摘要:

AbstractResults are presented of topological analyses of the electron density functions of cyclopropeniumyldiazonium dication,1, of its 2,3‐diamino derivative,2, and of their products3and4, respectively, formed by dediazoniative ring‐opening. The new CN‐bonding type in1and2, recently realized synthetically in derivatives of2, is compared to prototypical aliphatic diazonium ions with regard to electronic structure and thermodynamic stability, factors that both are crucial for the appreciation of the mechanisms of deamination reactions of chemical and biochemical significance. Association of the dication with N2involves density accumulation in the CN bonding region, occurs without major overall charge transfer, and leads to an electrostatically favorable quadrupolar charge distribution in the diazonium ion. The CN‐bonding model recently proposed for aliphatic diazonium ions also applies to these dications.1is thermodynamically stable while the dediazoniation of 2 is exothermic but kinetically hindered. Our best estimates for the reaction energies of the dediazoniations1→3+ N2and2→4+ N2, respectively, are 65.5 and −7.2 kcal/mol, respectively. We have found that, in general, the cation is destabilized and that N2is stabilized upon CN‐bond cleavage. Cations force N2to form diazonium ions. The remarkable difference between the stabilities of1and2is primarily due to the larger destabilization of the open dication3compared to4. Push‐pull interactions between the diazo‐ and the overall electron‐withdrawing amino‐functions characterize the electronic structure of2. CN‐Bonding and the overall electronic structure of2are incompatible with the usual Lewis resonance notations. Instead of dismissing the Lewis notations, it is shown that the topological description can be reconciled with the Lewis notations if the resonance forms are interpreted in a way that appropriately reflects the atom populations and first moments. Implications of the model with regard to

ISSN:0192-8651    

DOI:10.1002/jcc.540110602

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn algorithm for predicting the conformations of strained molecules using Artificial Intelligence techniques is described and illustrated with some typical examples.

ISSN:0192-8651    

DOI:10.1002/jcc.540110603

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe relationships between atomic hardness, atomic electronegativity, and electronic energy are considered and emphasized. A new method for calculating atomic hardness is described. The concept of local hardness is quantified through the calculation of a new variable named alfahardness. Atomic hardness and alfahardness are used for the calculation of both the mean molecular and local properties. The results obtained are discussed and a comparison made with the analogous quantities presented by Pearson. An algorithm has been realized and transformed into a computer routine for use within a CAOS program.

ISSN:0192-8651    

DOI:10.1002/jcc.540110604

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA combined quantum mechanical (QM) and molecular mechanical (MM) potential has been developed for the study of reactions in condensed phases. For the quantum mechanical calculations semiempirical methods of the MNDO and AM1 type are used, while the molecular mechanics part is treated with the CHARMM force field. Specific prescriptions are given for the interactions between the QM and MM portions of the system; cases in which the QM and MM methodology is applied to parts of the same molecule or to different molecules are considered. The details of the method and a range of test calculations, including comparisons withab initioand experimental results, are given. It is found that in many cases satisfactory results are obtained. However, there are limitations to this type of approach, some of which arise from the AM1 or MNDO methods themselves and others from the present QM/MM implementation. This suggests that it is important to test the applicability of the method to each particular case prior to its use. Possible areas of improvement in the methodology are discussed.

ISSN:0192-8651    

DOI:10.1002/jcc.540110605

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractWe describe a force field extending Boyd and Kesner's molecule mechanics treatment of alkyl chlorides, fluorides and bromides to alkyl iodides: accordingly, as well as parameters for the potential energy of bond stretching and bending and for interactions between nonbonded atoms, the field includes parameters taking into account permanent and induced C‐I dipoles, whose values have been estimated by fitting the model to experimental dipole moment data. The results of applying this force field to 22 organic iodides comprising several substitution series are reported, and the structural and energy trends in the various series are discussed. These results agree satisfactorily with available experimental data and with those obtained by A. Y. Meyer using a different kind of force fiel

ISSN:0192-8651    

DOI:10.1002/jcc.540110606

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractTwo approaches to the simulation of internal rotation potential energies in substituted ethanes are formulated for general applications. Called the vicinal Fourier coefficient and vicinal pair energy methods, they differ only in form. The latter procedure has the advantage of yielding energy terms that represent pairwise interactions between vicinal substitutents. As numerical examples, the potential energies of ethane and five of its simple methyl and chloro derivatives are employed to simulate the corresponding energies of two higher derivatives of the series. The initial energy data were calculated by the molecular mechanics method (MM2) with geometry optimizations and theab initioMO procedure (STO‐3G) with standard geometries. Results indicate that simulated energies are reasonably accurate for the flexible‐rotor model (MM2) and extremely accurate for the rigid‐rotor model (STO‐3G). Deviations appear to be systematic and may be rationalized on the basis of molecular st

ISSN:0192-8651    

DOI:10.1002/jcc.540110607

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractWe have performed all electron Hartree–Fock gradient calculations and geometry optimizations on systems composed of one to three palladium atoms and: CH3cation and anion, C2H4, C2H3NH2, C4H4, NH3, and (NH3) + (C2H4). Several basis set considerations are discussed and the binding energies of Pd to these small molecules are reported. We find the counterpoise correction to the binding energies of these systems to be large. We also present MP2 calculations of the palladium binding energy with a large uncontracted palladium spd basis set in the PdC2H4and PdNH3systems. The binding interaction between ethylene and palladium results in a mixing of the 4d–π* and 5s–π orbitals, and, is dissociative to the ethylene. The palladium‐butadiene and palladium‐cyclobutene relative stabilites and structures are interesting since these molecules could form from acetylene on a palladium surface. We find the Pd‐butadiene cyclic structure to be 43 kcal/mol more stable than the Pd‐cycl

ISSN:0192-8651    

DOI:10.1002/jcc.540110608

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

ISSN:0192-8651    

DOI:10.1002/jcc.540110601

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY