On the structure and chemical bonding of Si6(2-) and Si6(2-) in NaSi6(-) upon Na+ coordination

Photoelectron spectroscopy was combined with ab initio calculations to elucidate the structure and bonding in Si6 2- and NaSi6 -. Well-resolved electronic transitions were observed in the photoelectron spectra of Si6 - and NaSi6 - at three photon energies (355, 266, and 193 nm). The spectra of NaSi6 - were observed to be similar to those of Si6 - except that the electron binding energies of the former are lower, suggesting that the Si6 motif in NaSi6 - is structurally and electronically similar to that in Si6 -. The electron affinities of Si6 and NaSi6 were measured fairly accurately to be 2.23+/-0.03 eV and 1.80+/-0.05 eV, respectively. Global minimum structure searches for Si6 2- and NaSi6 - were performed using gradient embedded genetic algorithm followed by B3LYP, MP2, and CCSDT calculations. Vertical electron detachment energies were calculated for the lowest Si6 - and NaSi6 - structures at the CCSD(T)/6-311+G(2df), ROVGF/6-311+G(2df), UOVGF/6-311+G(2d), and time-dependent B3LYP/6-311+G(2df) levels of theory. Experimental vertical detachment energies were used to verify the global minimum structure for NaSi6 -. Though the octahedral Si6 2-, analogous to the closo form of borane B6H6 2-, is the most stable form for the bare hexasilicon dianion, it is not the kernel for the NaSi6 - global minimum. The most stable isomer of NaSi6 - is based on a Si6 2- motif, which is distorted into C2v symmetry similar to the ground state structure of Si6 -. The octahedral Si6 2- coordinated by a Na+ is a low-lying isomer and was also observed experimentally. The chemical bonding in Si6 2- and NaSi6 - was understood using natural bond orbital, molecular orbital, and electron localization function analyses.     

A photoelectron spectroscopic and computational study of sodium auride clusters, NanAun- (n = 1-3)

Negatively charged sodium auride clusters, NanAun- (n = 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun- (n = 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2- is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3- has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.     

Ab initio molecular orbital investigation of the amine-alanes (CH(3))(n)H(3)(-)(n)AlNX(3) and phosphane-alanes (CH(3))(n)H(3)(-)(n)AlPX(3) (X = H, F, and Cl; n = 0-3) complexes

We report on ab initio calculations at the G2(MP2) level of the structures and Al-N(P) bond complexation energies of the (CH(3))(n)H(3)(-)(n)AlNX(3) and (CH(3))(n)H(3)(-)(n)()AlPX(3) (X = H, F, and Cl; n = 0-3) donor-acceptor complexes. For the (CH(3))(3)AlNX(3) and (CH(3))(3)AlPX(3) complexes, the C(3)(v) symmetry is found to be favored, and for the other complexes the C(s) symmetry is found to be favored. The G2(MP2) calculated complexation energies show for the amine ligands the trend NH(3) > NCl(3) > NF(3). A similar trend PH(3) approximately PCl(3) > PF(3) is predicted for the phosphane ligands. The NBO partitioning scheme shows that there is no correlation between the stability and the charge transfer.     

Planar nitrogen-doped aluminum clusters AlxN- (x=3-5)

The electronic and geometrical structures of three nitrogen-doped aluminum clusters, Al(x)N(-) (x=3-5), are investigated using photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra have been obtained for the nitrogen-doped aluminum clusters at four photon energies (532, 355, 266, and 193 nm). Global minimum structure searches for Al(x)N(-) (x=3-5) and their corresponding neutrals are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structures and low-lying isomers are compared with the experimental observations. Planar structures have been established for all the three Al(x)N(-) (x=3-5) anions from the joint experimental and theoretical studies. For Al(5)N(-), a low-lying nonplanar isomer is also found to contribute to the experimental spectra, signifying the onset of two-dimensional to three-dimensional transition in nitrogen-doped aluminum clusters. The chemical bonding in all the planar clusters has been elucidated on the basis of molecular orbital and natural bond analyses.     

Global minimum structure search in Li(x)CoO(2) composition using a hybrid evolutionary algorithm

The global minimum structures for Li(x)CoO(2) compositions where 0 ≤ x ≤ 1 were probed by using a hybrid evolutionary algorithm with an underlying ab initio structural relaxation scheme. The method successfully predicted experimentally observed variants of layered configurations at various degrees of lithiation and the spinel (Fd3[combining macron]m) phase at x = 1/2. New low-energy non-layered host structures at x < 1/2 were also revealed. These structures can be formed from the usual layered configuration through coherent stacking faults along the c-axis and the migration of Co ions into the Li-poor intercalation layer.     

Al(6)(2-) - fusion of two aromatic Al(3)(-) units. A combined photoelectron spectroscopy and ab initio study of M(+)[Al(6)(2-)] (M = Li,Na, K,Cu, and Au)

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl(6)(-) (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl(6)(-) (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl(6)(-) clusters can be viewed as an M(+) cation interacting with an Al(6)(2-) dianion. Al(6)(2-) was found to possess an O(h) ground-state structure, and all of the MAl(6)(-) clusters possess a C(3v) ground-state structure derived from the O(h) Al(6)(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C(3v)ground-state structures for the MAl(6)(-) clusters. A detailed molecular orbital (MO) analysis is conducted for Al(6)(2-) and compared with Al(3)(-). It was shown that Al(6)(2-) can be considered as the fusion of two Al(3)(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al(3)(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)' ' MOs of Al(3)(-), provides the key bonding interactions for the fusion of the two Al(3)(-) into Al(6)(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the O(h)Al(6)(2-). It is shown that every face of the Al(6)(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al(3)(-), and that in fact Al(6)(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.     

Electronic structure and chemical bonding in MO(n)- and MO(n) clusters (M = Mo, W; n = 3-5): a photoelectron spectroscopy and ab initio study

Photoelectron spectroscopy (PES) and ab initio calculations are combined to investigate the electronic structure of MO(n)- clusters (M = W, Mo; n = 3-5). Similar PES spectra were observed between the W and Mo species. A large energy gap between the first and second PES bands was observed for MO3- and correlated with a stable closed-shell MO3 neutral cluster. The electron binding energies of MO4- increase significantly relative to those of MO3-, and there is also an abrupt spectral pattern change between MO3- and MO4-. Both MO4- and MO5- give PES features with extremely high electron binding energies (>5.0 eV) due to oxygen-2p-based orbitals. The experimental results are compared with extensive density functional and ab initio [CCSD(T)] calculations, which were performed to elucidate the electronic and structural evolution for the tungsten oxide clusters. WO3 is found to be a closed-shell, nonplanar molecule with C3v symmetry. WO4 is shown to have a triplet ground state (3A2) with D2d symmetry, whereas WO5 is found to be an unusual charge-transfer complex, (O2-)WO3+. WO4 and WO5 are shown to possess W-O* and O2-* radical characters, respectively.     

Photoelectron spectroscopy and ab initio study of the doubly antiaromatic B(6) (2-) dianion in the LiB(6) (-) cluster

A metal-boron mixed cluster LiB(6) (-) was produced and characterized by photoelectron spectroscopy and ab initio calculations. A number of electronic transitions were observed and used to compare with theoretical calculations. An extensive search for the global minimum of LiB(6) (-) was carried out via an ab initio genetic algorithm technique. The pyramidal C(2v) ((1)A(1)) molecule was found to be the most stable at all levels of theory. The nearest low-lying isomer was found to be a triplet C(2) ((3)B) structure, 9.2 kcal/mol higher in energy. Comparison of calculated detachment transitions from LiB(6) (-) and the experimental photoelectron spectra confirmed the C(2v) pyramidal global minimum structure. Natural population calculation revealed that LiB(6) (-) is a charge-transfer complex, Li(+)B(6) (2-), in which Li(+) and B(6) (2-) interact in a primarily ionic manner. Analyses of the molecular orbitals and chemical bonding of B(6) (2-) showed that the planar cluster is twofold (pi- and sigma-) antiaromatic, which can be viewed as the fusion of two aromatic B(3) (-) units.     

Density functional and Ab initio study of Cr(CO)n (n = 1-6) complexes

Cr(CO)n (n = 1-6) systems were studied for all possible spin states using density functional and high-level ab initio methods to provide a more complete theoretical understanding of the structure of species that may form during ligand dissociation of Cr(CO)6. We carried out geometry optimizations for each system and obtained vibrational frequencies, sequential bond dissociation energies (BDE), and total CO binding energies. We also compared the performance of various DFT functionals. Generally, the ground states of Cr(CO)6, Cr(CO)5, and Cr(CO)4, whose spin multiplicity is a singlet, are in good agreement with both previous theoretical results and currently available experimental data. Calculations on Cr(CO)3, Cr(CO)2, and CrCO provide new findings that the ground state of Cr(CO)3 might be a quintet with C2v symmetry instead of a singlet with C3v symmetry, and the ground state of Cr(CO)2 is not a linear quintet, as suggested by previous DFT calculations, but rather a linear septet. We also found that nonet states of Cr(CO)2 and CrCO display partial C-O bond breakage.     

Myths and reality of hypervalent molecules. The electronic structure of FClO(x), x = 1-3, Cl3PO, Cl3PCH2, Cl3CClO, and C(ClO)4

In the present work we examine a series of hypervalent molecules, namely, FClO(x) (x = 1-3), Cl(3)PO, Cl(3)PCH(2), Cl(3)CClO, and C(ClO)(4), through single-reference [CCSD(T)] and multireference (MRCI) ab initio methods, the principal aim being the deciphering of their binding pattern. Our electronic structure calculations consistently show that the bonding occurs through an electron pair transfer from the Cl or P atoms of the molecules considered to the (1)D state of the O atom(s). We strongly believe that the term "hypervalency" when viewed from an unbiased side and with a critical eye reveals a simple chemical bonding situation that is in conformity with a scientific parsimony that dissolves the mythology of an enormous class of molecular systems that are categorized under the term hypervalent.     

CAl2Be3(2-) and its salt complex LiCAl2Be3-: anionic global minima with planar pentacoordinate carbon

Following the isoelectronic relationship in global minima planar pentacoordinate carbon (ppC) species (cationic CAl(5)(+), neutral CAl(4)Be, and monoanionic CAl(3)Be(2)(-)), we designed a dianionic ppC species C(2v) CAl(2)Be(3)(2-) (1a) and its salt complex C(2v) LiCAl(2)Be(3)(-) (2a) in this work. In combination with DFT and high-level ab initio calculations (CCSD(T)), the extensive exploration on their potential energy surfaces indicates that they are the global minima. Their kinetic stability was proved by two sets of 100 ps ab initio Born-Oppenheimer molecular dynamic simulations at the B3LYP/6-31+G(d) level. The detailed analyses indicate that the introduction of Li(+) into 1a only influences the electrovalent bonding (through changing of the charge distribution) and the σ aromaticity (through changing of the in-plane ring current), while the structures, the bonding properties, the π aromaticity, and so forth are almost unchanged. Nevertheless, the MO energy levels, the HOMO-LUMO gaps, and the values of vertical detachment energies (VDEs) all verify that the lithiation significantly improves the stability. We think the ppC dianion 1a is possible to detect directly in the gas-phase experiments, but it can be detected as its salt complex 2a more easily.     

Structures and kinetic stabilities of the possible complexes of mononuclear Ni and (N2)x (x = 1-4)

Ab initio and density functional theory (DFT) methods have been applied to study the structures and kinetic stabilities of the possible products of the reactions of mononuclear nickel with (N(2))(x) (x = 1-4). Energy analyses show that end-on bound Ni(N(2))(x) (x = 1-4) complexes are preferred to side-on and N(4) bound ones. Several decomposition and isomerization pathways for Ni(N(2))(x) (x = 2-4) were investigated at the B3LYP/6-31G level of theory. The present study suggests that besides the four experimentally assigned complexes (NiN(2) (C(infinity)(v)), Ni(N(2))(2) (D(infinity)(h)), Ni(N(3))(2) (D(3)(h)), and Ni(N(2))(4) (T(d))), another two complexes (Ni(N(2))(4) (C(2)(v)) and Ni(N(2))(4) (D(4)(d))) are likely to be kinetically stable, while other complexes may be kinetically unstable with barrier heights of less than 30 kcal/mol. The present study also suggests that side-on bound N(2) ligand is ready to transform into the end-on bound one, while N(4) ligand is hard to transform into side-on or end-on bound N(2) ligand.     

Structures and energetics of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters

The structures and energies of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters have been examined in ab initio theoretical electronic structure calculations. Cluster geometries have been established in B3LYP/6-31G(2df) calculations and accurate relative energies determined by the G3XMP2 method. The two atoms readily bond to each other and to other atoms of their own kind. The result is a great variety of low-energy clusters in a variety of structural types.     

Studies of NO adsorption on Pt(110)-(1 x 2) and (1 x 1) surfaces using density functional theory

Adsorption of NO on Pt(110)-(1 x 2) and (1 x 1) surfaces has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full geometry optimization and without symmetry restriction. Adsorption energies, structures, and N-O stretching vibrational frequencies of NO are studied by considering multiple possible adsorption sites and comparing with the experimental data. Adsorption is strongly dependent on both coverage and surface phase. The assignment of adsorption sites has been carried out with precise calculation of vibrational frequencies for NO on various sites. We clearly show the NO site switching on both of the surfaces as found in the experiments: at low coverages, bridge species is formed on the surface, and at high coverages, NO switches to atop sites.     

Valence isoelectronic substitution in the B8(-) and B9(-) molecular wheels by an Al dopant atom: umbrella-like structures of AlB7(-) and AlB8(-)

The structures and the electronic properties of two aluminum-doped boron clusters, AlB(7)(-) and AlB(8)(-), were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB(7)(-) and AlB(8)(-) are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B(8)(-) and B(9)(-) planar clusters in AlB(7)(-) and AlB(8)(-). Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B(7) (C(6v)) and B(8) (C(7v)) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B(7)(3-) and planar B(8)(2-) molecular wheels derived from double aromaticity.     

Identification of conical structures in small aluminum oxide clusters: infrared spectroscopy of (Al2O3)1-4(AlO)+

The vibrational spectroscopy of the electronically closed-shell (Al 2O 3) n (AlO) (+) cations with n = 1-4 is studied in the 530-1200 cm (-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C 3 v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.     

Cu3C4-: a new sandwich molecule with two revolving C2(2-) units

A combined photoelectron spectroscopy (PES) and ab initio study was carried out on a novel copper carbide cluster in the gas phase: Cu(3)C(4)(-). It was generated in a laser vaporization cluster source and appeared to exhibit enhanced stability among the Cu(3)C(n)(-) series. Its PES spectra were obtained at several photon energies, showing numerous well-resolved bands. Extensive ab initio calculations were performed on Cu(3)C(4)(-), and two isomers were identified: a C(2) structure ((1)A) with a Cu(3)(3+) triangular group sandwiched by two C(2)(2-) units and a linear CuCCCuCCCu structure (D(infinity)(h), (1)Sigma(g)(+)). A comparison of ab initio PES spectra with experimental data showed that the sandwich Cu(3)C(4)(-) cluster was solely responsible for the observed spectra and the linear isomer was not present, suggesting that the C(2) structure is the global minimum in accordance with CCSD(T)/6-311+G predictions. Interestingly, a relatively low barrier (0.4-0.6 kcal/mol) was found for the internal rotation of the C(2)(2-) units in the sandwich Cu(3)C(4)(-). To test different levels of theory in describing the Cu(m)C(n)(-) systems and lay foundations for the validity of the theoretical methods, extensive calculations at a variety of levels were also carried out on a simpler copper carbide species CuC(2)(-), where two isomers were found to be close in energy: a linear one (C(infinity)(v), (1)Sigma(+)) and a triangular one (C(2)(v), (1)A(1)). The calculated electronic transitions for CuC(2)(-) were also compared with the PES data, in which both isomers were present.     

Structure of 1-(arylselanyl)naphthalenes. 2. G dependence in 8-G-1-(p-YC(6)H(4)Se)C(10)H(6).

The structures of 8-G-1-(p-YC(6)H(4)Se)C(10)H(6) (1 (G = Cl) and 2 (G = Br): Y = H (a), OMe (b), Me (c), Cl (d), Br (e), COOEt (f), and NO(2) (g)) were investigated by X-ray crystallographic analysis, NMR spectroscopy, and ab initio MO calculations. The structures of all members in 1 and 2 are concluded to be type B, which is in striking contrast to the type A structure for 4d-g (4 (g(n)), where G = H). The Se-C(i) bond of the p-YC(6)H(4)Se group in 8-G-1-(p-YC(6)H(4)Se)C(10)H(6) is almost perpendicular to the naphthyl plane in type A, and it is located on the plane in type B. The chlorine and bromine substitution at the 8-position in 1 and 2 dramatically changes the type A structure of 4 (g(n)) to type B. The nonbonded G- - -Se-C 3c-4e type interaction must contribute to stabilize the type B structure. The type B structure in 1 and 2 should also be more stabilized than the same structure in 4 by the 3c-4e type interaction: The structure of 4b is type B in the crystals and type B would be more stable for 4c and might be for 4a in solutions. Ab initio MO calculations are performed on 8-G-1-(p-YC(6)H(4)Se)C(10)H(6), 8-G-C(10)H(6)SeH-1, and models HG- - -SeH(2), where G = Cl, Br, and F, to clarify the reason for the dramatic change in the structures. The type B structure is optimized to be more stable than the type A for all species examined, which supports the observations. The energy differences between type B and type A are larger for the models than for the naphthalenes. While the superiority of the type B for the former is Br > Cl > F, that of the latter is Br approximately Cl >/= F. These results show that the main factor of the structural change from type A to type B is the nonbonded G- - -Se-C 3c-4e interaction. The electronic effect of halogens through the naphthalene pi-framework would also contribute to some extent, although the direct comparison of the evaluated values between the naphthalene systems and the models is not so easy. Factors to stabilize the two structures of 1, 2, 4, and 8-(MeSe)-1-(p-YC(6)H(4)Se)C(10)H(6) are reexamined from a viewpoint of the nonbonded G- - -Se-C 3c-4e interaction (G dependence), together with the electronic effect of Y (Y dependence).     

Atmospheric reactions Cl+CH3-(CH2)n-OH (n=0-4): a kinetic and theoretical study

The reactions of Cl with a series of linear alcohols: methanol (k1), ethanol (k2), 1-propanol (k3), 1-butanol (k4), and 1-pentanol (k5) were investigated as a function of temperature in the range of 264-382 K by laser photolysis-resonance fluorescence. The obtained kinetic data were used to derive the following Arrhenius expressions: k1=(3.55+/-0.22)x10(-10) exp[-(559+/-40)T], k2=(5.25+/-0.52)x10(-11) exp[(190+/-68)T], k3=(2.63+/-0.21)x10(-11) exp[(525+/-51)T], k4=(3.12+/-0.31)x10(-11) exp[(548+/-65)T], and k5=(3.97+/-0.48)x10(-11) exp[(533+/-77)T] (in units of cm(3) molecule(-1) s(-1)). To our knowledge, these are the first absolute kinetic data reported for 1-butanol and 1-pentanol and also the first kinetic study as a function of temperature for these two compounds. Results, mechanism, and tropospheric implications are discussed and compared with the reported reactivity with OH radicals. Moreover, a theoretical insight into the mechanisms of these reactions has also been pursued through ab initio M?ller-Plesset second-order perturbation treatment calculations with 6-311G** basis sets. Optimized geometries and vibrational frequencies have been obtained for transition states and molecular complexes appearing along the different reaction pathways. Furthermore, molecular energies have been calculated at quadratic configuration interaction with single, double, and triple excitations level in order to get an estimation of the activation energies.     

Semiempirical (AM1) and ab initio calculations of 2-X-adenine (X = H, F, Cl) and β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribofuranose

We performed semiempirical (AM1) and ab initio (3-21G and 4-31G^**) calculations, with complete optimization of geometry, for 2-X-adenine (X = H, F, Cl) and selected conformers of β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribo-furanose. Together (as 2-X-2',3'-didehydro-2',3'-dideoxyadenosine), or in separate conjunction with similar fragments, these structures belong to a group of 2',3'-dideoxynucleosides with potential anti-HIV activity.

The 2-X-adenine molecules are basically flat. For the halogenated species, especially the chlorine derivative, the results of the semiempirical method differ from the ab initio findings more widely than for adenine.

The results for the ribose derivative show the existence of conformers differing very little in energy. Ring puckering does not appear to depend on exocyclic torsion angles, structures with different conformations around the exocyclic C-C bond having similar ring conformations in all the cases analysed. The AMI method yielded geometries similar to those obtained ab initio with 3-21G.