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The first polyhedral aluminum chlorides are described, exhibiting unusual multicenter bonding properties.

The article discusses the re-evaluation of calculations used to assess the chemical benzene structure and the accuracy of those measurements.

Issues concerning the r(sub e) structure of cyclopropane are discussed. It was established that reliable results are gained for cyclopropane only if coupled-cluster methods with inclusion of triple excitations are used in combination with large basis sets.

Research was conducted to study the isomers of the carbon-rich molecule C5H2 utilizing coupled-cluster approximation-based techniques. The structures were optimized at the coupled-cluster singles and doubles level. Analytic calculation of second derivatives was utilized to measure harmonic vibrational frequencies. Results suggested that estimates of the rotational constants and vibrational frequencies should facilitate experimental identification of the compounds.

Superacids protonate bis(cyclopropylidene)methane to produce the cyclopropylcyclopropylidenemethyl cation, the first stable vinyl cation. Hyperconjugative interactions between the cation and the C,C-sigma-bonds stabilizes this radical. NMR spectroscopy proves that either C, C-hyperconjugation or the cyclopropyl ring induces charge delocalization. Ab initio calculations reveal the geometric consequences due to sigma C, C-hyperconjugation.

Analyses of the performance of B NMR chemical shift computations at a correlated GIAO-MP2/TZP' level of ab initio theory, rectified the large errors of non-correlated wave functions at GIAO-MP2 for 1,5-C2B3H5 and B4R4. Inconsistencies in the initial experimental delta-11B values were responsible for the irregularities evident in 1,2-C2B3H7.

Theoretical NMR shift calculations were carried out for a variety of Al compounds at the SCF and MP2 levels. The computed values were rationalized in terms of a molecular orbital picture of the bonding and compared with available experimental data for Al compounds. Results indicate that chemical shift calculations are very useful for the interpretation of experimental spectra of unknown compounds if there are no other experimental NMR data available.

High accuracy extrapolated ab initio thermochemistry (HEAT) scheme is used for studying the standard heat of formation of vinyl chloride. The enthalpies of the reactions [C.sub.2][H.sub.4] + [Cl.sub.2] -> C[H.sub.2]CHCl + HCl and [C.sub.2][H.sub.2] + HCl -> C[H.sub.2]CHCl are obtained.

The article describes the benchmark, near basis set limit fc-CCSD(T) equilibrium harmonic frequencies for a set of 12 closed-shell and five open-shell molecules, which is computed numerically by using the CCSD(T)-R12 method. The corrections due to core-valance correlation are used for assessing the accuracy of the benchmark data, along with corrections from scalar relativistic effects.

Disc-shaped hexa-peri-hexabenzocoronenes(HBCs) peripherally substituted by flexible dodecyl chains (molecule 1) or rigid polyphenylene dentrons (molecules 2a, b and 3) are efficiently synthesized. As disclosed by concentration and temperature-dependent H NMR spectroscopy, the hexadodecyl-substituted HBC 1 displayed a high tendency to aggregate in solution.

A theoretical estimation of the enthalpy of formation for the hydroperoxyl radical is presented. The results are based on CCSD(T)/aug-cc-pCV5Z calculations extrapolated to the basis-set limit with additional corrections.

The results of high-level quantum chemical calculations and the available experimental data are used to obtain the equilibrium geometry of the ethynyl (CCH) radical. The calculated vibrational corrections to the rotational constants are used to derive the equilibrium geometrical parameters from the available experimental rotational constants.

Examines a simple scheme for the direct calculation of ionization potentials with coupled-cluster theory that exploits established excitation energy methods. Vertical ionization potentials for the nitrogen, carbon monoxide and fluorine molecules at the neutral ground-state equilibrium geometries; Rigorous perturbation analysis and qualitative behavior.

Investigates the electronic spectrum of pyrrole by performing calculations using a hierarchy of coupled-cluster models. Basis-set effects; Characterization of the electronic transitions and final states; Structures and vibrational frequencies of selected states.

Reports that static and frequency-dependent polarizabilities and first hyperpolarizabilities have been calculated for HF and Ne using full configuration interaction and a hierarchy of coupled cluster models. Results from unrelaxed calculations; Orbital relaxed properties of NE, HF and BH.

Four different methods and various basis sets ranging from double to quadruple X1 quality are used to calculate the (super 17)O NMR chemical shift values of polyoxides HOnH, CH3OnH, and CH3OnCH3 (n=2,3,4). Averaging of NMR chemical shifts due to the rotational flexibility of the polyoxides has little influence on the measured (super 17)O NMR chemical shifts.

Proposes the application of the spin-restricted coupled-cluster theory as an approach for the treatment of high-spin open-shell systems in coupled-cluster theory. Spin constraints on the wave functions which allow a reduction in the number of independent amplitudes; Correspondence of the spin expectation value to the exact value; Equations derived for amplitudes, energy and first derivatives of energy.

Investigates the accuracy of molecular equilibrium structures of small closed-shell molecules containing first-row atoms as predicted by the standard electronic wave functions. Use of the correlation-consistent polarized valence double-zeta basis; Shortening the bond distances at all levels; Accuracy of the Moller-Plesset series oscillates.

Reports on the results of full configuration interaction benchmark calculations for the electric dipole and quadrupole moments and the electric field gradient at the nuclei of BH and HF to test the accuracy of perturbation theory. Indications that the coupled cluster models outperform the perturbation series for all the properties.

Studies the electronically excited states of Na[sub3]O. Formation of the compound; First measure of visible/NIR region photodissociation cross sections; Ground state dissociation energy.

Examines the conspicuous differences from ab initio harmonic frequencies and from an empirical harmonic force field. Calculation of fundamental frequencies; Speculation of strong anharmonicity of bridge stretching modes as a feature of electron-deficient molecules; Restriction to harmonic approximation.

Quantum mechanical ab initio methods are applied to the prediction of the predicted structure and vibrational spectra of the Oh and D3h forms of CrF6. Electron correlation effects are calculated via conventional configuration interaction with single and double excitations, while quadruple excitations are calculated by using the Davidson correction. Results indicate that CrF6 possesses an Oh geometry.

A quantum chemical ab initio study is presented demonstrating H NMR chemical shifts.

This review provides a computational chemist's perspective of rotational spectroscopy and discusses the theoretical background and application of state-of-the-art quantum-chemical methods for the accurate determination of the relevant spectroscopic parameters. [ABSTRACT FROM AUTHOR]

The paper describes how Born-Oppenheimer breakdown parameters for the rotational constants of diatomic molecules can be determined via quantum-chemical computations. The deviations from the Born-Oppenheimer equilibrium values are accounted for by considering the adiabatic correction to the equilibrium bond distances, the electronic contribution to the rotational constant via the rotational g tensor, and the so-called Dunham correction, which can be computed directly from a polynomial expansion of the potential curve around the equilibrium distance. Calculations for HCl, SiS, and HF demonstrate the accuracy that can be achieved in the theoretical treatment of the considered Born-Oppenheimer breakdown parameters and the usefulness of such calculations is illustrated for highly accurate spectroscopic measurements in the field of rotational spectroscopy. [ABSTRACT FROM AUTHOR]

The hyperfine structure in the rotational spectra of six isotopic species of bromofluoromethane, namely CH279BrF, CH281BrF, CDH79BrF, CDH81BrF, CD279BrF and CD281BrF, has been investigated using the Lamb-dip technique in the submillimeter-wave frequency range. Measurements and assignment procedures were supported by high-level quantum-chemical calculations of the hyperfine parameters at the coupled-cluster level. For all species, the accuracy of the determined rotational and centrifugal distortion constants and the bromine quadrupole-coupling constants were improved with respect to available literature data, whereas the full bromine quadrupole-coupling tensor for the monodeuterated species and the bromine spin-rotation constants for all isotopologues were determined for the first time. [ABSTRACT FROM AUTHOR]

Studies the vibrational structure in the vinylidene anion photoelectron spectrum. Equilibrium structures of vinylidene and its anion; Molecular structure and vibrational levels for vinylidene and its anion.

Reports on an estimation of the isomerization energy of acetylene. Electronic energies for acetylene and vinylidene; Determination of zero-point vibrational effects.

Reports on the synthesis and X-ray structural analysis of polyhedral aluminum chlorides, Al[sub 22]Cl[sub 20]...12L (L = tetrahydrofuran (THF), tetrahydropyran (THP)). Co-condensation of aluminum chloride with toluene and either THF of THP; Structural competition during expansion between face centered cubic- and alpha-B type- aluminum based on ab initio full-potential calculations.

We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T3 contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T4 (connected quadruple excitations) effects are included that superior performance is achieved. T4 effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T5 (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X 1Sigmag+ state of C2 but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems., (Copyright 2004 American Institute of Physics)