摘要:

AbstractA topological model which provides a unifying framework for chemical reactions and molecular structure is proposed. Such basic concepts as overlap, orthogonality, reaction continuity, reaction reversibility, and orbital correspondence are incorporated into the model in a logical fashion. A chemical reaction pathway is regarded as a function that transforms a reactant topological space into its equivalent product space. The unique character usually ascribed to reactants, products, and their wavefunctions is superfluous. The model also allows considerable approximation of the wavefunctions and the reaction pathway without affecting the overall result. A simple orbital mapping technique consistent with the model which traces the transformation of orbitals usingintermolecular overlaps of the orbitals is also proposed. The suitability of a given pathway (“allowed” or “forbidden”) can be deduced explicitly without invoking symmetry (or other) rules and without resorting to detailed calculation of reaction energy surfaces. The validity of the mapping procedure has been confirmed by several thermal electrocyclic reactions: the ring‐opening isomerizations of substituted cyclopropyl cations, cyclopropyl anion, cyclopropanone, cyclobutene, benzocyclobutene, Dewar benzenes, and 1,3‐cyclohexadiene. Orbital mapping with EHT and CNDO/2 MOs correctly predicts the reaction stereochemistry (conrotatory or disrotatory) in

ISSN:0192-8651    

DOI:10.1002/jcc.540010102

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractOrbital mapping analysis, based on EHT and CNDO/2 semiempirical molecular orbitals, has been used to survey the thermal, disrotatory, ring‐opening isomerizations of bicyclo[2.2.0]hexa‐2,5‐dienes (Dewar benzenes), bicyclo[2.1.0]pent‐2‐enes, and bicyclo[2.1.0]pent‐2‐en‐5‐yl ions to their planar isomers. Results indicate that isoelectronic substitution (CH replaced by C−, O+, N, NH+, etc.) in the molecular framework may favor allowed thermal reactions in some cases, in contrast to the disallowed reaction predicted for the

ISSN:0192-8651    

DOI:10.1002/jcc.540010103

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractA number of cyclobutane derivatives containing one or more double bonds in endo‐ or exocyclic positions have been studied by the molecular mechanics method within the context of the MM2 force field. Generally speaking, the structures and energies of these compounds are well calculated in cases in which they are known experimentally and are predicted in others. Examples are shown of the use of the moments of inertia of molecules, which are known from microwave studies, in conjunction with molecular mechanics calculations to yield better structures than could be obtained by either method alone. Compounds examined include cyclobutene, methylenecyclobutane, Dewar benzene, and related compound

ISSN:0192-8651    

DOI:10.1002/jcc.540010104

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe variable‐size simplex optimization method is used to reparametrize theI+Aand β parameters of an INDO approximation to the perturbed Hartree–Fock calculation of13C chemical shifts in hydrocarbons. The absolute shifts for 39 nuclei in a set of molecules containing up to four carbons are reproduced within a standard error of 9.9 ppm for an unconstrained optimization and to a standard error of 10.0 ppm for an optimization constrained to yield gross atomic charges in agreement with double‐zetaab initiocalcul

ISSN:0192-8651    

DOI:10.1002/jcc.540010105

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractCurrent efforts to determine the nature of the interactions that influence protein folding involve, among other things, minimization of an appropriate empirical conformational energy function (ECEPP, Emprical Conformational Energy Program for Peptides) to obtain the native structure. Because of the prohibitive cost of such a massive computational project, either on a conventional large‐scale machine at a self‐supporting installation or on a dedicated minicomputer, an alternative computer hardware system has been developed to aid in the conformational analysis of proteins. It consists of a Floating Point Systems AP‐120B array processor and a Prime 350 minicomputer host. A version of ECEPP has been adapted to run on the AP‐120B. The data structures and algorithms chosen for this version reflect the highly unusual parallel architecture of this machine. Benchmark comparisons with BPTI (Bovine Pancreatic Trypsin Inhibitor), a protein of 58 residues and a known structure, have been carried out on this system as well as on an IBM 370/168. They show a significant advantage in speed for the AP‐120B/Prime 350 system as well as a substantially lower cost. An energy minimization of BPTI with 154 variable dihedral angles is reported, an effort heretofore prohibited by the computer costs

ISSN:0192-8651    

DOI:10.1002/jcc.540010106

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractPrevious attempts to determine the strengths of multiple metal‐metal bonds are reviewed. Estimates of 73 and 97 kcal/mole for the MoMo bond energies in Mo2Cl 84−and Mo2(O2CH)4, respectively, are obtained by combining the known experimental bond energy in Mo2(96.5 ± 5 kcal/mole) with the results of SCF‐Xα‐SW calculations on Mo2, Mo2Cl 84−, and Mo2(O2CH)4. Possible errors in the estimates are discussed. It is noted that the quadruple bonds in the complexes are predicted strongerper componentthan the sextuple bon

ISSN:0192-8651    

DOI:10.1002/jcc.540010107

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractA recently developedab initiopseudopotential molecular orbital approach was applied to the Te 42+ion, a system outside the practical reach of conventional all‐electron treatments. Computations were carried out with a minimalSTO‐4G basis set. Results account reasonably well for the observed optical absorption spectrum and suggest the origin of a hitherto unassigned weak band. Ground‐state properties, which included the structure, force field, and vibrational frequencies, were also investigated. Treated as a free, gas‐phase ion, tetratellurium (II) yielded a bond length 0.05 Å shorter than the experimental value for the ion in a crystal lattice. Placement of static, point‐charge counterions in the Te 42+coordination sphere increased the bond length to a value 0.005 Å longer than derived by experiment. Calculations on neutral, cyclic Te4provided a theoretical single‐bond reference length 0.09 Å longer than that obtained for the ion in a counterion environment. Comparisons between observed and calculated frequencies suggest an assignment of the vibrational spectrum different from the provisional assignment

ISSN:0192-8651    

DOI:10.1002/jcc.540010108

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe use of modified virtual orbitals is studied in a systematic conventionalCIprocedure which offers considerable potential in regard to convergence and extension to larger systems. The method is applied to the HCN molecule by using 37 basis functions, and analysis of energy expectation values, together with the one‐electron density, yields some insight into the physical content of CI wavefunction

ISSN:0192-8651    

DOI:10.1002/jcc.540010109

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractEmpirical force field calculations were performed on hexamethylbenzene to elucidate the internal motions of the methyl groups. When the benzene ring is constrained to be planar (as in solid‐phase studies), the methyl groups undergo a geared, disrotatory motion. When this constraint is relaxed, results are force field dependent. Calculated barriers are in good agreement with experimentally determined value

ISSN:0192-8651    

DOI:10.1002/jcc.540010110

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe electron projection functionP(x,z) = ∫ ρ(x,y,z)dyis used to evaluate charge transfer and covalency in two series of molecules, LiX and CH3X (X = Li, BeH, BH2, CH3, NH2, OH, and F), with wavefunctions derived from STO‐3G, 4‐31G, and, in some cases, 6‐31*ab initiocalculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH3Li which by our criteria is whol

ISSN:0192-8651    

DOI:10.1002/jcc.540010111

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY