ISSN:0026-8976    

DOI:10.1080/002689797171274

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The electric field gradients, and hence nuclear quadrupole coupling constants, plus the dipole moments of the alkali halide molecules LiF, LiCl, LiBr, NaF, NaCl, NaBr, KF, KCl and KBr have been calculated usingab initioquantum chemical methods that range from SCF to coupled cluster techniques with perturbative corrections for triple excitations [CCSD(T)], employing basis sets of quadruple zeta quality with multiple sets of polarization functions. Zero-level vibrational corrections to these quantities and spectroscopic constants also were computed. The level of agreement with experimental data is generally good, especially in the case of the light molecules. The field gradients at the Na and Cl nuclei in NaCl were analysed using the constrained spatial orbital variation technique in order to quantify the importance of polarization, charge transfer and Pauli repulsion contributions, and to understand why a simple electrostatic interpretation of the field gradients fails in the case of alkali halides. The dipole polarizabilities, Sternheimer antishielding constants γ∞ and electric field gradient–electric field hyperpolarizabilities ϵ of the alkali and halide ions also were calculated at a correlated level of theory (ACPF).

ISSN:0026-8976    

DOI:10.1080/002689797171283

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A coupled Hartree-Fock method is used to compute the magnetizability tensors and carbon and hydrogen shielding tensors of the isoelectronic series of cyclic molecules consisting of the cations C3H3+(cyclopropenyl), C2H2N+(azirinyl), CHN2+(diazirinyl) and N3+(triazirinyl) and the neutral molecules C2H3B (borirene), CH2NB (borazirene), N2BH (boradiazirine), and NB2H3(azadiboridine). A distributed-origin method is also used to map the electron current densities induced in these by an external magnetic field. Comparisons are made with the benzene molecule. The computations show that whereas the anisotropy of the magnetizability of benzene is dominated by the contribution of the π current, that in the cyclopropenyl cation is due almost entirely to the valence σ current. The current density maps of the two molecules also differ sharply. The maps for benzene show a localized circulation of σ charge in each of the bonds, and a simple circulation of π charge around the principal axis with maximum density on the carbons of the ring: a π ring current. The maps for cyclopropenyl show a weak π ring current and a circulation of σ charge around the perimeter of the molecules that can be interpreted as a global σ ring current. The corresponding maps for the heterocyclic systems show that the π and σ ring currents are maintained with little change when CH in C3H3+is replaced by N, but that the introduction of B results in partial localization of these currents.

ISSN:0026-8976    

DOI:10.1080/002689797171292

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Three approaches to the calculation of torsional potentialsVϕ of internal rotations in molecules, namely rigid rotation, relaxed rotation and reaction path, are discussed. The rigid-rotation approach producesVϕ Values that are anomaly free but insufficiently accurate in general. In systems in which the hindrances that impede the internal rotation can be relieved or entirely avoided at relatively low cost in energy by relaxation of the other degrees of freedom, catastrophe points and hysteretic loops may be present in the relaxed-rotationVϕ. AlthoughVϕ obtained within the reaction-path approach are guaranteed to be continuous and to encompass all the relevant minima and transition states, they may possess bifurcation points. These anomalies all stem from the projection of the multivariate total energyU(R) onto the univariateVϕ. Their presence inVϕ of such simple systems as 2,6,2',6'-tetrabromobiphenyl and oxamide raises questions about the general validity and usefulness of the concept of torsional potentials.

ISSN:0026-8976    

DOI:10.1080/002689797171300

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

-Variable temperature (-60°C to -100°C) infrared spectra (3500–400 cm-1) of propenoyl fluoride, CH2CHCFO, dissolved in liquid xenon have been recorded. From these data the 2 enthalpy difference has been determined to be 113±15 cm-1(322±42 cal mol-1) with theanticonformer (carbonyl bondtransto the carbon-carbon double bond) more stable than thesynform. From this ΔHvalue along with assigned torsional fundamentals for both conformers and accompanying ‘hot bands’ the potential function governing the conformational interchange has been calculated. Utilizing the infrared data from the xenon solution, some new Raman data, andab initiofrequency predictions fromMP2/6-31G**calculations, a few reassignments of the fundamentals have been made.Ab initiocalculations have been carried out with several different basis sets up toMP2/6-311++G**from which structural parameters and conformational stabilities have been determined. With all the basis sets, theanticonformer is predicted to be the more stable conformer, which is in agreement with the experimental results. These spectroscopic and theoretical results are compared with the corresponding quantities for some similar molecules.

ISSN:0026-8976    

DOI:10.1080/002689797171319

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A second order Møller-Plesset (MP2) energy gradient algorithm for distributed memory parallel computers is described. A direct approach is used in that integrals are recalculated as required, but the degree of recalculation is minimized by exploiting the large global memory typically available on parallel machines. Results, obtained using up to 256 processors of the Cray T3D show very good scalability, with over 99·5% parallelism.

ISSN:0026-8976    

DOI:10.1080/002689797171328

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Ab-initioCCSD(T)/6-311G(3df, 2p) calculations of the molecular structure and the harmonic force field of methanol were carried out to obtain high-level benchmark data. These results are compared with calculations based on the second-order Møller-Plesset, the Hartree-Fock (HF) and the non-local density functional methods, and with experimental results. Special attention has been paid to evaluation of methanol Raman intensities and depolarization ratios. The CCSD(T) force field was combined with the Cartesian polarizability derivatives calculated using the HF and the density functional (B-LYP) methods.

ISSN:0026-8976    

DOI:10.1080/002689797171337

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Spin-rotation constants for H2,HF,N2, CO and F2singles and doubles level augmented by a perturbative correction for triple excitations together with large uncontracted basis sets. Explicit values for the lowest rovibrational states of these diatomics are obtained by solving the rovibrational Schrödinger equation with the finiteelement method. Good agreement between theory and experiment is obtained for H2, HF and F2, while for N2and CO a thorough comparison with experiment is hampered by inaccuracies in the experimental numbers and the calculated values should be considered more reliable. are calculated at the coupled-cluster

ISSN:0026-8976    

DOI:10.1080/002689797171346

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The high resolution infrared emission spectrum of lithium iodide has been recorded with a Fourier transform spectrometer. A total of 1024 transmissions with Δv= 1 for7LiI (fromv= 1 → 0 tov= 8 → 7), 387 transitions with Δv= 2(v= 2 → 0 tov= 6 → 4) and 504 transitions with Δv= 1 for6LiI (v= 1 → 0 tov= 5 → 4) have been assigned. The infrared data have been used to determine improved mass-dependent DunhamYlmcoefficients for the ground X1Σ+electronic state. The mass-independent DunhamUlmcoefficients were obtained along with Born-Oppenheimer breakdown constants. Finally, all of the data were fitted to the eigenvalues of the radial Schrödinger equation containing a modified Morse potential function to determine the parameters of an effective ground state potential.)

ISSN:0026-8976    

DOI:10.1080/002689797171355

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Making use of results derived from calorimetric, electron diffraction and microwave measurements, quantum chemical calculations at the MP2/6-31G*//MP2/6-31G* level (with zeropoint energies at the HF/6-31G* level) and assorted estimation methodologies, structures and enthalpies of formation are presented for some highly unsaturated alicyclic hydrocarbons (cyclopropene, cyclobutene, cyclopentadiene, 1,4- and 1,3-cyclohexadiene, and cycloheptatriene) and their keto- and exo-methylene derivatives.

ISSN:0026-8976    

DOI:10.1080/002689797171364

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor