摘要:

AbstractSemiempirical AM1, MNDO‐PM3, and MNDO/M, as well asab initioHartree–Fock and Möller–Plesset calculations using the 4‐31G, 6‐31G(d,p), and 6‐31 +G(d,p) basis sets have been done on the water‐assisted addition of water to formaldehyde.ab initiomethods predict a reduction of 30% of the bimolecular ΔG0,‡(from 48 to 33 kcal/mol) when an ancillary water molecule is used as catalyst. AM1 and MNDO‐PM3 predict anticatalysis instead; ΔG0,‡for the termolecular addition is larger than the one for the bimolecular reaction. MNDO/M predicts a modest decrease, from 42 to 39 kcal/mol. We conclude that, as predicted by theab initiocalculations, there is a bifunctional catalysis caused by the ancillary water. Two of the semiempirical methods are qualitatively (and all of them quatitatively) in disagreement with this conclusion. © 1992

ISSN:0192-8651    

DOI:10.1002/jcc.540130902

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractFour semiempirical methods (AM1, MNDO, PM3, and MINDO/3) are used to calculate the deformation angles of [n]paracyclophanes and their Dewar benzene isomers forn= 3… 10. The results obtained by all these methods are in good agreement with data from X‐ray studies. We have determined the strain energies that, in both series of compounds, are due to two components: (1) the strain energy of deformation of the cycle (aromatic or Dewar Benzene skeletons) and (2) the strain energy of the oligomethylene chain. In [6]paracyclophane, the strain energy [SEring(MNDO) ≈︁ 32.9 kcal/mol] almost compensates the resonance energy (Eresonance≈ 36 kcal/mol) so that its chemical properties are closer to alkenes than to benzenic compounds. To better reproduce the enthalpy of the valence isomerization [n]Dewar bezene → [n]paracyclophane, which is poorly calculated with these methods, a correction is proposed and the reaction enthalpy of [6]paracyclophane is estimated to be about ΔHr≈ 15 ± 15 kcal/mol. It is found that MNDO and MINDO/3 need the smallest corrections, but MNDO leads to better geometries than MINDO/3. In conclusion, MNDO seems to be the best technique for further studies of these compounds. © 1992 by John

ISSN:0192-8651    

DOI:10.1002/jcc.540130903

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractIn the present work we have carried out a Monte Carlo simulation of a dissociative electron transfer reaction in a polar solvent. In particular, we have chosen as a very simple model the electrochemical reduction of hydrogen fluoride to give a hydrogen atom and a fluoride anion in a dipolar solvent. From a classical point of view, the electron transfer occurs at the intersection regionS* of the diabatic potential hypersurfacesHppandHss, corresponding to the precursor and successor complexes, respectively. We have evaluated both diabatic surfaces using potential functions that have been built up withab initiomethods by us. For each of the obtained configurations the parameter ΔE=Hss–Hpphas been calculated. This parameter is then used as the reaction coordinate for obtaining the diabatic free energy curves of the reaction. Because the activation energy is high, a suitable mapping potential along with the statistical perturbation theory is employed to force the system to evolve toward the intersection regionS*. A total of 68,340,000 configurations have been generated. The main conclusion of this article is that Marcus' relationship seems to fail for this kind of inner‐sphere processes. © 1992 by John Wiley&Sons

ISSN:0192-8651    

DOI:10.1002/jcc.540130904

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAb initiocalculations of the potential energy surfaces of CH2N2, CHFN2, and CF2N2at MP4SDTQ/6‐31G*//MP2(full)/6‐31G* reveal several surprising features. While diazomethane is more stable than diazirine, only the three‐membered ring forms of the fluorine‐substituted isomers are known experimentally. We find fluorodiazomethane and difluorodiazomethane not to be viable species: They have no barriers toward exothermic dissociation into N2and CHF or CF2, respectively. In contrast, the three‐membered ring isomers, fluorodiazirine and difluorodiazirine, have high barriers toward dissociation despite being high in energy. Diazomethane bends easily; a nonplanar Csminimum is found at MP2(full)/6‐31G* but C2vsymmetry is preferred at QCISD/6‐31G*. © 1992 by John W

ISSN:0192-8651    

DOI:10.1002/jcc.540130905

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAn algorithm is introduced that can generate reaction‐pathway hypotheses for computer‐assisted elucidation. A key aspect of the algorithm is its ability to conjecture reaction intermediates and products that are not input to the algorithm. The formal basis for the conjecture is stoichiometry, i.e., species variables are used in the construction of a pathway, and their molecular formulas are inferred when sufficient stoichiometric constraint is placed on the variables. These conjectured species have a degree of plausibility when the algorithm is used systematically to search for thesimplestpathways consistent with given experimental evidence. The MECHEM system for computer‐assisted elucidation under development by the author adapts this algorithm to generate initial pathway hypotheses from experimental data. © 1992 by John Wiley&Son

ISSN:0192-8651    

DOI:10.1002/jcc.540130906

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractOur recently proposed scheme for including aqueous solvation free energies in parameterized NDDO SCF models is extended to the Parameterized Model 3 semiempirical Hamiltonian. The solvation model takes accurate account of the hydrophobic effect for hydrocarbons, as well as electric polarization of the solvent, the free energy of cavitation, and dispersion interactions. Eight heteroatoms are included (along with H and C), and the new model is parameterized accurately for the water molecule itself, which allows meaningful treatments of specifically hydrogen bonded water molecules. The unphysical partial charges on nitrogen atoms predicted by the Parameterized Model 3 Hamiltonian limit the accuracy of the predicted solvation energies for some compounds containing nitrogen, but the model may be very useful for other systems, especially those for which PM3 is preferred over AM1 for the solute properties of the particular system under study. © 1992 by John Wiley&Sons, Inc

ISSN:0192-8651    

DOI:10.1002/jcc.540130907

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe describe two algorithms for the parallel calculation of a CHARMm‐like force field in macromolecules. For a molecule with a given number of atoms, we show that there is an optimal number of processors leading to a minimum computation time. At the optimum, both the number of processors and the computation time are proportional to the number of atoms. © 1992 by John Wiley&Sons, I

ISSN:0192-8651    

DOI:10.1002/jcc.540130908

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe problem of the computation of the Centrifugal Distortion Constants (CDC) related to a diatomic potential is considered. The analytical expressions obtained from a reformulation of the Rayleigh‐Schrödinger perturbation theory are used [Kobeissi et al.,J. Mol. Spectrosc.,138, 1 (1989)]; these areen+1= 〈Φ0RΦn〉 – Σ m=1nem〈Φ0Φn−m〉 whereR= 1/r2, Φ0= ψvis the vibrational wave function (corresponding to the given energyEv=e0) and Φ1, Φ2,…, are the “rotational corrections” to Φ0, solutions of the rotational (nonhomogeneous) Schrödinger equations. These equations are integrated by using a recent integrator using a powerful local control allowing (for Φ0) a high accuracy. The integrals are computed by using another powerful technique tailored for matrix elements between numerical wave functions [Kobeissi et al.,J. Comp. Chem.,10, 358 (1989)]. This numerical treatment is applied to the model Lennard‐Jones potential and to the RKR potential of theI2ground state. In both applications the CDC are computed up toe6=Nvande7=Ov(these two are published for the first time), and up to the dissociation [up tov= 23 for the Lennard–Jones potential, and tov= 108 for theXΣ –I2(RKR) p

ISSN:0192-8651    

DOI:10.1002/jcc.540130909

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe protein–dipole Langevin–dipole (PDLD) model developed by Warshel and co‐workers is an approach to evaluate electrostatic interactions in protein systems from microscopic sights. This model grasped the main physical factors and required little computations. But it might need the tests from every aspect. In the present work, we have chosen the solvation energies of Asp3, Glu7, Glu49, and Asp50 in bovine pancreatic trypsin inhibitor (BPTI) as a calibration to discuss the influences of parameters and conditions on the simulation results in the PDLD model. Some improvements have been proposed. The calculated solvation energies associated with ionizing the four acidic groups in BPTI and aspartic acid in solution are found in good agreement with the corresponding observed results if the improved PDLD approach and computational methods are used. © 1992 by John Wiley&Son

ISSN:0192-8651    

DOI:10.1002/jcc.540130910

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Poisson–Boltzmann equation can be used to calculate the electrostatic potential field of a molecule surrounded by a solvent containing mobile ions. The Poisson–Boltzmann equation is a non‐linear partial differential equation. Finite‐difference methods of solving this equation have been restricted to the linearized form of the equation or a finite number of non‐linear terms. Here we introduce a method based on a variational formulation of the electrostatic potential and standard multi‐dimensional maximization methods that can be used to solve the full non‐linear equation. © 1992 by John W

ISSN:0192-8651    

DOI:10.1002/jcc.540130911

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY