摘要:

AbstractThe geometries and energies of beryllium clusters up to Be5are examined usingab initiomolecular orbital theory. Allowances are made for electron correlation with Møller—Plesset perturbation theory to fourth order. Correlation is found to have a dramatic effect on the relative energies of the several structures examined for Be4and Be5. Furthermore, the effect ofd‐type basis functions on the correlation energy results in an increased binding energy for the clusters. Be2is only weakly bound. For Be3, the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be4is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be5is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory

ISSN:0192-8651    

DOI:10.1002/jcc.540010402

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe numerical description of functionality allows a rigorous generation of all reactions from one structure (or partial structure) to another. The minimum number of steps required can be calculated and this “reaction distance” used to guide computer generation of sequences to converge on the target structure. A computer algorithm is described for efficient generation of all shortest synthetic routes between two given molecules as starting material and product. The program is seen as a prototype of a complete synthesis design prog

ISSN:0192-8651    

DOI:10.1002/jcc.540010403

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractEquilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with theab initioand experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with theab initiovalues. With respect to dipole moments, the CNDO/2 values are in better agreement with theab initioresults than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with theab initiopredictions.

ISSN:0192-8651    

DOI:10.1002/jcc.540010404

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractCNDO/Scalculations are carried out on 1,3,5‐cycloheptatriene (I), 1,3,5,7‐cyclooctatetraene (II), all‐cis‐1,3,5,7‐cyclononatetraene (III), and all‐cis‐1,4,7‐cyclononatriene (IV), for which π‐σ separability is a questionable matter. When disparate choices are available for the geometries, preferences are based on the agreement between calculated and experimental spectra. The microwave structure for I and an unsymmetrical conformation for III are suggested. In the electronic spectra, conjugative contributions are assigned to the lowest‐energy weak band, while an ethylenic nature is to be attributed to the intense band in the range of 6 eV. The possibility of through‐space π interactions is examined qualitatively. An appreciable degree of such in

ISSN:0192-8651    

DOI:10.1002/jcc.540010405

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractElectron density maps for the heterocycles thiophene, furan, and pyrrole are determined fromab initio4‐31G wavefunctions. The charge distributions in these molecules are analyzed in terms of the total molecular density and difference density maps and their profiles. The atomiclike core, especially theLcore of sulfur, is found to play an important role, via its polarization and interaction, in determining the extent and direction of valence density transfer from the carbon to the heteroatom. The changes in the charge distributions that occur in the immediate vicinity of the heteroatoms and the relation of density quantities to binding and antibinding characteristics are discussed. The quantum topological features of the molecular charge distributions of the three heterocycles are analyzed and discussed and the different bonding situations, e.g., ring strain, ionic and covalent binding, etc., are compared in a model‐independent

ISSN:0192-8651    

DOI:10.1002/jcc.540010406

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe localized molecular orbitals (LMOs) of thiophene, furan, and pyrrole are derived fromab initio4‐31G wavefunctions using Boys' criteria for localization. From the transferability point of view, these LMOs are classified as (i) completely different and nontransferable LMOs (these are the lone‐pair orbitals on O and N on one hand and those on S on the other hand), (ii) chemically similar lone pairs and inner shells on O and N (of furan and pyrrole, respectively), and (iii) chemically equivalent CC, CC, and CH LMOs in the three heterocycles. Thesp3hybridization of theLcore of sulfur, its appreciable polarization, and considerable involvement in bonding in the CS bond region have been discussed. The present investigation indicates the limitation of the application of semiempirical MO methods to molecules that contain second‐row atoms due to both the appreciable core—valence and π‐σ interactions involving such atoms. Qualitative investigation of aromaticity and reactivity of the studied heterocycles agrees satisfactorily with experimental observations and shows that conclusions drawn based solely on static factor considerations (charge distribution in the noninteracting molecules) might very well be misleading and such factors determine the ease rather than the final orientation

ISSN:0192-8651    

DOI:10.1002/jcc.540010407

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractPrediction of the known crystal structure of cyclic‐L‐Ser(O‐t‐Bu)‐β‐Ala‐Gly‐L‐β‐Asp(OMe) has been attempted by establishing the low‐energy conformations of the isolated molecule by conformational analysis, and then regarding each of these as a rigid molecule, by establishing the favorable crystal packing arrangements by molecular packing analysis. The theoretical model closest to the observed structure was one of the lowest‐energy solutions and was recognized as essentially correct by reference to the x‐ray data. The limitations of the model follow from the fact that the side chain conformations are somewhat affec

ISSN:0192-8651    

DOI:10.1002/jcc.540010408

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractAb initiocalculations with full geometry optimization have been carried out on the planarcCc,cTc,tTc,tCt,tTt, andcCtconformers of β‐hydroxyacrolein using the 4‐21G basis set, and on thecCcandcCtconformers using the 4‐31G basis set. The hydrogen‐bondedcCcconformer is the most stable and thecCtconformer the least stable, with the other conformers following the above sequence. β‐Hydroxy substitution has scarcely any influence on the geometry of thetrans‐acrolein structure, whereas the geometry of thecis‐acrolein structure shows significant changes which depend on whether the OH group is cis or trans with respect to the CHO group about the CC bond. The ΔETvalues for cis → trans isomerization about the CC bond incCtandcTcsupport the hypothesis that these changes in geometry are the result of a destabilizing interaction incCtand a stabilizing interaction incTc.The geometry of the hydrogen‐bonded structurecCcsets it apart from all the other conformers: it has by far the longest CC, the longest CO, the longest OH, the shortest CC, and the shortest CO. Its formation fromcCtinvolves a lengthening of CC, CO, and OH and a shortening of CC and CO, indicating a delocalization of charge within the ring. 4‐21G calculations have also been made for a distortedcCtstructure that has the same bond lengths and angles as the equilibriumcCcstructure, and the distortion energy,cCt(equm. geom.) →cCt(distorted geom.), is found to be +13.1 kJ mole−1. Taking the energy of this distortedcCtstructure as the baseline, the hydroge

ISSN:0192-8651    

DOI:10.1002/jcc.540010409

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractA characterization of atomic environments based on counting random walks in a molecular skeleton is outlined. To each atom in a molecule a sequence of integersw1,w2,w3,…,wnis assigned, wherewirepresents the number of self‐returning walks of lengthk, the length being defined by the number of bonds traversed. Properties of the derived atom codes are discussed. The codes display an impressive diversity and are superior to atomic codes based on enumeration of self‐avoiding walks (or paths) in discriminating atomic environments. In certain cases the codes of individual atoms are not unique and the same codes appear in different molecules or even within the same molecule. The occurrence of the nonunique codes can be related to special structural situations, associated with the occurrence of isospectral graphs. These isospectral graphs which have atoms with identical codes can generate additional isospectral structures by attaching any arbitrary group to such points. If nonequivalent atoms of a single molecule have identical random walk codes, substitution at the singular points alternatively will produce isospectral graphs. Examples of such situations are

ISSN:0192-8651    

DOI:10.1002/jcc.540010410

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe results ofab initio“supermolecule” calculations of the charge transfer between formamide and the enol forms of methylglyoxal, ethylglyoxal, dimethylglyoxal, and propenylglyoxal are compared for several different conformations of the constituent molecules. The enols were found to be poorer electron acceptors than their respective keto isom

ISSN:0192-8651    

DOI:10.1002/jcc.540010411

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY