Molecular orbital study for Na, Mg, and Al adsorption on the Si (111) surface

We investigated the interactions between the Si(111) surface and the Na, Mg, and Al atoms using cluster model calculations. Calculations were performed at levels of complete-active-space self-consistent-field (CASSCF) and multi-reference singly and doubly excited configuration interaction (MRSDCI) calculations using the model core potential method. Our calculations revealed that the most favorable sites of Na, Mg, and Al adsorption on Si(111) are on top (T₁), bridge (B₂), and 3-fold filled (T₄) sites, respectively. The nature of chemical bonds between these metal atoms and the dangling bonds of the surface Si atoms are found to be essentially covalent.     

Orbital disproportionation and spin crossover as a pseudo Jahn-Teller effect

It is shown that in systems with electronic half-closed-shell configurations of degenerate orbitals, e(2) and t(3) (which have totally symmetric charge distribution), ground state distortions from high-symmetry geometries may occur due to a strong pseudo Jahn-Teller effect (PJTE) in the excited states, resulting also in a novel phenomenon of PJT-induced spin crossover. There is no JTE neither in the ground state term nor in the excited terms (including degenerate terms) of these configurations but a strong PJT mixing between two excited states [((1)E+(1)A) [cross-filled circle] e and ((2)T(1)+(2)T(2)) [cross-filled circle] e in the e(2) and t(3) cases, respectively] pushes down the lower term to cross the ground state of the undistorted system and to form the global minimum with a distorted geometry. The analysis of the electronic structure of this distorted configuration shows that it is accompanied by orbital disproportionation: instead of proportional population of all degenerate orbitals by one electron each (as in the ground state of the undistorted system that follows Hund's rule), two electrons with opposite spins occupy one orbital, resulting in transformations of the type (e(theta);e(epsilon))-->(e(theta)e(theta)) for e(2) and (t(x);t(y);t(z))-->(t(x);t(x);t(z)) for t(3) systems. Since the two geometry configurations, undistorted and distorted, appertain to different electronic terms that have different spin states, the formation of the global minimum with the distorted configuration is accompanied by a spin crossover. Distinguished from the known spin-crossover phenomenon in some transition metal compounds, the two states with different spin in the PJT-induced spin crossover have also different nuclear configurations, undistorted and distorted, that coexist with a relatively small energy difference. The change of configuration reduces significantly the rate of relaxation between the two states; the relaxation is further reduced by the lower spin-orbital coupling in the light-atom systems as compared with transition metal compounds. This means that there may be systems for which the switch between the two states (in both directions) under perturbations may be observed as a single-molecule phenomenon. Systems with half-closed-shell electronic configurations e(2) and t(3) are available in a variety of molecules from different classes, organic and inorganic; the theory is illustrated here by ab initio calculations for a series of molecular systems, including Si(3), Si(3)C, CuF(3), Na(3), Si(4), Na(4), Na(4) (-), and C(60) (3-), which are in agreement with the experimental data available.     

Au12M (M=Na,Mg, Al,Si, P,S, Cl)团簇的结构、稳定性和电子性质

采用基于密度泛函理论的第一性原理方法系统地研究了Au12M(M=Na,Mg,Al,Si,P,S,Cl)团簇的结构、稳定性和电子性质.对团簇的平均结合能、镶嵌能、垂直离化势、最高占据分子轨道(HOMO)和最低未占据分子轨道(LUMO)的能级差、电荷布居分析、自然键轨道(NBO)进行了计算和讨论.对于Au12M(M=Na,Mg,Al)团簇,它们形成了内含M原子的最稳定的笼状结构.然而对于Au12M(M=Si,P,S,Cl)团簇,它们却形成了以M元素为顶点的稳定锥形结构.在这些团簇中发现Au12S团簇相对是最稳定的,这是由于Au12S团簇形成了稳定的满壳层的电子结构.自然电荷布居分析表明:对于所有的Au12M(M=Na,Mg,Al,Si,P,S,Cl)团簇电荷总是从Au原子转向M原子.自然键轨道和HOMO分析表明Au12M团簇中发生了Au原子的s-d轨道和M原子的p轨道间的杂化现象.     

Theoretical Study on the Structure and Stability of Si5X （X = Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl） Clusters

1 INTRODUCTION In the later 60s of last century, silicon substituted for germanium to present as mainstream in semicon- ductor. The semi-conductive devices made by silicon have many advantages, for example, refractory pro- perty, high radioresistance, simple and stable process- ing technic, high machinability and low cost. So it was widely used to manufacture large power appara- tuses, for instance, digit and linear integrated circuit, large scale integrated circuit (LSI), etc. Thus, th…     

Spectroscopic properties of novel aromatic metal clusters: NaM4 (M=Al,Ga,In) and their cations and anions

The ground- and several excited states of metal aromatic clusters, namely NaM(4) and NaM(4) (+/-) (M=Al,Ga,In) clusters have been investigated by employing complete active-space self-consistent-field followed by multireference singles and doubles configuration interaction computations that included up to 10 million configurations and other methods. The ground states NaM(4) (-) of aromatic anions are found to be symmetric C(4nu) ((1)A(1)) electronic states with ideal square pyramid geometries. While the ground state of NaIn(4) is also predicted to be a symmetric C(4nu) ((2)A(1)) square pyramid, the ground state of the NaAl(4) cluster is found to have a C(2nu) ((2)A(1)) pyramid with a rhombus base, and the ground state of NaGa(4) possesses a C(2nu) ((2)A(1)) pyramid with a rectangle base. In general, these structures exhibit two competing geometries, viz., an ideal C(4nu) structure and a distorted rhomboidal or rectangular pyramid structure (C(2nu)). All of the ground states of the NaM(4) (+) (M=Al,Ga,In) cations are computed to be C(2nu) ((3)A(2)) pyramids with rhombus bases. The equilibrium geometries, vibrational frequencies, dissociation energies, adiabatic ionization potentials, adiabatic electron affinities for the electronic states of NaM(4) (M=Al,Ga,In), and their ions are computed and compared with experimental results and other theoretical calculations. On the basis of our computed excited states energy separations, we have tentatively suggested assignments to the observed X and A states in the anion photoelectron spectra of Al(4)Na(-) reported by Li et al. [X. Li, A. E. Kuznetov, H. F. Zheng, A. I. Boldyrev, and L. S. Wang, Science 291, 859 (2001)]. The X state can be assigned to a C(2nu) ((2)A(1)) rhomboidal pyramid. The A state observed in the anion spectrum is assigned to the first excited state ((2)B(1)) of the neutral NaAl(4) with the C(4nu) symmetry. The assignments of the excited states are consistent with the experimental excitation energies and the previous Green's function-based methods for the vertical transition energy separations between the X and A bands.     

A comparative study of the pseudo Jahn–Teller instability of linear molecules Ag3 and I3 and their positive and negative ions

In confirmation of the general idea of pseudo Jahn–Teller origin of instability of high-symmetry configurations of polyatomic systems that determines their geometry, a series of six molecules, Ag₃ⁿ and I₃ⁿ, n=−1,0,+1, were investigated. The electronic structure of the ground state of all the six molecules in the linear configuration was calculated by the extended-Hückel method with atomic charge and electron configuration self-consistency, while the excited state energy levels and wavefunctions were estimated in the single transition approximation. Then, the orbital vibronic constants, bare force constants and the vibronic contribution of the appropriate excited states to the instability of the linear configuration were evaluated.

The obtained results show that in both series, silver and iodine, the curvature of the adiabatic potential of the linear configuration, in the direction of the bending distortions, decreases from the negative ions to the neutral atoms to the positive ions and becomes negative in the latter two cases, thus explaining the origin of the experimentally observed geometries. The numerical data give a detailed insight into: (1) the origin of the linear→bent distortions as being due to the additional covalency created by the σ–π overlap in the bent configuration; (2) the specific excited states that contribute to this process of geometry formation; and (3) the difference between the silver and iodine series.     

Pseudo Jahn-Teller origin of nonplanarity and rectangular-ring structure of tetrafluorocyclobutadiene

It is shown that the pseudo Jahn-Teller effect (PJTE) in combination with ab initio calculations explains the origin of instability of the planar configuration of tetrafluorocyclobutadiene, C(4)F(4), with respect to a puckered structure and square-to-rectangle distortion of the carbon ring, and rationalizes its difference from the planar-rectangular geometry of C(4)H(4) and nonplanar (puckered) structure of Si(4)H(4). The two types of instability and distortion of the high-symmetry D(4h) configuration in these systems emerge from the PJT coupling of the ground B(2g) state with the excited A(1g) term producing instability along the b(2g) coordinate (elongation of the carbon or silicon square ring), and with the excited E(g) term resulting in e(g) (puckering) distortion. A rhombic distortion b(1g) of the ring is also possible due to the coupling between excited A(1g) and B(1g) terms. For C(4)F(4), ab initio calculations of the energy profiles allowed us to evaluate the PJTE constants and to show that the two instabilities, square-to-tetragonal b(2g) and puckering e(g) coexist, thus explaining the origin of the observed geometry of this system in the ground state. The preferred cis-trans (e(g) type) puckering in C(4)F(4) versus trans-trans puckering (b(2u) distortion) in Si(4)H(4) follows from the differences in the energy gaps to their excited electronic E(g) and A(1u) terms causing different PJTE in these two cases.     

On the origin of alternating bond distortions and the emergence of chirality in polyoxometalate anions

Alternating short and long bond length (ABL) distortions observed within the ring structures of molecular metal oxide anions or polyoxometalates (POMs) are reminiscent of the cooperative linear ABL distortions in perovskite d(0) metal oxides. We show herein that these distortions have a common origin: a pseudo Jahn-Teller (PJT) vibronic instability. Four POM structural types with different M(n)O(n) ring sizes are investigated herein using density functional theoretical methods: Lindqvist [M6O19](q-) (n = 4), Keggin alpha-[XM12O40](q-) (n = 6), Wells-Dawson alpha-[X2M18O62](q-) (n = 8), and Preyssler [(Na)P5W30O110](14-) (n = 10), where M = Mo(VI) and W(VI) and X = Si(IV), Ge(IV), P(V), As(V), S(VI), and Se(VI). Chirality is induced within the latter three structural types by the ABL ring distortions. The calculations confirm the PJT vibronic origin of the ABL distortions with good agreement between calculated geometries and published single-crystal X-ray diffraction data. Both theory and experiment show that the vibronic interaction and distortion magnitude increase for (1) molybdates relative to that of tungstates, (2) larger M(n)O(n) ring sizes, (3) increases in negative charge of the internalized fragments (O(2-) or XO4(q-)), and (4) d(0) versus d(n) metal oxidation states. The PJT vibronic coupling model explains these observations in terms of the energy gap between Kohn-Sham frontier molecular orbitals (MOs) concomitant with the propensity for metal-oxygen pi-bonding within the M(n)O(n) rings. The frontier MOs for the undistorted nuclear configurations are largely nonbonding pi-O(p) (occupied) and pi-M(d) (unoccupied) in character, where smaller HOMO-LUMO (H-L) gap energies lead to greater metal-oxygen pi-orbital mixing under the influence of the nuclear distortion. A reduction in pi-bond order decreases the distortion in mixed-valence POMs. Of the tungstates examined, only the Preyssler anion shows pronounced ABL ring distortions, which derive from its large ring size and concomitant small H-L gap.     

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs)

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.     

SiOM(M=Li,Be,B,Na,Mg,Al)分子结构的DFT研究

利用Gaussian-94计算程序,B3LYP方法,6-311+G(2d)6d基组,对SiOM(M=Li,Be,B,Na,Mg,Al)诸体系的几何结构进行优化.结果表明,M既可与SiO中的Si键合,也可与O键合.第一和第二主族的SiOM体系以折线形构型为最稳定构型,而第三主族则以近直线形或直线形构型为最稳定构型.从Si-O间键长R_SiO、力常数f_SiO及自然键轨道分析可知,第一主族的SiOLi和SiONa的最稳定构型中SiO-M间的离子键成分较大,可近似看作离子键;而对SiOLi,SiOBe,SiOB和SiOMg体系的以Si为中心的构型,M-SiO间的离子键成分很小,不能看作离子键,可认为M与SiO之间存在着弱相互作用     

Multiple aromaticity and antiaromaticity in silicon clusters.

A series of silicon clusters containing four atoms but with different charge states (Si4(2+), Si4, Si4(2-), and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si4(2+) is square-planar, analogous to the recently discovered aromatic Al4(2-) cluster. Upon of two electrons, neutral Si4 becomes sigma-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si4(2-): either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si4(2-), a stabilizing cation (Na+) was used to produce Si4(2-) in the gas phase in the form of Na+[Si4(2-)], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si4(2-)] species is highly unusual.     

Hydrogen bridging in the compounds X2H (X=Al,Si,P,S)

X2H hydrides (X=Al, Si, P, and S) have been investigated using coupled cluster theory with single, double, and triple excitations, the latter incorporated as a perturbative correction [CCSD(T)]. These were performed utilizing a series of correlation-consistent basis sets augmented with diffuse functions (aug-cc-pVXZ, X=D, T, and Q). Al2H and Si2H are determined to have H-bridged C2v structures in their ground states: the Al2H ground state is of 2B1 symmetry with an Al-H-Al angle of 87.6 degrees, and the Si2H ground state is of 2A1 symmetry with a Si-H-Si angle of 79.8 degrees. However, P2H and S2H have nonbridged, bent Cs structures: the P2H ground state is of 2A' symmetry with a P-P-H angle of 97.0 degrees, and the S2H ground state is of 2A' symmetry with a S-S-H angle of 93.2 degrees. Ground state geometries, vibrational frequencies, and electron affinities have been computed at all levels of theory. Our CCSD(T)/aug-cc-pVQZ adiabatic electron affinity of 2.34 eV for the Si2H radical is in excellent agreement with the photoelectron spectroscopy experiments of Xu et al. [J. Chem. Phys. 108, 7645 (1998)], where the electron affinity was determined to be 2.31+/-0.01 eV.     

Theoretical studies of structures and stabilities of endohedral fullerenes X0/n+ @C32 (X = H,Li, Na,K, Be,Mg, Ca,B, Al,C, Si,N, P,n = 1–3)

Based on the D₃ C₃₂ fullerene, the equilibrium geometries, electronic structures, and binding energies of the endohedral fullerenes X^0/n+@C₃₂ (X = H, Li, Na, K, Be, Mg, Ca, B, Al, C, Si, N, P, n = 1–3) have been calculated using the DFT/B3LYP/6‐31G(d) method. The results show that the C₃₂ cages are slightly enlarged due to encapsulation, and the sizes of non‐neutral molecules are smaller than the corresponding neutral ones. Cages containing Li, Na, and Ca and most of the cations, except Na⁺ and K⁺, are energetically favorable. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Different approaches for the calculation of electronic excited states of nonstoichiometric alkali halide clusters: the example of Na3F

The electronic structure and excited states of the Na(3)F cluster are investigated using different approximate, but numerically efficient, computational schemes, such as a 2e hybrid quantum/classical pseudopotential model with full-configuration interaction or time-dependent density-functional theory. Various quantities such as geometries and transition energies are compared with results previously obtained by multireference configuration interaction calculations, taken as reference data. The potential energy surfaces of the lowest excited states are investigated and the finite-temperature absorption spectra are calculated. The good agreement with recent beam experiments [J.-M. L'Hermite, V. Blanchet, A. Le Padellec, B. Lamory, and P. Labastie, Eur. Phys. J. D 28, 361 (2004)] leads to the conclusion that the absorption spectrum observed experimentally corresponds to the lowest energy isomer which has a C(2v) planar rhombic geometry.     

Copper adsorption and Si, Al, Ca, Mg, and Na release from clinoptilolite

Copper adsorption onto clinoptilolite (natural zeolite), Al/Si dissolution, and Mg, Ca, and Na release from the substrate were the subjects of the investigation described here. Experimental variables were Cu and electrolyte concentrations and solution pH. Copper adsorption was found to increase with increased pH and with decreased electrolyte concentration. Large amounts of K were also adsorbed from electrolyte. Since solution pH was assumed as a variable, the effects of [H(+)] differentiation on Cu adsorption and on Al/Si dissolution were also examined. Al dissolution was affected mainly by electrolyte concentration, whereas Si dissolution was affected mainly by adsorbed Cu amount. It was assumed that the release of Mg, Ca, and Na occurs through ion-exchange reactions with solution K(+), because their release is affected more by electrolyte concentration than by adsorbed Cu. From the study of FTIR spectra for various samples used in the present investigation, we observed that the removal of framework Si/Al shifts the band which was attributed to O-T-O stretching vibration toward higher frequency. Significant changes were observed for the bands assigned to Si-OH-Al bridges and to monomeric and polymeric hydrogen bonds at the region between 3650 and 3200 cm(-1). It is proposed that the Cu species caused the destruction of H-bonded structures, whereas K adsorbed species were located at exchangeable sites after an ion-exchange process between K and Ca, Mg, and Na from the zeolite's surface. An expansion of the zeolite framework was detected from XRD patterns under acid conditions.     

Li2Mg2Si4O10F2、H2Mn8O16•1.4H2O和Li1.3Ti1.7Al0.3(PO4)3在高浓度LiCl水溶液中的离子交换行为

研究了用功能材料Li2Mg2Si4O10F2 (LHT)、H2Mn8O16•1.4H2O (CRYMO)和Li1.3Ti1.7Al0.3(PO4)3 (LTAP)分别去除高浓度氯化锂水溶液中的杂质Fe3＋、K＋和Na＋.实验结果表明，这几种功能材料分别对溶液中的杂质Fe3＋、K＋和Na＋有很高的选择性，除杂效果明显.分析和研究了这几种功能材料在高浓度氯化锂水溶液中分别与Fe3＋、K＋和Na＋的交换行为.结果表明，在高浓度氯化锂溶液中这几种功能材料与杂质交换的动力学行为可近似用JMAK方程描述.     

Photodissociation of Mg+-XCH3 (X=F, Cl, Br, and I) complexes. I. Electronic spectra and dissociation pathways

Photodissociation spectra of Mg+-XCH3 (X=F, Cl, Br, and I) complexes have been measured in the ultraviolet region (225-415 nm). Several fragment ions with and without charge transfer (CT), Mg+, XCH3+, MgX+, MgCH3+, CH3+, and X+, were formed by evaporation (intermolecular bond dissociation) and intracluster reaction (intramolecular bond dissociation) via excited electronic states. Branching ratios of these ions were found to depend both on absorption bands and on halogen atoms. The ground states of the complexes were calculated to have geometries in which the Mg atom lies next to X atom of methyl halide molecules. Positive charges of the complexes are confirmed to be almost localized on Mg. Observed absorption bands were assigned to the transitions of the Mg+2P-2S atomic line perturbed by interactions with methyl halide molecules. Branching ratios of fragment ions can be partly explained by the stability of fragment ions and neutral counterparts. From the excited state potential energy curves along the Mg-X bond distance, dissociation reaction after CT was concluded to proceed predissociatively; potential curve crossings between the initially excited states and repulsive CT states may have a crucial role in the formation of CH3+, XCH3+, and X+. In particular, XCH3+ ions were formed via repulsive CT states having a character of electron excitation from Xnp to Mg+3s.     

Electrochemical investigation of the anodic hydrogen evolution on MgZn2, Mg2Si,and Al4Cu2Mg8Si7 intermetallic phases

Journal of Solid State Electrochemistry - The anodic hydrogen evolution (AHE) on Mg, MgZn2 (η-phase), Al4Cu2Mg8Si7 (Q-phase), and Mg2Si (β-phase) intermetallic compounds has been...     

Synthesis and characterization of mesoporous silica from selectively acid-treated saponite as the precursors

Mesoporous silicas were synthesized by hydrothermal treatment of selectively acid-treated saponite (an ideal structural formula: Na(1/3)Mg(3)(Si(11/3)Al(1/3))O(10)(OH)(2)), having a 2:1 type layered structure as the silica source and its porous properties were examined and compared with that from kaolinite (an ideal structural formula: Al(2)Si(2)O(5)(OH)(4)), having a 1:1 type layered structure. Synthetic saponite was selectively leached in H(2)SO(4) solutions with various concentrations (0.05-1 M) at 70 degrees C for 0.5 h. The resulting products (precursors) were mixed with cetyltrimethylammonium bromide (CTABr), NaOH and H(2)O, hydrothermally treated at 110 degrees C and removed the CTABr by calcining at 560 degrees C. A hexagonal mesoporous phase was obtained with higher Si/(Al(+Mg)) ratios of the resulting precursors. The XRD patterns of these products show the peaks assigned by a hexagonal lattice with a(0)=4.0-4.6 nm and the crystallinity becomes higher with higher Si/(Al(+Mg)) ratios of the precursors. The specific surface area (S(BET)) values of the present mesoporous silicas range from 800 to 1100 m(2)/g at CTABr/precursor=0.1 and although they are not as high as those from precursors prepared from calcining and acid-treatment of kaolinite (1420 m(2)/g), they are increased to 1400-1500 m(2)/g by increasing the ratio CTABr/precursor 0.2. The reason for the difference in the optimum preparation conditions between saponite and kaolinite may be attributed to the difference in the linkage of the SiO(4) tetrahedra in these precursors (i.e. layered or framework structures), which result in great differences in the selective leaching rates and structures of the resulting silica-rich products.     

Particulate matter (PM10) from São Carlos-SP (Brazil): spectroanalytical techniques to evaluate and determine chemical elements

Chemical composition of PM10 was studied during the period of 2014–2015 in the city of São Carlos, Brazil (‘Dos Voluntários’ Square). PM10 samples were directly analysed by wavelength dispersive X-ray fluorescence – WD-XRF (Al, Ca, Ce, Cl, Fe, K, Mg, P, S, Si, Ti, V, and Zn), and by laser-induced breakdown spectroscopy – LIBS (Ba, Ca, Fe, K, Mg, Na, Si and Ti) both for qualitative purposes. For quantitative analysis of Al, Ba, Ca, Fe, K, Na, S, and Zn, the analytes were extracted from samples of PM10 collected, in filters of glass fibre, with an extractive acid solution (HNO₃ and HCl) and determined by inductively coupled plasma optical emission spectrometry – ICP OES. Spatial variations of elemental concentrations (ng m^–3) were significantly higher in winter Al (19.0), Ba (13.6), Ca (20.0), Na (27.0), S (37.1), and Zn (9.5), autumn showed the highest concentration of Na (26.4), spring showed the highest concentration of Fe (11.6), K (13.1) and also S (25.3) and summer did not show a high concentration in the comparison between the seasons at the site. Using principal component analysis (PCA), as a data interpretation tool, with the data obtained by the WD-XRF and LIBS it was not possible to obtain a good correlation, but with the data of ICP OES, it was possible to verify correlations between identified and determinate elements, with samples collected in the autumn, winter, spring and summer seasons in the city of São Carlos. These associated analytical techniques were excellent tools in environmental monitoring, through the analysis of PM10 samples, presenting reliable and efficiency strategy, and based on the PCA and the EF equation was possible to draw the profile of the possible origins of these elements in the city.