Roaming atoms and radicals: a new mechanism in molecular dissociation.

Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
Accounts of Chemical Research (Impact Factor: 24.35). 08/2008; 41(7):873-81. DOI: 10.1021/ar8000734
Source: PubMed

ABSTRACT The detailed description of chemical reaction rates is embodied in transition state theory (TST), now recognized as one of the great achievements of theoretical chemistry. TST employs a series of simplifying assumptions about the dynamical behavior of molecules to predict reaction rates based on a solid foundation of quantum theory and statistical mechanics. The study of unimolecular decomposition has long served as a test bed for the various assumptions of TST, foremost among which is the very notion that reactions proceed via a single well-defined transition state. Recent high-resolution ion imaging studies of formaldehyde unimolecular decomposition, in combination with quasiclassical trajectory calculations from Bowman and coworkers, have shown compelling evidence, however, for a novel pathway in unimolecular decomposition that does not proceed via the conventional transition state geometry. This "roaming" mechanism involves near dissociation to radical products followed by intramolecular abstraction to give, instead, closed shell products. This phenomenon is significant for a number of reasons: it resists easy accommodation with TST, it gives rise to a distinct, highly internally excited product state distribution, and it is likely to be a common phenomenon. These imaging studies have provided detailed insight into both the roaming dynamics and their energy-dependent branching. The dynamics are dominated by the highly exoergic long-range abstraction of H from HCO by the "roaming" hydrogen atom. The energy-dependent branching may be understood by considering the roaming behavior as being descended from the radical dissociation; that is, it grows with excess energy relative to the conventional molecular dissociation because of the larger A-factor for the radical dissociation. Recent work from several groups has identified analogous behavior in other systems. This Account explores the roaming behavior identified in imaging studies of formaldehyde and considers its implications in light of recent results for other systems.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Direct dynamics simulations were performed to understand the mechanisms of the N(4S) + CH3 reaction on both the triplet and singlet potential energy surfaces using the B3LYP/6-31(d, p) method with modified hybrid parameters for the exchange-correlation functional. The hybrid parameters were determined so as that the stationary-point energy levels on the potential energy surfaces reproduce those obtained at the CASPT2 results. The trajectories starting from the N(4S) + CH3 reactants on the triplet potential surface mostly lead to the H + H2CN products via the CH3N intermediate. The relative translation energy distribution of this channel shows that the reaction dynamics is nonstatistical. The trajectories on the singlet potential energy surface were also propagated from the CH2=NH intermediate structure. We found that about 10% of the reactive trajectories showed “roaming dynamics,” which lead to the H2 + HCN/HNC products through intramolecular hydrogen abstraction.
    Computational and Theoretical Chemistry 03/2015; 1061. DOI:10.1016/j.comptc.2015.03.015 · 1.37 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH3NO2 decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH3 and NO2 radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3 with NO2.
    The Journal of Physical Chemistry A 04/2015; DOI:10.1021/acs.jpca.5b01563 · 2.78 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Non-adiabatic processes play an important role in photochemistry, but the mechanism for conversion of electronic energy to chemical energy is still poorly understood. To explore the possibility of vibrational control of non-adiabatic dynamics in a prototypical photoreaction, namely, the A-band photodissociation of NH3(X̃(1)A1), full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH3(X̃(1)A1) is investigated on recently developed coupled diabatic potential energy surfaces. The experimentally observed H atom kinetic energy distributions are reproduced. However, contrary to previous inferences, the NH2(Ã(2)A1)/NH2(X̃(2)B1) branching ratio is found to be small regardless of the initial preparation of NH3(X̃(1)A1), while the internal state distribution of the preeminent fragment, NH2(X̃(2)B1), is found to depend strongly on the initial vibrational excitation of NH3(X̃(1)A1). The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH2(X̃(2)B1) fragment, rather than in NH2(Ã(2)A1) as previously proposed. The high internal excitation of the NH2(X̃(2)B1) fragment is attributed to the torques exerted on the molecule as it passes through the conical intersection seam to the ground electronic state of NH3. Thus in this system, contrary to previous assertions, the control of electronic state branching by selective excitation of ground state vibrational modes is concluded to be ineffective. The juxtaposition of precise quantum mechanical results with complementary results based on quasi-classical surface hopping trajectories provides significant insights into the non-adiabatic process.
    The Journal of Chemical Physics 03/2015; 142(9):091101. DOI:10.1063/1.4913633 · 3.12 Impact Factor