Three methods to measure RH bond energies

Abstract

This report was prepared as an account of work sponsored-by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not neoessarily state or reflect those of the United States Government or any agency thereof.-3-3/21/93 molecules: a) the study of radical kinetics, b) the use of negative ion thermochemical cycles, and c) photoionization mass spectroscopic techniques. It is essential to stress the complcmc#tarity ofthese three experimental methods; theyarcall interrelated. Our goal in this essayistodissect eachofourmethodstodescribe how themeasurements arecarried out, whatthelimitations are, andtodemonstrate by direct comparison that all givethesame bondenergies. An introduction tothese three experimental programs isnow inorder. a)Radical Kinetics Supposeone measuresthekinetics of equilibrium of a halogenatom,X, witha substrate, RH. RH + X = R +XH (I) By monitoring thetime dependenceof [X] and [R] after flash photolysis, by atomic fluorescence, and/orresonance lamp photoionization detection, one can determine the absolute rateconstants kI and k.1. These rateconstants fixtheequilibrium constant, Kequi(1), which permits one todetermine AGrxn(1), fromwhichthecnthalpy, AHrxn(1), can bc extracted. Iftheheatsof formation (AHf°(RH),AHf°(X),and AHf°(XH)) are known,AHrxn(1) permits one tofind AHf°(R)whichfixes thebond energy, BDE(R-H). b)Negative IonCycles Ionchemistry canbc usedtodeducethegasphaseacidity ofa target molecule, RH. The acidity, AHacid, isthecnthalpy for theproton abstraction reaction.

Theoretical study of the reaction mechanism and kinetics of low-molecular-weight atmospheric aldehydes (C1–C4) with NO2

“…The reaction enthalpies of each channel are calculated and listed in Tables S3eS4. The calculated reaction enthalpies are 10.09 (formaldehyde) and 11.16 (acetaldehyde) kcal mol À1 for the syn-R O-abs 1 channel in this paper, which are in excellent agreement with the corresponding energies [9.9 AE 0.9 and 12.41 AE 1.6 kcal mol À1 , respectively (Table S3)] calculated by the experimental standard heats of formation (Berkowitz et al, 1994;Chase, 1998;Frenkel et al, 1994;Lias et al, 1998;NIST). For other channels, it is difficult to make a direct comparison of the theory with the experiment for the enthalpies due to lack of the experimental data.…”

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…The absence of the abstraction product in the case of acetone may be explained by the higher bond energy of the methyl C−H bond (388.3 ± 9.2 kJ·mol -1 ) relatively to that of the acetyl C−H bond in acetaldehyde (358.6 ± 9.6 kJ mol -1 ). 90 Insertion of the CH radical into a methyl group C−H bond of acetaldehyde is consistent with formation of ethyl + CO, ketene + methyl, and acrolein + H. However, this mechanism does not explain the formation of either the formyl + ethene channel or the methylketene + H channel. Ultimately, no evidence for an insertion mechanism is found for the CH + acetone reaction.…”

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

“…Quantifying the contribution of these photodissociation products of the alkyne to the ion signals is therefore crucial in our experiment for accurate isomer-specific product detection. We have identified methyl (CH 3 , IE = 9.843 eV) 60 However, it is unlikely that the products of the reaction will contribute to the ion signal at either m/z = 64 (C 5 H 4 ) or 78 (C 6 H 6 ). On the other hand, C 3 H 3 radicals could react with C 4 H 6 to produce C 6 H 6 (e.g.…”