Determination of Kinetic Properties in Unimolecular Dissociation of Complex Systems from Graph-Theory Based Analysis of an Ensemble of Reactive Trajectories.

<div>We describe and apply a general approach based on graph-theory to obtain kinetic and structural properties from direct dynamics simulations. In particular, we focus on the unimolecular fragmentation of complex systems in which, prior to dissociation, different events can take place, and notably isomerizations and formation of ion-molecule complex.</div><div>3-state and 4-state kinetic models are thus obtained and rate constants for global or specific pathways are obtained from direct counting and flux calculation, both being in agreement.<br />Finally, we show how a theoretical mass spectrum can also be obtained automatically.<br /></div>

Watching a hydroperoxyalkyl radical (•QOOH) dissociate

A prototypical hydroperoxyalkyl radical (•QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of •QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate—again increased by heavy-atom tunneling—which are required for global models of atmospheric and combustion chemistry.

Noble Gas in a Ring

We have designed a new type of molecule with a noble gas (Ng = Kr and Xe) atom in a six-membered ring. Their structures and stability have been studied by density functional theory and by correlated electronic structure calculations. The results showed that the six-membered ring is planar with very short Ng–O and Ng–N polar covalent bonds. The calculated energy barriers for all the unimolecular dissociation pathways are higher than 20 and 35 kcal/mol for Ng = Kr and Xe, respectively. The current study suggests that these molecules and their derivatives might be synthesized and observable at cryogenic conditions.

Determination of Kinetic Properties in Unimolecular Dissociation of Complex Systems from Graph-Theory Based Analysis of an Ensemble of Reactive Trajectories.

<div>We describe and apply a general approach based on graph-theory to obtain kinetic and structural properties from direct dynamics simulations. In particular, we focus on the unimolecular fragmentation of complex systems in which, prior to dissociation, different events can take place, and notably isomerizations and formation of ion-molecule complex.</div><div>3-state and 4-state kinetic models are thus obtained and rate constants for global or specific pathways are obtained from direct counting and flux calculation, both being in agreement.<br>Finally, we show how a theoretical mass spectrum can also be obtained automatically.<br></div>

Determination of Kinetic Properties in Unimolecular Dissociation of Complex Systems from Graph-Theory Based Analysis of an Ensemble of Reactive Trajectories.

<div>We describe and apply a general approach based on graph-theory to obtain kinetic and structural properties from direct dynamics simulations. In particular, we focus on the unimolecular fragmentation of complex systems in which, prior to dissociation, different events can take place, and notably isomerizations and formation of ion-molecule complex.</div><div>3-state and 4-state kinetic models are thus obtained and rate constants for global or specific pathways are obtained from direct counting and flux calculation, both being in agreement.<br>Finally, we show how a theoretical mass spectrum can also be obtained automatically.<br></div>