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Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin (1a) and acetone, catalyzed by L-leucinol (3a), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn-enamine 6, catalyzed by isatin, was identified as the rate-determining step by both the calculations (G=26.1kcalmol(-1) for the analogous L-alaninol, 3b) and the kinetic isotope effect (k(H)/k(D)=2.7 observed for the reaction using [D-6]acetone). The subsequent reaction of the syn-enamine 6 with isatin produces (S)-2a (calculated G=11.6kcalmol(-1)). The calculations suggest that the overall stereochemistry is controlled by two key events: 1)the isatin-catalyzed formation of the syn-enamine 6, which is thermodynamically favored over its anti-rotamer 7 by 2.3kcalmol(-1); and 2)the high preference of the syn-enamine 6 to produce (S)-2a on reaction with isatin (1a) rather than its enantiomer (G=2.6kcalmol(-1)).

Chirped-pulse (CP) Fourier transform rotational spectroscopy is uniquely suited for near-universal quantitative detection and structural characterization of mixtures that contain multiple molecular and radical species. In this work, we employ CP spectroscopy to measure product branching and extract information about the reaction mechanism, guided by kinetic modeling. Pyrolysis of ethyl nitrite, CH[subscript 3]CH[subscript 2]ONO, is studied in a Chen type flash pyrolysis reactor at temperatures of 1000–1800 K. The branching between HNO, CH[subscript 2]O, and CH[subscript 3]CHO products is measured and compared to the kinetic models generated by the Reaction Mechanism Generator software. We find that roaming CH[subscript 3]CH[subscript 2]ONO → CH[subscript 3]CHO + HNO plays an important role in the thermal decomposition of ethyl nitrite, with its rate, at 1000 K, comparable to that of the radical elimination channel CH[subscript 3]CH[subscript 2]ONO → CH[subscript 3]CH[subscript 2]O + NO. HNO is a signature of roaming in this system., Donors of the American Chemical Society Petroleum Research Fund (Grant 50650-ND6), United States. Dept. of Energy. Office of Basic Energy Sciences (Award DEFG0287ER13671), United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001198), Swiss National Science Foundation (Postdoctoral Research Grant PBEZB2-140081)

Following our previous study of prototypical insertion reactions of energetically asymmetric type with the RPMD (Ring-Polymer Molecular Dynamics) method [Y. Li, Y. Suleimanov, and H. Guo, J. Phys. Chem. Lett. 5, 700 (2014)], we extend it to two other prototypical insertion reactions with much less exothermicity (near thermoneutral), namely, X + H[subscript 2] → HX + H where X = C([superscript 1] D), S([superscript 1] D), in order to assess the accuracy of this method for calculating thermal rate coefficients for this class of reactions. For both chemical reactions, RPMD displays remarkable accuracy and agreement with the previous quantum dynamic results that make it encouraging for the future application of the RPMD to other barrier-less, complex-forming reactions involving polyatomic reactants with any exothermicity., United States. Dept. of Energy. Office of Basic Energy Sciences (Massachusetts Institute of Technology. Energy Frontier Research Center for Excitonics. Combustion Energy Frontier Research Center. Award DE-SC0001198), MIT Energy Initiative

Quantum effects play a crucial role in chemical reactions involving light atoms at low temperatures, especially when a light particle is exchanged between two heavier partners. Different theoretical methodologies have been developed in the last decades attempting to describe zero-point energy and tunneling effects without abandoning a classical or semiclassical framework. In this work, we have chosen the D + HMu → DMu + H reaction as a stress test system for three well-established methods: two representative versions of transition state theory (TST), canonical variational theory and semiclassical instanton, and ring polymer molecular dynamics (RPMD). These calculations will be compared with accurate quantum mechanical results. Despite its apparent simplicity, the exchange of the extremely light muonium atom (0.114 u) becomes a most challenging reaction for conventional methods. The main result of this work is that RPMD provides an overall better performance than TST-based methods for such a demanding reaction. RPMD might well turn out to be a useful tool beyond TST applicability., Spanish Ministry of Science and Innovation (Grant CTQ2012-37404-C02), Spanish Ministry of Science and Innovation (Grant Consolider Ingenio 2010 CSD2009-00038), United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001198)

The thermal rate constant of the O([superscript 3]P) + CH[subscript 4] → OH + CH[subscript 3] reaction is investigated with ring polymer molecular dynamics on a full-dimensional potential energy surface. Good agreement with experimental and full-dimensional quantum multiconfiguration time-dependent Hartree results between 300 and 1500 K was obtained. It is shown that quantum effects, for example, tunneling and zero-point energy, can be effectively and efficiently included in this path-integral based approach implemented with classical trajectories. Convergence with respect to the number of beads is rapid, suggesting wide applicability for other reactions involving polyatomic molecules., United States. Dept. of Energy (DE-FG02-98ER14914), United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Fellowship Award DE-SC0001198)

Using a recently developed full-dimensional accurate analytical potential energy surface [Gonzalez-Lavado, E., Corchado, J. C., and Espinosa-Garcia, J. J. Chem. Phys. 2014, 140, 064310], we investigate the thermal rate coefficients of the O([superscript 3]P) + CH[subscript 4]/CD[subscript 4] reactions with ring polymer molecular dynamics (RPMD) and with variational transition-state theory with multidimensional tunneling corrections (VTST/MT). The results of the present calculations are compared with available experimental data for a wide temperature range 200–2500 K. In the classical high-temperature limit, the RPMD results match perfectly the experimental data, whereas VTST results are smaller by a factor of 2. We suggest that this discrepancy is due to the harmonic approximation used in the present VTST calculations, which leads to an overestimation of the variational effects. At low temperatures the tunneling plays an important role, which is captured by both methods, although they both overestimate the experimental values. The analysis of the kinetic isotope effects shows a discrepancy between both approaches, with the VTST values smaller by a factor about 2 at very low temperatures. Unfortunately, no experimental results are available to shed any light on this comparison, which keeps it as an open question., Gobierno de Extremadura, Spain, La Federacion Espanola de Enfermedades Raras (Project IB10001), United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Center Award DE-SC0001198)

The thermal rate coefficients and kinetic isotope effects have been calculated using ring polymer molecular dynamics (RPMD) for the prototypical reactions between methane and several hydrogen isotopes (H, D, and Mu). The excellent agreement with the theoretical rate coefficients of the H + CH[subscript 4] reaction obtained previously from a multi-configuration time-dependent Hartree calculation on the same potential energy surface provides strong evidence for the accuracy of the RPMD approach. These quantum mechanical rate coefficients are also in good agreement with the results obtained previously using the transition-state theory with semi-classical tunneling corrections for the H/D + CH[subscript 4] reactions. However, it is shown that the RPMD rate coefficients for the ultralight Mu reaction with CH[subscript 4] are significantly smaller than the experimental data, presumably suggesting inaccuracies in the potential energy surface and/or experimental errors. Significant discrepancies between the RPMD and transition-state theory results have also been found for this challenging system., United States. Dept. of Energy (Grant DE-FG02-98ER14914), United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Fellowship Award DE-SC0001198)

The kinetic isotope effect (KIE) of the seven-atom reactions OH + CH[subscript 4] → CH[subscript 3] + H[subscript 2]O and OH + CD[subscript 4] → CD[subscript 3] + HDO over the temperature range 200–1000 K is investigated using ring polymer molecular dynamics (RPMD) on a full-dimensional potential energy surface. A comparison of RPMD with previous theoretical results obtained using transition state theory shows that RPMD is a more reliable theoretical approach for systems with more than 6 atoms, which provides a predictable level of accuracy. We show that the success of RPMD is a direct result of its independence of the choice of transition state dividing surface, a feature that is not shared by any of the transition state theory-based methods. Our results demonstrate that RPMD is a prospective method for studies of KIEs for polyatomic reactions for which rigorous quantum mechanical calculations are currently impossible., United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Fellowship Award DE-SC0001198), United States. Dept. of Energy (Grant DE-SC0001198)