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We have investigated the effect of F12 geminals on the hasis set convergence of harmonic frequencies calculated using two representative double-hybrid density functionals, namely, B2GP-PLYP and revDSD-PBEP86-D4. Like previously found for energetics (Mehta N.; Martin J. M. L. J.Chem. Theory Comput. 2022, 18, 5978), one sees an acceleration by two zeta steps, such that even the cc-pVDZ-F12 basis set is quite close to the complete basis set (CBS) limit. However, the basis set convergence problem is not as acute as for energetics, and compared with experimental harmonic frequencies, conventional orbital calculations with augmented triple zeta quality basis set are acceptably close to the CBS limit, and can be carried out using analytical second derivatives. An efficient implementation of double-hybrid F12 analytical derivatives would make the F12 approach attractive in the sense that even an spd orbital basis set would be adequate. For the accurate revDSD-PBEP86-D4 functional, the role of differing local correlation terms (Perdew-Zunger 1981 versus VWN5) in different electronic structure programs has been investigated, while optimal double-hybrid parameters and performance statistics for energetics as well as frequencies differ slightly between the two implementations; these differences are insignificant for practical purposes. Key words: explicitly correlated, double hybrid, density functional theory, vibrational spectroscopy, 1. Introduction Among the various ground-state molecular properties of a molecule that can be calculated through wavefunction ab initio methods (WFT) or density functional theory (DFT), harmonic frequencies are second [...]

Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes (1,2). For example, the catalytic oxidation of organic molecules (3,4), some oxidative enzymatic transformations (5,7), [...]

The description of two novel double-hybrid functionals, B2K-PLYP and mPW2K-PLYP, is presented. The proposed functionals depict excellent results for main-group thermochemistry, main-group thermochemical kinetics, and late transition metal reactions.

An extensive ab initio and density functional theory (DFT) methods are applied to characterize and study Rozen's epoxidation reagent, C[H.sub.3]CN.HOF, and a prototype epoxidation reaction employing it. The formation of alternative hydrogen bond networks is considered and DFT anharmonic force fields of the C[H.sub.3]CN.HOF dimer and its monomers are obtained for further spectroscopic studies on the dimer.

Geometric parameters, atomization energies, and heats of formative are calculated for a series of small calcium-containing molecules using high-level quantum chemical methods. The W2C-best heats of formation are generally in good agreement with experimental values however there are discrepancies for a small number of molecules.

The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical chromophores are determined by time-dependent density functional theory. The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model.

Various metallabenzene complexes, analogues of benzene where one CH unit is replaced by an organometallic fragment are reported. An examination of the formation, reactivity, and stability of the different metallabenzenes suggests that the high valent formalism is more plausible.

The reaction mechanism of the Rh(I) catalyzed hydrogenation of acetone to 2-propanol as investigated by DFT methods at the mPW1K/SDB-cc pVDZ//mPW1K/SDD level is reported. The results suggest that an enol-type catalytic cycle occurs and that cooperativity between the two metal centers is responsible for the acceleration of the reaction in the monolayer system.

A comparison of C-H bond activation/reductive elimination in aliphatic and aromatic PC-Rh systems is presented through a combined experimental and theoretical study. Fine-tuning of the electron density of the metal center and the reaction media can be utilized for selective C-H activation by various mechanisms.

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.

Research was conducted to examine the formation of eta-2 C-H rhodium arene complexes and their importance to electrophilic bond activation. The X-ray crystal structure and NMR data revealed strong interaction between the agostic C-H bond and the metal center which leads to the high acidity of the agostic proton. Results indicate that the deprotonation of eta-2 aromatic C-H agostic complexes can be proposed as an alternative route to electrophilic metalation of aromatic compounds.

A study was conducted to analyze association reactions associated with 12-crown-4-ether, 12C4H+, with NH3, CH3OH, CH3NH2, (CH3)2NH, and (CH3)3N using the B3LYP density functional technique and a range of basis sets. The Gaussian 94 package was utilized to carry out density functional computations. Experimental results indicated that the geometric properties of the optimized clusters correlated with experimental findings and a priori expectations.

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.

A study is conducted on the geometry of the protein, crambin using ab initio HF/4-21G geometry optimization. The results are then compared to the crystal structure of the compound and with HF/4-21G phi, psi-conformational geometry maps obtained for the model peptide N-acetyl-N'-methylalaninamide. Root-mean-square deviations between the calculated and crystallographic backbone structural parameters are determined. A helix compression in elongated helical chains compared to isolated residues to smaller oligomers is also observed.

The ammonia dimer (NH[sub 3])[sub 2] has been investigated using high-level ab initio quantum chemistry methods and density functional theory. The structure and energetics of important isomers are obtained to unprecedented accuracy without resorting to experiment. The global minimum of eclipsed C[sub s] symmetry is characterized by a significantly bent hydrogen bond which deviates from linearity by as much as ≈20°. In addition, the so-called cyclic C[sub 2h] structure, resulting from further bending which leads to two equivalent “hydrogen bonding contacts,” is extremely close in energy on an overall flat potential energy surface. It is demonstrated that none of the currently available [generalized gradient approximation (GGA), meta-GGA, and hybrid] density functionals satisfactorily describe the structure and relative energies of this nonlinear hydrogen bond. We present a novel density functional, HCTH/407+, which is designed to describe this sort of hydrogen bond quantitatively on the level of the dimer, contrary to, e.g., the widely used BLYP functional. This improved generalized gradient approximation functional is employed in Car–Parrinello ab initio molecular dynamics simulations of liquid ammonia to judge its performance in describing the associated liquid. Both the HCTH407+ and BLYP functionals describe the properties of the liquid well as judged by analysis of radial distribution functions, hydrogen bonding structure and dynamics, translational diffusion, and orientational relaxation processes. It is demonstrated that the solvation shell of the ammonia molecule in the liquid phase is dominated by steric packing effects and not so much by directional hydrogen bonding interactions. In addition, the propensity of ammonia molecules to form bifurcated and multifurcated hydrogen bonds in the liquid phase is found to be negligibly small. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) is used for studying the geometric and electronic structure of [[(PCN)Pt[double bond]O].sup.+], composition in solution and reactivity. Stabilization of the Pt-oxo bonding is ascribed to bending of the terminal oxo ligand out of the plane of the complex and to the transfer of electron density from compact low lying Pt 5d orbitals to more diffuse 6s and 6p orbitals.

The exclusive activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) in the presence of aryl-I containing substrates is described. The reaction has obeyed first-order kinetics in the [[eta].sup.2]-coordination complexes with identical activation parameters for Br and I derivatives and the ring-walking process is shown to be kinetically accessible.

The importance of [Gd.sup.3+] complexes as the potential spin labels for high field (HF) pulsed EPR measurements is demonstrated. The complexes are found to be perfect for the job because of their high sensitivity and magnetic field values.

The effect of basis set on second order Moller-Plesset perturbation (MP2) of several hydrogen-bonded complexes is studied. The possibilities of utilizing basis set extrapolations for geometries and dissociation energies are discussed along with the results of R12 methods and density functional and local correlation methods.

The applicability of density functional theory (DFT) to the polarizability of [C.sub.n.sup.-], where n = 3-9, cluster anions is examined. The results have shown that polarizability has increased monotonically with chain size, which is consistent with the trend obtained from electron detachment experiments.

The selective [sp.sup.3] C-H activation of alkyl ketones at the B position by Ir(1) is reported. Calculations reveal that the selectivity is both kinetically and thermodynamically controlled, the latter as a result of carbonyl oxygen coordination in the product and that water has a strong influence on the regioselectivity.

A selective chemical transformation in which metal coordination to a stilbazole double bond is followed by a ring-walking process prior to intramolecular aryl-halide oxidative addition to a low-valent metal center is demonstrated. The metal center walks over the pi-conjugated ring system until it reaches and activates the aryl-halide bond.

The heats of the formation for a number of key C1 and C2 bromoalkanes and radicals are calculated ab initio, both directly using an all-relativistic variant of W2 theory and indirectly using Douglas-Kroll relativistic CCSD(T)/Aug-VTZ reaction energies. The accurate thermodynamic parameters can be used to develop kinetic rate parameters.

The density functional theory (DFT) with the B97-1 exchange-correlation functional and a triple-zeta plus double polarization (TZ2P) basis set are used to obtain the anharmonic force fields and vibrational spectra of the azabenzene series and benzene. The B97-1/TZ2P quartic force fields combined with second-order rovibrational perturbation theory shows great promise for the assignment of vibrational spectra of medium-sized organic molecules.

Presents methods for computing molecular total atomization energies and heats of formation of first- and second-row compounds. Atomic electron affinities used to compare computed results with experiment; Results for molecules; W2 theory and its performance.

Density functional theory (DFT) is used for examining the mechanistic aspects of cycloaddition reactions of acetone, CO2 and CS2 to the iridium metalloaromatic complexes [(C4H4YIr)(PH3)3](super +n) and their rhodium analogues. The size of the ring, length of the substrate C=X bond, ring strain and double bond conjugation significantly affect the reaction barriers.

High-level ab initio calculations, including variants of the Wn and G3 procedures, are used to determine the structures and heats of formation of the alkali metal and alkaline earth metal oxides and hydroxides. The best calculated heats of formation came from a procedure designated W2C//ACQ, while G3[CC](dir,full) is the best of the G3-type procedures.

Examines the basis set convergence study of sulfur dioxide in terms of atomization energies, geometries and anharmonic fields. Importance of high-exponent polarization functions to ensure convergence; Effects of harmonic frequencies on the bond distance of sulfur dioxide; Indispensability of inner polarization functions for benchmark calculations.

Assesses the spectroscopic quality of quartic force field for acetylene. Details on coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected Triple excitations method; Bending modes of carbon-carbon multiple bonds; Computations on quartic force field and harmonic frequencies.

Focuses on benchmark quality total atomization energies (TAE) of small polyatomic molecules. Successive coupled-cluster calculations in basis sets; Separate Schwartz-type extrapolations of the self consistent field (SCF) and correlation energies; Mean absolute error; Valence correlation contribution to the (TAE).

The reactivity of TpPtMe(H)2 (Tp = hydrido-tris(pyrazolyl)borate) is investigated. This complex is remarkably resistant to methane loss, and heating it in methanol at 55 degree Celsius does not lead to either methane or hydrogen loss. Investigations of similar sized systems should be examined at the density functional theory or the mPW1k/SDB-cc-pVDZ/mPW1k/SDD level for improved accuracy of the energetic calculations.

A study demonstrates that cyclopentadienyl (Cp) complex formation can proceed from metallabenzene complexes through a carbine migratory insertion regardless of the type of transition metal and ancillary ligands involved. Density functional theory (DFT) investigation reveals that certain palladiabenzenes and rhodiabenzenes might be isolated prior to Cp complex formation.

The synthesis and structure of the first stable metallaquinone is reported.

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.

We have developed and benchmarked a new extended basis set for explicitly correlated calculations, namely cc-pV5Z-F12. It is offered in two variants, cc-pV5Z-F12 and cc-pV5Z-F12(rev2), the latter of which has additional basis functions on hydrogen not present in the cc-pVnZ-F12 (n = D,T,Q) sequence. A large uncontracted ‘reference’ basis set is used for benchmarking. cc-pVnZ-F12 (n = D–5) is shown to be a convergent hierarchy. Especially the cc-pV5Z-F12(rev2) basis set can yield the valence CCSD (coupled cluster with all single and double substitutions) component of total atomisation energies,without any extrapolation, to an accuracy normally associated with aug-cc-pV{5,6}Z extrapolations. Hartree-Fock self-consistent field (SCF) components are functionally at the basis set limit, while the MP2 limit can be approached to as little as 0.01 kcal/mol without extrapolation. The determination of (T) appears to be the most difficult of the three components and cannot presently be accomplished without extrapolation or scaling. (T) extrapolation from cc-pV{T,Q}Z-F12 basis sets, combined with CCSD-F12b/cc-pV5Z-F12 calculations, appears to be an accurate combination for explicitly correlated thermochemistry. For accurate work on noncovalent interactions, the basis set superposition error with the cc-pV5Z-F12 basis set is shown to be so small that counterpoise corrections can be neglected for all but the most exacting purposes. [ABSTRACT FROM PUBLISHER]

C-C and C-H bond activation requirements in terms of steric and electronic comparisons with regard to chelating vs nonchelating, are examined.

The oxidative addition of C-C vs C-H bonds to rhodium(I) using PCP ligands was undertaken.

We present the results of a coupled-cluster basis-set convergence study on 12 eight-valence-electron systems. These small systems span a wide range of correlation character (from purely dynamical correlation to pathological static correlation) as well as of chemical bonding (from ionic with some backdonation to quadruple covalent bonding). Indicators such as the correlation entropy and the CASSCF 1-C02, which primarily sample type A static correlation, are good predictors for the energetic importance of connected quintuple excitations, T5, and to a lesser extent of higher order triple excitations, T3–(T). The percentage of the total atomisation energy recovered by connected triple excitations, %TAE[(T)], is however a fair predictor for the importance of connected quadruples, T4. Of the various ansätze for quasiperturbative connected quadruples, CCSDT(Q)/B appears to be the most robust, even as it is still outperformed by the much more expensive CCSDT(Q)Λ. [ABSTRACT FROM AUTHOR]

A study was conducted to analyze the structures of furanosides. Equilibrium geometries of the compounds were calculated using density functional methods supporting different exchange-correlation functionals and a sequence of basis sets. Neutron diffraction data were then determined using a four-circle diffractomer. Results indicated the inadequacy of molecular mechanics and small basis set SCF treatments.