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Comparison of the potential energy surface including the ZPVE corrections of the highly exothermic hydrogen abstraction reaction H2Te + OH with the H2Se + OH, H2S + OH, and H2O + OH reactions at the CCSD(T)/5Z level.

The DFT study suggests that the polarization of CC bonds is not the controlling factor of outer-sphere bifunctional CC bond hydrogenations. Instead, the “push–pull” type π-conjugative effect can contribute to these reactions.

Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H2O)3 → HCl + (H2O)2OH using a standard DFT method. As found for the analogous fluorine reaction, the geometrical and energetic results for the four chlorine pathways are closely related. However, the energetics for the Cl reaction are very different from those for fluorine. In the present paper, we investigate the lowest-energy chlorine pathway using the “gold standard” CCSD(T) method in conjunction with correlation-consistent basis sets up to cc-pVQZ. Structurally, the stationary points for the water trimer reaction Cl + (H2O)3 may be compared to those for the water monomer reaction Cl + H2O and water dimer reaction Cl + (H2O)2. Based on the CCSD(T) energies, the title reaction is endothermic by 19.3 kcal mol−1, with a classical barrier height of 16.7 kcal mol−1 between the reactants and the exit complex. There is no barrier for the reverse reaction. The Cl⋯(H2O)3 entrance complex lies 5.3 kcal mol−1 below the separated reactants. The HCl⋯(H2O)2OH exit complex is bound by 8.6 kcal mol−1 relative to the separated products. The Cl + (H2O)3 reaction is somewhat similar to the analogous Cl + (H2O)2 reaction, but qualitatively different from the Cl + H2O reaction. It is reasonable to expect that the reactions between the chlorine atom and larger water clusters may be similar to the Cl + (H2O)3 reaction. The potential energy profile for the Cl + (H2O)3 reaction is radically different from that for the valence isoelectronic F + (H2O)3 system, which may be related to the different bond energies between HCl and HF.

The cyano radical (CN) is an abundant, open-shell molecule found in a variety of environments, including the atmosphere, the interstellar medium and combustion processes. In these environments, it often reacts with small, closed-shell molecules via hydrogen abstraction. Both carbon and nitrogen atoms of the cyano radical are reactive sites, however the carbon is more reactive with reaction barrier heights generally between 2–15 kcal mol−1 lower than those of the analogous nitrogen. The CN + HX → HCN/HNC + X, with X = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, C2H, CN reactions have been studied at a high-level of theory, including CCSD(T)-F12a. Furthermore, kinetics were obtained over the 100–1000 K temperature range, showing excellent agreement with those rate constants that have been determined experimentally.

While the 1 : 1 reaction of 3 with an N-heterocyclic carbene ({(Me)CN(i-Pr)}2C:) in THF resulted in ligand-substituted product 4, the corresponding 1 : 2 reaction (in the presence of H2O) gives the first structurally characterized germanium tris(dithiolene)dianion 5 as the major product and the "naked" dithiolene radical 6˙ as a minor by-product. The structure and bonding of 4 and 5 were probed by experimental and theoretical methods. Our study suggests that carbene-mediated partial hydrolysis may represent a new method to access tris(dithiolene) complexes of main-group elements.

All important stationary points on the potential energy surface (PES) for the reaction OH + (H2O)3 → (H2O)2OH + H2O have been fully optimized using the “gold standard” CCSD(T) method with the large Dunning correlation-consistent cc-pVQZ basis sets. Three types of pathways were found. For the pathway without hydrogen abstraction, the barrier height of the transition state (TS1) is predicted to lie 5.9 kcal mol−1 below the reactants. The two major complexes (H2O)3⋯OH (CP1 and CP2a) are found to lie 6.3 and 11.0 kcal mol−1, respectively, below the reactants [OH + (H2O)3]. For one of the H-abstraction pathways the lowest classical barrier height is predicted to be much higher, 6.1 kcal mol−1 (TS2a) above the reactants. For the other H-abstraction pathway the barrier height is even higher, 15.0 (TS3) kcal mol−1. Vibrational frequencies and the zero-point vibrational energies connected to the PES are also reported. The energy barriers for the H-abstraction pathways are compared with those for the OH + (H2O)2 and OH + H2O reactions, and the effects of the third water on the energetics are usually minor (0.2 kcal mol−1).

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H2O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H2 and CO2 (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FAwater complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol-1 plus 6.07, 3.79, 2.60, and 2.55 kcal mol-1 for the trans-FAwater and cis-FAwater complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol-1 and the barrier to decarboxylation by up to 15.90 kcal mol-1. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FAH2O system as well as provide insight into future studies of similar atmospheric systems.

The 1-norbornyl (nor) ligand is known experimentally to form stable transition metal alkyl derivatives through direct metal–carbon bond formation. This appears to be related to its steric bulk and inaccessibility towards β-hydrogen elimination, as exemplified by the tetraalkyls (nor)4M, some of which are very stable. In this connection we have used density functional theory and the DLPNO-CCSD(T) method to investigate the 1-norbornylcobalt carbonyl derivatives (nor)Co(CO)n (n = 4, 3, 2, 1) and (nor)2Co2(CO)n (n = 7, 6, 5). Low-energy structures of the unsaturated systems (nor)Co(CO)n (n = 3, 2) and (nor)2Co2(CO)n (n = 6, 5) are found to have agostic hydrogen atoms from a CH2 group adjacent to the Co–C bond. Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than ∼2 A. In such structures unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH2 group. In addition, structures in which carbonyl migration from cobalt to carbon has occurred to form acyl norCO ligands are among the lowest energy structures. The resulting acyl carbonyl groups of the norCO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety. Furthermore, such neutral norCO acyl ligands can either be one-electron donors to a cobalt atom, bonding solely through the carbonyl carbon, or three-electron donor η2-μ-norCO groups bridging a central Co2 unit through both the acyl carbon and oxygen atoms. The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient (RDG) values.

Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol−1, −5.56 kcal mol−1, and −4.97 kcal mol−1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol−1 and −3.03 kcal mol−1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size.

Density functional theory studies show that the lowest energy C4F8Fe(CO)4 structure is not the very stable experimentally known ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4 obtained from Fe(CO)12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO)4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol−1. However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO)4 complexes from tetrafluoroethylene are very high (e.g., ∼70 kcal mol−1). Therefore the ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4, which does not require a fluorine shift to form from Fe3(CO)12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C4F8)2Fe2(CO)n (n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands.

The lowest lying singlet and triplet states of HBCN−, HCCN, HNCN+, HAlCN−, HSiCN, HPCN+, HGaCN−, HGeCN, and HAsCN+ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet–triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet–triplet gap (ΔEST = Esinglet − Etriplet) of 3.5 kcal mol−1, 11.9 kcal mol−1, and 22.6 kcal mol−1, respectively, for HBCN−, HCCN, and HNCN+. The second row increasing favors the singlet state, with singlet–triplet gaps of −20.4 kcal mol−1 (HAlCN−), −26.6 kcal mol−1 (HSiCN), and −26.8 kcal mol−1 (HPCN+). The third row also favors the singlet state, with singlet–triplet gaps of −26.8 kcal mol−1 (HGaCN−), −33.5 kcal mol−1 (HGeCN), and −33.1 kcal mol−1 (HAsCN+). The HXCN species have larger absolute singlet–triplet energy gaps compared to their parent species XH2 except for the case of X = N+. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO.

Three constitutional isomers of both Ca2H2 and Ca2H4 have been characterized with molecular electronic structure theory. Correlation methods as complete as CCSDT(Q) and basis sets as large as cc-pwCV5Z have been used to converge the relative energies within chemical accuracy (≤1 kcal mol−1). Anharmonic vibrational frequencies were computed using second-order vibrational perturbation theory employing CCSD(T)/cc-pwCVTZ cubic and quartic force-fields and a CCSD(T)/cc-pwCVQZ quadratic force field. The monobridged [Ca(μ2-H)CaH] and dibridged [Ca(μ2-H)2Ca] isomers of Ca2H2 were predicted to lie 6.5 and 12.9 kcal mol−1 below the energy of the classical HCaCaH linear isomer, respectively. Despite the energetic favorability of the bridged Ca2H2 isomers, we conclude (surprisingly) that only the higher energy linear structure has been observed in the laboratory. At 0 K, the tribridged [Ca(μ2-H)3CaH] isomer of Ca2H4 is predicted to be enthalpically favored by 0.9 kcal mol−1 in comparison to the enthalpy of the dibridged [HCa(μ2-H)2CaH] structure. Comparison of experiment with our computed frequencies suggests that the observed vibrational features arise from both the dibridged and the tribridged Ca2H4 structures.

The RSSH + H2S → RSH + HSSH reaction has been suggested by numerous labs to be important in H2S-mediated biological processes. Seven different mechanisms for this reaction (R = CH3, as a model) have been studied using the DFT methods (M06-2X and ωB97X-D) with the Dunning aug-cc-pV(T+d)Z basis sets. The reaction of CH3SSH with gas phase H2S has a very high energy barrier (>45 kcal mol−1), consistent with the available experimental observations. A series of substitution reactions R1–S–S–H + −S–R2 (R1 = Me, tBu, Ad, R2 = H, S–Me, S–tBu, S–Ad) have been studied. The regioselectivity is largely affected by the steric bulkiness of R1, but is much less sensitive to R2. Thus, when R1 is Me, all −S–R2 favorably attack the internal S atom, leading to R1–S–S–R2. While for R1 = tBu, Ad, all −S–R2 significantly prefer to attack the external S atom to form −S–S–R2. These results are in good agreement with the experimental observations.

Bimolecular reactions involving stabilized Criegee intermediates (SCI) have been the target of many studies due to the role these molecules play in atmospheric chemistry. Recently, kinetic rates for the addition reaction of the simplest SCI (formaldehyde oxide) and its methylated analogue (acetone oxide) with methanol were reported both experimentally and theoretically. We re-examine the energy profile of these reactions by employing rigorous ab initio methods. Optimized CCSD(T)/ANO1 geometries are reported for the stationary points along the reaction path. Energies are obtained at the CCSD(T)/CBS level of theory. Contributions of full triple and quadruple excitations are computed to assess the convergence of this method. Rate constants are obtained using conventional canonical transition state theory under the rigid rotor harmonic oscillator approximation and with the inclusion of a one-dimensional hindered rotor treatment. These corrections for internal rotations have a significant impact on computed kinetic rate constants. With this approach, we compute rate constants for the addition of methanol to formaldehyde oxide (H2COO) and acetone oxide [(CH3)2COO] at 298.15 K as (1.2 ± 0.8) × 10-13 and (2.8 ± 1.3) × 10-15 cm3 s-1, respectively. Additionally, we investigate the temperature dependence of the rate constant, concluding that the transition state barrier height and tunneling contributions shape the qualitative behaviour of these reactions.

The redox chemistry of the first stable anionic dithiolene radical 1˙ was investigated by both reactivity and cyclic voltammetry studies. While one-electron reduction of 1˙ by Cp2Co or KC8 affords the corresponding dithiolate dimers 2 and 3, respectively, one-electron oxidation of 1˙ by Ph3C+BF4- (or O2) conveniently gives 4, the neutral dithiolene dimer.

This research presents an ab initio characterization of the potential energy surface for the methylamine plus 1D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH2 containing species in the interstellar medium.

Quantitative conformational analysis (Eliel, Stereochemistry of Organic Compounds, Wiley-Interscience, 1994) has been with us at least as long as Pitzer's landmark 1937 paper (J. Am. Chem. Soc., 1937, 59, 276) on ethane. Cyclohexanes have played a critical role in the quest for understanding. Notably, 1,2,3,4,5,6 hexachlorocyclohexane (C6H6Cl6) was apparently synthesized for the first time by Michael Faraday in 1825 (Philos. Trans. R. Soc., B, 1825, 115, 440). The γ-1,2,3,4,5,6 hexachlorocyclohexane molecule subsequently acquired the common name lindane. Although banned or limited by many countries in 2006, nearly one billion tons of lindane has been manufactured and employed, mostly in agriculture, but also for treatment of human diseases such as lice. Although not as well characterized as lindane, other chlorocyclohexanes have been made and to some degree characterized. The pioneering experimental conformational studies by LeFevre and coworkers (J. Chem. Soc. B, 1970, 1608) of 1,2 dichlorocyclohexane, 1,1,2 trichlorocyclohexane, and 1,2,3,4,5,6 hexachlorocyclohexane are particularly noteworthy. The chlorocyclohexanes have also played a role in the development of molecular mechanics methods by Allinger and coworkers (J. Am. Chem. Soc., 1983, 105, 1716 and 1723). In the present research, we report the first systematic studies of all the chlorocyclohexanes, excluding those with two chlorines attached to a single carbon atom. We make careful comparisons with previous experimental and computational studies. A simple system is established to estimate the relative energies of the different isomers of a particular molecular species. Predicted dipole moments range from identically zero to 5.7 Debye.

The known sandwich compound [η5-(CH2)3N2(BPh)2CMe]2Fe in which adjacent C2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B3N3H6, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me4B2N2CH)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me4C5H)2M. Low-energy sandwich structures having parallel B2N2C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η5-Me4B2N2CH)2M sandwich compounds relative to the corresponding metallocenes (η5-Me4C5H)2M. Thus (η5-Me4B2N2CH)2Cr has a quintet ground state in contrast to the triplet ground state of (η5-Me4C5H)2Cr. Similarly, the sextet ground state of (η5-Me4B2N2CH)2Mn lies ∼18 kcal mol−1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me4C5H)2Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η5-(CH2)3N2(BPh)2CMe]2Fe.

The spectroscopic identification of Bi4 has been very elusive. Two constitutional Bi4 isomers of Td and C2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the Td isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi4 (Td) into two Bi2 and the adiabatic energy difference between the C2v and Td isomers of Bi4. These quantities are predicted to be +65 kcal mol−1 and +39 kcal mol−1, respectively. Two electron vertical excitation energies between the Td and C2v electronic configurations are computed to be 156 kcal mol−1 for the Td isomer and 9 kcal mol−1 for the C2v isomer. The most probable approach to laboratory spectroscopic identification of Bi4 is via an infrared spectrum. The predicted fundamentals (cm−1) with harmonic IR intensities in parentheses (km mol−1) are 94(0), 123(0.23), and 167(0) for the Td isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi4 has yet to be observed. Through natural bond orbital analysis, the C2v isomer of Bi4 was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C2v isomer.

Theoretical methods show that the lowest energy bis(butadiene)metal structures (C4H6)2M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four CC double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium.

Many experimental studies have been reported on alkali metal trifluorides MF3 (M = Li, Na, K, Rb, and Cs), and several controversies remain. In the present research, we systematically study the MF3 systems using both coupled-cluster and multireference methods. New predictions and explanations are provided for some known experimental and theoretical challenges, including identification of the true MF3 minima and global minima, the unclear existence of light alkali metal trifluorides MF3 (M = Li and Na), and assignment of the F–F–F symmetric stretch frequencies for the heavier alkali metal trifluorides MF3 (M = K, Rb, and Cs). With several new structures located, we predict a preference of Cs minima for MF3 (M = Li and Na) and C2v minima for MF3 (M = K, Rb, and Cs). For the species where multiple minima were located, near degeneracies of those minima can be found in most cases. The endothermicities (∼3–4 kcal mol−1) for the favored MF3 → MF + F2 fragmentations suggest that MF3 (M = Li and Na) are weakly bonded complexes. The existence of those species at low temperatures cannot be ruled out, and vibrational frequencies are reported to guide future experiments. Most importantly, significant differences between the coupled-cluster and multireference results were found in predicting the F–F–F symmetric stretch frequencies (νs) of the C2v MF3 (M = K, Rb, and Cs) structures, although both methods show good performance in predicting most structures and antisymmetric stretch frequencies (νas). The coupled-cluster [CCSD(T), CCSDT, and CCSDT(Q)] results agree with the recent experimental assignment of Redeker, Beckers, and Riedel [389 cm−1, RSC Adv., 2015, 5, 106568] to the νs fundamental of CsF3. In contrast, the multireference (CASPT2, CASPT3, and MRCISD+Q) results support the original experimental assignment of Ault and Andrews [461 cm−1, J. Am. Chem. Soc., 1976, 98, 1591; Inorg. Chem., 1977, 16, 2024]. The F–F–F symmetric stretch frequencies for the MF3 molecules (M = K, Rb, and Cs) continue to provide a great challenge to theory and experiment.

Despite the interest in sulfur monoxide (SO) among astrochemists, spectroscopists, inorganic chemists, and organic chemists, its interaction with water remains largely unexplored. We report the first high level theoretical geometries for the two minimum energy complexes formed by sulfur monoxide and water, and we report energies using basis sets as large as aug-cc-pV(Q+d)Z and correlation effects through perturbative quadruple excitations. One structure of SO⋯H2O is hydrogen bonded and the other chalcogen bonded. The hydrogen bonded complex has an electronic energy of −2.71 kcal mol−1 and a zero kelvin enthalpy of −1.67 kcal mol−1, while the chalcogen bonded complex has an electronic energy of −2.64 kcal mol−1 and a zero kelvin enthalpy of −2.00 kcal mol−1. We also report the transition state between the two structures, which lies below the SO⋯H2O dissociation limit, with an electronic energy of −1.26 kcal mol−1 and an enthalpy of −0.81 kcal mol−1. These features are much sharper than for the isovalent complex of O2 and H2O, which only possesses one weakly bound minimum, so we further analyze the structures with open-shell SAPT0. We find that the interactions between O2 and H2O are uniformly weak, but the SO⋯H2O complex surface is governed by the superior polarity and polarizability of SO, as well as the diffuse electron density provided by sulfur's extra valence shell.

The first magnesium-based dithiolene, 2, was prepared by reaction of the lithium dithiolene radical, 1˙, with 2-mesitylmagnesium bromide. Reaction of 2 with N-heterocyclic carbenes (in toluene) gave a carbene-stabilized magnesium monodithiolene complex, 3. Complex 3, in turn, is readily converted to a THF-solvated magnesium bis-dithiolene dianion, 4, via partial hydrolysis in polar solvents (i.e., THF/CH3CN). Compounds 2, 3 and 4 have been spectroscopically and structurally characterized and probed by DFT computations.

Fullerenes, in particular C60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. However, chemical transformations have to be done in order to transform C60 in added-value compounds with increased applicability. The most common procedure corresponds to the classical Diels-Alder cycloaddition reaction. In this research, a comprehensive study of the electronic activity that takes place in the cycloaddition between C60 and cyclopentadiene toward the [5,6] and [6,6] reaction pathways is presented. These are competitive reaction mechanisms dominated by σ and π fluctuating activity. To better understand the electronic activity at each stage of the mechanism, the reaction force (RF) and the symmetry-adapted reaction electronic flux (SA-REF, JΓi(ξ)) have been used to elucidate whether π or σ bonding changes drive the reaction. Since the studied cycloaddition reaction proceeds through a Cs symmetry reaction path, two SA-REF emerge: JA'(ξ) and JA''(ξ). In particular, JA'(ξ) mainly accounts for bond transformations associated with π bonds, while JA''(ξ) is sensitive toward σ bonding changes. It was found that the [6,6] path is highly favored over the [5,6] with respect to activation energies. This difference is primarily due to the less intensive electronic reordering of the σ electrons in the [6,6] path, as a result of the pyramidalization of carbon atoms in C60 (sp2 → sp3 transition). Interestingly, no substantial differences in the π electronic activity from the reactant complex to the transition state structure were found when comparing the [5,6] and [6,6] paths. Partition of the kinetic energy into its symmetry contributions indicates that when a bond is being weakened/broken (formed/strengthened) non-spontaneous (spontaneous) changes in the electronic activity occur, thus prompting an increase (decrease) of the kinetic energy. Therefore, contraction (expansion) of the electronic density in the vicinity of the bonding change is expected to take place.

Vinyl alcohol and acetaldehyde are isoelectronic products of incomplete butanol combustion. Along with the radicals resulting from the removal of atomic hydrogen or the hydroxyl radical, these species are studied here using ab initio methods as complete as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)], with basis sets as large as cc-pV5Z. The relative energies provided herein are further refined by including corrections for relativistic effects, the frozen core approximation, and the Born-Oppenheimer approximation. The effects of anharmonic zero-point vibrational energies are also treated. The syn conformer of vinyl alcohol is predicted to be lower in energy than the anti conformer by 1.1 kcal mol-1. The alcoholic hydrogen of syn-vinyl alcohol is found to be the easiest to remove, requiring 84.4 kcal mol-1. Five other radicals are also carefully considered, with four conformers investigated for the 1-hydroxyvinyl radical. Beyond energetics, we have conducted an overhaul of the spectroscopic literature for these species. Our results also provide predictions for fundamental modes yet to be reported experimentally. To our knowledge, the ν3 (3076 cm-1) and ν4 (2999 cm-1) C-H stretches for syn-vinyl alcohol and all but one of the vibrational modes for anti-vinyl alcohol (ν1-ν14) are yet to be observed experimentally. For the acetyl radical, ν6 (1035 cm-1), ν11 (944 cm-1), ν12 (97 cm-1), and accounting for our changes to the assignment of the 1419.9 cm-1 experimental mode, ν10 (1441 cm-1), are yet to be observed. We have predicted these unobserved fundamentals and reassigned the experimental 1419.9 cm-1 frequency in the acetyl radical to ν4 rather than to ν10. Our work also strongly supports reassignment of the ν10 and ν11 fundamentals of the vinoxy radical. We suggest that the bands assigned to the overtones of these fundamentals were in fact combination bands. Our findings may be useful in constructing improved combustion models of butanol and in spectroscopically characterizing these molecules further.

Interest in peroxy radicals derives from their central role in tropospheric and low-temperature combustion processes; however, their transient nature limits the scope of possible experimental characterization. As a result, theoretical methods (notably, coupled-cluster theory) have become indispensable in the reliable prediction of properties of such ephemeral open-shell systems. Herein, the and A state conformers of ethylperoxy radical (C2H5O2) have been structurally optimized at the CCSD(T)/ANO2 level of theory. Relative enthalpies at 0 K [including A ← transition origins (T0)] are reported, incorporating CCSD(T) electronic energies extrapolated to the complete basis set limit via the focal point approach. Higher-level computations, employing basis sets as large as cc-pV5Z and post-HF methods up to CCSDT(Q), prove essential in achieving predictions to within 10 cm−1 for experimental T0; we predict 7363 and 7583 cm−1 for the trans and gauche conformers, respectively. Furthermore, predictions of state fundamental transitions incorporating CCSD(T)/ANO0 anharmonic contributions are given. For each conformer, all 21 modes were characterized, improving upon the 16 modes reported in the experimental literature, and providing predictions for the 5 remaining modes.

The stationary points, including the entrance complex, transition states, and the exit complex, for the reaction OH + (H2O)2 → (H2O)OH + H2O have been carefully examined using the “gold standard” CCSD(T) method with the correlation-consistent basis sets up to cc-pVQZ. The complex (H2O)2⋯OH is found to lie 10.8 kcal mol−1 below the separated reactants. This complex should be observable in the gas phase via vibrational or microwave spectroscopy. Seven unique transition states were found. One pathway for the title reaction has no barrier, in which the OH radical captures a whole water molecule from the water dimer. For the hydrogen abstraction pathways the lowest classical barrier height is predicted to be 5.9 kcal mol−1 (TS1) relative to separated reactants, and the other pathways are of higher barriers, i.e., 17.8 (TS2) and 18.4 (TS3) kcal mol−1. The harmonic vibrational frequencies and the zero-point vibrational energies of the stationary points for the reaction are also reported.

The unexpected 2004 discovery of decamethyldizincocene suggested some new possibilities for organometallic compounds in which an M2 unit is sandwiched between two planar carbocyclic rings. Density functional theory shows that the low-energy structures for the dizincocenes Zn2(C5X5)2 (X = H, F) are singlet coaxial structures having two (η5-C5X5)Zn units linked by a Zn–Zn single bond of length ∼2.3 A. However, the low-energy M2(C5H5)2 (M = Cu, Ni, Co, Fe) structures have perpendicular configurations with bridging C5H5 ligands. They exhibit increasingly higher spin states from singlet (Cu, Ni) to triplet (Ni,Co), quintet (Co,Fe), and septet (Fe). The low-energy structures of the perfluorinated systems M2(C5F5)2 (M = Co, Fe) have irregular geometries with one bent bridging C5F5 ring and one planar terminal pentahapto η5-C5F5 ring, with the metal atom bearing the terminal η5-C5F5 ligand has the favored 18-electron configuration while the other metal atom has only a lower electron configuration and vacant coordination sites.

The F + HOCH3 → HF + OCH3 reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F-(HOCH3) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.80 eV accesses both the reactant and product channels for this reaction. In the product channel (HF + OCH3 + e-) of the neutral potential energy surface, vibrationally excited HF products in addition to the stable product-channel hydrogen-bonded complex (FH-OCH3) are observed in the PPC and photoelectron spectra. In addition, experimental evidence is observed for the reactant-channel van der Waals complex (F-HOCH3), in good agreement with the theoretical predictions. The relative stability of these long-lived complexes was probed by reducing the ion beam energy, increasing the product time-of-flight, indicating lifetimes on the microsecond timescale for the reactant- and product-channel complexes as well as providing evidence for long-lived vibrational Feshbach resonances associated with the HF(v > 0) + OCH3 product states. This system will provide a model for extending full-dimensionality quantum dynamics to larger numbers of degrees of freedom.

A new series of unsaturated stannylenes is studied computationally. The singlet and triplet states of acyclic and cyclic stannylenes are fully optimized using the B3LYP, BHLYP, OPBE, and M06 functionals. The basis sets used are of double-ξ plus polarization quality with additional s- and p-type diffuse functions denoted DZP++. All cyclic and most acyclic stannylenes have been found to have triplet ground states. The most favored triplet state is that for the NHC (NMeCHCHNMe)SnSn: system, where the triplet state lies ∼20 kcal mol−1 below the singlet. The saturated cyclic systems are expected to be easier to synthesize, but the unsaturated cyclic counterparts have larger singlet–triplet splittings. A preparative outline based on retro-synthetic routes is briefly described.

Reaction of the anionic NHDC ligand, [:C{[N(2,6-Pri2C6H3)]2CHCLi}]n (1), with SiCl4 gives the trichlorosilyl-substituted NHC ligand (7). Abnormal carbene–SiCl4 complex (8) can be conveniently synthesized by combining 7 with HCl·NEt3. Meanwhile, 7 may react with CH2Cl2 in warm hexane, giving the abnormal carbene-complexed SiCl3+ cation (9). The structure and bonding of 9 have also been probed by DFT computations.

The CO2-reduction activity of two Re(I)–NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H2 formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

Thialene (C8H6S) is an isomer of benzothiophene and related to azulene by replacement of a CC unit in the seven-membered ring by a sulfur atom. The geometries of binuclear iron carbonyl complexes of thialene (C8H6S)Fe2(CO)n (n = 6, 5, 4) have been investigated using density functional theory for comparison with the corresponding azulene derivatives. The lowest energy (thialene)Fe2(CO)6 structures have a bis(tetrahapto)-η4,η4-thialene ligand bonded to two separate Fe(CO)3 units without involvement of the sulfur atom. This differs from the isomeric (benzothiophene)Fe2(CO)6 structure known experimentally in which an iron atom has inserted into a carbon–sulfur bond to give a ferrathianaphthalene system. The only low-energy (thialene)Fe2(CO)5 structure has a pentahapto-trihapto-η5,η3 thialene ligand bonded to an Fe2(CO)5 unit, again without involvement of the sulfur atom. This structure is related to the experimental structure of azulene diiron pentacarbonyl, (η5,η3-C10H8)Fe2(CO)5 by replacement of the uncomplexed CC double bond in the seven-membered ring with the sulfur atom in the thialene six-membered ring. The potential energy surface of the unsaturated (thialene)Fe2(CO)4 is very complicated but does not include any low-energy structures with formal FeFe double bonds. Thermochemical considerations suggest (thialene)Fe2(CO)5 as a realistic synthetic objective, which is potentially accessible from reactions of thialene with Fe2(CO)9 or (benzalacetone)Fe(CO)3 under mild conditions.

Theoretical studies are reported on methylphosphepine, methylbenzophosphepine, their norcaradiene isomers, and their metal complexes with Fe(CO)3 and CpCo (Cp = η5-C5H5) units. Both methylphosphepine and methylphosphanorcaradiene are C6H6PCH3 species existing as anti/syn stereoisomer pairs with the methylphosphepine structures at slightly higher energies. The transition states for the interconversion of these isomers and their benzo derivatives lie ∼20 kcal mol−1 in energy above the methylphosphanorcaradiene isomers. Complexation of either C6H6PCH3 ligand with the transition metal units Fe(CO)3 and CoCp leads to energetically closely spaced η4-tetrahapto and η3-trihapto isomers of the methylphosphepine complexes and η4-tetrahapto isomers of the methylphosphanorcaradiene complexes. However, the bis(dihapto) (η2,2-C6H6PCH3)Fe(CO)3 and (η2,2-C6H6PCH3)CoCp complexes involving coordination of non-adjacent CC double bonds lie at significantly higher energies. Fusion of a benzene ring to the C6H6PCH3 rings in methylphosphepine and methylphosphanorcaradiene leads to significantly different structures of their lowest energy metal complexes. Thus the lowest energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures are η2,2 and η4 methylbenzophosphepine complexes, which avoid using any carbon atoms of the benzene ring in the ligand for metal complexation. Higher energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures have tetrahapto ligands with one or both CC double bonds of the benzene ring complexed with the metal atom.

The superoxide radical anion O2(-) is involved in many important chemical processes spanning different scientific disciplines (e.g., environmental and biological sciences). Characterizing its interaction with various substrates to help elucidate its rich chemistry may have far reaching implications. Herein, we investigate the interaction between O2(-) (X[combining tilde] (2)Πg) and the hydrogen halides (X[combining tilde] (1)Σ) with coupled-cluster theory. In contrast to the short (1.324 A) hydrogen bond formed between the HF and O2(-) monomers, a barrierless proton transfer occurs for the heavier hydrogen halides with the resulting complexes characterized as long (>1.89 A) hydrogen bonds between halide anions and the HO2 radical. The dissociation energy with harmonic zero-point vibrational energy (ZPVE) for FHO2(-) (X[combining tilde] (2)A'') → HF (X[combining tilde] (1)Σ) + O2(-) (X[combining tilde] (2)Πg) is 31.2 kcal mol(-1). The other dissociation energies with ZPVE for X(-)HO2 (X[combining tilde] (2)A'') → X(-) (X[combining tilde] (1)Σ) + HO2 (X[combining tilde] (2)A'') are 25.7 kcal mol(-1) for X = Cl, 21.9 kcal mol(-1) for X = Br, and 17.9 kcal mol(-1) for X = I. Additionally, the heavier hydrogen halides can form weak halogen bonds H-XO2(-) (X[combining tilde] (2)A'') with interaction energies including ZPVE of -2.3 kcal mol(-1) for HCl, -8.3 kcal mol(-1) for HBr, and -16.7 kcal mol(-1) for HI.

Open metallocenes containing a metal sandwiched between two U-shaped pentadienyl or substituted pentadienyl ligands have been synthesized by Ernst and co-workers using reactions of pentadienyl or substituted pentadienyl anion reagents with metal halides. The complete series of such open metallocenes (2,4-Me2C5H5)2M of the first row transition metals (M = Ti to Ni) has now been examined using density functional theory. The experimentally known lowest energy structures of the early transition metals are open metallocene structures with two pentahapto U-shaped η5-2,4-Me2C5H5 ligands. Such structures can exist as syn-eclipsed, gauche-eclipsed, and/or anti-eclipsed conformations of similar energies. The theoretical predictions of the lowest energy conformations of the open metallocenes are in excellent agreement with available experimental data. Singlet and triplet spin state structures are preferred energetically for the Ti and Cr derivatives, respectively. For (η5-2,4-Me2C5H5)2V, doublet and quartet spin state structures are spaced in energy by 7.6 kcal mol−1. The (2,4-Me2C5H5)2Mn energy surface is complicated with doublet, quartet, and sextet spin state structures closely spaced in energy. In the sextet spin state of (2,4-Me2C5H5)2Mn only the end carbon atoms of the 2,4-Me2C5H5 ligand are within bonding distance of the manganese atom. This corresponds to tetrahedral C4Mn coordination similar to that found experimentally in the sextet ground state t-butyl derivative (2,4-tBu2C5H5)2Mn as well as the related Mn(CN)42−. The low energy structures for the “open ferrocene” (η5-2,4-Me2C5H5)2Fe are singlet spin state structures with two U-shaped pentahapto η5-2,4-Me2C5H5 ligands but otherwise analogous to ferrocene. The low energy (2,4-Me2C5H5)2M structures of the late transition metals Co and Ni have at least one trihapto η3-2,4-Me2C5H5 ligand. The lowest energy (2,4-Me2C5H5)2Ni structure is a singlet spin state structure with two trihapto η3-2,4-Me2C5H5 ligands and a 16-electron nickel environment analogous to the well-known bisallylnickel, (η3-C3H5)2Ni.

Togni's reagents have become very popular trifluoromethylating reagents in organic synthesis. The existing form of Togni's reagent I is a hypervalent iodine compound which lies much higher in energy than its ether isomer. The high-energy hypervalent iodine form makes Togni's reagent I very effective and versatile. The energy differences between the two forms correlate with the trans influence of the substituents. The five-membered ring in the benziodoxole-based scaffold is an important reason for its existence in the higher-energy form. The relation to Buchwald's 2014 research is discussed.

Alkali metal salts of the 2-phosphaethynolate anion PCO− synthesized from reactions of CO with NaPH2 or K3P7 have recently become available in quantities for the synthesis of transition metal complexes of the potentially ambidentate PCO ligand (Angew. Chem., Int. Ed., 2013, 38, 10064). This is exemplified by the recently reported rhenium carbonyl complex (triphos)Re(CO)2(PCO) (triphos = MeP(CH2PPh2)3). Density functional theory studies on the related manganese carbonyl complexes Mn(CO)n(PCO) (n = 5, 4, 3) and Mn2(CO)n(PCO)2 (n = 8, 7, 6, 5) are now reported. For the binuclear systems the low-energy Mn2(CO)8(PCO)2 structures are singlet spin state structures having two bridging P-bonded phosphaketenyl μ-PCO ligands without a direct Mn–Mn bond. Carbonyl loss from Mn2(CO)8(μ-PCO)2 is predicted to lead to migration of CO groups from phosphorus to manganese resulting in Mn2(CO)n+2(μ-P2) structures with bridging diphosphido groups as the lowest energy Mn2(CO)n(PCO)2 isomers (n = 7, 6, 5). Isomeric Mn2(CO)6(PCO)2 structures with dihapto bridging η2-μ-PCO ligands at ∼30 kcal mol−1 above the global minimum are also found representing intermediates in the migration of CO groups from phosphorus to manganese. For the mononuclear systems the P-bonded Mn(CO)n(PCO) (n = 5, 4) phosphaketenyl structures are found to lie 20 to 28 kcal mol−1 in energy below the isomeric O-bonded Mn(CO)n(PCO) phosphaethynoxy isomers consistent with previously reported results by Grutzmacher and coworkers on R3E(PCO)–R3E(OCP) systems (R = iPr, Ph; E = Si, Sn, Ge, Pb). The lowest energy structure for the tricarbonyl Mn(CO)3(PCO) is a singlet structure with an unusual trihapto η3-PCO ligand. However, higher energy isomeric Mn(CO)3(PCO) structures with P-bonded phosphaketenyl or O-bonded phosphaethynoxy ligands and tetrahedral Mn coordination are also found.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

The cyclopentadienyliron trifluorophosphine hydride CpFe(PF3)2H, in contrast to CpFe(CO)2H, is a stable compound that can be synthesized by reacting Fe(PF3)5 with cyclopentadiene. Theoretical studies on the binuclear Cp2Fe2(PF3)n (n = 5, 4, 3, 2) derivatives derived from CpFe(PF3)2H indicate the absence of viable structures having PF3 ligands bridging Fe–Fe bonds solely through the phosphorus atom. This contrasts with the analogous Cp2Fe2(CO)n systems for which the lowest energy structures have two (for n = 4 and 2) or three (for n = 3) CO groups bridging an iron–iron bond. Higher energy singlet Cp2Fe2(PF3)3 structures have a novel four-electron donor bridging η2-μ-PF3 ligand bonded to one iron atom through its phosphorus atom and to the other iron atom through a fluorine atom. Other higher energy triplet and singlet Cp2Fe2(PF3)2 structures are of the Cp2Fe2F2(μ-PF2)2 type having terminal fluorine atoms and bridging μ-PF2 ligands. The lowest energy Cp2Fe2(PF3)5 structure is actually Cp2Fe2(PF3)3(PF4)(μ-PF2) with a bridging PF2 group and a terminal PF4 group. Such structures are derived from a Cp2Fe2(PF3)4(μ-PF3) precursor by migration of a fluorine atom from the bridging PF3 group to a terminal PF3 group with a low activation energy barrier.

Acylperoxy radicals [RC(O)OO˙] play an important catalytic role in many atmospheric and combustion reactions. Accordingly, the prototypical formylperoxy radical [HC(O)OO˙] is characterized here using high-level ab initio coupled-cluster theory. Important experiments have been carried out on this system, but have not comprehensively described the properties of even the ground electronic state. We report cis and trans geometries for the ground ( 2A′′) and first excited (A 2A′) state equilibrium conformers and the torsional saddle point on the ground state surface at the CCSD(T)/ANO2 level of theory. Relative energies of these ground- and excited-state stationary points were obtained using coupled cluster theory with up to perturbative quadruple excitations, extrapolated from the sextuple zeta basis set to the complete basis set limit. These methods predict conformational energy differences ΔE(trans- → cis-) = 2.35 kcal mol−1 and ΔE(trans-A → cis-A) = −2.95 kcal mol−1. On the surface, the transition state for the conformational change lies 8.42 kcal mol−1 above the trans ground state minima. The adiabatic electronic excitation energies from the ground state isomers are predicted to be 18.17 ± 0.10 (trans) and 13.03 ± 0.10 kcal mol−1 (cis). The former is in excellent agreement with the 18.1 ± 1.4 kcal mol−1 transition found by Lineberger and coworkers. Additionally, transition properties between the 2A′′ and A 2A′ states are reported for the first time, using the equation of motion (EOM)-CCSD method, which predicts lifetimes for trans-A 2A′ HC(O)OO˙ of 5.4 ms and cis-A 2A′ HC(O)OO˙ of 20.5 ms. Second-order vibrational perturbation theory was utilized to determine the fundamental frequencies at the CCSD(T)/ANO2 level of theory for the cis and trans conformers of the and A states and five ground state isotopologues of both conformers: H13C(O)OO˙, HC(18O)OO˙, HC(O)18O18O˙, DC(O)OO˙, and DC(O)18O18O˙. This study provides high accuracy predictions of vibrational frequencies, helping to resolve large uncertainties and disagreements in the experimental values. Furthermore, we characterize experimentally unassigned vibrational frequencies and transition properties.

Thiophene is known experimentally to be desulfurized by Fe3(CO)12 under mild conditions to give the tricarbonyl ferrole (η4,η2-C4H4)Fe2(CO)6. A similar reaction of benzothiophene (thianaphthene) with Fe3(CO)12 gives a (C8H6S)Fe2(CO)6 complex in which an iron carbonyl moiety has inserted into the thiophene ring to give a thiaferranaphthalene ligand. Density functional theory shows this experimental structure to be the lowest energy structure. Furthermore, the lowest energy structures of the diiron pentacarbonyl (C8H6S)Fe2(CO)5 are simply derived from this (C8H6S)Fe2(CO)6 by loss of a CO group retaining the thiaferranaphthalene ligand. However, a higher energy isomeric (η6,η2-C8H6S)Fe2(CO)5 structure retains the original benzothiophene ligand with the C6 ring bonded to an Fe(CO)2 moiety as a hexahapto ligand and the CC double bond of the C4S ring bonded to an Fe(CO)3 moiety as a dihapto ligand with an Fe→Fe dative bond between the iron atoms. Similar insertion of an iron atom into a thiophene ring to give a thiaferrabenzene ring is predicted to occur in the lowest energy (C6H4S2)Fe2(CO)6 structure derived from either the anti or syn isomers of thienothiophene. However, the bonding of the exocyclic iron atom to the resulting thiaferrabenzothiophene ligand involves atoms in both rings in contrast to the (C8H6S)Fe2(CO)6 complex where the benzene ring is not involved in the ligand–iron bonding.

The SiOOH potential energy surface has become central to the understanding of recent experiments (Science 2013, 342, 463) by Chakraborty associated with nebular meteorite formation. The entrance complex, transition states, and exit complex for the title reaction SiO + OH→ SiO2 + H have been studied using the CCSD(T) method with correlation consistent basis sets as large as cc-pV(Q+d)Z. Reported here are characteristics of the reactants, products, six transition states, and four intermediate complexes for this reaction. These show four previously undiscovered stationary point geometries. The entrance complex OH⋯OSi is predicted to lie 28.6 kJ mol−1 below the separated reactants. The classical barriers cis-TS1 and trans-TS1 are predicted to lie 21.8 kJ mol−1 and 6.8 kJ mol−1, respectively, below the reactants. The exit complex HSiO2 is bound by 115.3 kJ mol−1 relative to the separated products. After zero-point vibrational energy corrections, the reaction energy is predicted to be −1.4 kJ mol−1. Vibrational frequencies of the stationary points are reported and compared with the limited available experimental results. The SiOOH potential surface is found to be very different from that for COOH, contrary to the analogy drawn by Chakraborty. Notwithstanding, the assumption of Chakraborty appears justified, because all the stationary points for the SiO + OH reaction have lower relative energies than known for the analogous carbon system.

The chemistry of trinuclear osmium carbonyl hydrides is a rich area with the three H2Os3(CO)n derivatives (n = 12, 11, 10) all being known stable compounds ultimately obtained from Os3(CO)12 and hydrogen under various conditions. Density functional theory studies on the H2Os3(CO)n systems (n = 12, 11, 10, 9, 8) correctly predict the structures previously reported experimentally for n = 12, 11, and 10. These include a linear structure for H2Os3(CO)12 and triangular structures for H2Os3(CO)11 and H2Os3(CO)10. However, the H2Os3(CO)11 system is predicted to be a fluxional system with the four low energy isomers lying within 2 kcal mol−1 of energy. Three of these H2Os3(CO)11 isomers, all with one terminal hydrogen and one bridging hydrogen, have been observed experimentally by NMR. In addition, the lowest energy isomer has been isolated and structurally characterized by X-ray crystallography. In contrast to H2Os3(CO)11, the lowest energy H2Os3(CO)10 structure, namely the known structure with an OsOs edge bridged by both hydrogen atoms and all terminal CO groups, lies ∼10 kcal mol−1 below the next lowest energy isomer. The predicted CO dissociation energies of the H2Os3(CO)n derivatives (n = 12, 11, 10) suggest this H2Os3(CO)10 structure to be the “thermodynamic sink” in the H2Os3(CO)n systems, consistent with its synthesis from Os3(CO)12 and H2 at 120 °C and atmospheric pressure. The lowest energy structures of the more highly unsaturated H2Os3(CO)n (n = 9, 8) can be derived from this (μ-H)2Os3(CO)10 structure by removal of CO groups from the osmium atom remote to the doubly bridged OsOs edge of the Os3 triangle, with relatively little change in the central (μ-H)2Os3 triangle geometry.

Oxidative degradation of the octahedral dianion [Fe6C(CO)16]2− with an interstitial carbon atom leads eventually to the neutral Fe4C(CO)13 cluster with a butterfly-shaped central Fe4C unit. The complete series of related Fe4C(CO)n (n = 16, 15, 14, 13, 12, 11) derivatives have now been investigated using density functional theory. For the lowest energy Fe4C(CO)n (n = 16, 15, 14, 13) structures the geometries obey the n + f = 18 rule where f is the number of Fe–Fe bonds. This leads to a spiropentane geometry with two Fe–Fe bonds for Fe4C(CO)16, a central bent Fe–Fe–Fe–Fe chain for Fe4C(CO)15, a distorted trigonal pyramidal structure with four Fe–Fe bonds for Fe4C(CO)14, and the experimentally observed butterfly structure with five Fe–Fe bonds for Fe4C(CO)13. A symmetrical higher energy centered tetrahedral structure for Fe4C(CO)12 with six Fe–Fe bonds also follows the n + f = 18 rule. However, the lowest energy Fe4C(CO)n (n = 12, 11) structures are derived from the lowest energy Fe4C(CO)13 structure by removal of CO groups with retention of the central Fe4C butterfly unit.

The interaction of carbon monoxide with organoactinides has recently been shown experimentally, particularly by Cloke and co-workers, to result in coupling to give the oligomeric anions CnOn2− (n = 2, 3, 4). In order to model possible intermediates in reactions of this type, we have used density functional theory to explore the systems (C8H8)Th(CO)n (n = 1 to 5) and (C8H8)2Th2(CO)n (n = 2 to 7) related to the known “thorocene”, (η8-C8H8)2Th. Thorium was chosen as the actinide for this work since its chemistry almost entirely involves the single diamagnetic +4 oxidation state. All of the binuclear (C8H8)2Th2(CO)n structures found in this work have long Th⋯Th distances ranging from 4.4 to 5.0 A suggesting the absence of direct Th–Th bonds. Two (C8H8)2Th2(CO)2 isomers of similar energies in which the two CO groups have coupled to form trans and cis isomers of a bridging η4-μ-C2O2 ligand are low energy structures. These bridging η4-μ-C2O2 ligands exhibit ultralow ν(CO) frequencies around 1000 cm−1 indicating strong back donation of thorium d and f electrons into C–O antibonding orbitals. Most of the carbonyl richer (C8H8)2Th2(CO)n (n = 3 to 7) structures are derived from one of these basic (C8H8)2Th2(CO)2 structures by addition of terminal CO groups. An exception is the lowest energy (C8H8)2Th2(CO)4 structure which has C4v symmetry with four equivalent separate η2-μ-CO groups bridging the thorium atoms. The thermochemistry of these systems suggest (C8H8)Th(CO)4 and (C8H8)2Th2(CO)n (n = 2, 4) to be the most promising synthetic objectives, which are potentially obtainable by reductive carbonylation of the known (C8H8)ThCl2.

Density functional theory (DFT) studies on Mn2(CS)2(CO)8 using the B3LYP and BP86 methods show that no less than eight different unbridged structures are of significantly lower energies than the lowest energy doubly bridged structure. The Mn–Mn single bonds in these Mn2(CS)2(CO)8 structures range from 2.99 ± 0.02 A for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 A for the four structures with eclipsed equatorial CO/CS groups. The six lowest energy Mn2(CS)2(CO)7 structures all have four-electron donor bridging η2-μ-CE groups (E = S, O) and formal Mn–Mn single bonds of lengths 2.95 ± 0.01 A, rather than only two-electron donor CO and CS groups and formal MnMn double bonds. The Mn2(CS)2(CO)7 structures with an η2-μ-CS group are of lower energy than those with an η2-μ-CO group. These Mn2(CS)2(CO)7 structures are similar to the lowest energy structure for Mn2(CO)9 predicted previously as well as (Ph2PCH2PPh2)2Mn2(CO)4(η2-μ-CO), which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy Mn2(CS)2(CO)6 structures are predicted have a single four-electron donor bridging η2-μ-CS group and a formal MnMn double bond. However, at only slightly higher energies, Mn2(CS)2(CO)6 structures are found with two η2-μ-CS groups and a formal Mn–Mn single bond. A formal MnMn triple bond of length 2.36 ± 0.03 A is found in an even higher energy unbridged Mn2(CS)2(CO)6 structure, similar to the lowest energy Mn2(CO)8 structure found in a previous theoretical study. The lowest energy structures for Mn2(CS)2(CO)5 have two η2-μ-CS groups and a formal MnMn double bond of length 2.57 ± 0.03 A.

The tetranuclear iron carbonyls Fe4(CO)n (n = 16, 15, 14) have been investigated using density functional theory (DFT). Low-energy structures are predicted for Fe4(CO)16, Fe4(CO)15, and Fe4(CO)14 in which the Fe4 units are rhombi, planar butterflies, and tetrahedra with four, five, and six Fe–Fe single bonds, respectively, to give all four iron atoms the favored 18-electron configuration. However, for Fe4(CO)14 an alternative low energy structure is predicted with a planar Fe4 butterfly and one of the five iron–iron distances short enough to correspond to a formal FeFe double bond, thereby also leading to the favored 18-electron configuration. The dissociation energy of Fe4(CO)16 into Fe(CO)5 + Fe3(CO)11 or Fe3(CO)12 + Fe(CO)4 is predicted to be very low (

We have investigated the reaction of Re(dmb)(CO)(3)COOH with CO(2) using density functional theory, and propose a mechanism for the production of CO. This mechanism supports the role of Re(dmb)(CO)(3)COOH as a key intermediate in the formation of CO. Our new experimental work supports the proposed scheme.

The chemical reaction dynamics to form d1-diacetylene, DCCCCH (X 1Σ+), and the d1-butadiynyl radical, DCCCC, via the reaction of d1-ethinyl, C2D (X 2Σ+), with acetylene, C2H2 (X 1Σg+), are explored in a crossed molecular beam experiment at an average collision energy of 26.1 kJ mol−1. The experiments show that the reaction follows indirect scattering dynamics via a C4H2D intermediate. The calculations confirm that the reaction has no entrance barrier and that it proceeds via an attack of the ethinyl radical on the π electron density of the acetylene molecule. The initially formed trans-1-d-ethinylvinyl-2 (HCCHC2D) intermediate rearranges to its cis form; the latter is found to fragment predominantly via H atom emission to form d1-diacetylene, HCCCCD (X 1Σ+) and H (2S1/2) (channel 1). A second involves a [1,2]-H shift in trans-HCCHC2D to yield a 1-d-ethinylvinyl-1 radical. The latter channel then shows two fragmentation pathways: a molecular hydrogen elimination to form the d1-butadiynyl radical (DCCCC) (channel 2) and an atomic hydrogen loss to yield d1-diacetylene (HCCCCD) (channel 3). Compared to the C4HD/H products (98–99%), the C4D/H2 channel presents only a minor pathway (1–2%). The solid identification of diacetylene under single collision conditions is the first experimental proof of a long-standing hypothesis that the title reaction can synthesize diacetylene in dark, molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as in the atmospheres of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.