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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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Journal ArticleDOI
TL;DR: In this paper, the origin of enantioselectivity in dihydroxylation of H2CCH(Ph) catalyzed by (DHQD)2PYDZ·OsO4 (dihydroquinidine)-3,6-pyridazine) is analyzed theoretically by means of hybrid QM/MM calculations with the IMOMM(Becke3LYP:MM3) method.
Abstract: The origin of enantioselectivity in the dihydroxylation of H2CCH(Ph) catalyzed by (DHQD)2PYDZ·OsO4 ((DHQD)2PYDZ = bis(dihydroquinidine)-3,6-pyridazine) is analyzed theoretically by means of hybrid QM/MM calculations with the IMOMM(Becke3LYP:MM3) method. Twelve different possible reaction paths are defined from the three possible regions of entry of the substrate and its four possible orientations and characterized through their respective transition states. The transition state with the lowest energy leads to the R product, in agreement with experimental results. The decomposition of the interaction energy between catalyst and substrate shows how the selectivity is essentially governed by stacking interactions between aromatic rings, with a leading role for the face-to-face interaction between the substrate and one of the quinoline rings of the catalyst.

79 citations

Journal ArticleDOI
TL;DR: In this paper, the G2 method is applied to the study of the hydroxyl radical oxidation of SO2 to SO3 as well as the hydrolysis of SO 2 to H2SO3.
Abstract: The G2 computational method is applied to the study of the hydroxyl radical oxidation of SO2 to SO3 as well as the hydrolysis of SO2 to H2SO3. A key intermediate in the oxidation process is the HOSO2 radical, which is predicted to have a S−OH bond enthalpy (ΔH298) of 26.2 kcal/mol, 4.3 kcal/mol lower than the currently accepted value of 30.5 kcal/mol. The radical is characterized by a 2c-2e S−OH bond with an unpaired electron delocalized into the π* orbital of the SO2 moiety. The hydrolysis of SO2 to H2SO3 was computed with and without a catalytic water. The SO2·H2O and SO2·2H2O complexes and transition states are very similar to those computed for SO3 plus water. The uncatalyzed reaction has an activation barrier of 33.9 kcal/mol, which is reduced to 20.0 kcal/mol with one catalytic water. Since the reaction of SO2 with two waters is nearly thermoneutral (4.5 kcal/mol endothermic), the reaction may be more amenable to thermodynamic study compared to the SO3 + 2H2O reaction, which is much more exothermic.

79 citations

Journal ArticleDOI
TL;DR: The second-order rate constants for reaction of substituted phenoxide ions with 4-nitrophenyl diphenyl phosphate obey a linear equation over a range of 18 substituents as mentioned in this paper.
Abstract: The second-order rate constants (k ArO ) for reaction of substituted phenoxide ions with 4-nitrophenyl diphenyl phosphate obey a linear equation over a range of 18 substituents. The linear plot is consistent with a mechanism involving a single transition state or a two-step process with a very reactive intermediate with two almost identical transition states for its formation and breakdown; the value of the exponent (0.53) is also not consistent with a regular stepwise process with a discrete intermediate. The symmetrical reaction of 4-nitrophenolate ion with the 4-nitrophenyl ester is slightly imbalanced whereby bond formation does not keep up with bond fission in the transition states; the transition state, therefore, has some phosphorylium ion character. Transfer of the diethylphosphoryl group between weakly basic oxyanion nucleophiles is probably a concerted process with a transition state with more of the character of the pentacoordinate intermediate than it has in the corresponding diphenylphosphoryl group transfer

78 citations

Journal ArticleDOI
TL;DR: The key interactions responsible for the unique reactivity of (-)-sparteine-PdX(2) complexes in the enantioselective oxidation of secondary alcohols have been elucidated using quantum mechanics and reveal an important role of the anion, namely to communicate the steric interaction of the ligand on one side of the Pd(II) square plane and the substrate on the other side.
Abstract: The key interactions responsible for the unique reactivity of (−)-sparteine−PdX_2 complexes (X = chloride, acetate) in the enantioselective oxidation of secondary alcohols have been elucidated using quantum mechanics (B3LYP DFT with the PBF polarizable continuum solvent model). From examining many possible pathways, we find the mechanism involves: (1) substitution of the alcohol in place of an X-group, (2) deprotonation of the bound alcohol by the deposed anion and free sparteine, (3) β-hydride elimination through a four-coordinate transition state in which the second anion is displaced but tightly associated, (4) replacement of the ketone product with the associated anion. The enantioselectivities observed under base-rich reaction conditions follow directly from calculated energies of diastereomeric β-hydride elimination transition states incorporating (R) and (S) substrates. This relationship reveals an important role of the anion, namely to communicate the steric interaction of the ligand on one side of the PdII square plane and the substrate on the other side. When no anion is included, no enantioselectivity is predicted. Locating these transition states in different solvents shows that higher dielectrics stabilize the charge separation between the anion and metal and draw the anion farther into solution. Thus, the solvent influences the barrier height (rate) and selectivity of the oxidation.

78 citations

Journal ArticleDOI
TL;DR: The mechanism for the toxic superoxide radical disproportionation to molecular oxygen and hydrogen peroxide by copper-zinc superoxide dismutase (CuZnSOD) has been studied using the B3LYP hybrid density functional and the importance of Asp122 for catalysis is estimated.
Abstract: The mechanism for the toxic superoxide radical disproportionation to molecular oxygen and hydrogen peroxide by copper-zinc superoxide dismutase (CuZnSOD) has been studied using the B3LYP hybrid density functional. On the basis of the X-ray structure of the enzyme, the molecular system investigated includes the first-shell protein ligands of the two metal centers as well as the second-shell ligand Asp122. The substrates of the model reaction are two superoxide radical anions, approaching the copper center at the beginning of two half-reactions: the first part of the catalytic cycle involving Cu + oxidation and the second part reducing Cu 2 + back to its initial state. The quantitative free energy profile of the reaction is obtained and discussed in connection with the experimental data on the reduction potentials and CuZnSOD kinetics. The optimized structures are analyzed and compared to the experimental ones. The two transition states alternate the protonation state of His61 and correspond to histidine Cu-His61-Zn bridge rupture/reformation. Modifications applied to the initial model allow the importance of Asp122 for catalysis to be estimated.

78 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202364
2022136
2021148
2020155
2019145
2018147