ISSN:0021-2148    

DOI:10.1002/ijch.199300042

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

ISSN:0021-2148    

DOI:10.1002/ijch.199300043

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe history and evolution of Gaussian basis sets in molecular electronic structure calculations are reviewed, from the original proposals by McWeeny and Boys to the current high‐quality generally‐contracted basis sets of the atomic natural orbital and correlation‐consistent types. Various explorations of modified forms, generalizations, and alternatives are mentioned, but the focus is on the mainstream developments whose origin can largely be traced to the work of Reeves and his students. The introduction of various standard sequences of basis sets, such as those of Pople and coworkers, with well‐documented and extensive test results, and the wide distribution of easy‐to‐use computer codes, such as the GAUSSIAN series, have contributed greatly to the explosion in the use of ab initio calculations of molecular structure in the chemistry researc

ISSN:0021-2148    

DOI:10.1002/ijch.199300044

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractNMR chemical shift calculations provide the basis for an intensive collaboration between quantum chemists and experimentalists. Calculated shift data can be used to describe the magnetic properties of a molecule, to identify unknown compounds by comparison of experimental and theoretical shift values, to determine equilibrium geometries, to investigate conformational changes, to elucidate the mechanism of molecular rearrangements, to determine solvent effects on NMR data, to identify complexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to describe chemical bonding. This is demonstrated by three examples, namely the determination of the equilibrium structure of the homotropylium cation, the description of BH3NH3in solution or condensed phases, and the investigation of stannyl cation complexes in solution. IGLO calculations of13C,11B,15N, and119Sn chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium cation possesses an equilibrium 1,7 distance of 2 Å that is indicative of strong through‐space interactions and, as a consequence, homoaromatic character. (2) In solution, the charge transfer from NH3to BH3is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental δ(11B) and δ(15N) values can be reproduced when the geometry effect and the direct solvent effect are included in the shift calculations. (3) Stannyl cations form strongly‐bounded coordination complexes with solvent molecules (binding energy: ≥ 50 kcal/mol) that make the cation properties, in particular δ(119Sn) values, similar to those of covalently‐bounded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive solvent

ISSN:0021-2148    

DOI:10.1002/ijch.199300045

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe hydride transfer reactions from simple silanes to carbenium ions are studied by ab initio calculations. The simplest reaction, H4Si + CH3+→ H3Si++ CH4, is also studied with inclusion of the solvent effect (with the SCRF method) in the ab initio scheme. Under all conditions the preferred mechanism is the synchronous hydride transfer (SHT), which is barrierless in the gas phase but possesses small barriers in solution. The mechanistic alternative involving a rate‐determining single electron transfer (SET) step followed by H‐atom abstraction is found to be of very high energy. Modelling of the primary isotope effect for the SHT process of H3SiH(D) + CH3* → H3Si++ H3CH(D) shows that the primary isotope effect is small, between ca. 1.1 and 2.7, for the entire relevant range of Si—H(D) distances (1.5–2.3 Å). Furthermore, the pattern ofthe computed primary isotope effect shows it to be an insensitive probe of the SHT mechanism.The curve‐crossing method is used to model the mechanistic dichotomy. It is shown that the reaction profiles for both SHT and SET arise from an avoided crossing between the ground state and a charge transfer state of the R3SiH//R′3C+reactant pair. Thus, in the SHT mechanism a single electron switches sites in synchronicity with bond reorganization, while in SET the electron switch precedes the bond coupling. This avoided bond coupling is the foremost disadvantage of the SET mechanism. The common origin of the avoided crossing elucidates the reason whySHT exhibits characteristics of an electron transfer process without actually bei

ISSN:0021-2148    

DOI:10.1002/ijch.199300046

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe energy interaction curves of a number of diatomic and polyatomic dication systems were calculated in order to study their energy‐trapping properties. Generally, the ab initio complete active space multiconfiguration self‐consistent field method was used in an extended valence + polarization basis set, with compact effective potentials replacing the core electrons. The diatomic dications include all ten possible binary combinations of oxygen, sulphur, selenium, and tellurium. O22+shows the largest exothermicity, measured from equilibrium to the monocation combination asymptote, and highest barrier to dissociation. The calculated equilibrium bond length and harmonic vibrational frequency agree very well with experiment. The O22+, SO2+, SeO2+, and TeO2+series show progressively decreasing exothermicities but similar barrier heights. The non‐oxides, in contrast, show similar exothermicities but decreasing barriers with increasing size of the atom constituents. These trends are interpreted in terms of both valence bond curve‐crossing and molecular orbital bonding models.The ozone dication, O32+, is found to have a number of low‐lying singlet and triplet stationary state structures spanning near‐linear to D3h2+symmetries. Although the calculated exothermicity is even larger than for O22+, the barrier to O2++ O+dissociation is predicted to be low in each case.O22+surrounded by six argon atoms to model an isolating environment shows increased equilibrium O–O bond length, decreased exothermicity, and increased barrier to dissociation, relative to the bare dication. O22+flanked at each end by a perpendicularly oriented H2molecule in a staggered conformation is obstructed from direct conversion to the water dimer dication by a high barrier. However, [(H2O)2]2+dissociates smoothly from equilibrium to two water monocations with a large exothermicity but a

ISSN:0021-2148    

DOI:10.1002/ijch.199300047

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractAb initio calculations at the QCISD(T)/6–311G**level have been carried out to study the addition of the methyl radical to a series of substituted alkenes, and the results analyzed with the aid of the curve‐crossing model. It is found that (a) reaction exothermicity is the main factor that dominates reactivity, (b) polar contributions to the transition states are generally small and of minor energetic consequences, and (c) the general observation that π‐electron‐accepting substituents in the alkene enhance reactivity is a secondary correlation that is a consequence of the effect of these substituents on reaction exothermicity. There is no evidence for the prevalent view that the methyl radical is generally nucleophilic towards

ISSN:0021-2148    

DOI:10.1002/ijch.199300048

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractWe outline a continuum solvation model for aqueous solution similar to the recent SMx models of Cramer and Truhlar.1–3As in those models, the solvation energy is given as a sum of polarization and cavity‐dispersion contributions. The numerical integration required for calculation of the polarization energy in the SMx models has been replaced by a scaled two‐center Coulomb integral. The cavity‐dispersion energy is based on molecular volume and includes a term involving the molecular dipole moment. The present method is more accurate than the SMx models and is considerably less time‐

ISSN:0021-2148    

DOI:10.1002/ijch.199300049

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractNDDO‐based (AM1) configuration interaction (CI) calculations have been used to calculate the wavelength and oscillator strengths of electronic absorptions in organic molecules and the results used in a sum‐over‐states treatment to calculate second‐order hyperpolarizabilities. The results for both spectra and hyperpolarizabilities are of acceptable quality as long as a suitable CI‐expansion is used. We have found that using an active space of eight electrons in eight orbitals and including all single and pair‐double excitations in the CI leads to results that agree well with experiment and that do not change significantly with increasing active space for most organic molecules. Calculated second‐order hyperpolarizabilities using this type of CI within a sum‐over‐states calculation appear to be o

ISSN:0021-2148    

DOI:10.1002/ijch.199300050

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractA linear synchronous transit or quadratic synchronous transit approach is used to get closer to the quadratic region of the transition state and then quasi‐newton or eigenvector following methods are used to complete the optimization. With an empirical estimate of the hessian, these methods converge efficiently for a variety of transition states from a range of starting structure

ISSN:0021-2148    

DOI:10.1002/ijch.199300051

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY