摘要:

AbstractA variable force field model for radical dimerization was developed. It uses MM2 force constants for most atoms and new parameters only for the core atoms involved in bonding changes. The change in hybridization from sp2to sp3is modeled using distance‐dependent switch functions. The validity of the model has been tested by calculating the minimum energy path of the dimerization of di‐tert‐butylmethyl radicals. The calculated and experimental values for the enthalpy of activation of both dimerization of the radicals and dissociation of the dimer are in excellent agreement. The model has also been successfully applied to the stereoselective dimerization of 1‐phenylneopentyl radicals: The form of the potential energy surface yields an explanation for the observed stereoselectivity. Another common feature in radical dimerization seems to be the formation of adsorption complexes prior to dimerization that can lead to increased reactivity. The results suggest that it is important to analyze the whole reaction path and not only the transition stat

ISSN:0192-8651    

DOI:10.1002/jcc.540130502

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA direct method for theab initiocalculation of the magnetic susceptibility and chemical shielding tensors based on the individual gauge for localized molecular orbitals (IGLO) formalism is introduced. “Direct” in this context means we avoid storing the two‐electron repulsion integrals in favor of recalculating them whenever necessary. In conjunction with the Direct‐SCF package TURBOMOLE Direct IGLO (DIGLO) permits calculation of magnetic second‐order properties for large molecules by minimizing peripheral disc storage requirements. The size of the molecules to be treated is limited only by the amount of CPU time available. The performance of DIGLO is demonstrated for some selected

ISSN:0192-8651    

DOI:10.1002/jcc.540130503

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAmong some 2‐substituted 4H‐1,3,2‐benzodioxaphosphorin 2‐sulfides, the 2‐OMe derivative has the sulfur atom mainly in the quasiequatorial position. On the other hand, the conformation in which the sulfur is quasiaxial is favored in the 2‐NHMe and 2‐Et derivatives. We have doneab initiomolecular orbital calculations for the model compounds, and such a substituent effect was reproduced. Moreover, the effect was discussed in terms of the intramolecular electrostatic interactions and the degree of charge

ISSN:0192-8651    

DOI:10.1002/jcc.540130504

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractModifications to the distance geometry algorithm as embodied in the program DGEOM have been made to improve sampling capabilities. Specifically, torsion angle sampling replaces distance sampling for 1,4 atomic relationships and correlated distance sampling is disabled. The effects of these modifications are illustrated by comparing the different sets of conformations produced for butane. In addition, these changes are shown to increase the conformational sampling of two medium‐sized rings, cycloheptadecane and caprylolactam. The current results for these molecules are compared to those of other conformational searching method

ISSN:0192-8651    

DOI:10.1002/jcc.540130505

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Tyson‐Fife reaction‐diffusion equations are solved numerically using a locally implicit approach. Since the variables evolve at very different time scales, the resulting system of equations is stiff. The reaction term is responsible for the stiffness and the time step is increased by using an implicit method. The diffusion operator is evaluated explicitly and the system of implicit nonlinear equations is decoupled. The method is particularly useful for parameter values in which the equations are very stiff, such as the values obtained directly from the experimental reaction rate constants. Previous efforts modified the parameters on the equations to avoid stiffness. The equations then become a simplified model of excitable media and, for those cases, the locally implicit method gives a faster although less accurate solution. Nevertheless, since the modified equations no longer represent a particular chemical system an accurate solution is not as important. The algorithm is applied to observe the transition from simple motion to compound motion of a spiral

ISSN:0192-8651    

DOI:10.1002/jcc.540130506

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractProtein‐folding potentials, designed with the explicit goal that the global energy minimum correspond to crystallographically observed conformations of protein molecules, may offer great promise toward calculating native protein structures. Achieving this promise, however, depends on finding an effective means of dealing with the multiple‐minimum problem inherent in such potentials. In this study, a protein‐folding‐potential test system has been developed that exhibits the properties of general protein‐folding potentials yet has a unique well‐defined global energy minimum corresponding to the crystallographically determined conformation of the test molecule. A simulated‐annealing algorithm is developed that locates the global minimum of this potential in four of eight test runs from random starting conformations. Exploration of the energy‐conformation surface of the potential indicates that it contains the numerous local minima typical of protein‐folding potentials and that the global minimum is not easily located by conventional minimization procedures. When the annealing algorithm is applied to a previously developed actual folding potential to analyze the conformation of avian pancreatic polypeptide, a new conformer is located that is lower in energy than any conformer located in previous studies using a variety of minimi

ISSN:0192-8651    

DOI:10.1002/jcc.540130507

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have derived alternative expressions for computing the energies and forces associated with angle bending and torsional energy terms commonly used in molecular mechanics and molecular dynamics computer programs. Our expressions address the problems of singularities that are intrinsic in popular angle energy functions and that occur from other chain rule derivations of force expressions. Most chain rule derivations of expressions for Cartesian forces due to angle energies make use of relations such as\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{\partial E}}{{\partial x}} = \frac{{\partial E}}{{\partial \phi }}\frac{{\partial \phi }}{{\partial \cos \phi }}\frac{{\partial \cos \phi }}{{\partial x}} $$\end{document}where ϕ is a bond or torsion angle,E(ϕ) is energy, and ∂/∂xrepresents a derivative with respect to some Cartesian coordinate. This expression leads to singularities from the middle term, −1/sin ϕ, when ϕ is 0 or π. This is a problem that prevents the use of torsional energy expressions that have phase angles, ϕ°, other than 0 or π, such as inE(ϕ) = κ[1 + cos(nϕ − phsi;°)]. Our derivations make use of a different, but equivalent, form of the chain rule:\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{\partial E}}{{\partial x}} = \frac{{\partial E}}{{\partial \phi }}\frac{{\partial \phi }}{{\partial x}} $$\end{document}This form still possesses singularities for the bond angle forces since the last factor is undefined when ϕ is 0 or π. However, the alternate form may be used to great advantage for the torsional angle forces where no such problem arises. The new expressions are necessary if one desires the use of torsional energy expressions with general phase angles. Even for energy expressions in common use, i.e., with phase angles of 0 or π, our force expressions are as computationally efficient as the standard ones. The new expressions are applicable to all molecular simulations that employ restrained, or phase‐shifted, torsional

ISSN:0192-8651    

DOI:10.1002/jcc.540130508

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA two‐component Kramers' restricted Hartree–Fock method (KRHF) has been developed for the polyatomic molecules with closed shell configurations. The present KRHF program utilizes the relativistic effective core potentials with spin–orbit operators at the Hartree–Fock (HF) level and produces molecular spinors obeying the double group symmetry. The KRHF program enables the variational calculation of spin–orbit interactions at the HF level. KRHF calculations have been performed for theHX,X2,XY(X,Y= I, Br), and CH3I molecules. It is demonstrated that the orbital energies from KRHF calculations are useful for the interpretation of spin‐orbit splittings in photoelectron spectra. In all molecules studied, bond lengths are only slightly expanded, harmonic vibrational frequencies are reduced, and bond energies are significantly decreased by the spin–orbit

ISSN:0192-8651    

DOI:10.1002/jcc.540130509

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAn analytical derivation of multicenter and multiparticle integrals for explicitly correlated Cartesian Gaussian‐type cluster functions is demonstrated. The evaluation method is based on the application of raising operators that transform spherical cluster Gaussian functions into Cartesian Gaussian function

ISSN:0192-8651    

DOI:10.1002/jcc.540130510

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA computational chemistry study of the artificial redox enzyme synthesized by covalently attaching flavin to cyclodextrins explains some of its properties. Calculations indicate that the flavin moiety covalently attached to cyclodextrin is not within the cavity of cyclodextrin. This result is consistent with the UV‐vis spectrum of the artificial enzyme. The calculations also indicate hydrogen bonds formed between the carbonyl groups of the catalytic functionality and the hydroxyl groups of cyclodextrin play a role in their most stable conformation. This explains the observed overall stability of these artificial enzymes compared to riboflavin. Electrostatic energies and solvation energies play a major role in the stability of the hosts and the orientation of guests included within the artificial enzymes. The rates of oxidation of various thiols catalyzed by the artificial enzyme can be explained by the relative distances between the sulfur atom of the substrates and C(4a) of the flavin moiet

ISSN:0192-8651    

DOI:10.1002/jcc.540130511

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY