Transition state

About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.

Transition state geometry prediction using molecular group contributions

Abstract: Detailed kinetic models to aid the understanding of complex chemical systems require many thousands of reaction rate coefficients, most of which are estimated, some quite approximately and with unknown uncertainties. This motivates the development of high-throughput methods to determine rate coefficients via transition state theory calculations, which requires the automatic prediction of transition state (TS) geometries. We demonstrate a novel approach to predict TS geometries using a group-additive method. Distances between reactive atoms at the TS are estimated using molecular group values, with the 3D geometry of the TS being constructed by distance geometry. The estimate is then optimized using electronic structure theory and validated using intrinsic reaction coordinate calculations, completing the fully automatic algorithm to locate TS geometries. The methods were tested using a diisopropyl ketone combustion model containing 1393 hydrogen abstraction reactions, of which transition states were found for 907 over two iterations of the algorithm. With sufficient training data, molecular group contributions were shown to successfully predict the reaction center distances of transition states with root-mean-squared errors of only 0.04 A.

An ab initio determination of the rate constant for H 2 +C 2 H --> H+C 2 H 2

Abstract: The first ab initio determination of the rate constants for the reaction of ethynyl radical C2H with molecular hydrogen is presented. The potential energy surface was determined in the seagent and Saddle point regions using configuration interaction methods and the rate constant was evaluated using transition state theory. (AIP)

Theoretical and experimental study of the product branching in the reaction of acetic acid with OH radicals.

Abstract: The product distribution of the reaction of acetic acid, CH3COOH, with hydroxyl radicals, OH, was studied experimentally and theoretically. Mass-spectrometric measurements at 290 K and 2 Torr of He of the CO2 yield versus the loss of acetic acid yielded a branching fraction of 64 ± 14% for the abstraction of the acidic hydrogen as follows: CH3COOH + OH → CH3COO + H2O → CH3 + CO2 + H2O. A quantum chemical and theoretical kinetic analysis showed that the abstraction of the acidic hydrogen is enhanced relative to the abstraction of −CH3 hydrogens because of the formation of a strong pre-reactive H-bonded complex, where the H-bonds are retained in the H-abstraction transition state. The potential energy surface of the reaction is explored in detail, and the reaction products of the individual channels are identified. The theoretical product branching is found to be critically dependent on the energetic and rovibrational differences between the H-abstraction transition states.

Semiautomated Transition State Localization for Organometallic Complexes with Semiempirical Quantum Chemical Methods

Abstract: We present an efficient computational protocol for robust transition state localization that can be routinely applied to complex (organometallic) reactions. The capabilities of the combination of extended tight-binding semiempirical methods (GFNn-xTB) with a state-of-the-art transition state localization algorithm (mGSM) is demonstrated on a modified version of the MOBH35 benchmark set, consisting of 29 organometallic reactions and transition states. Furthermore, for three examples we demonstrate how error-prone the conventional (manual) approach based on chemical intuition can be and how errors are avoided by a semiautomated generation of reaction profiles. The performance of the GFNn-xTB methods is carefully assessed and compared with that of the widely used PM6-D3H4 and PM7 semiempirical methods. The GFNn-xTB methods show much higher success rates of 89.7% (GFN1-xTB) and 86.2% (GFN2-xTB) compared with 72.4% for PM6-D3H4 and 69.0% for PM7. The barrier heights and reaction energies are computed with much better accuracy at reduced computational cost for the GFNn-xTB methods compared with the PMx methods, allowing a semiquantitative assessment of possible reaction pathways already at a semiempirical level. The mean error of GFN2-xTB for the barrier heights (8.2 kcal mol-1) is close to what low-cost density functional approximations provide and substantially smaller than the corresponding error of the competitor methods.

Interplay of structure and reactivity in a most unusual furan Diels-Alder reaction

Abstract: Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidly and in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the same conditions. These reactions have been explored using density functional theory (DFT) calculations. They reveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl4 and by polar solvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding to the stepwise formation of the two new carbon−carbon bonds via transition states which have similar energies in all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predicted by our calculations. The calculated free energy barriers generally correlate we...