Thiophene adsorption and activation on MoP(001), gamma-Mo2N(100), and Ni2P(001): density functional theory studies

The adsorption and dissociation of thiophene on the MoP(001), gamma-Mo(2)N(100), and Ni(2)P(001) surfaces have been computed by using the density functional theory method. It is found that thiophene adsorbs dissociatively on MoP(001), while nondissociatively on gamma-Mo(2)N(100) and Ni(2)P(001). On MoP(001), the dissociation of the C-S bonds is favored both thermodynamically and kinetically, while the break of the first C-S bond on gamma-Mo(2)N(100) has an energy barrier of 1.58 eV and is endothermic by 0.73 eV. On Ni(2)P(001) there are Ni(3)P(2)- and Ni(3)P-terminated surfaces. On the Ni(3)P(2)-terminated surface, the dissociation of the C-S bonds of adsorbed thiophene is endothermic, while it is exothermic on the Ni(3)P-terminated surface.     

Accurate adsorption energies for small molecules on oxide surfaces: CH4/MgO(001) and C2H6/MgO(001)

A hybrid method is applied that combines second order Møller–Plesset perturbation theory (MP2) for cluster models with density functional theory for periodic (slab) models to obtain structures and energies for methane and ethane molecules adsorbed on the MgO(001) surface. Single point calculations are performed to estimate the effect of increasing the cluster size on the MP2 energies and to evaluate the difference between coupled cluster (CCSD(T)) and MP2 energies. The final estimates of the adsorption energies are 12.9 ± 1.3 and 18.9 ± 1.8 kJ/mol for CH₄ and C₂H₆, respectively. © 2016 Wiley Periodicals, Inc.     

Different adsorption structures of pyridine on Si(001) and Ge(001) surfaces

The adsorption and reaction of pyridine on the Si(001) and Ge(001) surfaces are investigated by first-principles density-functional calculations within the generalized gradient approximation. On both surfaces the N atom of pyridine initially reacts with the down atom of the dimer, forming a single bond between the N atom and the down atom. On Ge(001) such an adsorption configuration is most favorable, but on Si(001) a further reaction with a neighboring dimer occurs, resulting in formation of a bridge-type configuration. Especially we find that on Ge(001) the bridge-type configuration is less stable than the gas phase. Our results provide an explanation for a subtle difference in the adsorption structures of pyridine on Si(001) and Ge(001), which was observed from recent scanning tunneling microscopy experiments.     

Ordering of TiO2-based nanostructures on SrTiO3(001) surfaces

A class of nanostructured surface phases on SrTiO3(001) is reported and characterized through atomic-resolution scanning tunneling microscopy and Auger electron spectroscopy. These surface phases are created via argon ion sputtering and UHV annealing and form close-packed domains of highly ordered nanostructures. Depending on the type of nanostructures present, the domain ordering exhibit either (6 x 2), (9 x 2), (12 x 2), (6 x 8), or (7 x 4) surface patterning. The nanostructures are composed of TiO2-derived complexes surrounded by a TiO2 surface termination. Such surface ordering phenomena introduce another level of complexity in the chemistry of perovskite oxide surfaces and provide a basis from which potential photocatalytic and molecular-ordering applications may be developed.     

Local structural models of complex oxygen- and hydroxyl-rich GaP/InP(001) surfaces

We perform density-functional theory calculations on model surfaces to investigate the interplay between the morphology, electronic structure, and chemistry of oxygen- and hydroxyl-rich surfaces of InP(001) and GaP(001). Four dominant local oxygen topologies are identified based on the coordination environment: M-O-M and M-O-P bridges for the oxygen-decorated surface; and M-[OH]-M bridges and atop M-OH structures for the hydroxyl-decorated surface (M = In, Ga). Unique signatures in the electronic structure are linked to each of the bond topologies, defining a map to structural models that can be used to aid the interpretation of experimental probes of native oxide morphology. The M-O-M bridge can create a trap for hole carriers upon imposition of strain or chemical modification of the bonding environment of the M atoms, which may contribute to the observed photocorrosion of GaP/InP-based electrodes in photoelectrochemical cells. Our results suggest that a simplified model incorporating the dominant local bond topologies within an oxygen adlayer should reproduce the essential chemistry of complex oxygen-rich InP(001) or GaP(001) surfaces, representing a significant advantage from a modeling standpoint.     

Bis(phosphanylamino)benzene ligands: a zinc(II) complex and an unusual nickel(I) complex with a Dewar-benzene-type Ni2P2N2 backbone

ZnPr(2) reacts with 1,2-(NHPPh(2))(2)C(6)H(4) (1) to give the bis-amido complex [Zn(THF){1-N(PPh(2))-2-N(mu-PPh(2))C(6)H(4)-kappa(3)N,N',P}](2) (3), while monolithiated 1 (prepared in situ from 1 and LiBu(n)) reacts with NiCl(2) with formation of the unusual nickel(I) complex [Ni{1-NH(PPh(2))-2-N(micro-PPh(2))C(6)H(4)-kappa(2)N,P}](2) (4), which has a Ni-Ni bond. Complexes 3 and 4 were structurally characterised. Furthermore, the structure of the sterically demanding bis-aminophosphine 1,2-(NHPMes(2))(2)C(6)H(4) (2, Mes = 2,4,6-Me(3)C(6)H(2)) is compared with that of the corresponding phenyl-substituted derivative 1,2-(NHPPh(2))(2)C(6)H(4) (1). B3LYP/LANL2DZ molecular orbital calculations on 4 indicate that a two-electron reduction should convert the Dewar-benzene-like six-membered Ni(2)N(2)P(2) ring 4 in to a benzene-like structure, a structure which is observed for the isoelectronic Zn(II) complex 3.     

Instability and (2×1) reconstruction of Si(001) and (111) surfaces: a simple approach

Recently, the present author introduced a simple qualitative theory of the instability and reconstruction of surfaces of solids exhibiting a certain covalent component of bonding. This theory is now applied to the (2×1) Si(001) and (111) surfaces, with a possible extension to analogous surfaces of germanium and GaAs(001). It is proposed that Shockley surface states from the vicinity of the Fermi energy play a key role in the electron-phonon coupling.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Reactions of ammonia on stoichiometric and reduced TiO(2)(001) single crystal surfaces

The reaction of NH(3) on the surface of the 011-faceted structure of the TiO(2)(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH(3) adsorption at 300 K shows only molecular desorption at 340 K. Modeling of TPD signals as a function of surface coverage indicated that the activation energy, E(d), and pre-exponential factor, v(eff), decrease with increasing coverage. Near zero surface coverage, E(d) was found to be equal to 92 kJ/mol and v(eff) to be close to 10(13) /s. Both parameters decreased to approximately 52 kJ/mol and approximately 10(7) /s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH(3) molecules on the surface. Computing of the TPD results show that saturation is obtained at 1/2 monolayer coverage (referred to Ti atoms). Both the amount and shape of NH(3) peak change on the reduced (Ar(+)-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiO( approximately )(1.9)) and has totally disappeared on the heavily reduced surfaces (TiO(1.6)(-)(1.7)), where the main desorption peak is found at 440 K. This 440-K desorption is most likely due to NH(x) + H recombination resulting from ammonia dissociation upon adsorption on Ti atoms in low oxidation states.     

Probing the reaction pathways of DL-proline on TiO2 (001) single crystal surfaces

The reaction of DL-Proline on O2-annealed (stoichiometric) and O-defected (sub-stoichiometric) TiO2 (001) single-crystal surfaces has been investigated. This is of significance in trying to understand the concept of how biomolecules interact with the surfaces of biomedical implants (molecular recognition). On an O2-annealed TiO2 surface, proline is found to largely adsorb then desorb intact at approximately 350 K. DFT (B3LYP) calculations of proline bound to a Ti(OH)4 cluster suggest a binding through the carboxylate functional group rather than through the NH group of the ring. In contrast, proline reaction was considerably different on the O-defected surface. First, proline was further stabilized, evidenced by a shift of its desorption temperature (during temperature-programmed desorption) to approximately 530 K. Along with proline desorption, two distinctive sets of reaction processes occurred at 530 and 630 K, respectively. The first pathway (alpha) at 530 K shows desorption of large amounts of m/e 55 (attributed to 1-azetine) and m/e 42 (attributed to ketene). At still higher temperature, 630 K, a pathway (beta) dominated by the appearance of low masses, mainly m/e 28, 27, and 26, is seen. These masses are tentatively attributed to desorption of HCN, ethylene, and/or acetylene as they represent the logical further decomposition of the different fragments of proline.     

New insight into reactions of Ni(S2C2(CF3)2)2 with simple alkenes: alkene adduct versus dihydrodithiin product selectivity is controlled by [Ni(S2C2(CF3)2)2]- anion

The nickel bis(dithiolene) complex Ni(S2C2(CF3)2)2 employs its sulfur centers in reactions with alkenes, and stable interligand S-bonded alkene adducts can be formed. The present study shows that the selectivity of alkene binding to charge-neutral Ni(S2C2(CF3)2)2 is influenced by the anion [Ni(S2C2(CF3)2)2]-. In the absence of anion, formation of substituted dihydrodithiins (intraligand addition) dominates, whereas the presence of anion allows for the formation of stable interligand adducts. Mechanistic implications are discussed. The X-ray crystal structure of the ethylene adduct of Ni(S2C2(CF3)2)2 is presented, displaying interligand binding of ethylene to sulfur centers in the bis(dithiolene) complex.     

氢原子在Ni(100), Ni(111)和Ni(110)面上吸附扩散势能面的结构

本文构造了氢-镍相互作用的5参数Morse势, 用经典的对势方法研究氢原子在Ni(100), Ni(111)和Ni(110)面上的吸附和扩散, 得到氢原子在三个表面上的吸附位、吸附几何、结合能及本征振动等数据, 和实验结果符合得很好。同时, 系统地研究了三个体系的吸附扩散势能面结构。     

Catalysis of dimerization and oligomerization reactions of lower alkenes by systems based on Ni(PPh3)2(C2H4) and Ni(PPh3) n Cl (n = 2 or 3)

The catalytic characteristics of the individual complex Ni(PPh₃)₂(C₂H₄) and Ni(PPh₃) _n Cl (n = 2 or 3) and those of systems based on these complexes in combination with Brönsted and Lewis acids in ethylene and propylene oligomerization have been determined. A correlation between the BF₃ · OEt₂ solution storage time and the catalytic properties of the nickel systems has been established for the reactions of the lower alkenes. The observed increase in the turnover frequency and turnover number of the catalyst is due to the increase in the Brörsted acid concentration as a result of irreversible conversions of BF₃ · OEt₂ caused by its interaction with impurity water in the solvent. The formation of the Ni(PPh₃)₂(C₂H₄)-BF₃ · OEt₂ catalytic system in the presence of a substrate dramatically extends the system’s service life. The interaction of the nickel precursors with boron trifluoride etherate has been investigated using a complex of physical methods, and the main reactions yielding catalytically active species have been revealed.     

A CASSCF and CCI study of the formation of the Ni2(C2H4) complex

The authors thank Dr. M. R. A. Blomberg for suggesting that the original results were in error.     

A CASSCF and CCI study of the formation of the Ni2(C2H4) complex

Complete active space SCF and contracted CI calculations have been performed on the potential surface of the Ni₂-C₂H₄ complex in the singlet state. The ethene geometry and position relative to Ni₂ was optimized while the Ni-Ni distance was kept fixed at 2.5 Å.Four possible symmetric geometric arrangements were considered, yielding only an end on -bonded structure as bound. This is a consequence of the charge buildup between the nickel atoms and charge depletion at the ends, coupled with electron mobility along the bond axis, in the nickel dimer.The energy minimum corresponds to a C₂H₄ moiety distorted 21% towards a C₂H₆ geometry, with a bond energy o 24.6 kcal/mol at the CCI level and 28.1 kcal/mol with cluster corrections included. The binding is described by a donation backdonation mechanism. These results are discussed in connection with earlier work on Ni(C₂H₄) and Ni₂(C₂H₄) and in connection with experimental work.     

Vibrational frequencies and normal modes of Co adsorbed on Ni(001)

The vibrational modes of CO adsorbed on a Ni(001) metal surface have been obtained using two different cluster models to represent the on-top and bridge-bonding situations. New modes, not previously available in the literature, are obtained for the bridge cluster.     

Electrochemical self-assembly of alkanethiolate molecules on Ni(111) and polycrystalline Ni surfaces

In this work, the electrochemical formation of alkanethiolate self-assembled monolayers (SAMs) on Ni(111) and polycrystalline Ni surfaces from alkanethiol-containing aqueous 1 M NaOH solutions was studied by combining Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that alkanethiolates adsorb on Ni concurrent with NiO electroreduction. The resulting surface coverage depends on the applied potential and hydrocarbon chain length. Electrochemical and XPS data reveal that alkanethiolate electroadsorption at room temperature takes place without S-C bond scission, in contrast to previous results from gas-phase adsorption. A complete and dense monolayer, which is stable even at very high cathodic potentials (-1.5 V vs SCE), is formed for dodecanethiol. DFT calculations show that the greater stability against electrodesorption found for alkanethiolate SAMs on Ni, with respect to SAMs on Au, is somewhat related to the larger alkanethiolate adsorption energy but is mainly due to the larger barrier to interfacial electron transfer present in alkanethiolate-covered Ni. A direct consequence of this work is the possibility of using electrochemical self-assembly as a straightforward route to build stable SAMs of long-chained alkanethiolates on Ni surfaces at room temperature.     

Reactivity of anatase TiO(2) nanoparticles: the role of the minority (001) surface

Methanol adsorption on clean and hydrated anatase TiO(2)(001)-1 x 1 is studied using density functional theory calculations and first principles molecular dynamics simulations. It is found that (i) dissociative adsorption is favored on clean TiO(2)(001) at both low and high methanol coverages; (ii) on the partially hydrated surface, methanol dissociation is not affected by the coadsorbed water and can still occur very easily; (iii) the dissociative adsorption energy of methanol is always larger than that of water under similar conditions. This implies that water replacement by methanol is energetically favored, in agreement with recent experimental observations on colloidal anatase nanoparticles.