Probing the structures and chemical bonding of boron-boronyl clusters using photoelectron spectroscopy and computational chemistry: B(4)(BO)(n) (-) (n = 1-3)

The electronic and structural properties of a series of boron oxide clusters, B(5)O(-), B(6)O(2) (-), and B(7)O(3) (-), are studied using photoelectron spectroscopy and density functional calculations. Vibrationally resolved photoelectron spectra are obtained, yielding electron affinities of 3.45, 3.54, and 4.94 eV for the corresponding neutrals, B(5)O, B(6)O(2), and B(7)O(3), respectively. Structural optimizations show that these oxide clusters can be formulated as B(4)(BO)(n) (-) (n = 1-3), which involve boronyls coordinated to a planar rhombic B(4) cluster. Chemical bonding analyses indicate that the B(4)(BO)(n) (-) clusters are all aromatic species with two π electrons.     

Association patterns in (HF)(m)(H2O)(n) (m + n = 2-8) clusters

In an attempt to understand the phase behavior of aqueous hydrogen fluoride, the clustering in the mixture is investigated at the molecular level. The study is performed at the mPW1B95/6-31+G(d,p) level of theory. Several previous studies attempted to describe the dissociation of HF in water, but in this investigation, the focus is only on the association patterns that are present in this binary mixture. A total of 214 optimized geometries of (HF)n(H2O)m clusters, with m + n as high as 8, were investigated. For each cluster combination, several different conformations are investigated, and the preferred conformations are presented. Using multiple linear regressions, the average strengths of the four possible H-bonding interactions are obtained. The strongest H-bond interaction is reported to be the H2O...H-F interaction. The most probable distributions of mixed clusters as a function of composition are also deduced. It is found that the larger (HF)n(H2O)m clusters are favored both energetically and entropically compared to the ones that are of size m + n < or = 3. Also, the clusters with equimolar contributions of HF and H2O are found to have the strongest interactions.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

Structures and stabilities of small lead oxide clusters PbmOn (m=1-4,n=1-2m)

The structures and stabilities of small lead oxide clusters PbmOn with m=1-4, n=1-2m are systematically studied using density functional theory. It is found that the lowest-energy structures of all these clusters can be obtained by the sequential oxidation of small "core" lead clusters. For Pb-rich clusters (oxygen-to-lead ratio<1), oxygen atoms favor bridge sites for Pb2On and Pb3On and surface sites for Pb4On. The lead-monoxide-like clusters (PbO)i (i=1-4) have great stability because of their significant dissociation energies and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps. This suggests that they could be adopted as the building blocks of cluster-assembled materials. For O-rich clusters (oxygen-to-lead ratio>1), the grouping of oxygen atoms usually appears. It is found that the structures with a grouping of more than two oxygen atoms are unstable.     

Probing the electronic structure and band gap evolution of titanium oxide clusters (TiO(2))(n)(-) (n = 1-10) using photoelectron spectroscopy

TiO2 is a wide-band-gap semiconductor, and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n- clusters for n = 1-10 have been obtained at 193 nm (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n- clusters for n > 1 appears to be localized in a tricoordinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.     

Structure and bonding in cyclic isomers of B2AlH(n)(m) (n = 3-6, m = -2 to +1): a comparative study with B3H(n)(m), BAl2H(n)(m) and Al3H(n)(m)

The structure, bonding and energetics of B(2)AlH(n)(m) (n = 3-6, m = -2 to +1) are compared with corresponding homocyclic boron, aluminum analogues and BAl(2)H(n)(m) using density functional theory (DFT). Divalent to hexacoordinated boron and aluminum atoms are found in these species. The geometrical and bonding pattern in B(2)AlH(4)(-) is similar to that for B(2)SiH(4). Species with lone pairs on the divalent boron and aluminum atoms are found to be minima on the potential energy surface of B(2)AlH(3)(2-). A dramatic structural diversity is observed in going from B(3)H(n)(m) to B(2)AlH(n)(m), BAl(2)H(n)(m) and Al(3)H(n)(m) and this is attributable to the preference of lower coordination on aluminum, higher coordination on boron and the higher multicenter bonding capability of boron. The most stable structures of B(3)H(6)(+), B(2)AlH(5) and BAl(2)H(4)(-) and the trihydrogen bridged structure of Al(3)H(3)(2-) show an isostructural relationship, indicating the isolobal analogy between trivalent boron and divalent aluminum anion.     

IR spectroscopy on isolated Co(n)(alcohol)m cluster anions (n=1-4, m=1-3): structures and spin states

Isolated cobalt-alcohol cluster anions containing n=1-4 cobalt and m=1-3 alcohol molecules (alcohol=methanol, ethanol, propanol) are produced in a supersonic beam by using a laser ablation source. By applying IR photodissociation spectroscopy vibrational spectra in the OH stretching region are obtained. Several structures in different spin states are discussed for the (n,m) clusters. In comparison with density functional theory calculations applied to both the Co/alcohol clusters and the naked Co cluster anions, an unambiguous structural assignment is achieved. It turns out that structures are preferred with a maximum number of hydrogen bonds between the OH groups and the Co···Co units. These hydrogen bonds are typical for anionic species leading to an activation of the OH groups which is indicated by large red-shifts of the OH stretching frequencies compared to the naked alcohols. For each (n,m) cluster, the frequency shifts systematically with respect to the different alcohols, but the type of structure is identical for all alcohol ligands. The application of IR spectroscopy turns out to be an ideal tool not only as a probe for structures but also for spin states which significantly influence the predicted OH stretching frequencies.     

Structures of [(CO2)n(CH3OH)m](-) (n = 1-4, m = 1, 2) cluster anions

The infrared photodissociation spectra of [(CO 2) n (CH 3OH) m ] (-) ( n = 1-4, m = 1, 2) are measured in the 2700-3700 cm (-1) range. The observed spectra consist of an intense broad band characteristic of hydrogen-bonded OH stretching vibrations at approximately 3300 cm (-1) and congested vibrational bands around 2900 cm (-1). No photofragment signal is observed for [(CO 2) 1,2(CH 3OH) 1] (-) in the spectral range studied. Ab initio calculations are performed at the MP2/6-311++G** level to obtain structural information such as optimized structures, stabilization energies, and vibrational frequencies of [(CO 2) n (CH 3OH) m ] (-). Comparison between the experimental and the theoretical results reveals the structural properties of [(CO 2) n (CH 3OH) m ] (-): (1) the incorporated CH 3OH interacts directly with either CO 2 (-) or C 2O 4 (-) core by forming an O-HO linkage; (2) the introduction of CH 3OH promotes charge localization in the clusters via the hydrogen-bond formation, resulting in the predominance of CO 2 (-).(CH 3OH) m (CO 2) n-1 isomeric forms over C 2O 4 (-).(CH 3OH) m (CO 2) n-2 ; (3) the hydroxyl group of CH 3OH provides an additional solvation cite for neutral CO 2 molecules.     

Structure and stability of (TiO2)n, (SiO2)n, and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] clusters

Structures, energetics, and vibrational spectra are investigated for small pure (TiO(2))(n), (SiO(2))(n), and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] oxide clusters by density functional theory (DFT). The BP86/ATZP level of theory is employed to obtain constitutional isomers of the oxide clusters. In accordance with previous studies, our calculations show three-dimensional compact structures are preferred for pure (TiO(2))(n) with oxo-stabilized higher hexavalent states, and linear chain structures are favored for pure (SiO(2))(n) with tetravalent states. However, the herein theoretically first reported mixed Ti(m)Si(n-m)O(2n) oxide clusters prefer either three-dimensional compact or linear chain structures depending upon the stoichiometry of the compound. Vibrational analysis of the important modes of some highly stable structures is provided. Coupled-cluster single and double excitation (with triples) [CCSD(T)] computed energy gaps for the TiO(2) dimers compare well with results from previous study. Excitation energies are computed by use of time-dependent (TD) DFT and equation-of-motion coupled-cluster calculations with singles and doubles (EOM-CCSD) for the most stable isomers.     

Structures of [(CO2)n(H2O)m]- (n=1-4, m=1,2) cluster anions. I. Infrared photodissociation spectroscopy

The infrared photodissociation spectra of [(CO(2))(n)(H(2)O)(m)](-) (n=1-4, m=1, 2) are measured in the 3000-3800 cm(-1) range. The [(CO(2))(n)(H(2)O)(1)](-) spectra are characterized by a sharp band around 3570 cm(-1) except for n=1; [(CO(2))(1)(H(2)O)(1)](-) does not photodissociate in the spectral range studied. The [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2) species have similar spectral features with a broadband at approximately 3340 cm(-1). A drastic change in the spectral features is observed for [(CO(2))(3)(H(2)O)(2)](-), where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm(-1). Ab initio calculations are performed at the MP2/6-311++G(**) level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the [(CO(2))(n)(H(2)O)(m)](-) species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in [(CO(2))(n)(H(2)O)(m)](-): (1) the incorporated H(2)O is bonded to either CO(2) (-) or C(2)O(4) (-) through two equivalent OH...O hydrogen bonds to form a ring structure in [(CO(2))(n)(H(2)O)(1)](-); (2) two H(2)O molecules are independently bound to the O atoms of CO(2) (-) in [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2); (3) a cyclic structure composed of CO(2) (-) and two H(2)O molecules is formed in [(CO(2))(3)(H(2)O)(2)](-).     

Probing the electronic properties of dichromium oxide clusters Cr2On- (n=1-7) using photoelectron spectroscopy

In an effort to elucidate the variation of the electronic structure as a function of oxidation and composition, we investigated an extensive series of dichromium oxide clusters, Cr2On- (n=1-7), using photoelectron spectroscopy (PES). Well-resolved PES spectra were obtained at several photon energies. While low photon energy spectra yielded much better spectral resolution, high photon energy data allowed both Cr 3d- and O 2p-derived detachment features to be observed. The overall spectral evolution of Cr2On- exhibits a behavior of sequential oxidation with increasing oxygen content, where low binding energy Cr 3d-based spectral features diminish in numbers and the spectra shift towards higher binding energies as a result of charge transfer from Cr to O. Evidence was obtained for the population of low-lying isomers for Cr2O2-, Cr2O3-, and Cr2O6-. The current data are compared with previous studies and with related studies on W2On- and Mo2On-.     

Module-based assembly of copper(II) chloranilate compounds: syntheses, crystal structures, and magnetic properties of [[Cu(2)(CA)(terpy)(2)][Cu(CA)(2)]](n)() and [[Cu(2)(CA)(terpy)(2)(dmso)(2)][Cu(CA)(2)(dmso)(2)](EtOH)](n)(H(2)CA = chloranilic acid, terpy = 2,2':6',2' '-terpyridine, dmso = dimethyl sulfoxide)

Two new copper(II) compounds of chloranilate and 2,2':6',2' '-terpyridine have been synthesized, and the structures have been solved by the single-crystal X-ray diffraction method. The crystal structure of [[Cu(2)(CA)(terpy)(2)][Cu(CA)(2)]](n)(1), where H(2)CA = chloranilic acid and terpy = 2,2':6',2' '-terpyridine, consists of two modules, the dimer unit [Cu(2)(CA)(terpy)(2)](2+) and the anionic mononuclear unit [Cu(CA)(2)](2)(-), forming an alternated chain. The chain is stabilized by semicoordinating and additional but efficient secondary bonding interactions. The crystal structure of [[Cu(2)(CA)(terpy)(2)(dmso)(2)][Cu(CA)(2)(dmso)(2)](EtOH)](n)(2), where dmso = dimethyl sulfoxide, consists of solvent molecules and two discrete modules, the dimer unit [Cu(2)(CA)(terpy)(2)(dmso)(2)](2+) and the anionic mononuclear unit [Cu(CA)(2)(dmso)(2)](2)(-). The dimer units form a layer by secondary bonding interactions, and the monomer units and ethanol molecules are introduced between the layers. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. A weak ferromagnetic interaction was observed in 1, J(a) = 2.36 cm(-)(1) and zJ(b) = -0.68 cm(-)(1) while no exchange coupling was observed in 2.     

B_n(BO)_n~(2-),CB_(n-1)(BO)_n~-,and C_2B_(n-2)(BO)_n (n=5-12):Cage-like boron oxide clusters analogous to closo-B_nH_n~(2-),CB_(n-1)H_n~-,and C_2B_(n-2)H_n

A density functional theory (DFT) investigation has been performed in this work on the cage-like boron-rich boron oxide clus-ters Bn(BO)2n-,CBn-1(BO)-n,and C2Bn-2(BO)n (n = 5-12) which are boronyl analogues of the closo-boranes BnHn2-,monocarbo-ranes CBn-1H-n,and dicarboranes C2Bn-2Hn. These boron oxide clusters possess similar geometrical and electronic structures with the corresponding boranes and carboranes and prove to be three-dimensional (3D) aromatic compounds,consistent with the previously proposed ...     

In search of CS2(H2O)(n=1-4) clusters

Gaussian-3 and MP2/aug-cc-pVnZ methods have been used to calculate geometries and thermochemistry of CS(2)(H2O)n, where n=1-4. An extensive molecular dynamics search followed by optimization using these two methods located two dimers, six trimers, six tetramers, and two pentamers. The MP2/aug-cc-pVDZ structure matched best with the experimental result for the CS(2)(H2O) dimer, showing that diffuse functions are necessary to model the interactions found in this complex. For larger CS(2)(H2O)n clusters, the MP2/aug-cc-pVDZ minima are significantly different from the MP2(full)6-31G* structures, revealing that the G3 model chemistry is not suitable for investigation of sulfur containing van der Waals complexes. Based on the MP2/aug-cc-pVTZ free energies, the concentration of saturated water in the atmosphere and the average amount of CS(2) in the atmosphere, the concentrations of these clusters are predicted to be on the order of 10(5) CS(2)(H2O) clusters.cm(-3) and 10(2) CS(2)(H2O)(2) clusters.cm(-3) at 298.15 K. The MP2/aug-cc-pVDZ scaled harmonic and anharmonic frequencies of the most abundant dimer cluster at 298 K are presented, along with the MP2/aug-cc-pVDZ scaled harmonic frequencies for the CS(2)(H(2)O)(n) structures predicted to be present in a low-temperature molecular beam experiment.     

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.     

Comparison of hydrated hydroperoxide anion (HOO-)(H2O)n clusters with alkaline hydrogen peroxide (HOOH)(OH-)(H2O)(n-1) clusters, n = 1-8, 20: an ab initio study

Hydroperoxide anion (HOO(-)), the conjugate base of hydrogen peroxide (HOOH), has been relatively little studied despite the importance of HOOH in commercial processes, atmospheric science, and biology. The anion has been shown to exist as a stable species in alkaline water. This project explored the structure of gas phase (HOO(-))(H(2)O)(n) clusters and identified the lowest energy configurations for n ≤ 8 at the B3LYP/6-311++G** level of theory and for n ≤ 6 at the MP2/aug-cc-pVTZ level of theory. As a start toward understanding equilibration between HOO(-) and HOOH in an alkaline environment, (HOOH)(OH(-))(H(2)O)(n-1) clusters were likewise examined, and the lowest energy configurations were determined for n ≤ 8 (B3LYP/6-311++G**) and n ≤ 6 (MP2/aug-cc-pVTZ). Some studies were also done for n = 20. The two species have very different solvation behaviors. In low energy (HOOH)(OH(-))(H(2)O)(n-1) clusters, HOOH sits on the surface of the cluster, is 4-coordinated (each O is donor once and acceptor once), and donates to the hydroxide ion. In contrast, in low energy (HOO(-))(H(2)O)(n) clusters, (HOO(-)) takes a position in the cluster center surrounded on all sides by water molecules, and its optimum coordination number appears to be 7 (one O is donor-acceptor-acceptor while the other is a 4-fold acceptor). For n ≤ 6 the lowest (HOOH)(OH(-))(H(2)O)(n-1) cluster lies 1.0-2.1 kcal/mol below the lowest (HOO(-))(H(2)O)(n) cluster, but the lowest clusters found for n = 20 favor (HOO(-))(H(2)O)(20). The results suggest that ambient water could act as a substantial kinetic brake that slows equilibration between (HOOH)(OH(-)) and (HOO(-))(H(2)O) because extensive rearrangement of solvation shells is necessary to restabilize either species after proton transfer.     

Structures and electronic properties of small FeB n (n=1-10) clusters

The geometries, stabilities, electronic properties, and magnetism of FeB(n) clusters up to n=10 are systematically studied with density functional theory. We find that our optimized structures of FeB(2), FeB(3), FeB(4), and FeB(5) clusters are more stable than those proposed in previous literature. The results show that it is favorable for the Fe atom to locate at the surface, not at the center of the cluster, and that FeB(4) and FeB(9) clusters exhibit high stability. For all the FeB(n) clusters studied, we find the charge transfer from Fe to B site and the coexistence of ionic and covalent bonding characteristics. The computed total magnetic moments of the lowest-energy structures oscillate with the cluster size and are quenched at n=4, 6, 8, and 10.     

Tin-capped trinuclear nickel clusters: redox isomerism between micro(3)-stannyl and micro(3)-stannylene clusters of the class [Ni(3)(dppm)(3)(micro3-I)(micro3-SnCl(x)](n+) (x = 2, n = 1; x = 3, n = 0)

The reaction of Ni(3)(dppm)(3)(micro(3)-I)(2)) with sodium trichlorostannate affords the first tin-capped nickel cluster Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3) (1). A site of coordinative unsaturation at tin can be introduced by the reaction of 1 with Tl[PF(6)] yielding the stannylene-capped cluster [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+) (2). Clusters 1 and 2 were characterized by 31P NMR, X-ray diffraction, and cyclic voltammetry (CV). Clusters 1 and 2 exhibit single electron redox chemistries, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl3](0/*-), [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+/0), that together comprise a redox equilibrium. Thus, electrochemical reduction of 1 produces first the 49e- cluster radical anion [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3)](*-) which then yields the reduced form of 2, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)], upon chloride dissociation.