Showing papers on "Transition state published in 1989"
01 Jan 1989
TL;DR: In this article, a qualitative valence bond model for organic reactions is proposed for the Diels Alder reaction, which is based on the potential energy surface topology of potential energy surfaces.
Abstract: Multidimensional theoretical stereochemistry and conformational potential energy surface topology.- Some practical suggestions for optimizing geometries and locating transition states.- Reaction topology and quantum chemical molecular design on potential energy surfaces.- Topology of molecular shape and chirality.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. I. Theory.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. II. An illustrative application to the Diels Alder reaction.- A qualitative valence bond model for organic reactions.- Solvent effects on potential energy surfaces and chemical kinetics.- Modifications of potential energy surfaces by solvation and catalysis.- Computational tests of potential energy surfaces from dynamical properties.- Dynamical formulation of transition state theory: variational transition states and semiclassical tunneling.- Theoretical models for reaction dynamics in polyatomic molecular systems.- Practical applications of new theoretical concepts in organic chemistry.
215 citations
TL;DR: In this article, the electronic structure and geometry of alkoxy groups on the surface of zeolites with a high silica content were analyzed and the net positive charge of the alkyl fragments was found to be low and almost constant when passing from methoxyl to isopropoxyl substituents.
168 citations
TL;DR: It is concluded that enzyme active sites provide specific polar environments that do not resemble the gas phase but that are designed for electrostatic stabilization of ionic transition states and that "solvate" these states more than water does.
Abstract: Considerable attention has recently been drawn to the hypothesis that enzymes catalyze their reactions by displacing solvent and creating an environment similar to the gas phase for the reacting substrates. This "desolvation hypothesis" is reexamined in this paper by defining a common reference energy for reactions in various environments. It is argued that consistent attempts to describe the actual energetics of enzymatic reactions, taking either gas phase or solution as a reference, would contradict the above hypothesis. That is, the enzyme does remove water molecules from its substrate, but substitutes these molecules for another polar environment (namely, its active site). By taking amide hydrolysis as an example, we use experimentally estimated solvation energies and analyze the reaction profile in the gas phase, in solution, and in enzyme active sites. We show that the gas-phase reaction is characterized by an enormous activation barrier (associated with forming the charged nucleophile from neutral fragments), although the nucleophilic attack is essentially barrierless. On the other hand, the enzyme and solution reactions are found to have similar reaction profiles, with a lower activation barrier for the enzymatic reaction. Presumably, the fact that previous analyses of this problem did not involve the construction of the relevant thermodynamic cycles (and quantitative estimates of the corresponding solvation energies) led to the desolvation hypothesis. Our conclusion is that enzyme active sites provide specific polar environments that do not resemble the gas phase but that are designed for electrostatic stabilization of ionic transition states and that "solvate" these states more than water does.
131 citations
TL;DR: In this paper, transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclop-polycyclopene were determined, and the zero-point energies were calculated, and activation energies for pertinent reactions were evaluated.
Abstract: SCF, MCSCF, and CI calculations have been carried out to study thermal interconversions occurring on the singlet C{sub 3}H{sub 4} surface. Transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclopropylidene were determined. In addition, the zero-point energies were calculated, and the activation energies for pertinent reactions were evaluated. The thermal rearrangement of allene to methylacetylene was found to proceed in four steps via vinylmethylene, cyclopropene, and propenylidene with the activation energy of 65.8 kcal/mol, which is in good agreement with the observed values of 60.5 and 63.8 kcal/mol. The same reaction paths can also apply to pyrolysis of cyclopropene, in which it undergoes conversion to methylacetylene via propenylidene more easily than to allene via vinylmethylene; the calculated activation energies are 38.1 and 43.4 kcal/mol, respectively. These are again in excellent agreement with the observed values of 37.5 and 43.3 kcal/mol. The activation energies for the allene to cyclopropylidene and the reverse conversions were calculated to be 72.2 and 10.2 kcal/mol. This indicates that cyclopropylidene may not be involved in the interconversion of allene, cyclopropene, and methylacetylene. One of the significant findings in this study is the reaction path for the cyclopropenemore » to methylacetylene conversion via propenylidene, which is less energy demanding than that via vinylmethylene. This made the calculated mechanisms in accord with the experimental data. Furthermore, we will present and discuss reaction mechanisms for pyrolysis of singly and doubly substituted cyclopropene in which this particular reaction path is expected to play a dominant role.« less
86 citations
TL;DR: In this article, the authors investigated the dynamics of water binding structure reorganization by analyzing the potential energy surfaces involved and found that the energy barrier heights, the transition state energies, along the reaction coordinates are mostly distributed in the range of 0.2-6 kcal/mol.
Abstract: Dynamics of water binding structure reorganization is investigated by analyzing the potential energy surfaces involved. The water structures in a trajectory are quenched to their local minima, called the inherent structures. The reaction coordinates, which connect the inherent structures successively visited by the system, are determined. It is found that the energy barrier heights, the transition state energies, along the reaction coordinates are mostly distributed in the range of 0.2–6 kcal/mol. The classification of inherent structures is made to groups of ‘‘overall‐inherent structures’’; successive inherent structures are most often not so geometrically distinct. It is found that transitions between the overall‐inherent structures, involving large collective motions, occur in the subpicosecond time scale. Individual molecular motions in these collective motions are stongly correlated, not yielding large transition energies. The transition state energy sometimes reaches up to 20 kcal/mol, when the system goes through the ridge between deep minima, yielding ballistic dynamical behavior. Temperature dependence of the collective motions is also investigated.
62 citations
TL;DR: In this article, the behavior of α-alkoxyallyltributyltins has been studied in terms of chemo-, regio- and stereoselectivity, and an interpretation of the stereochemical trends is given on the basis of the interactions occurring in usually accepted transition states.
37 citations
TL;DR: In this paper, a method of estimating the strength of binding of transition states to catalysts is applied to reactions mediated by cyclomalto-oligosaccharides (cyclodextrins, CDs).
36 citations
27 citations
TL;DR: The geometries and energies of the reactants, transition state, and products for the gas phase reaction were obtained from abinitio calculations using a closed shell as mentioned in this paper, where the transition state and products were derived from ab initio calculations.
Abstract: The geometries and the energies of the reactants, transition state, and products for the gas phase reaction: Cl− + CH3Br = ClCH3 + Br−, were obtained from abinitio calculations using a closed shell...
24 citations
TL;DR: In this article, various mechanisms leading to the dissociation of ethynylsilane have been investigated by ab initio molecular orbital methods and various configurations corresponding to the reactant, transition states, and products have all been optimized at the HF/3-21G and HF/6-31 G* levels of theory.
Abstract: Various mechanisms leading to the dissociation of ethynylsilane have been investigated by ab initio molecular orbital methods. Geometries corresponding to the reactant, transition states, and products have all been optimized at the HF/3-21G and HF/6-31 G* levels of theory. Heats of reaction and barrier heights have been obtained at the MP4SDTQ/6-31G* level. Zero-point energy corrections and harmonic vibrational frequencies have been computed at the HF/3-21G level. These results have been used to calculate unimolecular dissociation rate constants by RRKM theory. This information is then used to reexamine the mechanisms of dissociation for ethynylsilane from previous experimental shock-tube and stirred-flow studies.
23 citations
TL;DR: In this article, a 3-21+G energy surface corresponding to the proton transfer reaction in the hydroperoxyl anion solvated by one water molecule is presented.
TL;DR: The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex.
Abstract: The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.
TL;DR: In this article, various modifications of the Marcus equation have been applied to the problem of photochemical proton transfer, using available data on general acid-catalyzed photohydration reactions, and the overall conclusion from all of these approaches is that the reactions are characterized by somewhat asymmetric and loose transition states, with a small but significant degree of charge development on the in-flight proton at the transition state.
TL;DR: In this article, the rate constants for the reaction of nitrate radical (NO3·) with alcohols have been determined by flash photolysis method in aqueous and nonaqueous solutions.
Abstract: The rate constants for the reaction of nitrate radical (NO3·) with alcohols have been determined by flash photolysis method in aqueous and nonaqueous solutions. The reactivity of NO3· in acetonitrile was higher than that in aqueous solution, suggesting that hydrogen bonds between solvent and NO3· or alcohols decrease the reaction rate. From the deuterium-isotope effect, the main reaction path was confirmed to be hydrogen atom abstraction from the carbon atom bonded with the OH group. For 2-methyl-2-propanol, the hydrogen atom abstraction from the OH group was presumed. These reaction paths were confirmed, since the reaction rate constants decreased with an increase in the corresponding bond-dissociation energies. From strong dependency of the rate constants upon the ionization energies of alcohols, the polar transition states of the reactions were suggested.
TL;DR: The results of a microcanonical variational transition state theory study of the HO2 and HeH+2 systems were reported in this paper, where two transition states were found for HO2→OH+O half-reaction.
Abstract: The results of a microcanonical variational transition state theory study of the HO2 and HeH+2 systems are reported. The calculations were carried out using a modification of the Wardlaw–Marcus flexible transition state theory method in which the sum of states N(E,R) is calculated directly. For the HO2 system, the results obtained using the Melius–Blintpotential surface are in excellent agreement with previously reported variational transition state theory calculations of Rai and Truhlar on this system. In particular, two transition states were found for the HO2→OH+O half‐reaction. However, calculations using the Lemon–Hase surface produced only one transition state. From calculations carried out on surfaces constructed by combining the two original surfaces and calculations carried out using an orbiting transition state variational model (i.e., ignoring the angular part of the potential), it is shown conclusively that the two transition states found for the Melius–Blint surface arise from the peculiar sh...
TL;DR: The large kH/kD isotope effects obtained in the hydride transfer reactions of triarylmethanes with triarylmethyl cations support linear rather than nonlinear transition states as discussed by the authors.
TL;DR: In this article, a nother approach based on an expansion of configuration of the transition states is presented, and the r ates are estimated in terms of the following parameters: reaction energy, force constants, equilibrium bond lengths, transition state bond order and capacity to store energy.
Abstract: The current approach of Marcus theory to interpret electron transfer is questioned. A nother approach based on an expansion of configuration of the transition states is presented, and the r ates are estimated in terms of the following parameters: reaction energy, force constants, equilibrium bond lengths, transition state bond order and capacity to store energy. The model can interpret several anomalous features of these reactions, namely electron-exchang es where the Marcus theory e stimates rates several orders of magnitude slower and faster than experiment, "cross-relations ", solvent effects, t he i nverted region and the asymmetry of the T afel plots of metal-aquo ions, and can a ssess the nonadiabatic character of some outer-sphere processes. Electron transfer reactions play an essential role in many physical, chemical and biological processes. The investigation of the mechanisms of these reactions rests essentially on the systematic investigation of structure-reacti vity relationships that results from the geometric rearrangements which accompany the change in oxidation s tates of the coordination compounds. Although many theories have been proposed, it is no surprise that the more simpler ones such as Marcus theory are t he most popular.1 In spite of the great success of the theory of Marcus in interpreting several of these structure-reacti vity relations namely in terms of the reaction energy (AG), changes in equilibrium bond lengths (lred-lox) and metal-ligand force constants (fox and fred), several problems r emain.
Journal Article•
TL;DR: In this article, an ab initio 3-21G study of the direct addition of HF to C2HnF(4-n), with n = 0 to 4, has been performed to investigate the effect of the substituent on the reaction.
Abstract: An ab initio 3-21G study of the direct addition of HF to C2HnF(4–n), with n = 0 to 4, has been performed to investigate the effect of the substituent on the reaction. Geometry optimization of all charge-transfer complexes and transition states has been done. Standard analysis of activation energies of addition reactions, vibrational and thermodynamical analysis, as well as Morokuma energy decomposition, BSSE correction, PMO analysis, and Pauling bond orders were used to explain the results. A subset of the reactions, including that of C2H4 as reference one and the two most favorable cases, was also studied at the MP2/6–31G(d,p)//HF/6–31G(d,p) level. The barriers so obtained are in agreement with the indirectly found from experimental data. It was found that the effect of the substituent is not monotonic for the additions. Decomposition of the interaction energy is shown to be adequate to explain this nonmonotonic behavior. The implications for laser chemistry of the addition of hydrogen halides to fluorosubstituted olefins is briefly discussed.
TL;DR: In this paper, the authors presented the details of the mechanisms of the dehydration and decarboxylation processes of acetic acid using ab initio MO calculations, and confirmed the full vibrational analysis at the HF/STO-3G level.
Abstract: This paper presents the details of the mechanisms of the dehydration and decarboxylation processes of acetic acid proposed recently by Nguyen and Ruelle, using ab initio MO calculations. The two transition states corresponding to the first and second step, the intermediate, 1,1-dihy-droxyethylene(1,1-ethenediol), of the dehydration process and one transition state of the decarboxylation reaction were optimized by gradient techniques and Powell's method using STO-3G and 3-21G basis sets. All of the transition states have been confirmed by full vibrational analysis at the HF/STO-3G level. For the first and second steps of the dehydration reaction, the activation energies calculated at the MP4 level were 359.8 and 186.0 kJ mol −1 respectively, while the decarboxylation reaction pathway is found to be via a four-centre transition state with 325.2 kJ mol −1 activation energy.
TL;DR: In this article, the authors investigated the kinetics of the hydrolysis of the title compound in moderately concentrated aqueous sulphuric acid media at 30 °C and determined the activation parameters.
Abstract: The kinetics of the hydrolysis of the title compound, (3), have been investigated in moderately concentrated aqueous sulphuric acid media at 30 °C. Activation parameters have also been determined. Rate correlation by the Cox–Yates excess acidity method shows that hydrolysis occurs from the monoprotonated substrate by the A-SE2 mechanism of the SNAr type. Nucleophilic attack by water at the aryl carbon and a subsequent proton-transfer equilibrium are fast processes which precede the electrophilically catalysed separation of the leaving group, in which the functional catalysts are all general acids in solution. An abnormal value of 1.4 is obtained for the slope parameter m‡(Kresge's αA); this is discussed in terms of differential solvation of the initial and transition states. The Bunnett–Olsen slope parameter (φ‡–φe) of –2.0 indicates that the transition state of the reaction is substantially less solvated than its initial state. The values of ΔH‡ and ΔS‡ vary to compensate each other and the decreasing ΔS‡ values accord with the ordered transition state proposed.
TL;DR: In this article, the potential energy surfaces for the Diels-Alder reactions between substituted butadienes and ethenes have been investigated by intensive RHF- and UHF-grid searches using the AM1 method.
Abstract: The potential-energy surfaces for the Diels–Alder reactions between substituted butadienes and ethenes have been investigated by intensive RHF- and UHF-grid searches using the AM1 method. The RHF/AM1 potential surface show a concerted, synchronous path, but the UHF/AM1 potential surface has four transition states, two intermediates and two maxima with two stepwise, diradical reaction paths. The diagonal, concerted path in the UHF/AM1 potential surface vanishes owing to the coalescence of two maxima induced by the two stable intermediates on the two-dimensional contour diagram. Substituent effects on the activation barriers are of electronic and steric origin, and only the UHF/AM1 results agreed with experiments.
TL;DR: A mixture of tetrachiral deca-1, 7E, 9-triene 5 and its 7z isomer 6 was prepared from D-glucose by the use of two Wittig olefination reactions as discussed by the authors.
TL;DR: In this article, the activation entropies and the statistical A factors of these two reactions were estimated by the use of RRKM theory, and they were shown to be formed from both HOMO-LUMO and HO-HOMO interactions.
Abstract: The ab initio calculation has been performed with the addition pathways of HLi and its dimer to acetylene at the RHF/3-21G basis set. It shows that the reaction mechanisms of these two reactions are rather similar. In either of two reaction pathways, there is a meta-stable molecular complex near the isolated reactant state. This kind of addition can be treated approximately as the unimolecular reaction in which the molecular complex rearranges into the product. We have estimated the activation entropies and the statistical A factors of these two reactions by the use of RRKM theory. Frontier molecular orbital analysis of these two transition states reveals their HOMOS to be formed from both HOMO-LUMO and HOMO-HOMO interactions.
TL;DR: In this article, second-order rate coefficients and activation parameters are reported for the reaction in solution of triphenylphosphine with a series of diazoalkanes (Ar2CN2) having two aromatic groups attached to the diaz-carbon atom, 9-diazofluorene (DAF), 5-diazo-5H-dibenzo[a,d]cycloheptene (DBSE), its 10,11-dihydro analogue (DBSA), and diazodiphenylmeth
Abstract: Second-order rate coefficients and activation parameters are reported for the reaction in solution of triphenylphosphine with a series of diazoalkanes (Ar2CN2) having two aromatic groups attached to the diazo-carbon atom, 9-diazofluorene (DAF), 5-diazo-5H-dibenzo[a,d]cycloheptene (DBSE), its 10,11-dihydro analogue (DBSA), and diazodiphenylmethane (DDM). Although the product phosphazine Ar2CNNPPh3 arises formally by nucleophilic attack of phosphorus on the terminal nitrogen atom of the diazoalkane, the pattern of reactivity approximates to that for proton transfer from acetic acid to the diazoalkane under similar conditions, highlighting the biphilic nature of the reaction. The results can be rationalised in terms of qualitative FMO theory taking into account both HOMO(Ph3P)–LUMO (Ar2CN2) and HOMO(Ar2CN2)–LUMO (Ph3P) interactions. This is lent qualified support by MNDO calculations on Ar2CN2 based on molecular parameters from the X-ray crystal structure of DAF itself and of the ketones corresponding to DBSE, DBSA, and DDM.The reactivity of DDM and the effect of 4,4′-disubstitution by CH3O–, CH3–, and Cl–, which gives rise to a U-shaped Hammett ρσ correlation, is not easily interpreted within the FMO framework. Conformational changes which affect the interacting orbitals as the reactants are transformed into transition states offer an alternative interpretation.
TL;DR: In this article, the energy profile and location of ground and transition states for the process of internal rotation of the formyl group in oxazole- and thiazole-2-carbaldehyde were performed with ab initio MO calculations by employing the 3-21G (3- 21G*) basis set and molecular geometry relaxation.
Abstract: The analysis of the energy profile and location of ground and transition states for the process of internal rotation of the formyl group in oxazole- and thiazole-2-carbaldehyde was performed with ab initio MO calculations by employing the 3-21G (3-21G* for the sulphur derivative) basis set and molecular geometry relaxation. For both compounds two ground-states are found, corresponding to very nearly coplanar, formyl group and heterocyclic ring, the X,O-cis (X = O,S) conformer being the more stable. The difference in energy content between the two conformers is considerably higher in the thiazole derivative. The conformer populations were estimated from vibro-rotational analysis of the potential energy profile; that of the X,O-cis conformer amounts to 0.88 and nearly 1.00 for the oxazole and thiazole derivatives, respectively. The transition state approximates to the perpendicular orientation of the formyl group, yet in the thiazole derivative this group is bent towards the nitrogen atom of the ring. While the minima and maxima of the potential energy for internal rotation seem to be mainly determined by the extent of π-electron conjugation, distortion of the transition state of the thiazole derivative from the perpendicular situation should also account for significant contributions from nuclear-nuclear interactions.
15 Jan 1989
TL;DR: In this article, several methods are proposed for determining the differences in structure and free energy for homologous transition states in a condensed phase, based upon the well-known simplex optimization scheme.
Abstract: Several methods are proposed for determining the differences in structure and free energy for homologous transition states in a condensed phase. The first of these methods is based upon the well-known simplex optimization scheme. The second is a variant of the simplex method. The third method uses first and second derivatives of the free energy with respect to the molecular composition and the reaction coordinate. Free energy difference calculations are used on a simple model reaction in a Lennard-Jones liquid to display the problem of finding condensed phase transition states and to test the proposed algorithms.
TL;DR: In this article, the fully optimized transition states for hydrogen abstraction from methane by formaldehyde and acetaldehyde have been computed using UMP2/6-31G(d)//6 -31G (d) calculations.
TL;DR: In this article, the authors studied the thermal ring-splitting reactions of diarylcyclobutanes (1-5) and showed that the stability and reactivities depend on the stable conformations and rotational mobilities of the aryl substituents.
Abstract: Regiochemistry and reactivities in the thermal ring-splitting reactions of diarylcyclobutanes (1–5) have been studied and shown to depend on the stable conformations and rotational mobilities of the aryl substituents. The reactions of 1 and 2 result in a regiospecific symmetric cleavage to give indene or styrene along with significant isomerization of 2 to 3. In the cases of 3–5 both the symmetric and unsymmetric cleavages competitively occur with decreasing symmetric-to-unsymmetric ratios with an increase in methyl substitution. The olefin products from 4 are mixtures of cis- and trans-2-butene, cis- and trans-β-methylstyrene, and trans-stilbene. Thermochemical analyses combined with product analyses indicate that the symmetric cleavage of 1 and the unsymmetric cleavage of 3 proceed with a concerted mechanism, whereas 1,4-biradicals are involved in the other reactions. Structure-reactivity relationships of the present reactions are discussed in terms of mixing of the σ* character in a bonding MO by speci...
TL;DR: Searches for the exo and endo transition states for the ene reaction of methyl acrylate with propene, using split-valence 3-21G and 6-31G basis sets, suggest less asynchronous character in the bond reorganization as compared with the parent reaction between propene and ethylene as discussed by the authors.
Abstract: Searches for the exo and endo transition states for the ene reaction of methyl acrylate with propene, using split-valence 3-21G and 6-31G basis sets, suggest less asynchronous character in the bond reorganization as compared with the parent ene reaction between propene and ethylene.
TL;DR: In this paper, the 3-21 + G basis set has been used to model the alkylation process of acetaldehyde enolate enolate in the presence of thermal corrections and electron correlation.
Abstract: The alkylation processes of the reactions X–+ CH3F → XCH3+ F–, where X = OH, CH3 and H2CCHO, have been studied theoretically by means of 3–21 + G ab initio calculations. Transition states have been directly located. The use of a diffuse function-augmented basis set allows kinetically (O-alkylation) and thermodynamically (C-alkylation) controlled reactions of acetaldehyde enolate to be clearly separated. The changes of potential surfaces by the use of the 3–21 + G basis set instead of the 3–21G one are discussed, with special attention to the relevant modifications of transition structures. The influences of thermal corrections and electron correlation have also been considered.