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Functional group contributions from ab initio calculations were the basis for zero-point vibrational effects on proton shieldings.

A theoretical study of the optical characteristics of the chromone molecule using multireference perturbation theory and density functional theory, is presented.

It has been possible to compute the scalar relativistic contribution to the atomization energies of BF3. Scalar relativistic effects cut the atomization energy of BF3 by around 0.7 kcal/mol.

Research was conducted to address questions on the heat of vaporization of silicon through a benchmark calculation of the total atomization energy of SiF4 molecule. Results supported the suggestion of Grev and Schaefer that the uncertain JANAF/CODATA value of 106.5+1-1.9 kcal/mol must be revised to a lower value. Future studies on heats produced by silicon compound formation could take advantage of the revision.

The total atomization energy of BF3 was accurately obtained ab initio in a comprehensive study employing coupled cluster techniques to compute the heat of formation of boron atom. Basis sets up to (8s7p6df4g3h2i) quality were analyzed and an extrapolation for further basis set incompleteness was used. Inner-shell correlation, anharmonicity in the zero-point energy and atomic spin-orbit splitting were all considered. The best heat of formation for gaseous boron at 0 K was 136.0 kcal/mol.

Aluminum reacted with certain carbon atoms to yield dialuminum acetylide, while certain energized aluminum atoms reacted with acetylene to produce aluminum acetylide. These linear molecules were similar to acetylene as they contained similar carbon triple bonds. A pi complex was obtained when thermal Al atoms were added to acetylene.

The boron-acetylene reaction was studied to analyze the C-H insertion and C=C addition interreactions between the boron and acetylene atoms. Mixtures of argon/acetylene were deposited with laser-evaporated boron atoms at the target using 40 milliJoules/pulse. Calculations were performed at the MP2 level using the DZP basis set. Results indicate the presence of a strong absorption band in the mixture near the ultraviolet range.

The full quartic force field, heat of formation, geometry and harmonic frequencies of fluoroacetylene (FCCH) are examined using coupled cluster methods with basis sets. It is shown that FCCH fundamentals can be reproduced to about 2/cm and the rotational constants to four decimal places without empirical correction. The best computed geometry, harmonic frequencies and heat of formation are discussed.

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C3H3+ molecular cation, referred to as c-C3H3+ and 1-C3H3+. Specifically, the singles and doubles coupled-cluster method that includes a perturbation a! estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schrödinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C3H3+, obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C3H3+ and 1-C3H3+ are the most reliable available for the free gas-phase species, and it is hoped that these will be useful in the assignment of future high- resolution laboratory experiments or astronomical observations. [ABSTRACT FROM AUTHOR]

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C(3)H(3)(+) molecular cation, referred to as c-C(3)H(3)(+) and l-C(3)H(3)(+). Specifically, the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schrödinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C(3)H(3)(+), obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C(3)H(3)(+) and l-C(3)H(3)(+) are the most reliable available for the free gas-phase species, and it is hoped that these will be useful in the assignment of future high-resolution laboratory experiments or astronomical observations.

This paper analyses the behavior of different density functionals in the description of the most stable structures of Cu(2+)-(H(2)O)(n) complexes (n = 1-6). From n = 3 to n = 6, different coordination numbers of the metal cation were considered. The structures and energies of the complexes were theoretically determined by means of density functional methods that include different amounts of exact exchange: the BLYP functional (0% of exact exchange), the B3LYP functional (20% of exact exchange), the MPWB1K functional (44% of exact exchange), and BHLYP functional (50% of exact exchange). In addition, CCSD(T) calculations with a large basis set were carried out. It has been found that the functionals with lesser amount of exact exchange, especially BLYP, fail to describe the relative energies between the different structures of each cluster because these functionals tend to overestimate the stability of low-coordinated structures. The inclusion of the exact exchange into the functional improves the results, those obtained with MPWB1K and BHLYP being in very good agreement with the CCSD(T) ones. This behavior is related to the poor description of the second ionization energy of Cu by pure functionals, which leads to a too delocalized spin density in the complex with GGA functionals.

The weak interaction between unpaired electrons in polynuclear transition-metal complexes is often described by exchange and spin polarization mechanisms. The resulting intrinsic multiconfigurational electronic structure for such complexes may be calculated with wave function-based methods (e.g., complete active space configuration interaction and complete active space self-consistent field), but computations become extremely demanding and even unfeasible for polynuclear complexes with a large number of open-shells. Here, several levels of selection of configurations and symmetry considerations that still capture the essential physics of exchange and spin polarization mechanisms are presented. The proposed approximations result in significantly smaller configuration interaction expansions and are equally valid for ab initio and semiempirical methods. Tests are performed in simple molecular systems and in small transition-metal complexes that cover a range of valence and charge states. In particular, superexchange contributions can be calculated to good accuracy using only single ionic excitations. Further reduction in the size of the configuration expansions is possible but restricts the description to low-lying spin ladders. The proposed configuration interaction schemes may be used to resolve space and spin symmetries in the calculation of electronic structures, exchange coupling constants, and other properties pertinent to polynuclear transition-metal complexes.

An analysis of the major factors affecting the accuracy of harmonic force field computations of water is presented. By systematically varying the level of approximation in the basis set, treatment of electron correlation, core electron correlation, and relativistic correction, the underlying sources of error in the computation of harmonic vibrational frequencies for water were quantified. The convergence error due to wavefunction description with a cc-pVQZ basis set in the absence of electron correlation was 1.6 cm(-1), as determined from extending the Hartree-Fock computations to larger basis sets. The convergence error due to neglecting higher-order electronic correlation terms than are included at the CCSD(T) level using the cc-pVTZ basis set was estimated to be 4.7 cm(-1), as determined from frequency calculations up to CCSDTQ for water and literature results up to CCSDTQP for diatomic molecules. The convergence error due to omitting higher-order diffuse functions than included in aug-cc-pVQZ was found to be 3.7 cm(-1), as determined by adding more diffuse functions in larger basis sets. The error associated with neglecting core electron correlation effects (i.e., "freezing" core electrons) was 5.0 cm(-1) and with neglecting relativistic effects was 2.2 cm(-1). Due to a cancellation among these various sources of error, the harmonic frequencies for H2O computed using the CCSD(T)/aug-cc-pVQZ model chemistry were on average within 2 cm(-1) of experimentally inferred vibrational frequencies.

DFT (B3LYP as well as a number of common exchange-correlation functionals) predicts a low-spin Fe(IV) ground state for Fe(P)F2 (P = porphyrinato), whereas electrochemical evidence has apparently indicated an Fe(III) porphyrin π-cation radical formulation for such a species. Ab initio CASPT2 calculations favor a high-spin porphyrin π-cation radical as the ground state by a significant energetic margin, thus dramatically overturning the DFT results. In contrast, both DFT and CASPT2 calculations correctly indicate a true Mn(IV) ground state for Mn(P)F2. The remarkable failure of DFT to correctly predict the metal- versus ligand-oxidized nature of Fe(P)F2 may have significant ramifications for the theoretical modeling of heme protein reaction pathways where until now the performance of DFT has raised little concern.