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Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of MnxMoOy− (x = 1, 2; y = 3, 4) clusters toward H2O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H2 production, specifically for MnxMoO3− (x = 1, 2) clusters. Unlike previously studied MoxOy− clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoOy− and Mn2MoOy− preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the MnxMoOy (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster–water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

The 3.49 eV photoelectron spectra of HNiCO−, PdCO−, and PtCO− generated from the gas-phase reaction of atomic anions with methanol are presented and interpreted in the context of new density functional calculations. The calculated electron affinities (EAs) for PdCO (0.58 eV) and PtCO (1.13 eV) are in excellent agreement with experimental values [0.606(10) and 1.212(10) eV, respectively] while for HNiCO, the calculated EA (1.64 eV) is 0.48 eV lower than the experimental value. However, in all three cases, the vibrational frequencies and spectral profiles are consistent with calculated structures, frequencies, and normal coordinates. HNiCO− and HNiCO are predicted to be linear, with 1Σ+ and 2Δ ground states, respectively. Calculations on PdCO− agree with recent calculations by Andrews and co-workers [J. Phys. Chem. A 104, 3905 (2000)] which predict a bent 2A′ ground state. The ground state of PtCO− is calculated to be the linear 2Σ+ state. Both PdCO and PtCO are predicted to have 1Σ+ ground states. For all ...

We present the results of a combined theoretical and experimental study of the growth of extended surface vibrational modes during the initial oxidation of Si(100)-(2×1). Controlled incorporation of oxygen into the first two layers of the silicon surface is achieved via sequential water (H2O) exposures and stepwise annealing in ultrahigh vacuum. Transmission infrared spectroscopy is used to monitor the transition from vibrational modes characteristic of isolated Si–O structures, so-called silicon epoxides, to extended modes with the character of transverse optical and longitudinal optical phonons in an extended SiO2 film. Quantum chemical calculations on two-dimer silicon clusters identify these modes as arising predominantly from coupling between species on adjacent dimer units, consistent with the thermodynamic driving force for local oxygen agglomeration. The vibrational signature of this surface is proposed to offer a link between well-characterized local surface Si–O structures and the extended SiOx ...

Aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) is currently under investigation for use as a high-κ gate dielectric alternative to SiO2. Cluster calculations employing hybrid density functional theory have been carried out to examine the chemical reaction pathways between the ALD precursors, trimethylaluminum (TMA) and H2O, with the H/Si(100)-2×1 surface. Results obtained using Si9H14 and Si15H20, dimer and double dimer clusters to represent the surface active site are in good agreement, providing a consistent view of reaction energetics on the H/Si(100)-2×1 surface. The adsorption energies for TMA and H2O on the surface are calculated to be 0.02 and 0.15 eV, respectively. For the reaction between H2O and the H/Si(100)-2×1 surface, hydroxylation of the surface accompanied by loss of H2 was found to be the preferred pathway having an activation energy and overall reaction enthalpy of 1.60 eV and −0.75 eV, both of which are ⩾0.70 eV lower than the corresponding values for the possible H/D exc...

An atomic scale model of thermal oxidation of Si(100) has been developed based on a kinetic Monte Carlo approach. This method makes it possible to analyze the effects of elementary mechanistic steps of oxidation on macroscopic surfaces. The initial thermal decomposition of chemisorbed hydroxyl groups resulting from water adsorption on Si(100)-(2×1) is investigated by utilizing extensive IR data and ab initio calculations.

Surface infrared spectroscopy and density functional cluster calculations are used to study the thermal and atomic hydrogen-induced decomposition of water molecules on the clean Si(100)-(2×1) surface. We report the first observation of the Si–H bending modes associated with the initial insertion of oxygen into the dimer and backbonds of a silicon dimer. We find that, while one and two oxygen-containing dimers are formed almost simultaneously during the thermal decomposition of water on this surface, atomic H can be used to drive the preferential formation of the singly oxidized dimer. This work highlights the sensitivity of Si–H bending modes to the details of local chemical structure in an inhomogeneous system, suggesting that the combined experimental and theoretical approach demonstrated herein may be extremely useful in studying even more complex systems such as the hydrogenation of defects in SiO2 films.

The G2/97 test set [J. Chem. Phys. 106, 1063 (1997)] for assessing quantum chemical methods used to predict thermochemical data is expanded to include 75 additional enthalpies of formation of larger molecules. This new set, referred to as the G3/99 test set, includes enthalpies of formation, ionization potentials, electron affinities, and proton affinities in the G2/97 set and 75 new enthalpies of formation. The total number of energies in the G3/99 set is 376. Overall, G3 theory has a mean absolute deviation of 1.07 kcal/mol for the G3/99 test set and does about as well for the new hydrocarbons and substituted hydrocarbons as it does for those in the G2/97 test. However, G3 theory has large deviations for several of the new nonhydrogen systems in the G3/99 test set such as SF6 and PF5. Part of the source of error is traced to the inadequate geometries used in G3 theory for these molecules. Other variations of G3 theory are also assessed such as G3(MP2), G3(MP3), and the versions of G3 theory using scaled...

A modification of Guassian-3 (G3) theory using multiplicative scale factors, instead of the additive higher level correction, is presented. In this method, referred to as G3S, the correlation energy is scaled by five parameters and the Hartree–Fock energy by one parameter. The six parameters are fitted to the G2/97 test set of 299 energies and the resulting mean absolute deviation from experiment is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. The G3S method has the advantage compared to G3 theory in that it can be used for studying potential energy surfaces where the products and reactants have a different number of paired electrons. In addition, versions of the computationally less intensive G3(MP3) and G3(MP2) methods that use scaled energies are also presented. These methods, referred to as G3S(MP3) and G3S(MP2), have mean absolute deviations of 1.16 and 1.35 kcal/mol, respectively.

First-principles quantum chemical techniques on cluster models have been used to investigate the photoabsorption of the peroxide linkage defect in silicate glasses. The effects of geometry, basis sets, and cluster size have been considered carefully to derive converged values for the low-lying excitation energies. The lowest singlet-to-singlet transition is a weak absorption at 5.5 eV. A stronger valence absorption occurs at 6.8 eV.

The structures and coalescence behavior of size-selected, matrix-isolated silicon clusters have been studied using surface-plasmon-polariton ~SPP! enhanced Raman spectroscopy. The cluster ions were produced in a laser vaporization source, mass selected then deposited into a co-condensed matrix of Ar, Kr or N2 on a liquid He cooled substrate. Raman spectra from monodisperse samples of the smaller clusters studied, Si 4 ,S i 6and Si7, show sharp, well-resolved, vibrations which are in good agreement with predictions based on ab initio calculations. From these comparisons we confirm that Si 4 is a planar rhombus, and assign Si6 as a distorted octahedron and Si7 as a pentagonal bypyramid. Si5 depositions down to 5 eV did not reveal a measurable Raman spectrum under our experimental conditions. Evidence for cluster‐cluster aggregation ~or fragmentation! was observed under some conditions, even for a ‘‘magic number’’ cluster such as Si 6. The spectra of the aggregated small clusters were identical to those observed for directly deposited larger cluster ‘‘bands,’’ such as Si 25‐35. The Raman spectra of the aggregated clusters bear some similarity to those of bulk amorphous silicon. Cluster-deposited thin films were prepared by sublimating the matrix material. Even under these ‘‘soft landing’’ conditions, changes in the Raman spectrum are observed with the thin films showing even greater similarity to amorphous silicon. © 1999 American Institute of Physics.@S0021-9606~99!70521-0#

A variation of Gaussian-3 (G3) theory is presented in which the basis set extensions are obtained at the second-order Mo/ller–Plesset level. This method, referred to as G3(MP2) theory, is assessed on 299 energies from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The average absolute deviation from experiment of G3(MP2) theory for the 299 energies is 1.30 kcal/mol and for the subset of 148 neutral enthalpies it is 1.18 kcal/mol. This is a significant improvement over the related G2(MP2) theory [J. Chem. Phys. 98, 1293 (1993)], which has an average absolute deviation of 1.89 kcal/mol for all 299 energies and 2.03 kcal/mol for the 148 neutral enthalpies. The corresponding average absolute deviations for full G3 theory are 1.01 and 0.94 kcal/mol, respectively. The new method provides significant savings in computational time compared to G3 theory and, also, G2(MP2) theory.

The surface plasmon polariton-enhanced Raman spectra of size-selected C16, C18, and C20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm−1 region for C16, C18, and C20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C20. The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions.

Gaussian-3 theory (G3 theory) for the calculation of molecular energies of compounds containing first (Li–F) and second row (Na–Cl) atoms is presented. This new theoretical procedure, which is based on ab initio molecular-orbital theory, modifies G2 theory [J. Chem. Phys. 94, 7221 (1991)] in several ways including a new sequence of single point energy calculations using different basis sets, a new formulation of the higher level correction, a spin–orbit correction for atoms, and a correction for core correlation. G3 theory is assessed using 299 energies from the G2/97 test set including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. This new procedure corrects many of the deficiencies of G2 theory. There is a large improvement for nonhydrogen systems such as SiF4 and CF4, substituted hydrocarbons, and unsaturated cyclic species. Core-related correlation is found to be a significant factor, especially for species with unsaturated rings. The average absolute devi...

A set of 146 well-established ionization potentials and electron affinities is presented. This set, referred to as the G2 ion test set, includes the 63 atoms and molecules whose ionization potentials and electron affinities were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 83 new atoms and molecules. It is hoped that this new test set combined with the recently published test set of enthalpies of neutral molecules [J. Chem. Phys. 106, 1063 (1997)] will provide a means for assessing and improving theoretical models. From an assessment of G2 and density functional theories on this test set, it is found that G2 theory is the most reliable method. It has an average absolute deviation of 0.06 eV for both ionization potentials and electron affinities. The two modified versions of G2 theory, G2(MP2,SVP) and G2(MP2) theory, have average absolute deviations of 0.08–0.09 eV for both ionization potentials and electron affinities. The hybrid B3LYP density functional method has the smallest...

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm−1 was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on ...

Three complete basis set models of Petersson et al. [J. Chem. Phys. 104, 2598 (1996)], CBS-Q, CBS-q, and CBS-4, have been assessed on the G2 neutral test set of 148 molecules [J. Chem. Phys. 106, 1063 (1997)]. The average absolute deviations with experiment of the calculated enthalpies of formation from the three CBS methods are 1.57 kcal/mol (CBS-Q), 2.13 kcal/mol (CBS-q), and 3.06 kcal/mol (CBS-4). The maximum deviations of the methods are 11.2, 10.3, and 14.4 kcal/mol. respectively. The most accurate method, CBS-Q, has an average absolute deviation similar to that of G2 theory. The three CBS methods have also been assessed on a 40 molecule set using isodesmic bond separation reactions to calculate enthalpies of formation. There is a significant improvement in the accuracy of the enthalpies compared to those calculated using atomization energies, although not as much as for G2 theory. In a test on naphthalene, enthalpies calculated using the CBS methods have large deviations. The CBS-Q method has a deviation of 28.7 kcal/mol and, surprisingly, the deviation increases to 34.3 kcal/mol when isodesmic bond separation reaction energies are used.

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (

A set of 148 molecules having well-established enthalpies of formation at 298 K is presented. This set, referred to as the G2 neutral test set, includes the 55 molecules whose atomization energies were used to test Gaussian-2 ~G2! theory @J. Chem. Phys. 94, 7221 ~1991!# and 93 new molecules. The G2 test set includes 29 radicals, 35 nonhydrogen systems, 22 hydrocarbons, 47 substituted hydrocarbons, and 15 inorganic hydrides. It is hoped that this new test set will provide a means for assessing and improving new theoretical models. From an assessment of G2 and density functional theories ~DFT! on this test set it is found that G2 theory is the most reliable method both in terms of average absolute deviation ~1.58 kcal/mol! and maximum deviation ~8.2 kcal/mol!. The largest deviations between experiment and G2 theory occur for molecules having multiple halogens. Inclusion of spin‐orbit effects reduces the average absolute deviation to 1.47 kcal/mol and significantly improves the results for the chlorine substituted molecules, but little overall improvement is seen for the fluorine substituted molecules. Of the two modified versions of G2 theory examined in this study, G2~MP2,SVP! theory ~average absolute deviation51.93 kcal/mol! performs better than G2~MP2! theory ~2.04 kcal/mol!. The G2~MP2,SVP! theory is found to perform very well for hydrocarbons, radicals, and inorganic hydrides. Of the seven DFT methods investigated, the B3LYP method has the smallest average absolute deviation ~3.11 kcal/mol!. It also has a significantly larger distribution of error than the G2 methods with a maximum deviation of 20.1 kcal/mol. © 1997 American Institute of Physics.@S0021-9606~97!02202-2#

The performance of Gaussian‐2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero‐point energies. A higher level of correlation treatment is examined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than quadratic configuration interaction [QCISD(T)]. The use of geometries optimized at the QCISD level rather than the second‐order Mo/ller–Plesset level (MP2) and the use of scaled MP2 zero‐point energies rather than scaled Hartree–Fock (HF) zero‐point energies have also been examined. The set of 125 energies used for validation of G2 theory [J. Chem. Phys. 94, 7221 (1991)] is used to test out these variations of G2 theory. Inclusion of higher levels of correlation treatment has little effect except in the cases of multiply‐bonded systems. In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall performance. The use of QCISD geometries yields significantly better agreement with experiment for several cases including the ionization potentials of CS and O2, electron affinity of CN, and dissociation energies of N2, O2, CN, and SO2. This leads to a slightly better agreement with experiment overall. The MP2 zero‐point energies gives no overall improvement. These methods may be useful for specific systems.

Accepted: 1995-02-15, 資料番号: SA1002026000

Results of a systematic comparison of the MoxOy (-) + H2O and WxOy (-) + H2O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. WxOy (-) clusters undergo more direct oxidation by water to yield WxOy+1 (-) + H2, while for MoxOy (-) clusters, production of MoxOyH2 (-) (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, MoxOy (-) clusters generally have higher rate coefficients than analogous WxOy (-) clusters if MoxOy+1H2 (-) formation is included. Results of calculations on the M2Oy (-) + H2O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to MxOy+1H2 (-) formation. The results predict that the Lewis acid-base complex is more strongly bound for MoxOy (-) clusters than for WxOy (-) clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most MoxOy (-) + H2O reactions, and the WxOy (-) + H2O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H2 production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.

Ab initio molecular orbital investigations of the electronic structures, bonding, and stabilities of Ga4As4, Al4P4, and Mg4S4 are reported. The effects of polarization functions and electron correlation are included in these calculations. Our results indicate that the electronegativity difference between the constituents of a mixed cluster plays a very important role in determining its ground state structure. In A4B4 mixed clusters, a distorted cubic structure with alternating atomic charges leads to a particularly stable ionic form. This Td structure consisting of two interpenetrating tetrahedra of the two constituents is the ground state structure for both Al4P4 and Mg4S4. For Ga4As4, there also exists a more covalent Si8‐like Ci structure that is 3 kcal/mole more stable than the Td form. The structures and relative stabilities of these mixed clusters are compared with the results for valence‐isoelectronic Si8 and Na4Cl4 clusters.

Two variations of Gaussian‐2 (G2) theory are presented. In the first, referred to as G2 (MP2) theory, the basis‐set‐extension energy corrections are obtained at the 2nd order Mo/ller–Plesset (MP2) level and in the second, referred to as G2(MP3) theory, they are obtained at the MP3 level. The methods are tested out on the set of 125 systems used for validation of G2 theory [J. Chem Phys. 94, 7221 (1991)]. The average absolute deviation of the G2(MP2) and G2(MP3) theories from experiment are 1.58 and 1.52 kcal/mol, respectively, compared to 1.21 kcal/mol for G2 theory. The new methods provide significant savings in computational time and disk storage.

Three additivity approximations used in GAUSSIAN‐2 (G2) theory to compute effective QCISD(T)/6‐311+G(3df,2p) energies for molecular systems are investigated by carrying out full QCISD(T)/6‐311+G(3df,2p) calculations on the set of 125 systems used for validation [J. Chem. Phys. 94, 7221 (1991)]. The results indicate that the approximations work very well. The average absolute deviation of the full results from experiment is 1.17 kcal/mol which is close to that of G2 theory (1.21 kcal/mol). The average absolute deviation with G2 theory is only 0.30 kcal/mol. However, the cpu cost increases by a factor of 2–3.

The Gaussian‐2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of Sin and Si−n (n=1–5) clusters. The G2 energies are used to derive accurate binding energies and electron affinities of these clusters. The calculated electron affinities of Si2–Si4 are in agreement to within 0.1 eV with results from recent photoelectron spectroscopic measurements.

The reaction mechanism for the interaction of SiH+2 with SiH4 has been studied by means of accurate molecular orbital techniques including polarized basis sets, effects of electron correlation, and zero‐point corrections. The reaction leads to the formation of the disilene cation (H2Si–SiH+2) without any overall energy barrier. In addition, the formation of the hydride transfer product (SiH+3) is calculated to be exothermic. Further reactions of the stable ion H2Si–SiH+2 with silane require significant endothermic energy barriers. The calculated results are in good agreement with the experimental observations of Reents and Mandich.

Vibrationally resolved photoelectron spectra of Si−3 and Si−4, recently reported by Kitsopoulos, Chick, Weaver, and Neumark, are interpreted using ab initio quantum chemical calculations of the ground and excited electronic states of the corresponding neutral clusters. The calculated electron affinities as well as the low‐lying excitation energies agree within 0.1–0.2 eV of the experimental values, thus confirming the theoretically predicted structures of neutral and anionic Si3 and Si4 reported previously.

Electronic structures and stabilities of small AlnPn and MgnSn clusters (n=1–3) are explored by means of accurate quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Ionic factors are clearly dominant for MgS clusters. Thus, both Mg2S2 and Mg3S3 have planar ground state geometries where charge alternation is utilized effectively. AlP clusters, on the other hand, behave intermediate between the ionic MgS clusters and the covalent Si clusters. Thus, while the ground state structures of Al2P2 and Al3P3 are both analogous to those of the isoelectronic silicon clusters Si4 and Si6, other low‐lying minima which are similar to those of MgS clusters are also present. The hybridization and bonding in the different structures are discussed.

Using a first-principles density functional approach, we investigate the chemisorption mechanism for the interaction of spherosiloxane (H8Si8O12) clusters on Si(100). Our transition state studies reveal that the chemisorption pathway with the lowest activation barrier corresponds to attachment via Si-O bond cleavage across a surface dimer. Using the relaxed surface structure from this "cracked cluster" model, we calculate Si 2p core-level shifts, including core-hole relaxation effects, and show that the calculated values are in excellent agreement with the positions and intensities of all the experimentally observed core-level shifts. (C) 2000 American Institute of Physics. [S0003-6951(00)00526-X].

The mechanisms for the sequential clustering reactions of SiH+ with SiH4 have been studied by means of accurate quantum chemical techniques. Detailed reaction pathways including the necessary transition structures have been investigated to understand the structures and stabilities of the reaction intermediates and products. In accordance with the experimental observations of Mandich and Reents, the reactions lead to the formation of Si2H+3, Si3H+5, Si4H+7, and Si5H+11 without overall endothermic barriers. As seen experimentally, both reactive (dibridged) and unreactive (tribridged) isomers of Si2H+3 can be formed in the first reaction step. While Si2H+3 and Si3H+5 involve reactive structures with bridging hydrogens, Si4H+7 corresponds to a cyclic isomer (the second row analog of the cyclobutyl cation) which only forms a complex with silane. Silicon isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism.

The Gaussian‐2 (G2) theoretical procedure, based on ab initio molecular‐orbital theory, is used to calculate the energies of Si2Hn (n=0–6) and Si2H+n (n=0–7). The resulting G2 energies are used to calculate appearance potentials and ionization potentials. The results are in general agreement with the recent photoionization studies of Ruscic and Berkowitz [J. Chem. Phys. 95, XXXX (1991); 95, XXXX (1991)], supporting their observation of the species Si2H2, Si2H3, Si2H4, and Si2H5 for the first time. Atomization energies of the neutrals are calculated and used to derive enthalpies of formation. In most cases the results are in good agreement with previous theoretical values. In addition, new results for the structures of Si2H+6, Si2H3, and Si2H+, the enthalpies of formation of the cations, and proton affinities are presented.

The reaction mechanism for the interaction of Si+ with disilane has been studied by means of accurate ab initio molecular orbital techniques including polarized basis sets, effects of electron correlation, and zero‐point corrections. There are two main accessible channels for the reaction, via Si+ insertion into the Si–Si or the Si–H bonds. While both are exothermic and lead to the same products, the Si–Si insertion channel is the lower energy pathway. The insertion is followed by 1,2‐H shift and H2 elimination reactions. The reaction leads to the formation of two Si3H+4 isomers, a noncyclic isomer, H3Si–SiH–Si+, and a cyclic isomer, c(HSi–SiH2–SiH)+, with no overall activation barriers. Formation of silane and Si2H+2 is calculated to be significantly exothermic. Other fragmentation channels leading to the ions Si2H+5, Si2H+4, and Si2H+3 are calculated to be endothermic. Our results are in good agreement with the available experimental results. We compare our results to those from previous studies of the ...

The Gaussian‐2 theoretical procedure (G2 theory), based on a b i n i t i o molecular orbital theory, for calculation of molecular energies (atomization energies, ionization potentials,electron affinities, and proton affinities) of compounds containing first‐ (Li–F) and second‐row atoms (Na–Cl) is presented. This new theoretical procedure adds three features to G1 theory [J. Chem. Phys. 9 0, 5622 (1989)] including a correction for nonadditivity of diffuse‐s p and 2d f basis set extensions, a basis set extension containing a third d function on nonhydrogen and a second p function on hydrogen atoms, and a modification of the higher level correction. G2 theory is a significant improvement over G1 theory because it eliminates a number of deficiencies present in G1 theory. Of particular importance is the improvement in atomization energies of ionic molecules such as LiF and hydrides such as C2H6, NH3, N2H4, H2O2, and CH3SH. The average absolute deviation from experiment of atomization energies of 39 first‐row compounds is reduced from 1.42 to 0.92 kcal/mol. In addition, G2 theory gives improved performance for hypervalent species and electron affinities of second‐row species (the average deviation from experiment of electron affinities of second‐row species is reduced from 1.94 to 1.08 kcal/mol). Finally, G2 atomization energies for another 43 molecules, not previously studied with G1 theory, many of which have uncertain experimental data, are presented and differences with experiment are assessed.

Accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2–Si10. The effects of polarization functions, diffuse functions, and electron correlation have been included in these calculations. In most cases, there is a good correspondence between the ground state structures of the negative ions and those of the corresponding neutral species. Adiabatic electron affinities are computed and compared with recent experimental measurements. Si3, Si5, Si8, and Si9 are found to have electron affinities which are larger than their neighbors. This result is interpreted using our previous calculations on the low‐lying states of the corresponding neutral species.

The Gaussian‐1 theoretical procedure is extended and tested on compounds containing second‐row atoms (Na–Cl). This is a composite procedure based on ab initio molecular orbital theory, utilizing large basis sets (including diffuse‐sp, double‐d, and f‐polarization functions) and treating electron correlation by Mo/ller–Plesset perturbation theory and by quadratic configuration interaction. Total atomization energies for a set of 24 species agree with accurate experimental data to an accuracy of better than 3 kcal/mol in most cases, SO2 being the notable exception. Similar agreement is achieved for ionization energies, electron affinities, and proton affinities. The method is used to assess experimental data for a number of other compounds having less accurate atomization energies.

The mechanisms for the sequential clustering reactions of SiH+3 with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including zero‐point corrections and the effects of electron correlation. Detailed reaction pathways including the necessary transition structures have been investigated to understand the structures and stabilities of the reaction intermediates and products. In accordance with the experimental observations of Mandich, Reents, and Kolenbrander, the reactions lead to the formation of Si2H+5 and Si3H+7 without significant overall barriers. In addition, bimolecular attachment complexes (Si2H+7,Si3H+9 and Si4H+11) bound by bridging hydrogen atoms are involved as reaction intermediates and have also been observed by Mandich et al. as collisionally stabilized products. Unusual hydrogen bridged complexes play a key role in the termination of the growth sequence. The first bottleneck in the sequence of growth reactions is due to the formation of...

First-principles molecular orbital methods and gradient-corrected density functional calculations on silicon clusters are used to study possible pathways for the initial oxidation of Si (100)-2×1. In these reactions, the adsorbed hydroxyl oxygen inserts into the dimer Si–Si bond to form a suboxide (≡Si–O–Si≡) surface structure. The reaction typically follows a two-step pathway involving an intermediate energy minimum. In the case of an ideal surface with full water coverage, the reaction is exothermic by 1.3 eV and the overall reaction barrier is estimated at 2.4 eV. However, an alternative pathway involving a dangling bond site lowers the activation barrier to 2.1 eV. The implications for the oxidation reaction rates are discussed as well as possible alternative pathways.

The two lowest singlet excited states of divalent Si defects in silicate glasses are studied by ab initio methods. Excitation energies of 5.2 and 6.8 eV are obtained. Steady convergence to these values is shown with the increase in size both of the model cluster and of the basis set employed in the calculations. The results clearly demonstrate the viability of the quantum-chemical cluster approach for the study of local excitations in glass defects.

Microscopic mechanisms for the reactions of H2 with divalent Ge defects in hydrogen loaded germanosilicate optical fibers have been studied by ab initio quantum chemical techniques. The reaction with the lowest energy barrier corresponds to H2 breaking a Ge–O bond at the divalent Ge site forming a –Ge–H bond. Alternatively, H2 addition yielding a dihydride followed by hydrogen migration from Ge to an adjacent O also leads to this –Ge–H defect. We have assigned the vibrational frequency corresponding to this –Ge–H defect to a previously observed, but unassigned, Raman mode at 1875 cm−1. The reverse reaction barrier is very small, indicating that this –Ge–H defect is likely to be easily bleached at higher temperatures.

A computational investigation of the Mo2Oy− + H2O (y = 4, 5) reactions as well as a photoelectron spectroscopic probe of the deuterated Mo2O6D2− product have been carried out to understand a puzzling question from a previous study: Why is the rate constant determined for the Mo2O5− + H2O/D2O reaction, the terminal reaction in the sequential oxidation of Mo2Oy− by water, higher than the W2O5− + H2O/D2O reaction? This disparity was intriguing because W3Oy− clusters were found to be more reactive toward water than their Mo3Oy− analogs. A comparison of molecular structures reveals that the lowest energy structure of Mo2O5− provides a less hindered water addition site than the W2O5− ground state structure. Several modes of water addition to the most stable molecular and electronic structures of Mo2O4− and Mo2O5− were explored computationally. The various modes are discussed and compared with previous computational studies on W2Oy− + H2O reactions. Calculated free energy reaction profiles show lower barriers fo...

Transition metal sulfides play an important catalytic role in many chemical reactions. In this work, we have conducted a careful computational study of the structures, electronic states, and reactivity of metal sulfide cluster anions M2S(X)(-) (M = Mo and W, X = 4-6) using density functional theory. Detailed structural analysis shows that these metal sulfide anions have ground state isomers with two bridging sulfide bonds, notably different in some cases from the corresponding oxides with the same stoichiometry. The chemical reactivity of these metal sulfide anions with water has also been carried out. After a thorough search on the reactive potential energy surface, we propose several competitive, energetically favorable, reaction pathways that lead to the evolution of hydrogen. Selectivity in the initial water addition and subsequent hydrogen migration are found to be the key steps in all the proposed reaction channels. Initial adsorption of water is most favored involving a terminal metal sulfur bond in Mo2S4(-) isomers whereas the most preferred orientation for water addition involves a bridging metal sulfur bond in the case of W2S4(-) and M2S5(-) isomers. In all the lowest energy H2 elimination steps, the interacting hydrogen atoms involve a metal hydride and a metal hydroxide (or thiol) group. We have also observed a higher energy reaction channel where the interacting hydrogen atoms in the H2 elimination step involve a thiol (-SH) and a hydroxyl (-OH) group. For all the reaction pathways, the Mo sulfide reactions involve a higher barrier than the corresponding W analogues. We observe for both metals that reactions of M2S4(-) and M2S5(-) clusters with water to liberate H2 are exothermic and involve modest free energy barriers. However, the reaction of water with M2S6(-) is highly endothermic with a considerable barrier due to saturation of the local bonding environment.

Aqueous HF etching of silicon surfaces results in the removal of the surface oxide and leaves behind silicon surfaces terminated by atomic hydrogen. The effect of varying the solution pH on the surface structure is studied by measuring the SiH stretch vibrations with infrared absorption spectroscopy. Basic solutions ( pH=9–10) produce ideally terminated Si(111) surfaces with silicon monohydride ( 3/4 SiH) oriented normal to the surface. The surface is found to be very homogeneous with low defect density (