Electron delocalization in nickel metallic wires: a DFT investigation of Ni(3)(dpa)(4)Cl(2) and [Ni(3)(dpa)(4)](3+) (dpa = dipyridylamide) and extension to higher nuclearity chains

The electronic structure of Ni(3)(dpa)(4)Cl(2) (1) has been investigated within the framework of the density functional theory (DFT), using two types of exchange-correlation functionals and various basis sets. The "broken-symmetry" approach proposed by Noodleman for the characterization of electronic states displaying an antiferromagnetic coupling has been applied to 1. All calculations lead to the conclusion that the ground state results from an antiferromagnetic coupling between the terminal Ni atoms, both displaying a high-spin electronic configuration. The central Ni atom is in a low-spin configuration, but is involved in a superexchange interaction connecting the two magnetic centers. These results are in agreement with the assignments recently proposed by the group of F. A. Cotton on the basis of magnetic measurements. It is shown that the ground state electronic configuration calculated for 1 provides the trinickel framework with some delocalized sigma bonding character. The observed geometry of 1 is accurately reproduced by the broken-symmetry solution. The doublet ground state assigned to the oxidized species [Ni(3)(dpa)(4)](3+) (2) and the dramatic contraction of the coordination sphere of the terminal metals observed upon oxidation are also confirmed by the calculations. However, the formal Ni-Ni bond order is not expected to increase in the oxidized species. The contraction of the Ni-Ni distance in 2 is shown to result in part from the vanishing of the important trans influence originating in the axial ligands, and for the rest from a more efficient shielding of the metal nuclear charge along the Ni-Ni-Ni axis. The conclusions deduced from the analysis of the bonding in 1 and 2 can be extended to their homologues with higher nuclearity. More specifically, it is predicted that the single occupancy of the most antibonding sigma orbital, extending over the whole metal framework, will provide the (Ni(p))(2)(p)(/(2)(p)(+1)+) chains with some delocalized bonding character and, possibly, with electrical conduction properties.     

Photodissociation studies of the electronic and vibrational spectroscopy of Ni(+)(H2O)

The electronic spectrum of Ni?(H?O) has been measured from 16200 to 18000 cm?1 using photofragment spectroscopy. Transitions to two excited electronic states are observed; they are sufficiently long-lived that the spectrum is vibrationally and partially rotationally resolved. An extended progression in the metal-ligand stretch is observed, and the absolute vibrational quantum numbering is assigned by comparing isotopic shifts between ??Ni?(H?O) and ??Ni?(H?O). Time-dependent density functional calculations aid in assigning the spectrum. Two electronic transitions are observed, from the 2A? ground state (which correlates to the 2D, 3d? ground state of Ni?) to the 32A? and 22A? excited states. These states are nearly degenerate and correlate to the 2F, 3d?4s excited state of Ni?. Both transitions are quite weak, but surprisingly, the transition to the 2A? state is stronger, although it is symmetry-forbidden. The 3d?4s states of Ni? interact less strongly with water than does the ground state; therefore, the excited states observed are less tightly bound and have a longer metal-ligand bond than the ground state. Calculations at the CCSD(T)/aug-cc-pVTZ level predict that binding to Ni? increases the H-O-H angle in water from 104.2 to 107.5° as the metal removes electron density from the oxygen lone pairs. The photodissociation spectrum shows well-resolved rotational structure due to rotation about the Ni-O axis. This permits determination of the spin rotation constants ε(αα)' = -12 cm?1 and ε(αα)' = -3 cm?1 and the excited state rotational constant A' = 14.5 cm?1. This implies a H-O-H angle of 104 ± 1° in the 22A? excited state. The O-H stretching frequencies of the ground state of Ni?(H?O) were measured by combining IR excitation with visible photodissociation in a double resonance experiment. The O-H symmetric stretch is ν?' = 3616.5 cm?1; the antisymmetric stretch is ν?' = 3688 cm?1. These values are 40 and 68 cm?1 lower, respectively, than those in bare H?O.     

The Ni + O2 reaction: a combined IR matrix isolation and theoretical study of the formation and structure of NiO2

The reaction of Ni atoms with molecular oxygen has been reinvestigated experimentally in neon matrices and theoretically at the DFT PW91PW91/6311G(3df) level. Experimental results show that i) the nature of the ground electronic state of the superoxide metastable product is the same in neon and argon matrices, ii) two different photochemical pathways exist for the conversion of the superoxide to the dioxide ground state (involving 1.6 or 4 eV photons) and iii) an important matrix effect exists in the Ni + O(2)--> Ni(O(2)) or ONiO branching ratios. Theoretical results confirm that the electronic ground state of the metastable superoxide corresponds to the singlet state, in agreement with former CCSD(T) calculations, but in contradiction with other recent works. Our results show that the ground electronic state of the dioxide is (1)Sigma(+)(g) with the lowest triplet and quintet states at slightly higher energy, consistent with the observation of weak vibronic transitions in the near infrared. The potential energy profiles are modelled for the ground state and nine electronic excited states and a pathway for the Ni(triplet) + O(2)(triplet) --> Ni(O(2)) or ONiO (singlet) reaction is proposed, as well as for the Ni(O(2)) --> ONiO photochemical reaction, accounting for the experimental observations.     

Crystal structures of Ln4Ni3O8 (Ln = La, Nd) triple layer T'-type nickelates

Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined by Rietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni1+/Ni2+ phases belong to the T'-type and are built by intergrowth of LnO2 fluorite layers with triple NiO2 infinite-layer structural blocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln4Ni3O10-delta Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln4Ni3O8 phases is attributed to internal structural stress. This transformation allows longer Ni-O bonds in Ln4Ni3O8 without overstretching of the Ln-O bonds, especially in the equatorial plane. The observed displacement of Ni atoms from the outer NiO2 planes toward the Ni atom of the central NiO2 plane in Ln4Ni3O8 is ascribed to large electrostatic repulsion from the fluorite part of the structure. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type Lnn+1NinO2n+2 homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect the covalency of the Ni-O bond.     

理论研究V2O6-的结构和电子性质

The geometrical and electronic properties of the anionic and neutral V2O6 clusters were studied with the spin unrestricted hybrid density functional B3LYP method. The calculated ground states of both clusters are different from the previous theoretical results. The ground state of V2O6- is found to be a doublet with C2v symmetry, while a doublet with D2h symmetry was previously obtained by Vyboishchikov and Sauer. For neutral V2O6, the ground state is an open-shell singlet with D2h symmetry whose energy is very close to that of the triplet state. In contrast, a closed-shell singlet with D2h symmetry was obtained by Vyboishchikov and Sauer, and Calatayud et al. found a triplet ground state with Cs symmetry. Moreover,the calculated adiabatic and vertical detachment energies of the anion cluster are in much better agreement with the experimental results of photoelectron spectroscopy than previous theoretical values.     

Multireference configuration interaction studies on the ground and excited states of N2O2: the potential energy curves of N2O2 along N-N distance

In this paper, the ground and excited states of N2O2 were studied at the multireference configuration interaction (MRCI) level of theory with Dunning's [J. Chem. Phys. 90, 1007 (1985); 96, 6796 (1992)] correlation consistent basis sets augo-cc-pVDZ and aug-cc-pVTZ. The geometry optimizations were performed for the ground state of N2O2. The vertical excitation energies and transition moments were calculated for the low-lying singlet states of N2O2 including the lowest three 1A1 states, two 1B1 states, one 1B2 state, and two 1A2 states at the MRCI level of theory with Dunning's correlation consistent basis sets aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ. Furthermore, for the first time, the potential energy curves were calculated at the complete active space self-consistent-field and MRCI levels of theory for as many as 12 N2O2 singlet electronic states along the N-N distance. The dissociation asymptotes of these 12 N2O2 singlet electronic states were discussed.     

DFT-UX3LYP studies on the coordination chemistry of Ni2+. Part 1: Six coordinate [Ni(NH3)n(H2O)(6-n)]2+ complexes

DFT calculations with the UX3LYP hybrid functional and a medium-sized 6-311++G(d,p) basis set were performed to examine the gas-phase structure of paramagnetic (S = 1) six-coordinate complexes [Ni(NH3)n(H2O)(6-n)](2+), 0 < or = n < or = 6. Significant interligand hydrogen bonding was found in [Ni(H2O)6](2+), but this becomes much less significant as NH3 replaces H2O in the coordination sphere of the metal. Bond angles and bond lengths obtained from these calculations compare reasonably well with available crystallographic data. The mean calculated Ni-O bond length in [Ni(H2O)6](2+) is 2.093 A, which is 0.038 A longer than the mean of the crystallographically observed values (2.056(22) A, 108 structures) but within 2sigma of the experimental values. The mean calculated Ni-N bond length in [Ni(NH3)6](2+) is 2.205(3) A, also longer (by 0.070 A) than the crystallographically observed mean (2.135(18) A, 7 structures). Valence bond angles are reproduced within 1 degree. The successive replacement of H2O by NH3 as ligands results in an increase in the stabilization energy by 7 +/- 2 kcal mol(-1) per additional NH3 ligand. The experimentally observed increase in the lability of H2O in Ni(II) as NH3 replaces H2O in the coordination sphere is explained by an increase in the Ni-OH2 bond length. It was found from a natural population analysis that complexes with the highest stabilization energies are associated with the greatest extent of ligand-to-metal charge transfer, and the transferred electron density is largely accommodated in the metal 4s and 3d orbitals. An examination of the charge density rho bcp and the Laplacian of the charge density nabla(2)rho(bcp) at the metal-ligand bond critical points (bcp) in the series show a linear correlation with the charge transferred to the metal. Values of nabla(2)rho(bcp) are positive, indicative of a predominantly closed-shell interaction. The charge transferred to the metal increases as n, the number of NH3 ligands in the complex, increases. This lowers the polarizing ability of the metal on the ligand donors and the average metal-ligand bond length increases, resulting in a direct correlation between the dissociation energy of the complexes and the reciprocal of the average metal-ligand bond length. There is a strong correlation between the charge transferred to the metal and experimental DeltaH values for successive replacement of H2O by NH3, but a correlation with stability constants (log beta values) breaks when n = 5 and 6, probably because of entropic effects in solution. Nevertheless, DFT calculations may be a useful way of estimating the stability constants of metal-ligand systems.     

Reactivity of atomic cobalt with molecular oxygen: a combined IR matrix isolation and theoretical study of the formation and structure of CoO2

The reactivity of atomic cobalt toward molecular oxygen in rare gas matrices has been reinvestigated. Experiments confirm that Co atoms in their a(4)F ground state are inert toward O(2) in solid argon and neon but reactive in the b(4)F first excited state, in agreement with the previous gas-phase study of Honma and co-workers. The formation of CoO(2) starting from effusive beams of Co and O(2) has been followed by IR absorption spectroscopy, both in neon and argon matrices. Our observations show that only the dioxo form, OCoO, is stabilized in the matrix and that IR absorptions previously assigned to the peroxo and superoxo forms are due to other, larger species. The present data strongly support the linear geometry in rare gas matrices proposed by Weltner and co-workers. We report on measurements on all IR-active fundamental modes for (16)OCo(16)O, (18)OCo(18)O, and (16)OCo(18)O with additional combination transitions supplying anharmonicity correction. This allows for a 5.93 +/- 0.02 mdyne/A CoO harmonic bond force constant in solid neon. Using the empirical relationship previously optimized for the CoO diatomics, an approximate value for the CoO internuclear bond distance is proposed (1.615 +/- 0.01A). In light of recent theoretical studies predicting (2)A(1) or (6)A(1) electronic ground states, the geometry and electronic structure of the OCoO molecule has also been reconsidered. Calculations carried out at the CCSD(T)/6-311G(3df) level indicate a linear structure with an r(e) = 1.62 A bond distance, consistent with the experimental estimate. For later studies of larger systems, where CCSD(T) calculations become too time-consuming, an effective DFT-based method is proposed which reproduces the basic electronic and geometrical properties of cobalt dioxide. Quantitative results are compared to the experimental data and high-level results regarding bond length and frequencies. This DFT method is used to propose a reaction pathway.     

Trinuclear heterobimetallic Ni2Ln complexes [L2Ni2Ln][ClO4] (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er; LH3=(S)P[N(Me)NCH-C6H3-2-OH-3-OMe]3): from simple paramagnetic complexes to single-molecule magnet behavior

The reaction of LH3 with Ni(ClO4)(2).6H 2O and lanthanide salts in a 2:2:1 ratio in the presence of triethylamine leads to the formation of the trinuclear complexes [L2Ni2Ln][ClO4] (Ln=La (2), Ce (3), Pr (4), Nd (5), Sm (6), Eu (7), Gd (8), Tb (9), Dy (10), Ho (11) and Er (12) and L: (S)P[N(Me)NCH-C6H3-2-O-3-OMe]3). The cationic portion of these complexes consists of three metal ions that are arranged in a linear manner. The two terminal nickel(II) ions are coordinated by imino and phenolate oxygen atoms (3N, 3O), whereas the central lanthanide ion is bound to the phenolate and methoxy oxygen atoms (12O). The Ni-Ni separations in these complexes range from 6.84 to 6.48 A. The Ni-Ni, Ni-Ln and Ln-O phenolate bond distances in 2-12 show a gradual reduction proceeding from 2 to 12 in accordance with lanthanide contraction. Whereas all of the compounds (2-12) are paramagnetic systems, 8 displays a remarkable ST=(11)/2 ground state induced by an intramolecular Ni. . .Gd ferromagnetic interaction, and 10 is a new mixed metal 3d/4f single-molecule magnet generated by the high-spin ground state of the complex and the magnetic anisotropy brought by the dysprosium(III) metal ion.     

Structure, stability, electronic and magnetic properties of Ni4 clusters containing impurity atoms

Using a gradient-corrected density functional method, we studied computationally how single impurity atoms affect the structure and the properties of a Ni4 cluster. H and O atoms coordinate at a Ni-Ni bond, inducing small changes to the structure of bare Ni4 which is essentially a tetrahedron. For a C impurity, we found three stable structures at a Ni4 cluster. In the most stable geometry, the carbon atom cleaves a Ni-Ni bond of Ni4, binding to all Ni atoms. Inclusion of the impurity atom leads to a partial oxidation of the metal atoms and, in the most stable structures, reduces the spin polarization of the cluster compared to bare Ni4. An H impurity interacts mainly with the Ni 4s orbitals, whereas the Ni 3d orbitals participate strongly in the bonding with O and C impurity atoms. For these impurity atoms, Ni 3d contributions dominate the character of the HOMO of the ligated cluster, in contrast to the HOMO of bare Ni4 where Ni 4s orbitals prevail. We also discuss a simple model which relates the effect of a H impurity on the magnetic state of metal clusters to the spin character (minority or majority) of the LUMO or HOMO of the bare metal cluster.     

Electronic structure of oxidized complexes derived from cis-[Ru(II)(bpy)2(H2O)2]2+ and its photoisomerization mechanism

The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.     

MRCI investigation of different isomers of Ni2O2H2(+)

The geometrical and electronic structures of different isomers of Ni(2)O(2)H(2)(+) are investigated by multireference configuration interaction (MRCI) calculations using natural atomic orbital basis sets. The lowest-lying isomer, Ni(2)(OH)(2)(+), has a rhombic shape with two OH groups bridging the Ni atoms. The next isomer in energetic order with a relative energy of 0.29 eV consists of a linear NiONi(OH(2))(+) chain. Other structures with a rhombic shape, (NiH)(2)O(2)(+), with H bound to the Ni atoms have considerably higher energies, above 4 eV. Especially the low-lying isomers are characterised by a large number of low-lying electronic terms. The product Ni(2)O(2)H(2)(+) of the reaction of Ni(2)O(2)(+) with small alkanes is likely to have the rhombic Ni(2)(OH)(2)(+) structure. The reaction energy of the reaction Ni(2)O(2)(+) + H(2)→ Ni(2)(OH)(2)(+) is estimated to be about -3.5 eV.     

Spin-forbidden ligand binding to the ferrous-heme group: ab initio and DFT studies

The potential energy surfaces (PESs) and associated energy barriers that characterize the spin-forbidden recombination reactions of the gas-phase ferrous deoxy-heme group with CO, NO, and H2O ligands have been calculated using density functional theory (DFT). The bond energy for binding of O2 has also been calculated. Extensive large basis set CCSD(T) calculations on two small models of the heme group have been used to calibrate the accuracy of different DFT functionals for treating these systems. Pure functionals are shown to overestimate the stability of the low-spin forms of the deoxy-heme model, and to overestimate the binding energy of H2O and CO, whereas hybrid functionals such as B3PW91 and B3LYP yield accurate results. Accordingly, the latter functionals have been used to explore the PESs for binding. CO binding is found to involve a significant barrier of ca. 3 kcal mol-1 due to the need to change from the deoxy-heme quintet ground state to the bound singlet state. Binding of water does not involve a barrier, but the resulting bond is weak and may be further weakened in the protein environment, which should explain why water binding is not usually observed in heme proteins such as myoglobin. NO binding involves a low barrier, which is consistent with observed rapid geminate recombination. The calculated bond energies are in good agreement with previous reported values and in fair agreement with experiment for CO and O2. The value for NO is significantly lower than the experimentally derived bond energy, suggesting that B3LYP is less accurate in this case.     

Charge density measurement and bonding character in LiNiO2

Accurate low-order structure factors of LiNiO₂ were measured by quantitative convergent-beam electron diffraction (QCBED), and then transformed into X-ray structure factors with Mott formula. Combining the structure factors measured by electron diffraction with the structure factors from X-ray diffraction measurements, accurate charge density maps based on a multipole model were obtained. The parameters of the bond critical points (BCP) were calculated for topological analyses. It shows that closed-shell interactions exist between Ni and O atoms, and that the Ni-O and Ni-Ni bonds exhibit some covalent character. The calculated d-orbital occupancies show the charge deficiency at e_g(e_g) orbital and charge surplus at e_g(t_2g) orbital. The remaining 29.12% population of e_g(e_g) is also an indication of covalent component in the Ni-O bond. The unusual small κ_defv value of the O atom is also discussed.     

Theoretical study of CeO2 and Ce2O3 using a screened hybrid density functional

The predicted structures and electronic properties of CeO(2) and Ce(2)O(3) have been studied using conventional and hybrid density functional theory. The lattice constant and bulk modulus for CeO(2) from local (LSDA) functionals are in good agreement with experiment, while the lattice parameter from a generalized gradient approximation (GGA) is too long. This situation is reversed for Ce(2)O(3), where the LSDA lattice constant is much too short, while the GGA result is in reasonable agreement with experiment. Significantly, the screened hybrid HSE functional gives excellent agreement with experimental lattice constants for both CeO(2) and Ce(2)O(3). All methods give insulating ground states for CeO(2) with gaps for the 4f band lying between 1.7 eV (LSDA) and 3.3 eV (HSE) and 6-8 eV for the conduction band. For Ce(2)O(3) the local and GGA functionals predict a semimetallic ground state with small (0-0.3 eV) band gap but weak ferromagnetic coupling between the Ce(+3) centers. By contrast, the HSE functional gives an insulating ground state with a band gap of 3.2 eV and antiferromagnetic coupling. Overall, the hybrid HSE functional gives a consistent picture of both the structural and electronic properties of CeO(2) and Ce(2)O(3) while treating the 4f band consistently in both oxides.     

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.     

Metal-metal bonding in mixed valence Ni2(5+) complexes and spectroscopic evidence for a Ni2(6+) species

Dinickel(II) complexes of the ligands N,N'-di-p-anisylformamidinate (DAniF) and N,N',N'-triphenylguanidinate (TPG) have been synthesized and crystallographically characterized, along with their one-electron-oxidized analogues. In both systems, the Ni-Ni distances become shorter by approximately 0.1 A upon oxidation, in accord with the proposal that the resulting Ni2(5+) complexes are appropriately described as having one electron removed from a metal-based sigma orbital and an overall Ni-Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni2(HNCHNH)4 and [Ni2(HNCHNH)4]+ appear to predict that the lowest energy state of the latter species would have one unpaired electron in an essentially ligand-based orbital. A single-point calculation of Ni2(DAniF)4 employing the geometry of its crystal structure with the full ligand included reveals a reversal of the previously predicted order of the HOMO and HOMO-1, and suggests that the unpaired electron in [Ni2(DAniF)4]+ is in a metal-based orbital of sigma symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni2(DAniF)4 shows an unexpectedly rich cyclic voltammogram with four stepwise reversible oxidation waves. Coulometric experiments show that the doubly oxidized species has a significant lifetime at -25 degrees C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni2(6+) core with a single Ni-Ni sigma bond.     

Matrix isolation infrared spectroscopic and theoretical study of group IV metal oxide clusters: M2O2 and M2O4

Dinuclear titanium, zirconium, and hafnium oxide clusters, M2O2 and M2O4 (M = Ti, Zr, Hf) have been prepared and characterized by matrix isolation infrared spectroscopy and quantum chemical calculations. The M2O2 clusters were formed through the reactions of metal dimers and O2 in solid argon upon sample annealing. Theoretical calculations indicate that the Ti2O2 cluster has a singlet ground state with a nonplanar cyclic C(2v) structure with a strong Ti-Ti bond, while the Zr2O2 and Hf2O2 clusters have planar cyclic structures. The M2O4 clusters were characterized to have a closed-shell singlet ground state with a nonplanar C2h symmetry, which were formed from the dimerization of the metal dioxide molecules.     

Valence bond structures for the D2h isomer of N2O4 and some isomers of S3O2 and S3O

The construction of valence bond structures of the increased-valence type is re-described for the D2h isomer of N2O4 and applied to obtain the corresponding valence bond structures for isomers of S3O2 and S3O, each of which has at least one six-electron four-center bonding unit. It is discussed how the S-O and S-S bond properties that are associated with the S3O2 and S3O increased-valence structures are in qualitative accord with the calculated bond lengths. The qualitative six-electron four-center molecular orbital theory for the symmetrical O-S-S-O component of each S3O2 isomer is related to the increased-valence structure for the six electrons. An increased-valence structure for the lowest-energy S3O2 isomer is equivalent to a restricted form of resonance between 16 Lewis-type valence bond structures. The two types of S-S bond length are used to provide empirical estimates of the weights for these Lewis structures and are compared with those obtained from the results of STO-6G valence bond calculations for the 3Sigma- ground state of SO.     

Spin-orbit coupling in O2(upsilon)+O2 collisions: I. Electronic structure calculations on dimer states involving the X 3Sigmag-, a 1Deltag, and b 1Sigmag+ states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.