based on appearance potential measurements. I N T R O D U C T I O NTHIS paper is concerned with the interpretation of thermochemical data derived from appearance potential (P,) measurements. The particular example discussed is the ion [C,H,CO]+ generated in the gas phase by fragmentation of benzoyl compounds. Several previous studies on the energetics of ionic gas phase reactions have discussed benzoyl and substituted benzoyl ions.1-12 The present study attempts to refine traditional (P,) data in the light of measured kinetic energy releases accompanying metastable ion fragmentations.An elementary precaution in discussing thermochemical data on polyatomic ions is to ensure that the species being compared are structurally identical. Methods for determining ion structures in the gas phase have developed ra~id1y.l~ The choice of the benzoyl ion for study was based in part on the evidence (see below) that a single structure is involved. Moreover, the reactions leading to [C,H,O]+ formation are all apparently simple bond cleavages at the carbonyl group, making the formation of different product ions unlikely.The heats of formation of ions, as determined from P, measurements, have often been used to characterize ion structure^.^.^^.^^ Such measurements always represent upper limits to ion enthalpies, because they include (i) the internal energy of the activated complex and (ii) the reverse activation energy. In cases where the reaction involves simple bond cleavage, the reverse reaction is an ion-radical combination for which activation energies are often negligibly small. In such instances the major source of error is likely to be the internal energy.In spite of independent evidence for a single [C,H,O]+ structure from all but one of the benzoyl compounds studied here, the most striking feature of the standard compilation of heats of formation of gaseous ions is the extraordinary range of values found for [C,H,O]+. The literature values range from 165 to 204 kcal mol-l'16 One must infer that P, measurem...

Heat of Formation for the Benzoyl Cation by Photoionization Mass Spectrometry

“…transition-state calculations indicated no reverse activation energy, with the products being located just 2.5 kJ mol -1 higher on the potential energy surface. However, there is a kinetic energy release of 1.5 kJ mol -1 associated with the metastable fragmentation of ionized benzoyl chloride, 16 implying that the derived cationic heat of formation of 742.5 ± 2.3 kJ mol -1 (table 3) might be slightly higher than the true thermochemical value.…”

“…Again, it is possible that the derived cationic heat of formation of 741.1 ± 2.4 kJ mol -1 (table 3) might not represent a thermochemical value, given that there is a 0.8 kJ mol -1 kinetic energy release involved in the metastable dissociation. 16 It is expected, however, that any difference will be small.…”

The main beam correction term in kinetic energy release from metastable peaks

“…Borderline zero KERs are usually a result of electronic predissociation. Preceding examples include cations of α‐halogenated acetophenones, (s)‐triazine, ketene, the neon dimer, and dichloromethane . The methane cation is rotationally predissociated .…”

Distributions of kinetic energy release in benzoyl ion formation

“…In the accompanying paper, 8 (1) kinetic energy releases, which seem to point to the occurrence of more than one mechanism or type of potential energy surface for the reaction. Three categories of energy release could conveniently be distinguished: (i) energy releases (T50%) of 100 meV or inore; (ii) energy releases of between 6 and 40 meV, and (iii) very small energy releases of the order of 1 meV or less.…”