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.

Cinchona Alkaloid-Squaramide Catalyzed Sulfa-Michael Addition Reaction: Mode of Bifunctional Activation and Origin of Stereoinduction.

Abstract: The mechanism of the enantioselective sulfa-Michael addition reaction catalyzed by a cinchona alkaloid-squaramide bifunctional organocatalyst was studied using density functional theory (DFT). Four possible modes of dual activation mechanism via hydrogen bonds were considered. Our study showed that Houk’s bifunctional Bronsted acid–hydrogen bonding model, which works for cinchonidine or cinchona alkaloid-urea catalyzed sulfa-Michael addition reactions, also applies to the catalytic system under investigation. In addition, we examined the origin of the stereoselectivity by identifying stereocontrolling transition states. Distortion–interaction analysis revealed that attractive interaction between the substrates and catalyst in the C–S bond forming transition state is the key reason for stereoinduction in this catalytic reaction. Noncovalent interaction (NCI) analysis showed that a series of more favorable cooperative noncovalent interactions, namely, hydrogen bond, π-stacking, and C–H···π interaction and C...

Product energy deposition of CN + alkane H abstraction reactions in gas and solution phases.

Abstract: In this work, we report the first theoretical studies of post-transition state dynamics for reaction of CN with polyatomic organic species. Using electronic structure theory, a newly developed analytic reactive PES, a recently implemented rare-event acceleration algorithm, and a normal mode projection scheme, we carried out and analyzed quasi-classical and classical non-equilibrium molecular dynamics simulations of the reactions CN + propane (R1) and CN + cyclohexane (R2). For (R2), we carried out simulations in both the gas phase and in a CH2Cl2 solvent. Analysis of the results suggests that the solvent perturbations to the (R2) reactive free energy surface are small, leading to product energy partitioning in the solvent that is similar to the gas phase. The distribution of molecular geometries at the respective gas and solution phase variational association transition states is very similar, leading to nascent HCN which is vibrationally excited in both its CH stretching and HCN bending coordinates. This...

Acrolein hydrogenation on Pt(211) and Au(211) surfaces: a density functional theory study

Abstract: Partial hydrogenation of acrolein, the simplest α,β-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.

Potential energy surface for unimolecular dissociation and rearrangement reactions of the ground electronic state of HFCO

Abstract: The potential energy surface of the HFCO molecule in its electronic ground state has been investigated with ab initio method, at levels up to MP4(SDTQ)/6‐311G**//MP2/6‐31G*. At the highest level, the barrier height for molecular dissociation (HFCO→HF+CO) was calculated to be 46.9 kcal/mol with a zero‐point energy correction, in good agreement with an experimental estimate and a recent theoretical result. The intrinsic reaction coordinate (IRC) for molecular dissociation was traced and the coupling between the IRC and normal modes as well as that among the normal modes was analyzed along the IRC. The analysis is consistent with the mode specificity of recently observed quasistable vibrational states of HFCO above the dissociation limit. Almost all possible stationary points on the potential surface of the HFCO system have been located, including the rearrangement and atomic dissociation products and transition states, as well as van der Waals complexes. These are compared with the H2CO system. All the spec...

Rate-Equilibrium Relationships in Hydride Transfer Reactions: The Role of Intrinsic Barriers

Abstract: A literature survey on the kinetics of hydride abstractions from CH-groups by carbocations reveals a general phenomenon: Variation of the hydride acceptor affects the rates of hydride transfer to a considerably greater extent than an equal change of the thermodynamic driving force caused by variation of the hydride donor. The origin of this relationship was investigated by quantum chemical calculations on various levels of ab initio and DFT theory for the transfer of an allylic hydrogen from 1-mono- and 1,1-disubstituted propenes (XYCCH−CH3) to the 3-position of 1-mono- and 1,1-disubstituted allyl cations (XYCCH−CH2+). The discussion is based on the results of the MP2/6-31+G(d,p)//RHF/6-31+G(d,p) calculations. Electron-releasing substituents X and Y in the hydride donors increase the exothermicity of the reaction, while electron-releasing substituents in the hydride acceptors decrease exothermicity. In line with Hammond's postulate, increasing exothermicity shifts the transition states on the reaction co...