摘要:

AbstractA molecular mechanical force field for fluorinated ethylenes and propylenes is described. The field computes geometries, dipole moments, and energy differences between isomers and rotamers. Component analysis indicates that “classical” strains alone (skeletal, nonbonded, electrostatic) cannot account for energy relationships. The discussion is supplemented by a semiempirical quantum chemical comparison ofcis‐ andtrans‐FCHCHF and ofgauche‐ andan

ISSN:0192-8651    

DOI:10.1002/jcc.540010202

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractAb initiomolecular orbital calculations have been carried out for the neutrals XNH2, XOH, and XF and the anions XNH−and XO−with substituents X = Li, BeH, BH2, CH3, NH2, OH, and F. All structures have been fully optimized with the 4‐31G basis set which is found to perform considerably better than the minimal STO‐3G basis in predicting the lengths of strongly polar bonds. A quantitative analysis of interactions between the directly bonded groups, utilizing energy changes in hydrogenation reactions, is presented and rationalized with the aid of perturbation molecular orbital theory. Favorable interactions occur when electron‐donor groups bond to electron‐acceptor groups. This applies to both σ and π interactions, the relative importance of which depends on the

ISSN:0192-8651    

DOI:10.1002/jcc.540010203

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractMNDO and MINDO/3 calculations were used to study the photochemical formation, thermal rearrangements, and dissociation of bicyclo[2.1.0]pentenone. The “forbidden” thermal conversion to cyclopentadienone requires little activation, which accounts for the low kinetic stability of bicyclo[2.1.0]pentenone. The theoretical results seem to be compatible with the available experimental evidence for the tri‐tert‐butyl‐substituted systems and suggest an explanation for observed differences in r

ISSN:0192-8651    

DOI:10.1002/jcc.540010204

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractSeveral “core‐deficient” small Gaussian basis sets were constructed and analyzed in terms of the balance requirements of functions that contribute predominantly to the core. Variations in the conformational energy barriers and geometrical parameters for ammonia and ethane, calculated with these basis sets, were analyzed with a gradient technique. A scheme for the reduction of the size of molecular basis sets is pro

ISSN:0192-8651    

DOI:10.1002/jcc.540010205

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractA conformational study was carried out on the three ring‐opened structures of triplet methyloxirane with a minimal Gaussian basis set, within the unrestricted Hartree–Fock framework. For the two structuresenergy surfacesE(θ1, θ2) were generated, where θ1measures the methyl rotation and θ2is associated with the torsion about the other CC bond. For the third structurean energy hypersurfaceE(θ1, θ2, θ3) was generated, where energy was a function of methyl rotation θ1and two nonequivalent CO rotations θ2and θ3. Analysis of the surfaces revealed the locations and relative energies of the critical points (minima, saddle points, and maxima). The overall stereochemical finding was that these ring‐opened triplet C3H6O species possessed rather

ISSN:0192-8651    

DOI:10.1002/jcc.540010206

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractAn algorithm is developed for generating and characterizing carcinogenic catacondensed benzenoid hydrocarbons. The bay regions in these structures are identified by a technique that we developed at Johns Hopkins. Using the three‐digit code proposed by Balaban, and the concept of ring adjacency matrix expounded here, we generate catacondensed benzenoid hydrocarbons in the computer and identify the number of potentially carcinogenic bay regions in each of them. The results of computer generation agree with the combinatorial enumeration of Harary and Read. All structures containing up to five rings and some with six rings and the number of bay regions in these are presented. Computer results for the structures and bay regions of all seven‐, eight‐, and nine‐membered unbranched catacondensed benzenoid hydrocarbons and the number of bays are available from the

ISSN:0192-8651    

DOI:10.1002/jcc.540010207

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe transition states for fragmentation of dihydroxycarbene [C(OH)2] to H2and CO2and for the rearrangement of this carbene to formic acid were located byab initiocalculations. The relative energies of the transition states were determined at several levels of theory and the basis set dependence of the energies is discussed. At the best level of theory; using a basis set of double‐zeta quality augmented by polarization functions and with the inclusion of extensive CI, we found that the transition state for fragmentation was considerably higher in energy than that for rearrangement. This finding is at variance with the predictions of the Woodward‐‐Hoffmann rules because fragmentation represents an “allowed” reaction, whereas rearrangement is “forbidden.” In conformity with the Woodward–Hoffman rules, the transition state for rearrangement was found to be close in energy to H· + ·CO2H. The even higher energy of the transition state for concerted fragmentation to H2and CO2is attributed to the need for the latter fragment to remain substantially bent in order to permit H2formation while maintaining a modicum of OH bonding. Difficulties in locating the transition state for concerted fragmentation are discussed and a new method for finding transition s

ISSN:0192-8651    

DOI:10.1002/jcc.540010208

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies ofcis‐ andtrans‐thiolformic acid are studiedab initioin the 4‐31G basis set. An extensive comparison is made between changes in diagonal and off‐diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between SH and OC, compared with O–H and OC, in keeping with the weaker hydrogenbonding property of the SH

ISSN:0192-8651    

DOI:10.1002/jcc.540010209

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractUnrestricted Hartree–Fock (UHF) SCF–MO calculations on the doublet reaction surface for the addition of methylidyne (CH) to ethylene (C2H4) using the standard extrapolation techniques of the GAUSSIAN 70 program show erratic behavior. On the other hand, the potential energy surface calculated without extrapolation of the density matrix and by using the final density matrix of a neighboring point as the initial guess for the density matrix for the new point gave a smooth potential curve without any kinks or erratic pattern. Therefore, the density extrapolation technique should be used with particular caution in UHF calculati

ISSN:0192-8651    

DOI:10.1002/jcc.540010210

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY

摘要:

AbstractThe energies of four different conformations forn‐butane were calculated by theab initiomethod using an STO‐3G basis set. Fully relaxed molecular geometries obtained from molecular mechanics (MM2) were used. The two energy minima [anti (C2h), gauche (C2)] and the two maxima (C2,C2v) had the following relative energies: 0.0, 0.88, 3.56, 5.99 kcal/mole. These are approximate Hartree–Fock numbers. It is estimated that inclusion of electron correlation in the calculation would lower the last number to about 5.1 kcal/mole while leaving the first three values essentially unch

ISSN:0192-8651    

DOI:10.1002/jcc.540010211

出版商:John Wiley&Sons, Inc.    

年代:1980

数据来源: WILEY