In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

No 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.

Abstract. We suggest a chemical model for the composition, structure, and atmospheric processing of organic aerosols. This model is stimulated by recent field measurements showing that organic compounds are a significant component of atmospheric aerosols. The proposed model organic aerosol is an "inverted micelie" consisting of an aqueous core that is encapsulated in an inert, hydrophobic organic monolayer. The organic materials that coat the aerosol particles are surfactants of biological origin. We propose a chemical mechanism by which the organic surface layer will be processed by reactions with atmospheric radicals. The net result of an organic aerosol being exposed to an oxidizing atmosphere is the transformation of an inert hydrophobic film to a reactive, optically active hydrophilic layer. Consequently, processed organic aerosols can grow by water accretion and form cloud condensation nuclei, influencing atmospheric radiative transfer. Radiative transfer may be affected directly by the chromophores left on the surface of the aerosol after chemical transformation. The chemical model yields certain predictions which are testable by observations. Among them is a curve of the percent organic material as a function of particle diameter which predicts that a high fraction of the mass of the upper tropospheric aerosol will be organic. Atmospheric processing of organic aerosols will lead to the release of small organic fragments into the troposphere which will play a subsequent role in homogeneous chemistry. Organic aerosols are likely to act as a transport vehicle of organics and other water insoluble compounds into the atmosphere. We speculate that biomass burning will produce a similar coating of surfactants derived from land sources. Finally, it is pointed out that the radical-induced transformation of the surface layer of aerosol particles from hydrophobic to hydrophilic offers an additional means by which the biosphere, through atmospheric chemistry, can affect the radiative balance.