Thermochemistry and bond energies of nitro -alkanes, -alkenes, -carbonyls and corresponding nitrites

Density functional and ab initio theory based calculations were performed on a series of nitro -alkanes, -alkenes, carbonyl and corresponding nitrites representing large-scale primary, secondary and tertiary nitro compounds and their radicals resulting from the loss of skeletal hydrogen atoms. Geometries, vibration frequencies and thermochemical properties, ΔfH°298, S °(T) and C°p(T) (10K T 5 5000K) are calculated at the individual B3LYP/6-31G(d,p), B3LYP /6-31+G(2d,2p) and composite CBS-QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6-31G(d, p) level of theory and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro- and nitrite isomers include: -30.6 and -28.4 kcal moil for n-propane-, -33.9 and -32.3 kcal mol-1 for iso-propane-, -42.8 and -41.4 kcal mol-1 for tert-butane-nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane and corresponding nitrites. C--H bond energies are decreased by ~ 5 kcal moil alpha to the nitro or nitrite groups and increased by ~ 0.5 kcal moil beta to the nitro and nitrite groups. Recommended values for enthalpies of formation of the most stable conformers of nitroacetone, acetonitrite, nitroacetate and acetyl nitrite are -51.6 kcal mol-1, -51.26 kcal mol-1, -45.4 kcal mol-1 and -58.2 kcal mol-1, respectively. The calculated ΔfH°298 for nitroethylene is 7.6 kcal mol-1 and for vinyl nitrite is 7.2 kcal mol-1. The carbonyl and olefin groups retain the major influence on the C-H bond energies. Radicals on carbon adjacent to a nitrite (RC.ONO) group do not exist; they dissociate to the corresponding carbonyl (RC=O + NO) with 38 kcal mol-1 exothermic and no apparent barrier. This results from formation of the strong carbonyl (it bond ~ 80 kcal mol-1) with dissociation of the weak RO--NO bond (-43 kcal mol-1)