Topic
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.
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TL;DR: In this paper, the effect of water molecules on the polymerization of acrylamide (AA) and methacrylamides (MAA) was studied by an extensive set of computational methods with particular focus on the possible influence of water molecule on the propagation reaction.
Abstract: The polymerization of acrylamide (AA) and methacrylamide (MAA) was Studied by an extensive set of computational methods With it particular focus on the possible influence of water molecules on the propagation reaction. Ail extensive set of electronic structure methods was tested, consisting of B3LYP, BMK, MPWB1K, MP2, and B2-PLYP of which some include dispersion effects, The effect of water on the transition state is modeled in two different ways. Explicit water Molecules are added to the system, showing that replacing the hydrogen bond that dominates the transition State Structure by a water-mediated hydrogen bond, results in more stable, more feasible transition states. This effect is the largest for AA polymerization, a monomer that is known to experience a larger solvent effect than MAA. Additionally, a conductor-like polarizable continuum model (C-PCM) is applied on both the transition states in gas phase and the ones bearing explicit water molecules. This model has a dramatic effect on all the propagation rates, raising them by about 3 orders of magnitude. The inclusion of explicit water molecules gives insight into the role of water molecules and the formation of prereactive complexes. The relative rate of polymerization of AA with regard to MAA is well reproduced for a trimeric propagating radical with inclusion of explicit water molecules or by using an implicit solvation model at the BMK and MPWB1K level of theory.
50 citations
50 citations
TL;DR: This study establishes the importance of including explicit solvent molecules in situations in which the gas-phase and continuum models are inadequate in obtaining meaningful insight regarding experimental stereoselectivities.
Abstract: A proline-catalyzed asymmetric Michael addition between ketones and trans-beta-nitrostyrene was studied by using the density-functional theory with mPW1PW91 and B3LYP functionals. Improved insight into the enantio- and diastereoselective formation of gamma-nitroketones/-aldehydes is obtained through transition-state analysis. Consideration of the activation parameters obtained from gas-phase calculations and continuum solvation models failed to reproduce the reported experimental stereoselectivities for the reaction between cyclohexanone and 3-pentanone with trans-beta-nitrostyrene. The correct diastereo- and enantioselectivites were obtained only upon explicit inclusion of solvent molecules in the diastereomeric transition states that pertain to the C--C bond formation. Among the several transition-state models that were examined, the one that exhibits cooperative hydrogen-bonding interactions with two molecules of methanol could explain the correct stereochemical outcome of the Michael reaction. The change in differential stabilization that arises as a result of electrostatic and hydrogen-bonding interactions in the key transition states is identified as the contributing factor toward obtaining the correct diastereomer. This study establishes the importance of including explicit solvent molecules in situations in which the gas-phase and continuum models are inadequate in obtaining meaningful insight regarding experimental stereoselectivities.
50 citations
TL;DR: In this paper, three different pathways for ethylene decomposition were investigated and it was shown that due to the excess energy available in the photochemical experiments, all three mechanisms can give rise to rotationally hot H2 when proper account is taken of the transverse vibrational modes along the reaction paths.
Abstract: Ab initio quantum chemical methods are applied to the study of ethylene decomposition to acetylene and molecular hydrogen in the ground electronic state. Results are reported on three different pathways for ethylene decomposition—two stepwise processes involving a hydrogen transfer followed by 1,1 elimination of H2, or vice versa, and a 1,2 elimination. The latter proceeds through an energy maximum with two imaginary frequencies, rather than one as for conventional transition states. Ethylidene and vinylidene are predicted to be stationary points on the C2H4 and C2H2 potential energy surfaces, respectively. Recent photochemical studies have observed rotationally hot H2. It is shown that due to the excess energy available in the photochemical experiments, all three mechanisms can give rise to rotationally hot H2 when proper account is taken of the transverse vibrational modes along the reaction paths.
50 citations
TL;DR: In this article, the problem of intramolecular hydrogen atom exchange in the OsH3(BH4)(PR3)2 system was examined from both theoretical and experimental points of view.
Abstract: The problem of intramolecular hydrogen atom exchange in the OsH3(BH4)(PR3)2 system is examined from both theoretical and experimental points of view, through ab initio MO calculations on the OsH3(BH4)(PH3)2 system at the MP2, MP4, and CCSD(T) computational levels and variable-temperature 1H NMR studies on the OsH3(BH4)(PiPr3)2 complex. Three different exchange processes are fully characterized from a theoretical point of view through location of intermediates and transition states. Experimental results supporting the existence of these three different exchange processes and providing definitely its intramolecular nature are also presented.
50 citations