摘要:

AbstractAb initioHartree‐Fock calculations at the 6–31G*//3–21G level of theory are reported for bicyclo[2.2.2]‐2,5,7‐octatriene (barrelene),1, bicyclo[2.2.2]‐2,5‐octadiene,2, bicyclo[2.2.2]‐2‐octene,3, and bicyclo‐[2.2.2]octane,4. The stepwise heats of hydrogenation of1were found to be 38.1, 31.8, and 28.4 kcal/mol, respectively. The unusually large heat of hydrogenation for the first double bond is attributed to the destabilizing electronic effects involving the interaction of the thr

ISSN:0192-8651    

DOI:10.1002/jcc.540090402

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractA general method for calculating pi charges based on the Hückel molecular orbital (HMO) approach is incorporated into an existing scheme for calculating partial atomic charges in saturated systems. The parametrization of the Hückel coulomb and resonance integrals is given based on (a) the CNDO pi densities and (b) the observed molecular dipole moments. The results for these two schemes are compared both with other theoretical methods and also with the observed dipole moments of a variety of conjugated and aromatic compounds. The dipole moment (DM) parametrization gives in general more consistent results and is to be preferred. The compounds studied include substituted olefins and aromatics (with substituents R, F, Cl, Br, I, OR), aldehydes, ketones, acids, esters, amides, and the heterocyclic ring systems of furan, pyrrole, pyridine, and indol

ISSN:0192-8651    

DOI:10.1002/jcc.540090403

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractSemilocal pseudopotential operators can be expressed as a linear combination of nonlocal (projection) operators. Pseudopotential operator integrals over a molecular basis set are therefore reduced to linear combinations of overlap integrals products. Molecular calculations indicate that sufficient precision can be achieved with a limited number of nonlocal operators. Analytic derivatives of pseudopotential integrals are easily deduced and implemented in a standard quantum chemistry program.

ISSN:0192-8651    

DOI:10.1002/jcc.540090404

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractAn algorithm for the recognition of all isomorphisms for any pair of labeled graphs as well as for generation of the graph group is described. Some results obtained with computer realization of the suggested algorithm are briefly discussed.

ISSN:0192-8651    

DOI:10.1002/jcc.540090405

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractConformational preferences in a series of alkylbenzenes and protonated and neutral aryl‐alkylamines of biological interest have been examined. First, a general picture was obtained for the 25 compounds in the series by means of the CAMSEQ empirical potential software system. This provided solution as well as vacuum data. A low‐energy “folded” conformation (alkyl chain coiled toward aromatic ring) was observed in every case. The presence of an amino nitrogen atom in the alkyl chain did not significantly influence conformational preference. Secondly, a group of 14 compounds, representative of the subgroupings within the main series, was selected and the previously established (CAMSEQ) folded and extended minimum positions were further examined by means of a modified MM2 program. Energy differences between conformers were also calculated, and the presence of a stable folded form was confirmed. A feature common to many of the folded conformations is the positioning over the ring of a hydrogen atom from the terminal group of the chain. The MM2 folded/extended energy differences were in some cases smaller than those determined by CAMSEQ, with the “extended” form in many cases being about 1 or 2 kcal/mol more stable. Thirdly, three representative compounds from the series were examined by means of a molecular dynamics program which permitted the sampling of conformational space throughout the transition from extended to folded forms. This method gave energy differences between folded and extended conformations which agreed with the corresponding MM2

ISSN:0192-8651    

DOI:10.1002/jcc.540090406

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractWe present a numerical method for calculating the electrostatic potential of molecules in solution, using the linearized Poisson‐Boltzmann equation. The emphasis in this work is on applications to biological macromolecules. The accuracy of the method is assessed by comparisons with analytic solutions for the case of a single charge in a dielectric sphere (Tanford‐Kirkwood theory), which serves as a model for a macromolecule. We find that the solutions are generally accurate to within 5%. Larger errors occur close to the charge and the dielectric boundary, but the maximum error found at ion‐bonding distance (3 Å) from a charge close to the boundary (1 Å deep) is only ∼15%. Several algorithmic improvements, described here, contribute to the accuracy of the method. The programs involved compose a coherent software package, called Del Phi, which goes from a Brookhaven Protein Data Bank format file to calculated electrostat

ISSN:0192-8651    

DOI:10.1002/jcc.540090407

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractA variety of quantum mechanical models have guided the interpretation of the far infrared (IR) spectrum of easily deformed ring systems, but an explicit guide to such modelling would ease further analysis. The coordinates introduced by Cremer and Pople provide a starting point for description of puckering, separate from other internal motions of a ring system. For a ring ofNatoms there areN‐3 puckering modes, composed of [N‐ (2)/2] pseudorotation “amplitudes” and [(N‐ 3)/2] “angles.” (The brackets []mean “truncate to the integer.”) Separation of the Schroedinger equation is possible for the “free puckerer,” the “puckerer in a box,” and for puckering opposed by a separable harmonic potential; in this latter case the energy is determined and the state is labeled by a set of pseudorotation quantum numbersMkand radial quantum numbersnk:\documentclass{article}\pagestyle{empty}\begin{document}$$E_k = \sum\limits_k {hv_k (2n_k + M_k + 1)}$$\end{document}Herevkis the harmonic frequency for thek‐th mode, andhis Planck's constant. Since most ring systems are nonharmonic, and require a distinct “quartic‐puckering” potential for each puckering mode, a perturbation treatment of the quartic terms is required. We provide formulas and a symbolic algebra computer program to generate expressions for integrals needed for the perturba

ISSN:0192-8651    

DOI:10.1002/jcc.540090408

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractA general algorithm is described which exhaustively searches conformational space using an internal coordinate tree search. Using only geometrical operations and a set of criteria for eliminating chemically unreasonable structures, the algorithm generates starting geometries for optimization by molecular mechanics. An implementation of this algorithm is exceedingly fast and finds all known minima, as well as several new ones, for four test alkanes. This method makes feasible global conformational searches for molecules with up to 109conformational possibilities.

ISSN:0192-8651    

DOI:10.1002/jcc.540090409

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractA least‐squares procedure for the construction of four 2s‐2phybrid orbital direction vectors that deviate least from four given bond vectors emanating from a tetravalent atom is described. The procedure is used as a tool for the interpretation of structural data from X‐ray diffraction, electron diffraction, and microwave experi

ISSN:0192-8651    

DOI:10.1002/jcc.540090410

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY

摘要:

AbstractCharacteristics of the Watson‐Crick‐type hydrogen‐bonded base pairs, thymine‐adenine and cytosineguanine, are presented. These were established using anab initiomolecular orbital method. Base–base interactions are revealed to have dominant roles in the structures of nucleic acids and also in their functions. The most stable conformations in Watson‐Crick‐type base pairs are almost in accord with the structure observed in fiber X‐ray diffraction study. Explanations are given of the origin of the sequencedependent local structures which differ from one base pair to the next, as observed in single‐crystal X‐ray analyses. In the case of the thymine‐adenine base pair, it is shown that various geometrical parameters having almost the same stability are available. According to the propeller twist, the instability is not large enough for a base pair

ISSN:0192-8651    

DOI:10.1002/jcc.540090411

出版商:John Wiley&Sons, Inc.    

年代:1988

数据来源: WILEY