Computational Study of Dehydration and Dehydrogenation of Ethanol on (TiO2)n (n = 2-4) Nanoclusters.

Dehydration and dehydrogenation of an ethanol molecule on (TiO2)n, n = 2-4, nanoclusters were studied at the correlated molecular orbital theory CCSD(T)/aug-cc-pVDZ(-PP(Ti)) level using density functional theory B3LYP/DZVP2-optimized geometries. Physisorption and chemisorption of ethanol at the bridge Ti site on the trimer and tetramer are thermodynamically preferred over these reactions at the Ti site with a terminal Ti═O. Two possible lowest energy reaction coordinates of dehydration were predicted for the dimer and trimer where the ß hydrogen on ethanol transfers to the adjacent terminal oxygen, or to the adjacent bidentate oxygen. Only the latter reaction coordinate was predicted to be the lowest energy one for the tetramer. Removal of ethylene from the (TiO2)nOH2-C2H4 complex for n = 2-4 at 0 K requires 2-7 kcal/mol. For dehydrogenation, transfer of the α hydrogen to the adjacent Ti atom results in the lowest energy reaction coordinate following a proton-coupled electron-transfer (PCET) process. Removal of the acetaldehyde molecule requires 14-26 kcal/mol from the (TiO2)nH2-C2H4O complex. Loss of H2 from the (TiO2)nH2 complex requires 5-8 kcal/mol. Dehydration and dehydrogenation of one ethanol molecule occur below the reactant asymptote for (TiO2)n, n = 2-4, whereas for (WO3)3 and (MoO3)3, two ethanol molecules are required for this process to be below the reactant asymptote. Dehydration of ethanol is thermodynamically preferred over dehydrogenation on (TiO2)n, n = 2-4. There is an approximate linear correlation of metal Lewis acidity with physisorption of ethanol. A quadratic correlation is predicted between the chemisorption barrier of ethanol and the corresponding proton affinity of oxygen to which the proton is being transferred. There are linear correlations between the basicity of the oxygen site and the acidity of the OH group versus the energy to remove C2H4 from that site. The results for the nanoclusters for n = 3 and 4 are consistent with the experimental results for the reactivity of ethanol on Ti5c4+ rutile TiO2 (110) surface sites.

A Comparison of Solid Electrolyte Interphase Formation and Evolution on Highly Oriented Pyrolytic and Disordered Graphite Negative Electrodes in Lithium-Ion Batteries.

The presence and stability of solid electrolyte interphase (SEI) on graphitic electrodes is vital to the performance of lithium-ion batteries (LIBs). However, the formation and evolution of SEI remain the least understood area in LIBs due to its dynamic nature, complexity in chemical composition, heterogeneity in morphology, as well as lack of reliable in situ/operando techniques for accurate characterization. In addition, chemical composition and morphology of SEI are not only affected by the choice of electrolyte, but also by the nature of the electrode surface. While introduction of defects into graphitic electrodes has promoted their electrochemical properties, how such structural defects influence SEI formation and evolution remains an open question. Here, utilizing nondestructive operando electrochemical atomic force microscopy (EChem-AFM) the dynamic SEI formation and evolution on a pair of representative graphitic materials with and without defects, namely, highly oriented pyrolytic and disordered graphite electrodes, are systematically monitored and compared. Complementary to the characterization of SEI topographical and mechanical changes during electrochemical cycling by EChem-AFM, chemical analysis and theoretical calculations are conducted to provide mechanistic insights underlying SEI formation and evolution. The results provide guidance to engineer functional SEIs through design of carbon materials with defects for LIBs and beyond.

Computational Study of Triphosphine-Ligated Cu(I) Catalysts for Hydrogenation of CO2 to Formate.

The catalyzed hydrogenation of CO2 to formate via a triphosphine-ligated Cu(I) was studied computationally at the density functional theory level in the presence of a self-consistent reaction field. Of the four functionals benchmarked, M06 was generally in the best agreement with the available experimentally estimated values. Two bases, DBU and TBD, were studied in the context of two proposed mechanisms in the MeCN solvent. Activation of H2 was explored by using LCu(DBU)+ to form LCuH. Dissociation of a ligand arm results in higher barriers to form the key hydride complex, LCuH. The preferred mechanism passes through a transition state, where the H2 has one H atom interacting with the copper center and the other H atom interacting with the N atom of the base, similar to H2 insertion into a frustrated Lewis pair. There is no significant difference between the choice of a base, DBU or TBD, with respect to the proposed mechanisms. We propose that the experimentally observed differences between DBU and TBD reactivities for this mechanism are due to off-pathway changes.

Formation of Surface Impurities on Lithium-Nickel-Manganese-Cobalt Oxides in the Presence of CO2 and H2O.

Surface impurities involving parasitic reactions and gas evolution contribute to the degradation of high Ni content LiNixMnyCozO2 (NMC) cathode materials. The transient kinetic technique of temporal analysis of products (TAP), density functional theory, and infrared spectroscopy have been used to study the formation of surface impurities on varying nickel content NMC materials (NMC811, NMC622, NMC532, NMC433, NMC111) in the presence of CO2 and H2O. CO2 reactivity on a clean surface as characterized by CO2 conversion rate in the TAP reactor follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111. The capacity of CO2 uptake follows a different order: NMC532 > NMC433 > NMC622 > NMC811 > NMC111. Moisture pretreatment slows down the direct CO2 adsorption process and creates additional active sites for CO2 adsorption. Electronic structure calculations predict that the (012) surface is more reactive than the (1014) surface for CO2 and H2O adsorption. CO2 adsorption leading to carbonate formation is exothermic with formation of ion pairs. The average CO2 binding energies on the different materials follow the CO2 reactivity order. Water hydroxylates the (012) surface and surface OH groups favor bicarbonate formation. Water creates more active sites for CO2 adsorption on the (1014) surface due to hydrogen bonding. The composition of surface impurities formed in ambient air exposure is dependent on water concentration and the percentage of different crystal planes. Different surface reactivities suggest that battery performance degradation due to surface impurities can be mitigated by precise control of the dominant surfaces in NMC materials.

Understanding Reaction Networks through Controlled Approach to Equilibrium Experiments Using Transient Methods.

We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH3 and D2 which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH2D, NHD2, and HD at different temperatures and delay times and NH3/D2 pulsing order reveal the role of the N2 formation barrier in controlling the surface concentration of NHx* species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.

Formation of Cerium and Neodymium Isocyanides in the Reactions of Cyanogen with Ce and Nd Atoms in Argon Matrices.

Laser ablation of metallic Ce and Nd reacting with cyanogen in excess argon during codeposition at 4 K forms Ce(NC)x and Nd(NC)x for x = 1-3, which are identified from their matrix infrared spectra using cyanogen substituted with 13C and 15N. The electronic structure calculations were performed for isocyano and cyano Cd and Nd compounds for up to n = 4. The frequencies were calculated at the density functional theory level with three different functionals as well as correlated molecular orbital theory (MP2) and are consistent with the experimental assignments and the corresponding 12C/13C isotopic frequency ratios for the isocyano species. The computed frequencies for the analogous cyanide complexes are significantly higher than those for the isocyano isomers, and they are not observed in the spectra. The high spin isocyano complexes are the lowest energy structures. On the basis of the natural population analysis results, the bonding in 4CeNC and 6NdNC is essentially purely ionic with the Ce/Nd in the +I-oxidation state. The bonding for disocyano (3Ce(NC)2 and 5Nd(NC)2) and triisocyano (2Ce(NC)3 and 4Nd(NC)3) complexes is still quite ionic with the lanthanide in the +II and +III formal oxidation states, respectively. For 1Ce(NC)4, the oxidation state is best described as being between +III and +IV. Formation of 5Nd(NC)4 does not really change the electron configuration on the Nd from that in 4Nd(NC)3 and the oxidation state on the Nd remains at +III. Although Nd compounds with up to 3 NC- groups have more ionic binding than do the corresponding Ce compounds, Ce(NC)4 has more ionic binding than does Nd(NC)4. The ionic nature of isocyano Ce and Nd complexes decreases as the number of isocyano groups increases. The energetics of formation of the isocyano Ce and Nd complexes using cyanogen or CN radicals are calculated to be mostly due to exothermic processes, with the exothermicity decreasing as the number of isocyano groups increases.

Infrared Spectroscopic and Theoretical Studies of the 3d Transition Metal Oxyfluoride Molecules.

A collection of 3d transition metal (V, Mn, Fe, Co, and Ni) oxyfluorides were prepared via the reactions of laser-ablated metal atoms and OF2 in an argon matrix, and the products were identified by infrared spectroscopy together with 18OF2 substitution. OMF2 is the major product from the reactions of metal atoms and OF2. The tetravalent metal center is coordinated to two fluorine atoms and one oxygen atom. Triatomic OMF molecules were observed in the reactions of V, Mn, Fe, and Co with OF2. In addition to OMF and OMF2, OMnF3 and OFeF3 were also formed presumably via the reactions of OMnF and OFeF with F2 resulting from photodecomposition of OF2. The seldom observed OF radical was produced in all of these experiments. Electronic structure calculations at the density functional theory and molecular orbital theory including electron correlation effects (CCSD(T) and CASPT2) levels are used to aid in the assignment of the structures. For OMF (M = Sc-Mn), the structures are bent and those for M = Fe-Cu are linear. The OMF2 molecules are optimized to be C2v structures. Both OMF and OMF2 have a high spin ground state, with the exception of OCoF2 in which the ground state quartet is the lower energy structure. The M-O stretching frequency is a sensitive measure of the computational method in terms of the bond angle, the coupling of the M-O and M-F stretches, and the amount of spin on the oxygen. A bonding analysis in terms of the CAS orbitals shows that a number of the structures have a multireference character after M = Cr. Oxidation states of the metal are given based on the CASPT2 results. Heats of formation for the OMF and OMF2 are reported.

Tungsten Hydride Phosphorus- and Arsenic-Bearing Molecules with Double and Triple W-P and W-As Bonds.

Laser ablation of tungsten metal provides W atoms which react with phosphine and arsine during condensation in excess argon and neon, leading to major new infrared (IR) absorptions. Annealing, UV irradiation, and deuterium substitution experiments coupled with electronic structure calculations at the density functional theory level led to the assignment of the observed IR absorptions to the E≡WH3 and HE═WH2 molecules for E = P and As. The potential energy surfaces for hydrogen transfer from PH3 to the W were calculated at the coupled-cluster CCSD(T)/complete basis set level. Additional weak bands in the phosphide and arsenide W-H stretching region are assigned to the molecules with loss of H from W, E≡WH2. The electronic structure calculations show that the E≡WH3 molecules have a W-E triple bond, the HE═WH2 molecules have a W-E double bond, and the H2E-WH molecules have a W-E single bond. The formation of multiple E-W bonds leads to increasing stability for the isomers.

Computational Study of Molecular Hydrogen Adsorption over Small (MO2) n Nanoclusters (M = Ti, Zr, Hf; n = 1 to 4).

Hydrogen adsorption on small group 4 metal oxide clusters for both the singlet and the first excited triplet states have been investigated by density functional theory and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The reaction starts with hydrogen physisorption on a metal center followed by formation of metal hydride/hydroxides due to splitting H2 into H- and H+. The hydrogen physisorption energies are predicted to be -1 to -8 kcal/mol for the singlet and -1 to -26 kcal/mol for the triplet, respectively. The formation of metal hydride/hydroxides does not involve redox processes. Chemisorption leading to formation of metal hydride/hydroxides is exothermic by -10 to -50 kcal/mol for the singlet, and exothermic by up to -60 kcal/mol for the triplet. The predicted energy barriers are less than 20 kcal/mol. Formation of metal dihydroxides from the metal hydride/hydroxides is generally endothermic for the monomer and dimer and is exothermic for the trimer and tetramer. Formation of the dihydroxide is a proton coupled electron transfer (PCET) process. The singlet energy barriers for the H-→ H+ transfer process are predicted to be 35-60 kcal/mol, in comparison to triplet energy barriers of less than 15 kcal/mol for the H• → H+ transfer process. For trimers and tetramers, there exist two different pathways: the first is a direct pathway with PCET to a terminal oxygen and the second is a two-step pathway with initial formation of a bridge OH group followed by a proton transfer to generate a terminal OH group. For the singlet, the two-step pathway is preferred for M = Ti and the direct pathway is more favorable for M = Zr and Hf. The two-step pathway is always preferred for the triplet as one-electron transfer is always more likely than two-electron transfer in the direct pathway.

Laser-Ablated U Atom Reactions with (CN)2 to Form UNC, U(NC)2, and U(NC)4: Matrix Infrared Spectra and Quantum Chemical Calculations.

Laser-ablated U atoms react with (CN)2 in excess argon and neon during codeposition at 4 K to form UNC, U(NC)2, and U(NC)4 as the major uranium-bearing products, which are identified from their matrix infrared spectra using cyanogen substituted with 13C and 15N and from quantum chemical calculations. The 12/13CN and C14/15N isotopic frequency ratios computed for the U(NC)1,2,4 molecules agree better with the observed values than those calculated for the U(CN)1,2,4 isomers. Multiplets using mixed isotopic cyanogens reveal the stoichiometries of these products, and the band positions and quantum chemical calculations confirm the isocyanide bonding arrangements, which are 14 and 51 kJ/mol more stable than the cyanide isomers for UNC and U(NC)2, respectively, and 62 kJ/mol for U(NC)4 in the isolated gas phase at the CCSD(T)/CBS level. The studies further demonstrate that the isocyano nitrogen is a better π donor, so it interacts with U(VI) better than carbon. Although the higher isocyanides are more stable than the corresponding cyanides, U(NC)5 and U(NC)6 were not observed here most likely because unfavorable or endothermic routes are required for their production from U(NC)4. The computed U-NC bond dissociation energies decrease from 581 kJ/mol for 4[UNC] to 168 kJ/mol for 1[U(NC)6 ]. The ionic nature of U(NC)n decreases as the number of isocyano groups increases.

Electronic Structures of Small (RuO2)n (n = 1-4) Nanoclusters and Their Anions and the Hydrolysis Reactions with Water.

Group 8 (RuO2)n (n = 1-4) nanoclusters, their anions, and the hydrolysis reactions of the neutral clusters have been studied with the density functional theory (DFT) as well as coupled cluster CCSD(T) theory. The ground state is predicted to be a singlet and a doublet for the neutral RuO2 clusters and anionic clusters, respectively. The CCSD(T) method is required to predict the correct ground state. The calculated singlet-triplet gaps (<15 kcal/mol) and fluoride affinities (<95 kcal/mol) are smaller than those of the group 4 (MO2)n and group 6 (MO3)n metal oxide clusters. The electron affinities range from 2.2 to 3.4 eV, showing that the RuO2 clusters are quite reducible. Clustering energies and heats of formation are calculated. The water physisorption energies are predicted to be -10 to -20 kcal/mol with the adsorption energy for the singlet being generally more exothermic than that for the triplet. The hydrolysis reactions are exothermic for the monomer and dimer clusters and are slightly endothermic or neutral for the trimer and tetramer. H2O is readily dissociated on the monomer and dimer but not on the trimer and tetramer. The physisorption and chemisorption energies are less exothermic, and the barriers for the hydrolysis reactions are larger for RuO2 nanoclusters than for the corresponding group 4 ZrO2 nanoclusters.

Infrared Spectroscopic and Theoretical Studies on the OMF2 and OMF (M = Cr, Mo, W) Molecules in Solid Argon.

Group 6 metal oxide fluoride molecules in the form of OMF2 and OMF (M = Cr, Mo, W) were prepared via the reactions of laser-ablated metal atoms and OF2 in excess argon. Product identifications were performed by using infrared spectroscopy, 18OF2 samples, and electronic structure calculations. Reactions of group 6 metal atoms and OF2 resulted in the formation of ternary OCrF2, OMoF2, and OWF2 molecules with C2v symmetry in which the tetravalent metal center is coordinated by one oxygen and two fluorine atoms. Both OCrF2 and OMoF2 are computed to possess triplet ground states, and a closed shell singlet is the ground state for OWF2. Triatomic OCrF, OMoF, and OWF molecules were also observed during sample deposition. All three molecules were computed to have a bent geometry and quartet ground state. A bonding analysis showed that the OMF2 molecules have highly ionic M-F bonds. 3OCrF2 and 3OMoF2 have an M-O double bond composed of a σ bond and a π bond. 1OWF2 has an M-O triple bond consisting of a σ bond, a π bond, and a highly delocalized O lone pair forming the other π bond. The M-O bonds in the OMF compounds have triple-bond character for all three metals.

Formation and Characterization of Homoleptic Thorium Isocyanide Complexes.

Homoleptic thorium isocyanide complexes have been prepared via the reactions of laser-ablated thorium atoms and (CN)2 in a cryogenic matrix, and the structures of the products were characterized by infrared spectroscopy and theoretical calculations. Thorium atoms reacted with (CN)2 under UV irradiation to form the oxidative addition product Th(NC)2, which was calculated to have closed-shell singlet ground state with a bent geometry. Further reaction of Th(NC)2 and (CN)2 resulted in the formation of Th(NC)4, a molecule with a tetrahedral geometry. Minor products such as ThNC and Th(NC)3 were produced upon association reactions of CN with Th and Th(NC)2. Homoleptic thorium cyanide isomers Th(CN)x (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-1 at the B3LYP level, more than 120 cm-1 higher than the N-C stretches of isocyanides and with much weaker intensities. No experimental absorptions appeared where Th(CN)x should be observed.

Prediction of Bond Dissociation Energies/Heats of Formation for Diatomic Transition Metal Compounds: CCSD(T) Works.

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 T1 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.

Properties of Lanthanide Hydroxide Molecules Produced in Reactions of Lanthanide Atoms with H2O2 and H2 + O2 Mixtures: Roles of the +I, +II, +III, and +IV Oxidation States.

The reactions of laser-ablated lanthanide metal atoms with hydrogen peroxide or hydrogen plus oxygen mixtures have been studied experimentally in a solid argon matrix and theoretically with the ab initio MP2 and CCSD(T) methods. The Ln(OH)3 and Ln(OH)2 molecules and Ln(OH)2+ cations are the major products, and the reactions to form those hydroxides are predicted to be highly exothermic at the CCSD(T) level. Vibronic interactions are hypothesized to contribute to the abnormalities in deuterium shifts for Ln-OH(D) stretching modes for several hydroxides, consistent with CASSCF calculations. Additional new absorptions were assigned as HLnO or LnOH and OLnOH molecules. The tetrahydroxides of Ce, Pr, and Tb have also been observed. These reactive intermediates were identified from their matrix infrared spectra by using D2O2, HD, D2, 16,18O2, and 18O2 isotopic substitution, by matching observed frequencies with values calculated by electronic structure methods, and by following the trends observed in frequencies going through different lanthanide metal hydroxide series across the periodic table. The lanthanides are in the +II oxidation state for Ln(OH)2 and are in the +III oxidation state for Ln(OH)3 and Ln(OH)2+.

Use of Improved Orbitals for CCSD(T) Calculations for Predicting Heats of Formation of Group IV and Group VI Metal Oxide Monomers and Dimers and UCl6.

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.

Benchmark Calculations of Energetic Properties of Groups 4 and 6 Transition Metal Oxide Nanoclusters Including Comparison to Density Functional Theory.

The heats of formation and the normalized clustering energies (NCEs) for the group 4 and group 6 transition metal oxide (TMO) trimers and tetramers have been calculated by the Feller-Peterson-Dixon (FPD) method. The heats of formation predicted by the FPD method do not differ much from those previously derived from the NCEs at the CCSD(T)/aT level except for the CrO3 nanoclusters. New and improved heats of formation for Cr3O9 and Cr4O12 were obtained using PW91 orbitals instead of Hartree-Fock (HF) orbitals. Diffuse functions are necessary to predict accurate heats of formation. The fluoride affinities (FAs) are calculated with the CCSD(T) method. The relative energies (REs) of different isomers, NCEs, electron affinities (EAs), and FAs of (MO2)n (M = Ti, Zr, Hf, n = 1-4) and (MO3)n (M = Cr, Mo, W, n = 1-3) clusters have been benchmarked with 55 exchange-correlation density functional theory (DFT) functionals including both pure and hybrid types. The absolute errors of the DFT results are mostly less than ±10 kcal/mol for the NCEs and the EAs and less than ±15 kcal/mol for the FAs. Hybrid functionals usually perform better than the pure functionals for the REs and NCEs. The performance of the two types of functionals in predicting EAs and FAs is comparable. The B1B95 and PBE1PBE functionals provide reliable energetic properties for most isomers. Long range corrected pure functionals usually give poor FAs. The standard deviation of the absolute error is always close to the mean errors, and the probability distributions of the DFT errors are often not Gaussian (normal). The breadth of the distribution of errors and the maximum probability are dependent on the energy property and the isomer.

Energetic Properties and Electronic Structure of [Si,N,S] and [Si,P,S] Isomers.

Correlated molecular orbital theory at the coupled cluster CCSD(T) level with augmented correlation consistent basis sets has been used to predict the structure and energetic properties of the isomers of [Si,N,S] and [Si,P,S]. The predicted ground states are linear (2)SNSi and cyclic (2)SPSi. The other two isomers are predicted to be ∼20 to 50 kcal/mol less stable than the ground state. The excess spin is mainly on S for (2)SNSi and on P for (2)SPSi. The calculated total atomization energies with the CBS limits derived from different methods differ by ∼2 kcal/mol. The results provide the best available heats of formation for these species. The bond dissociation energies (BDEs) in (2)SNSi are comparable to those in the corresponding diatomic molecules. For cyclic (2)SPSi, the formation of (4)P + (2)SSi requires less energy than the other bond dissociation processes. The BDEs in the higher energy isomers are substantially smaller than the corresponding diatomic species.

1,2-Ethanediol and 1,3-Propanediol Conversions over (MO3)3 (M = Mo, W) Nanoclusters: A Computational Study.

The dehydration and dehydrogenation reactions of one and two 1,2-ethanediol and 1,3-propanediol molecules on (MO3)3 (M = Mo, W) nanoclusters have been studied computationally using density functional and coupled cluster (CCSD(T)) theory. The reactions are initiated by the formation of a Lewis acid-base complex with an additional hydrogen bond. Dehydration is the dominant reaction proceeding via a metal bisdiolate. Acetaldehyde, the major product for 1,2-ethanediol, is produced by α-hydrogen transfer from one CH2 group to the other. For 1,3-propanediol, the C-C bond breaking pathways to produce C2H4 and HCH═O simultaneously and proton transfer to generate propylene oxide have comparable barrier energies. The barrier to produce propanal from the propylene oxide complex is less than that for epoxide release from the cluster. On the Mo3O9 cluster, a redox reaction channel for 1,2-ethanediol to break the C-C bond to form two formaldehyde molecules and then to produce C2H4 is slightly more favorable than the formation of acetaldehyde. For W(VI), the energy barrier for the reduction pathway is larger due to the lower reducibility of W3O9. Similar reduction on Mo(VI) for 1,3-propanediol to form propene is not a favorable pathway compared with the other pathways as additional C-H bond breaking is required in addition to breaking a C-C bond. The dehydrogenation and dehydration activation energies for the selected glycols are larger than the reactions of ethanol and 1-propanol on the same clusters. The CCSD(T) method is required because density functional theory with the M06 and B3LYP functionals does not predict quantitative energies on the potential energy surface. The M06 functional performs better than does the B3LYP functional.

Properties of Cerium Hydroxides from Matrix Infrared Spectra and Electronic Structure Calculations.

Reactions of laser ablated cerium atoms with hydrogen peroxide or hydrogen and oxygen mixtures diluted in argon and condensed at 4 K produced the Ce(OH)3 and Ce(OH)2 molecules and Ce(OH)2(+) cation as major products. Additional minor products were identified as the Ce(OH)4, HCeO, and OCeOH molecules. These new species were identified from their matrix infrared spectra with D2O2, D2, and (18)O2 isotopic substitution and correlating observed frequencies with values calculated by density functional theory. We find that the amounts of Ce(OH)3 and of the Ce(OH)2(+) cation increase on UV (λ > 220 nm) photolysis, while Ce(OH)2, Ce(OH)4, and HCeO are photosensitive. The observed major species for Ce are in the +III or +II oxidation state, and the minor product, Ce(OH)4, is in the +IV oxidation state. The calculations for the vibrational frequencies with the B3LYP functional agree well with the experiment. The NBO analysis shows significant backbonding to the metal 4f and 5d orbitals for the closed shell species. Most open shell species have the excess spin in the 4f with paired spin in the 5d due to backbonding. The heats of formation of the observed species were derived from the available data from experiment and the calculated reaction energies. The major products in this study are different from similar reactions for Th where the tetrahydroxide was the major species.