摘要:

AbstractWe report a theoretical study of the stretching of chemical bonds and its implications on the force‐field parametrization. Computations were performed at the SCF and MCSCF levels by using minimal, split‐valence, and large extended and polarized basis sets. The stretching energy profiles were determined considering up to 25 perturbed geometries of 11 different bonds (6 singles, 2 doubles, and 3 triples). The energy profiles and stretching parameters are compared with the experimental data compiled in the most popular force fields. MCSCF stretching energy profiles are mainly anharmonic and can be only roughly reproduced by quadratic equations. The use of Allinger's MM2 quasiharmonic expression appears as the best choice because it fits with reasonable accuracy a large percentage of the stretching profile without increasing the complexity of the formalism and of the parametrization procedure. MCSCF computations are needed to obtain reliable stretching force parameters. In this respect, MCSCF calculations considering as active space only the bonded and nonbonded orbitals of the perturbed bond seems to be the best strategy to obtain good results at a minimum computational cost, especially if small split‐valence basis sets like the 3‐21G are used. Results obtained at this level of sophistication are completely comparable to stretching parameters compiled on empirical force fields. © 1993 John Wiley&S

ISSN:0192-8651    

DOI:10.1002/jcc.540140802

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe AM1 and PM3 potential energy surfaces of cyclopentane have been compared and the energy difference between two (boat and chair) cyclohexane conformers analyzed. The effects of reparameterization and different core‐repulsion functions (CRFs) have been studied. The HH, CH, and CC CRF energy contributions have been calculated separately. The results show that the CRF energy corrections stabilize the correct (MM2 andab initio) geometries. The PM3 equilibrium geometry of cyclopentane is closer to the MM2 andab initioresults than the AM1 geometry. It is known that above 190 pm the AM1 HH CRF curve is smooth and negative but the PM3 HH CRF curve has a trough and a shoulder. It has been shown that this latter oscillation is not damped by other parameters. As a result, the HH PM3 energy gradient curve is different from the quasilinear AM1 andab initiogradient curves. © 1993 John W

ISSN:0192-8651    

DOI:10.1002/jcc.540140803

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractComplexes of 18‐crown‐6 with alkali–metal cations (Na+, K+, and Rb+), urea, and the uncomplexed crown ether were studiedin vacuowith the molecular dynamics method. Conformational data from these calculations (simulation times in the range from 6–15 ns) was compared with information from the Cambridge Structural Database. Despite the differences in condition between the simulations and the solid state, a number of interesting similarities are observed. © 1993 John Wiley&S

ISSN:0192-8651    

DOI:10.1002/jcc.540140804

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA semidirect implementation of the closed‐shell MP2 gradient for efficient use on workstation computers is presented. The approach is based on the algorithm proposed by Frisch and coworkers but includes several modifications to reduce disk storage requirements and exploits nonabelian point group symmetry. The performance of the resulting program MPGRAD (BIOSYM Corp., San Diego, CA) is demonstrated in applications to the molecules [AlSi(CH3)3]4and ferrocene. The largest calculation involved 492 basis functions and was carried out on IBM RS/6000 workstations with memory sizes of 32 and 128 Mb. The ratio of CPU to wallclock time exceeds 90% in all typical applications. © 1993 John Wiley&Sons, I

ISSN:0192-8651    

DOI:10.1002/jcc.540140805

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractRelativistic and electron correlation effects in thallium halides TlXand TlX3(XF, Cl, Br, and I) are investigated by extensiveab initioconfiguration interaction calculations. Spin–orbit coupling is included at the Hartree–Fock level for the diatomic TlBr and TlI. At the best level of treatment of electron correlation (quadratic configuration interaction), the calculated molecular properties are in good agreement with experimental results, i.e., for the diatomic thallium halides deviations from experimental values are<0.06 Å for bond distances,<0.14 mdyn/Å for force constants,<35 kJ/mol for dissociation energies, and<0.3 D for dipole moments. The convergence of the Møller–Plesset series up to the fourth order is discussed. Two alternative structures of TlI3are compared. At the Møller–Plesset level of theory, the trigonal planar structure with thallium in the oxidation state + 3 is the preferred gas phase arrangement compared with the bent arrangement containing a linear I 3−unit and thallium in the oxidation state + 1, the difference being ca. 95 kJ/mol. Vibrational frequencies are predicted for all trigonal planar thallium(III) halides. © 1993 Jo

ISSN:0192-8651    

DOI:10.1002/jcc.540140806

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA quantitative comparative analysis of molecular electrostatic potential (MEP) distributions generated from different wave functions was carried out. Wave functions were computed by using MNDO, AMl, STO‐3G, 3‐21G, 4‐31G, 6‐31G, 4‐31G*, 6‐31G*, and 6‐31G** methods. Ten different compounds, which include usual atoms and groups of biomolecules, such as hydroxyl, carbonyl, amine, amide, imine, double and triple bonds, and heteroaromatic rings, were studied. For each compound, MEP values in the points of a common 3‐D grid were computed; thereafter, the similarity between each pair of MEP distributions generated by different methods was assessed. Similarities were measured using the Spearman rank correlation coefficient. A similarity matrix was obtained for each compound. Similarity matrices were averaged and a hierarchical cluster analysis was carried out to classify the different quantum chemical methods. In the compounds studied, the main conclusion is the negligible difference between the pattern of MEP distributions generated from all split valence basis sets (with and without polarization functions). © 1993 John

ISSN:0192-8651    

DOI:10.1002/jcc.540140807

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA new index suitable for computing molecular similarity based upon the similarity of molecular properties such as electrostatic potentials or electrostatic fields is presented in two forms. For one form of the present index, general conditions are established for which a linear measure of similarity is obtained. An illustrative example is provided in which the electrostatic field and electrostatic potential of guanine obtained from different wave functions are compared. © 1993 John Wiley&Sons, Inc

ISSN:0192-8651    

DOI:10.1002/jcc.540140808

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractUsing the MM2‐87 program and parameter set, conformational analyses have been performed on cocaine (1), the potent analog 2β‐carbomethoxy‐3β‐(4‐fluorophenyl)tropane (CFT,2), and a group of dopamine reuptake blockers that contain two phenyl rings. The latter includes LU 19‐005 (3), a 1‐amino‐4‐phenyltetralin (4), a hexahydropyrrolo[2,1‐a]isoquinoline (5), diclofensine (6), and a hexahydro[1,2‐b]pyridine (7). Using different values for the dielectric constant, the global minimum of1and2is a conformer in which there is a favorable electrostatic interaction between the ammonium hydrogen and the carbonyl of the carbomethoxy group. TheN‐methyl groups in1and2are found to strongly prefer the equatorial position of the piperidine ring. These results were also related to four crystal structures of1and its close derivatives. Compounds3–7are found to have a common conformation that was used to define the pharmacophore for dopamine reuptake blockers including the required orientation of the ammonium hydrogen. The pharmacophore provides an explanation for why the tertiary amine analogs of3and4are less potent than the secondary amines because the addedN‐methyl group occupies the position required for the ammonium hydrogen. This explanation, however, does not work for7, in which the tertiary amine is again less active than the secondary amine. However, this last series appears to have a number of anomalous features. Superposition of2with the pharmacophore suggests that its carbomethoxy may occupy the same region of the receptor as the second phenyl ring in compounds3

ISSN:0192-8651    

DOI:10.1002/jcc.540140809

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA modification of Allinger's MM2 force field for theanomeric effectin OCN systems is presented. For optimal consistency, it was parameterized by alternate use ofab initio(3‐21G level) and X‐ray results to account for the energetic and structural manifestations of the effect in the gas or condensed phase, respectively. The results obtained with the modified force field are in good agreement with those from both theoretical and experimental methods. The parameterization scheme explicitly treats all structural parameters of the COCNC moiety as well as CN bond lengths in tertiary amines contained within an anomeric unit. In addition, it includes directional H‐bond type interactions. A limited number of parameters is put forward, in accord with the general MM2 force‐field approach. © 199

ISSN:0192-8651    

DOI:10.1002/jcc.540140810

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractCalculations of binding energies and optimum geometries of compounds of the seriesM(H2O)+withM= Sc to Zn have been carried out and compared with gas‐phase experimental data and with the Rosi and Bauschlicher MCPF calculations. Hartree–Fock calculations and correlated calculations at MP2, MP4, and QCISD(T) levels were used to test the dependence of the results upon the level of correlation. A test of basis set dependence was also carried out, using parallel calculations on four basis sets ranging in size from a small DZ set to a TZ contraction. Correlation levels above MP2 and elaboration of the metald‐function basis set to (4d/3d) size or greater were both necessary for convergence with the most uniformly reliable results obtained from QCISD(T) calculations on a basis set with a (6d/4d) contraction for thed‐function space. However, MP2 or higher‐level calculations with a contracted four or fivedfunction set [(5d/3d) or (4d/3d)] are capable of yielding results on binding energies and geometries close to the current gas‐phase experimental uncertainty on electrostatically bound transition metal complexes. © 1993 John Wil

ISSN:0192-8651    

DOI:10.1002/jcc.540140811

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY