Composite ab initio and density functional theory (DFT) methods were used to explore internal hydrogen-atom transfers in a variety of primary, secondary, and tertiary alkyl and functionalized radicals with implications for combustion environments. The composite ab initio method G3MP2B3 was found to achieve the most reasonable balance between accuracy and economy in modeling the energetics of these reactions. Increased alkyl substitution reduced barriers to isomerization by about 10 and 20 kJ mol(-1) for secondary and tertiary radical formation, respectively, relative to primary radical reactions and was relatively insensitive to the transition-state ring size (extent of H-atom internal shift). Reactions involving alkenyl and alkanoyl radicals were also explored. Hydrogen-atom transfers involving allylic radical formation demonstrated barrier heights that were 15-20 kJ mol(-1) lower than those in corresponding alkyl radicals, whereas those involving oxoallylic species (alpha-site radicals of aldehydes and ketones) were 20-40 kJ mol(-1) lower. In the cases of the alkyl radicals, enthalpies of activation were seen to scale with enthalpies of reaction. This correlation was not seen, however, in the cases of the allylic and oxoallylic radicals; this fact has significant implications in combustion chemistry and mechanism development, considering that such Evans-Polanyi correlations are widely used in estimating barrier heights for rate expressions.

The conformational distribution and unimolecular decomposition pathways for the n-propylperoxy radical have been generated at the CBS-QB3, B3LYP/6-31+G and mPW1K/6-31+G levels of theory. At each of the theoretical levels, the 298 K Boltzmann distributions and rotational profiles indicate that all five unique rotamers of the n-propylperoxy radical can be expected to be present in significant concentrations at thermal equilibrium. At the CBS-QB3 level, the 298 K distribution of rotamers is predicted to be 28.1, 26.4, 19.6, 14.0, and 11.9% for the gG, tG, gT, gG', and tT conformations, respectively. The CBS-QB3 C-OO bond dissociation energy (DeltaH298 K) for the n-propylperoxy radical has been calculated to be 36.1 kcal/mol. The detailed CBS-QB3 potential energy surface for the unimolecular decomposition of the n-propylperoxy radical indicates that important bimolecular products could be derived from two 1,4-H transfer mechanisms available at T< 500 K, primarily via an activated n-propylperoxy adduct.

Cavity ringdown spectra of the A-X electronic transition of the 1-propyl and 2-propyl peroxy radicals are reported. Spectroscopic assignments are facilitated by implementing several production mechanisms, either isomer-specific or not. Assignments of specific spectral lines to particular conformers of a given isomer are suggested. Observations on the temporal decay of the various species are reported.

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Standardized electronic formats for data are needed to efficiently and transparently communicate the results of scientific studies. A format for the unique identification of chemical species is a requirement in the field of chemistry, and the IUPAC International Chemical Identifier (InChI) has been widely adopted for this purpose. The InChI identifier has proved to be very useful. The InChI identifier, however, is currently insufficient to uniquely specify some types of molecular entities at a detailed molecular level needed to fully characterize their chemical nature, to differentiate between chemically distinct conformers, to uniquely identify structures used in quantum chemical calculations, and to completely describe elementary chemical reactions. To address this limitation, we propose an augmented form of InChI, denoted as InChI–ER, which contains additional optional layers that allow the unique and unambiguous identification of molecules at a detailed molecular level. The new layers proposed herein are optional extensions of the existing InChI formalism and, like all other InChI layers, would not interfere with InChI identifiers currently in use. The focus of the present work is the better specification of required molecular entities such as rotational conformations, ring conformations, and electronic states. In companion articles, we propose additional reaction layers using an extended InChI format that will enable the unique identification of elementary chemical reactions, including specification of associated transition states, specification of the changes in bonds that occur during reaction, and classification of reaction types.