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.

Phase space prediction of product branching ratios: canonical competitive nonstatistical model.

Abstract: We present a new model for predicting branching ratios of chemical reactions when a branching of the reaction path occurs after the dynamical bottleneck, including the case where it occurs after an intermediate. The model is based on combining nonstatistical phase space theory for the direct component of a reaction with variational transition-state theory for an indirect component of reaction. The competition between direct and indirect processes is treated by an extension of the unified statistical model. This new method provides a way to understand the factors that control this kind of chemical reaction and to perform calculations using high-level electronic structure methods for complex systems. The model is based on quantized energy levels of transition states and products, and it involves the same information as required for calculating transition-state rate constants and equilibrium constants plus a phenomenological relaxation time, which was taken from previous work. For the textbook reaction of th...

The Element Effect Revisited: Factors Determining Leaving Group Ability in Activated Nucleophilic Aromatic Substitution Reactions

Abstract: The "element effect" in nucleophilic aromatic substitution reactions (S(N)Ar) is characterized by the leaving group order, F > NO(2) > Cl ≈ Br > I, in activated aryl halides. Multiple causes for this result have been proposed. Experimental evidence shows that the element effect order in the reaction of piperidine with 2,4-dinitrophenyl halides in methanol is governed by the differences in enthalpies of activation. Computational studies of the reaction of piperidine and dimethylamine with the same aryl halides using the polarizable continuum model (PCM) for solvation indicate that polar, polarizability, solvation, and negative hyperconjugative effects are all of some importance in producing the element effect in methanol. In addition, a reversal of polarity of the C-X bond from reactant to transition state in the case of ArCl and ArBr compared to ArF also contributes to their differences in reactivity. The polarity reversal and hyperconjugative influences have received little or no attention in the past. Nor has differential solvation of the different transition states been strongly emphasized. An anionic nucleophile, thiolate, gives very early transition states and negative activation enthalpies with activated aryl halides. The element effect is not established for these reactions. We suggest that the leaving group order in the gas phase will be dependent on the exact combination of nucleophile, leaving group, and substrate framework. The geometry of the S(N)Ar transition state permits useful, qualitative conceptual distinctions to be made between this reaction and other modes of nucleophilic attack.

The reaction electronic flux in chemical reactions

Abstract: The mechanism of a chemical reaction can be characterized in terms of chemical events that take place during the reaction. These events are bond weakening/breaking and/or bond strengthening/forming. The reaction electronic flux (REF), a concept that identifies and rationalizes the electronic activity taking place along the reaction coordinate, has emerged recently as a powerful tool for characterizing the mechanism of chemical reactions. A quantitative theory introducing new descriptors for characterizing reaction mechanisms is presented in detail and three illustrative examples are revisited. In nucleophilic substitution reactions the REF indicates that bond breaking or forming events may be leading the electronic activity whereas in the methanol decomposition reaction by copper oxide, the REF allows to discover that consecutive electronic reductions of copper together with bond breaking processes control the course of the reaction.

Entropy considerations in kinetic method experiments

Abstract: In extended kinetic method experiments, relative binding enthalpies ('affinities') and relative entropies are obtained based on unimolecular dissociation kinetics. A series of ion-bound dimers A-X-B(i) is formed, in which the sample (A) and structurally similar reference molecules (B(i)) are bridged by a central cation or anion (X). The branching ratios of the A-X-B(i) set to A-X and B(i)-X are determined at different internal energies, usually by subjecting A-X-B(i) to collisionally activated dissociation at various collision energies. The dependence of the natural logarithm of the branching ratios on the corresponding B(i)-X bond enthalpies (X affinities of B(i)) is evaluated as a function of internal energy to thereby deduce the A-X bond enthalpy (X affinity of A) as well as an apparent relative entropy of the competitive dissociation channels, Delta(DeltaS(app)). Experiments with proton- and Na(+)-bound dimers show that this approach can yield accurate binding enthalpies. In contrast, the derived Delta(DeltaS(app)) values do not correlate with the corresponding thermodynamic entropy differences between the channels leading to A-X and B(i)-X, even after scaling. The observed trends are reconciled by the transition state switching model. According to this model, the kinetics of barrierless dissociations, such as those encountered in kinetic method studies, are dominated by a family of tight transition states ('entropy bottlenecks') lying lower in energy than the corresponding dissociation thresholds. In general, the relative energies of these tight transition states approximately match those of the dissociation products, but their relative entropies tend to be much smaller, as observed experimentally.

Insertion Reaction of Mn+ Bare Metal Cation into the N−H and C−H Bonds of Ammonia and Methane

Abstract: The Potential Energy Surfaces of the dehydrogenation reaction of NH3 and CH4 molecules by the first-row transition metal cations Mn+ (7S, 5S) were investigated employing the Density Functional (B3LYP) and the CCSD(T) levels of theory. A close description of the reaction paths leading to three dissociation products was given, including several minima and key transition states. The reactions proceed to give dehydrogenation products by oxidative addition of the metal cation into one of the H−X bonds (X = N,C) and formation of the H−M+−XHn-1, hydrido intermediates, which in these cases are also confirmed to represent stable minima along the quintet surface. Because the spin state of the reactants is different from that of intermediates and products an intersystem crossing is proposed to occur. The binding energies of reaction products were calculated and compared with available experimental data to calibrate the quality of our approach.