It was recently reported ( J. Chem. Theory Comput. 2015 , 11 , 2036 - 2052 ) that the coupled cluster singles and doubles with perturbative triples method, CCSD(T), should not be used as a benchmark tool for the prediction of dissociation energies (heats of formation) for the first row transition metal diatomics based on a comparison with the experimental thermodynamic values for a set of 20 diatomics. In the present work the bond dissociation energies as well as the heats of formation for those diatomics have been calculated by the Feller-Peterson-Dixon approach at the CCSD(T)/complete basis set (CBS) level of theory including scalar relativistic corrections and correlation of the outer shell of core electrons in addition to the valence electrons. Revised experimental values for the hydrides are presented that are based on new heterolytic R-H bond dissociation energies, which are needed for analysis of the mass spectrometry experiments. The agreement between the calculated bond dissociation energies and the revised experimental values of the hydrides is good. Good agreement of the calculated bond dissociation energies/heats of formation is also found for most of the chlorides, oxides, and sulfides given the experimental error bars from experiment and those of the transition metal atoms in the gas phase. Thus, reliable results can be achieved by the CCSD(T) method at the CBS limit. The use of PW91 orbitals for the CCSD(T) calculations improves the predictions for some compounds with large T diagnostics at the HF-CCSD(T) level. The optimized bond distances and calculated vibrational frequencies for the diatomics also agree well with the available experimental values.

Density functional theory and coupled cluster theory are used to study the hydrolysis reactions of (TiO2) n (n = 1–4) nanoclusters to provide insight into H2O activation on TiO2. The singlet–triplet energy gaps of (TiO2) n are predicted to lie between 30 and 65 kcal/mol, depending on the cluster size and structure, consistent with our previous studies. The excitation energies for the various hydroxides, Ti n O2n–m (OH)2m (n = 1–4, 1 ≤ m ≤ n) are predicted to be, in general, higher than those for (TiO2) n . The partial charge on Ti increases as the Ti O bonds are replaced with the Ti–OH bonds. The TiO and Ti–O frequencies in the triplet state of (TiO2) n and Ti n O2n–m (OH)2m are, in general, lower than those in the singlet state. The first H2O adsorption (physisorption) energies for these TiO2 nanoclusters are predicted to be −10 to −35 kcal/mol for the singlet states and −10 to −50 kcal/mol for the triplet states. These physisorption energies depend on the cluster size and the site of adsorption, consistent with existing experimental studies. In general, H2O prefers to physisorb on the Ti site with one TiO bond and two Ti–O bonds and at the Ti site with no TiO bond and three Ti–O bonds. The first hydrolysis (dissociative chemisorption) reaction energies of the TiO2 nanoclusters are predicted to be −20 to −70 kcal/mol for the singlet states and −15 to −80 kcal/mol for the triplet states. Both singlet and triplet potential energy surfaces for the hydrolysis are calculated. Our calculations show that H2O readily reacts with both the singlet and the triplet states of the TiO2 nanoclusters to form the hydroxides with reaction barriers of 5–16 kcal/mol for the singlet states and 5–26 kcal/mol for the triplet states for the first hydrolysis steps, which are, in general, less than the H2O complexation energies. Because H2O splitting to form H2 and O2 is a strongly endothermic process by ∼116 kcal/mol, photocatalytic processes are necessary only in the subsequent steps.

The reactions of ethanol (CH 3 CH 2 OD) over cyclic (MO 3 ) 3 (M = Mo, W) clusters were studied experimentally and computationally. The cyclic clusters were prepared by sublimation of MoO 3 and WO 3 powders in a vacuum. To evaluate the cluster activity in dehydration, dehydrogenation, and condensation reactions, they were suspended in an ethanol matrix on an inert substrate, graphene monolayer on Pt(111). The reaction products formed upon heating were followed and quantified using temperature-programmed desorption. The experimental results were corroborated using coupled cluster CCSD(T) calculations at DFT optimized geometries that provide quantitative molecular-scale information on the reaction mechanisms. The dehydration and dehydrogenation of ethanol probe both the Lewis/Brønsted acid/ base and redox properties of the metal centers. The overall conversion of the alcohol is governed by the Lewis acidity of the metal center, and product selectivities, as determined by the relative weights of dehydrogenation and dehydration, are governed by the reducibility of the metal center.

The reactions of deuterated methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, 2-butanol, and tert-butanol over cyclic (MO3)3 (M = Mo, W) clusters were studied experimentally with temperature-programmed desorption and theoretically with coupled cluster CCSD(T) theory and density functional theory. The reactions of two alcohols per M3O9 cluster are required to provide agreement with experiment for D2O release, dehydrogenation, and dehydration. The reaction begins with the elimination of water by proton transfers and forms an intermediate dialkoxy species that can undergo further reaction. Dehydration proceeds by a β-hydrogen transfer to a terminal MO. Dehydrogenation takes place via an α-hydrogen transfer to an adjacent MoVIO atom or a WVI metal center with redox involved for M = Mo and no redox for M = W. The two channels have comparable activation energies. H/D exchange to produce alcohols can take place after olefin is released or via the dialkoxy species, depending on the alcohol and the cluster. The Lewis acidity of the metal center with WVI being larger than MoVI results in the increased reactivity of W3O9 over Mo3O9 for dehydrogenation and dehydration. However, the product selection of aldehyde or ketone and olefin is determined by the reducibility of the metal center. Our calculations are consistent with the experiment in terms of the dehydrogenation, dehydration, and H/D exchange reactions. The condensation reaction requires a third alcohol with the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas-phase Brønsted acid. This Brønsted acid-driven reaction is different from the dehydrogenation and dehydration reactions that are governed by the Lewis acidity of the metal center.

Density functional theory (DFT) has been used to study the hydrolysis reaction of (MO2) n (M = Zr, Hf, n = 1–4) nanoclusters in the ground singlet and first triplet states. The reactions for singlet n = 1 were benchmarked at the CCSD(T) level of theory. The reactions of H2O with the metal site having an MO bond and/or M–O bonds as well as H transfer to both terminal O atoms and bridge −O atoms have been studied. The partial charge on M increases as the MO bonds are replaced with M–OH bonds. The first H2O adsorption (physisorption) energies for these MO2 nanoclusters are calculated to be −20 to −30 kcal/mol for the singlet state and −15 to −48 kcal/mol for the triplet state. These physisorption energies depend on the cluster size and the adsorption site, consistent with existing experimental and computational studies. The first hydrolysis (dissociative chemisorption) reaction energies of the MO2 nanoclusters are calculated to have a much broader range, −30 to −80 kcal/mol for the singlet states and −30 to −100 kcal/mol for the triplet states. Steric effects play an important role in determining the physisorption and chemisorption energies, especially for the trimers and tetramers. The potential energy surfaces for hydrolysis in both the singlet and triplet states are calculated. The calculated Lewis acidities (fluoride affinities) correlate with the hydrolysis properties of the nanoclusters. Our calculations show that H2O readily reacts with both the singlet and triplet states of the MO2 nanoclusters to form the hydroxides. The reaction barriers are generally less than 10 kcal/mol for the singlet states, and because the H2O physisorption energies are large, the barriers occur below the (MO2) n asymptote.

Coupled cluster [CCSD(T)] theory and density functional theory (DFT) have been used to study the production of H2 and O2 from hydrolysis products generated from H2O addition to (MO2)n (M = Ti, Zr, Hf, n = 1-3) clusters on both the lowest singlet and triplet potential energy surfaces (PESs). H2 production occurs via the formation of an M-H containing intermediate followed by H-H recombination and H2 desorption from M(n)O(2n)(OH)2 and M(n)O(2n+2). The hydrogen transfer reactions to form the M-H bond are the rate determining steps and can be considered to be proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M-O bonds in an atomic oxygen saturated metal oxide from an M(n)O(2n)•O2 intermediate. On the triplet PES, the activation energies for the first and second H transfer to the metal are calculated to be ~10 to 50 kcal/mol and ~75 to 90 kcal/mol depending on the size of the clusters and the metal. The barriers on the singlet surface for the first and the second H transfer are predicted to be 110 to 140 kcal/mol, in general larger than the H-O bond dissociation energy. The activation barriers for the step of H-H recombination are 15 to 50 kcal/mol, and the H2 desorption energies are less than 10 kcal/mol on the singlet and triplet PESs. The oxygen desorption energies follow the order Ti < Zr < Hf for the triplets and Ti < Zr ≈ Hf for the singlets. The oxygen desorption energy is approximately independent of the size of the cluster for the same metal. The water splitting reactions prefer to take place on the triplet surface. A low excess potential energy is needed to generate 2H2 and O2 from 2H2O after the endothermicity of the reaction is overcome on the triplet PES.

Homoleptic thorium isocyanide complexes have been prepared via the reactions of laser-ablated thorium atoms and (CN) in a cryogenic matrix, and the structures of the products were characterized by infrared spectroscopy and theoretical calculations. Thorium atoms reacted with (CN) under UV irradiation to form the oxidative addition product Th(NC), which was calculated to have closed-shell singlet ground state with a bent geometry. Further reaction of Th(NC) and (CN) resulted in the formation of Th(NC), a molecule with a tetrahedral geometry. Minor products such as ThNC and Th(NC) were produced upon association reactions of CN with Th and Th(NC). Homoleptic thorium cyanide isomers Th(CN) (x = 1-4) are predicted to be less stable than the corresponding isocyanides. The C-N stretches of thorium cyanides were calculated to be between 2170 and 2230 cm at the B3LYP level, more than 120 cm higher than the N-C stretches of isocyanides and with much weaker intensities. No experimental absorptions appeared where Th(CN) should be observed.

The prediction of the heats of formation of group IV and group VI metal oxide monomers and dimers with the coupled cluster CCSD(T) method has been improved by using Kohn-Sham density functional theory (DFT) and Brueckner orbitals for the initial wave function. The valence and core-valence contributions to the total atomization energies for the CrO3 monomer and dimer are predicted to be significantly larger than when using the Hartree-Fock (HF) orbitals. The predicted heat of formation of CrO3 with CCSD(T)/PW91 is consistent with previous calculations including high-order corrections beyond CCSD(T) and agrees well with the experiment. The improved heats of formation with the DFT and Brueckner orbitals are due to these orbitals being closer to the actual orbitals. Pure DFT functionals perform slightly better than the hybrid B3LYP functional due to the presence of exact exchange in the hybrid functional. Comparable heats of formation for TiO2 and the second- and the third-row group IV and group VI metal oxides are predicted well using either the DFT PW91 orbitals, Brueckner orbitals, or HF orbitals. The normalized clustering energies for the dimers are consistent with our previous work except for a larger value predicted for Cr2O6. The prediction of the reaction energy for UF6 + 3Cl2 → UCl6 + 3F2 was significantly improved with the use of DFT or Brueckner orbitals as compared to HF orbitals.