BeCH2: the simplest metal carbene. High levels of theory.

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am.

Communication: Some critical features of the potential energy surface for the Cl + H2O → HCl + OH forward and reverse reactions.

The forward and reverse reactions Cl + H2O → HCl + OH are very important in atmospheric chemistry. The entrance complex, transition state, and exit complex for the endothermic reaction Cl + H2O → HCl + OH have been studied using the CCSD(T) method with the correlation consistent basis sets through cc-pVQZ. The vibrational frequencies and the zero-point vibrational energies of the five stationary points for the reaction are reported and compared to the limited available experimental results.

F + (H2O)2 reaction: the second water removes the barrier.

In light of controversy concerning the classical barrier for the F + H2O → HF + OH reaction, higher level theoretical methods are applied. Both aug-cc-pV5Z CCSD(T) and cc-pVQZ full CCSDT methods predict a low classical barrier of about 2 kcal/mol. For the analogous water dimer reaction, the presence of the second water molecule erases this barrier entirely.

Investigating the Effects of Basis Set on Metal-Metal and Metal-Ligand Bond Distances in Stable Transition Metal Carbonyls: Performance of Correlation Consistent Basis Sets with 35 Density Functionals.

Density functional theory (DFT) is a widely used method for predicting equilibrium geometries of organometallic compounds involving transition metals, with a wide choice of functional and basis set combinations. A study of the role of basis set size in predicting the structural parameters can be insightful with respect to the effectiveness of using small basis sets to optimize larger molecular systems. For many organometallic systems, the metal-metal and metal-carbon distances are the most important structural features.

Structures and transition states of Ge2CH2.

In this study a systematic theoretical investigation of Ge2CH2 is carried out. The singlet potential energy surface (PES) was explored using state-of-the-art theoretical methods including self-consistent field (SCF), coupled cluster theory incorporating single and double excitation (CCSD), perturbative triple [CCSD(T)] and full triples [CCSDT] with perturbative quadruple (Q), together with a variety of correlation-consistent polarized valence basis sets cc-pVXZ (where X = D, T, and Q). A total of eleven stationary points have been located on the Ge2CH2 singlet ground state PES.

Journey through the potential energy surfaces for the isomerization and decomposition reactions of the telluroformaldehyde analogues: H2A═Te and HFA═Te (A = C, Si, and Ge).

The unavailability of monomeric heavy ketone analogues has been ascribed to the evanescence of the very reactive A═E double bond (A and E are the heavier group 14 and group 16 elements, respectively). Can the isolation of any of the monomeric telluro-ketones be assisted by an energetic favorability on its potential energy surface (PES)? In this light, the reaction pathways for the isomerization and decomposition reactions of H2A═Te and HFA═Te (A = C, Si, and Ge) molecules on their singlet state PES have been studied using second-order Møller-Plesset perturbation theory (MP2). The barrier heights reported suggest that telluroformaldehyde, silanetellone, and germatellone are kinetically more resistant to unimolecular reactions than the corresponding lighter chalcogen analogues.

Diatomic silylynes, germylynes, stannylynes, and plumbylynes: structures, dipole moments, dissociation energies, and quartet-doublet gaps of EH and EX (E = Si, Ge, Sn, Pb; X = F, Cl, Br, I).

Systematic theoretical studies of the carbyne and halocarbyne analogues E-H and E-X (E = Si, Ge, Sn, Pb; X = F, Cl, Br, I) were carried out with ab initio coupled-cluster methods using very large basis sets. The (2)Π state is the ground electronic state for all these compounds. The quartet-doublet energy separations, equilibrium distances, and dissociation energies for these species are predicted.

Extreme metal carbonyl back bonding in cyclopentadienylthorium carbonyls generates bridging C2O2 ligands by carbonyl coupling.

Laboratory studies of the interaction of carbon monoxide with organoactinides result in the formation of isolable complexes such as Cp3UCO derivatives (Cp = cyclopentadienyl) as well as coupling reactions to give derivatives of the oligomeric anions C(n)O(n)(2-) (n = 2, 3, 4). To gain some insight into actinide carbonyl chemistry, binuclear cyclopentadienylthorium carbonyls Cp2Th2(CO)n (n = 2 to 5) as model compounds have been investigated using density functional theory. The most favorable such structures in terms of energy and thermochemistry are the tricarbonyl Cp2Th2(η(2)-μ-CO)3 having three four-electron donor bridging carbonyl groups and the tetracarbonyl Cp2Th2(η(4)-μ-C2O2)(η(2)-μ-CO)2 having not only two four-electron donor bridging carbonyl groups but also a bridging ethynediolate ligand formed by coupling two CO groups through C-C bond formation.

The ethyl radical in superfluid helium nanodroplets: rovibrational spectroscopy and ab initio computations.

The ethyl radical has been isolated and spectroscopically characterized in (4)He nanodroplets. The band origins of the five CH stretch fundamentals are shifted by < 2 cm(-1) from those reported for the gas phase species [S. Davis, D.

New potential energy surface features for the Li + HF → LiF + H reaction.

The existing potential energy surfaces for the Li + HF system have been challenged by the experiments of Loesch, Stienkemeier, and co-workers. Here a very accurate potential energy surface has been obtained with rather rigorous theoretical methods. Methods up to full CCSDT have been pursued with basis sets as large as core correlated quintuple ζ.

Tetragermacyclobutadiene: energetically disfavored with respect to its structural isomers.

Germanium has been a central feature in the renaissance of main-group inorganic chemistry. Herein, we present the stationary-point geometries of tetragermacyclobutadiene and its related isomers on the singlet potential energy surface at the CCSD(T)/cc-pVTZ level of theory. Three of these 12 structures are reported for the first time and one of them is predicted to lie only 0.

Explicitly correlated atomic orbital basis second order Møller-Plesset theory.

The scope of problems treatable by ab initio wavefunction methods has expanded greatly through the application of local approximations. In particular, atomic orbital (AO) based wavefunction methods have emerged as powerful techniques for exploiting sparsity and have been applied to biomolecules as large as 1707 atoms [S. A.

Formylmethylene: the triplet ground state and the lowest singlet state.

The ground triplet state and lowest singlet state of formylmethylene have been proposed as important intermediates in the Wolff rearrangement of α-diazo ketones into ketenes. The ground triplet state of formylmethylene has been examined experimentally, but the lowest singlet state has yet to be observed. We predict equilibrium geometries, energies, bonding, dipole moments, and harmonic vibrational frequencies for these two lowest states of formylmethylene at the cc-pVQZ CCSD(T) level of theory.

Oxidation of carbene-stabilized diarsenic: diarsene dications and diarsenic radical cations.

Oxidation of carbene-stabilized diarsenic, L:As-As:L [L: = :C{N(2,6-(i)Pr(2)C(6)H(3))CH}(2)] (1), with gallium chloride in a 1:4 ratio in toluene affords the dicationic diarsene complex [L:As═As:L](2+)([GaCl(4)](-))(2) (2(2+)[GaCl(4)](2)), while oxidation of 1 with GaCl(3) in a 1:2 ratio in Et(2)O yields the monocationic diarsenic radical complex [L:AsAs:L](•+)[GaCl(4)](-) (2(•+)[GaCl(4)]). Strikingly, complex 2(•+) is the first arsenic radical to be structurally characterized in the solid state. The nature of the bonding in these complexes was probed computationally and spectroscopically.

Density cumulant functional theory: the DC-12 method, an improved description of the one-particle density matrix.

Density cumulant functional theory (DCFT) is a theory that, in principle, can compute energies and properties exactly without a wavefunction. To accomplish this, the energy is expressed as an exact, known functional of the one-particle density matrix and two-particle density cumulant. The correlation contribution to the one-particle density matrix is obtained from the cumulant, to eliminate redundancy in the equations.

Characterization of the t-Butyl Radical and Its Elusive Anion.

The t-butyl radical and its anion are studied theoretically using state-of-the-art quantum mechanical methods including coupled cluster theory with full single, double, and triple excitations (CCSDT) and CCSDT with perturbative quadruple excitations [CCSDT(Q)], in concert with large correlation-consistent cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5) basis sets. The relative energies are extrapolated to the complete basis set limit (CBS). The lowest energy structure of the t-butyl radical has a nonplanar carbon backbone with overall C3v symmetry.

Dicyanogermylenes: a tale of isomers and interconversions.

A systematic investigation is carried out using the B3LYP, BLYP, and BHLYP functionals and MP2 level of theory to characterize the low-lying electronic singlet and triplet GeC2N2 isomers. The basis sets used are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, DZP++. Three bent isomers Ge(CN)2, CNGeCN, and Ge(NC)2 are located on the singlet and triplet potential energy surfaces.

Arbitrary order El'yashevich-Wilson B tensor formulas for the most frequently used internal coordinates in molecular vibrational analyses.

In recent years, internal coordinates have become the preferred means of expressing potential energy surfaces. The ability to transform quantities from chemically significant internal coordinates to primitive Cartesian coordinates and spectroscopically relevant normal coordinates is thus critical to the further development of computational chemistry. In the present work, general nth order formulas are presented for the Cartesian derivatives of the five most commonly used internal coordinates--bond stretching, bond angle, torsion, out-of-plane angle, and linear bending.

Conformational preferences of gas-phase helices: experiment and theory struggle to agree: the seven-residue peptide Ac-Phe-(Ala)5-Lys-H+.

Peptide conformations: IR/UV double-resonance spectroscopy suggested relative energies for four peptide conformations (see figure). These results cannot be reproduced by any of 19 density functional methods.

Analytic gradients for density cumulant functional theory: the DCFT-06 model.

Density cumulant functional theory (DCFT) is one of a number of nascent electron correlation methods that are derived from reduced density matrices and cumulants thereof, instead of the wavefunction. Deriving properties from the density cumulant naturally yields methods that are size extensive and size consistent. In this work, we derive expressions for the analytic gradient, with respect to an external perturbation, for the DCFT-06 variant of density cumulant functional theory.

Bis(heptalene) "submarine" metal dimer sandwich compounds (C12H10)2M2 (M = Ti, V, Cr, Mn, Fe, Co, Ni).

The bis(heptalene)dimetal complexes (C12H10)2M2 of the first row transition metals from Ti to Ni are predicted by density functional theory to exhibit "submarine" sandwich structures with a pair of metal atoms sandwiched between the two heptalene rings. For the early transition metal derivatives (C12H10)2M2 (M = V, Cr) there are two types of such structures. In one structural type the metals are sandwiched between two heptahapto heptalene rings with metal-metal distances (3.

Structural changes upon replacing carbonyl groups with thiocarbonyl groups in first row transition metal derivatives: new insights.

The chemistry of metal thiocarbonyls is much more limited than that of metal carbonyls because of the instability of CS as a synthetic reagent. In view of the many gaps remaining in experimentally realized metal thiocarbonyl chemistry, theoretical studies using density functional methods have been used to explore the possible future scope of metal thiocarbonyl chemistry. This paper reviews such theoretical studies on binuclear metal carbonyl derivatives of the types M(2)(CS)(2)(CO)(n) and Cp(2)M(2)(CS)(2)(CO)(n) (Cp = η(5)-C(5)H(5); M = V through Ni) as well as the trinuclear and tetranuclear iron carbonyls Fe(3)(CS)(3)(CO)(n) (n = 9, 8, 7, 6) and Fe(4)(CS)(4)(CO)(n) (n = 12, 11, 10, 9).