Minimum Energy Reaction Path Research Articles

The thioketone–enethiol tautomerism of aliphatic thiocarbonyls: An ab initio study

AbstractApproximate minimum energy reaction paths have been calculated for two thioketone–enethiol tautomeric systems using an ab initio SCF–MO method. The calculations indicate nearly equal stabil ties of the isolated tautomers in both systems and an energy barrier of ca. 85 kcal/mol for their interconversion. This barrier is expected to be significantly lower in solution as a result of solvent–solute interactions.

Quantum-chemical studies of the energy hypersurface for the Meyer—Schuster rearrangement STO-3G calculation of minimum-energy paths. Intermolecular mechanism

Minimum-energy (ME) paths have been calculated for an intermolecular mechanism of the acid-catalysed rearrangement of α-acetylenic tertiary alcohols. Hydrogen-bonding along the reaction path is examined. A detailed characterization of the saddle point has been obtained. Good agreement between calculated and experimental activation enthalpy is found. The hypersurface obtained here points out a possible major role of solvation along the ME reaction path.

On the Mechanism of the α‐Alkynone Cyclization: A Theoretical Study

AbstractThe acetylene → carbene rearrangement of three model a‐alkynones, namely propynal (1a), 2‐butynal (1b) and butynone (1c) is studied by ab initio double‐zeta, double‐zeta plus polarization and double‐zeta plus self‐consistent electron pairs calculations. Transition states are located by force‐gradient geometry optimization. Calculated minimum energy reaction paths reveal substituent effects on the activation parameters, which indicate that a H‐atom or an alkyl group competes favorably with an acyl group in the [1,2]‐shift from a‐alkynones 1 to the corresponding acylvinylidenes 5.

Minimum energy reaction paths by a modified Pauling relation

A simple procedure for adjustment of the constant in the Pauling relation between bond order and bond length to generate reaction coordinates is proposed by means of a proton transfer as an example. The magnitude of this constant reflects bond and reaction types.

Quantum-chemical study of energy surface and the minimum-energy reaction path of the proton transfer along O-H...N hydrogen bond in acetic acid-imidazole x 2 H2O system

The semi-empirical quantum-chemical PCILO method has been used for calculation of the energy surface of the proton transfer along the O-H...N hydrogen bond in acetic acid-imidazole . 2 H2O system. The PCILO calculations gave the energy surface with two minima. The most stable minimum corresponds to the O-H...N hydrogen bond and has been found at the distances RH...N = 0.149 nm and RO...N = 0.107 nm. According to the PCILO calculations the proton transfer is accompanied by significant changes in the O...N distance. The second energy minimum corresponding to the proton transfer O-...NH+ complex has been found at RH...N = 0.10 nm and RO...N = 0.30 nm. The approximative minimum energy reaction path for the proton transfer has been calculated by the procedure developed by Muller and Brown. The calculated energy barrier represents a value 376.15 kJ/mol. The second energy minimum lies higher by 246 kJ/mol.

Virial theorem implications for the minimum energy reaction paths

The virial theorem for the cut of the fixed-nuclei potential energy surface, along the minimum energy path (MEP) is discussed. The partitioning of such MEP profiles of the reaction potential barriers into kinetic ( T) and potential ( V) parts is qualitatively examined and the illustrative, quantitative examples of the virial theorem partitioning for the BEBO barrier profiles of the HH + H → H + HH, and the FH + H → F + HH, collinear exchange reactions are presented. The use of the MEP integral virial theorem as a criterion of the reliability of calculated shapes of potential barriers is suggested and variational definitions of the MEP in terms of the T and V surfaces, respectively, are given. The implications of the T and V smoothness condition at the saddle point, on the position of the barrier maximum between the initial and final states of chemical reactions are explored and illustrative applications to H-atom exchange reactions are presented.

Ab initio study of β-lactam antibiotics. I. Potential energy surface for the amidic CN bond breaking in the β-lactam + OH − reaction

The potential energy surface of the β-lactam + OH − reaction, related to the mode of action of β-lactam antibiotics, was investigated using the ab initio Hartree—Fock method with the STO-3G basis set. Three possible reaction paths for the B A C 2 breaking of the amidic CN bond were obtained and discussed. The minimum-energy reaction path is characterized by the following processes: (1) the formation of a tetrahedral intermediate, ≈ 121 kcal mol −1 more stable than the reagents; (2) a barrier, ≈ 15 kcal mol −1 above the intermediate, which is mainly due to the partial breaking of the amidic bond; (3) the complete breaking of the amidic bond concerted with a proton transfer till the formation of the final product, ≈ 34 kcal mol −1 more stable than the intermediate. The evolution of some molecular orbitals and of the electron population along the reaction path was also discussed.

Ab initio study of β-lactam antibiotics. II. Potential energy surface for the amidic CN bond breaking in the 3-cephem + OH − reaction and comparison with the β-lactam + OH − reaction

The potential energy surface for the β-lactam amidic CN bond breaking in the 3-cephem + OH − reaction was investigated by using the ab initio Hartree—Fock method with a 9s6p/7s3p/3s basis set. The investigated reaction is a model of the reaction between an antibiotic cephalosporin and an enzymatic nucleophilic group, this last reaction being related to the mode of action of β-lactam antibiotics. The minimum-energy reaction path is characterized by a tetrahedral intermediate ≈ 116 kcal mol −1 more stable than the reagents, by a barrier which corresponds to the partial breaking of the amidic bond and is ≈ 7 kcal mol −1 above the intermediate and by a product ≈ 31 kcal mol −1 more stable than the intermediate. The analysis of the wavefunction along the reaction path and the comparison with the β-lactam + OH − reaction pointed out the role of electron-withdrawing groups on the height of the barrier and the role of intramolecular hydrogen bonds on the structure and energy of the product. The calculations suggest a model of the antibiotic activity of cephalosporins which is compared with previous qualitative pictures.

Reactive domains of energy hypersurfaces and the stability of minimum energy reaction paths

Special properties of the Riemannian metric for energy hypersurfaces, defined within the framework of the Born-Oppenheimer approximation, are utilized in devising a partitioning scheme for domains of nuclear coordinates. The chemically important coordinate domains are distinguished from domains of lesser importance by their curvature properties. Conditions are derived for the stability of minimum energy reaction paths, and the effects of instability regions are investigated. Instability domains along minimum energy paths may allow small vibrational perturbations to alter the outcome of the chemical reaction.

Reactive domains of energy hypersurfaces and the stability of minimum energy reaction paths

Reactive domains of energy hypersurfaces and the stability of minimum energy reaction paths

Accuracy of tunneling corrections to transition state theory for thermal rate constants of atom transfer reactions

We consider quantum mechanical effects on transition-state-theory transmission coefficients and on generalized transition-state-theory rate constants for elementary atom-transfer reactions in the gas phase. We examine two treatments of tunneling in the context of conventional transition-state-theory, the lowest order correction of Wigner and the full solution of the one-dimensional scattering problem for the classical potential energy barrier. We also examine two additional methods of including quantum effects on the reaction-coordinate motion in the context of adiabatic transition-state theory. One corresponds to taking the reaction coordinate as the minimum-energy reaction path, and the other uses the Marcus--Coltrin tunneling path. To test the accuracy of these approximate theories we present calculations for four collinear reactions: H + H/sub 2/, D + D/sub 2/, Cl + H/sub 2/, and Cl + T/sub 2/. Thermal rate constants are computed using these approximate theories and compared with those calculated using conventional and adiabatic transition state theory in which no quantal correction is made to the separable reaction-coordinate motion. The results are also compared with thermal rate constants obtained from accurate quantal calculations. We find that for the H + H/sub 2/ and D + D/sub 2/ systems the method using the Marcus--Coltrin path is very accurate,more » leading to errors less than 17% over a temperature range 300 to 1500/sup 0/K. However the simple Wigner transmission coefficient is found to give the best overall agreement for the four systems at low temperature. 6 tables.« less

Ab initio studies of the protonation of CO, N2 and NO+: Calculation of the minimum energy reaction paths

Ab initio molecular orbital calculations employing a 4-31G basis set have been used to study the minimum energy paths for the formation of HCO+, COH+, and HCOH2+ from CO by protonation. The protonation of N2 to give NNH+ and HNNH2+ and of NO+ to form HNO2+ and NOH2+ have also been investigated. All species formed have linear equilibrium geometries and the minimum energy path for approach of the proton is along the line-of-centers of the heavy atoms. Energy barriers to the formation of the various species are given, where appropriate, and changes in geometry, ordering of molecular orbitals and orbital occupancy are discussed.