added 12 research items
The physical and chemical properties of transition metal oxide particles result from the subtle interplay between atomic ordering and electronic structure, the latter being determined by a complex interaction between the partially filled d subshell of the transition metal atoms and the oxygen 2p orbitals. In this article, the geometric ground state structures} of several experimentally synthesized cationic chromium oxide clusters CrmOn⁺ (m = 2, 3, 4; n ≤ m) are characterized through infrared photodissociation spectroscopy on cluster-rare gas atom complexes in combination with quantum chemical calculations. Computational analysis of the electronic and magnetic properties of the identified isomers demonstrated that the magnetic configuration of the clusters varies with the size and oxidation state. Superexchange interaction causes ferromagnetic coupling in Cr2O2⁺ and Cr3O3⁺, while 3d-3d bonding-like interaction between two chromium atoms underlies ferrimagnetic behavior in Cr3O⁺, Cr3O2⁺, and Cr4O4⁺. The highest possible total magnetic moments are obtained in suboxides that have Cr-O-Cr bridges with a unique oxygen atom between each pair of Cr atoms. The addition of more oxygen atoms enhances the delocalization of the Cr 3d electrons and reduces the magnetic moment.
Planar and quasi-planar boron clusters with a disk-like shape are investigated in search of common bonding characteristics. Methods used involve molecular orbital calculations based on Density Functional Theory (DFT), and valence bond partitioning using Adaptive natural Density Partitioning (AdNDP) analysis. For high-symmetry cases the proposed bonding schemes are confirmed using the group-theoretical induction method. The focus is on the electron occupation of delocalized in-plane 3-center and 4-center bonds. For disks consisting of concentric rings this inner electron count is found to be equal to a multiple of the vertex count of the inner polygon. For two concentric rings the multiplying factor is four, for three concentric rings it is eight. The appropriate bonding schemes are presented which explain these results. Some giant clusters with two hexagonal holes are also discussed.
Structures of the binary Al n Si m clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Al n cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al4Si2, Al12Si and Al11Si2 + as "magic" clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al13 -, Al12Si, and Al11Si2 + that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the "magic" 40 shell electrons.
Using density functional theory with the TPSSh functional and the 6-311+G(d) basis set, we extensively searched for the global minima of two metallic atoms doped boron clusters B6M2, B7M2, B12M2 and B14M2 with transition metal element M being Co and Fe. Structural identifications reveal that B7Co2, B7Fe2 and B7CoFe clusters have global minima in a B-cyclic motif, in which a perfectly planar B7 is coordinated with two metallic atoms placed along the C7 axis. The B6 cluster is too small to form a cycle with the presence of two metals. Similarly, the B12 cluster is not large enough to stabilize the metallic dimer within a double ring 2xB6 tube. The doped B14M2 clusters including B14Co2, B14Fe2 and B14CoFe have a double ring 2xB7 tubular shape in which one metal atom is encapsulated by the B14 tube and the other is located at an expose d position. Dissociation energies demonstrate that while bimetallic cyclic cluster B7M2 prefers a fragmentation channel that generates the B7 global minimum plus metallic dimer, the tubular structure B14M2 tends to dissociate giving a bimetallic cyclic structure B7M2 and a B@B6 cluster. The enhanced stability of the bimetallic doped boron clusters considered can be understood from the stabilizing interactions between the anti-bonding MOs of metal-metal dimers and the levels of a disk aromatic configuration (for bimetallic cyclic structures), or the eigenstates of the B14 tubular form (in case of bimetallic tubular structure).
ABSTRACT: Mass spectrometry experiments show an exceptionally weak bonding between Si7Mn+ and rare gas atoms as compared to other exohedrally transition metal (TM) doped silicon clusters and other SinMn+(n = 5−16) sizes. The Si7Mn+ cluster does not form Ar complexes and the observed fraction of Xe complexes is low. The interaction of two cluster series, SinMn+ (n = 6−10) and Si7TM+ (TM = Cr, Mn, Cu and Zn), with Ar and Xe is investigated by density functional theory calculations. The cluster−rare gas binding is for all clusters, except Si7Mn+ and Si7Zn+, predominantly driven by short-range interaction between the TM dopant and the rare gas atoms. A high s-character electron density on the metal atoms in Si7Mn+ and Si7Zn+ shields polarization toward the rare gas atoms and thereby hinders formation of short-range complexes. Overall, both Ar and Xe complexes are similar except that the larger polarizability of Xe leads to larger binding energies
The smallest triple ring tubular silicon cluster Mn2@Si15 is reported for the first time. Theoretical structural identification shows that the Mn2@Si15 tubular structure whose triple ring is composed by three five-membered Si rings in anti-prism motif, is stable in high symmetry (D5h) and singlet ground state (1A1’). The dimer Mn2 is placed inside the tubular along the C5 axis, and the Mn dopant form single Si- Mn bonds with Si skeleton, whereas the Mn-Mn is characterized as a triple bond. The effect of Mn2 on the stability of the Si15 triple ring structure arises from strong orbital overlap of Mn2 with Si15.
The prebiotic formation of nucleobases, the building blocks of RNA/DNA, is of current interest. Highly reactive radical species present in the atmosphere under irradiation have been suggested to be involved in the prebiotic synthesis of nucleobases from formamide (FM). We studied several free radical reaction pathways for the synthesis of pyrimidine bases (cytosine, uracil, and thymine) from FM under cold conditions. These pathways are theoretically determined using density functional theory (DFT) computations to examine their kinetic and thermodynamic feasibilities. These free radical reaction pathways share some common reaction types such as H-rearrangement, (•)H/(•)OH/(•)NH2 radical loss, and intramolecular radical cyclization. The rate-determining steps in these pathways are characterized with low energy barriers. The energy barriers of the ring formation steps are in the range of 3-7 kcal/mol. Although DFT methods are known to significantly underestimate the barriers for addition of (•)H radical to neutral species, many of these reactions are highly exergonic with energy release of -15 to -52 kcal/mol and are thus favorable. Among the suggested pathways for formation of cytosine (main route, routes 7a and 1a), uracil (main route, routes 7b and 1b), and thymine (main route and route 26a), the main routes are in general thermodynamically more exergonic and more kinetically favored than other alternative routes with lower overall energy barriers. The reaction energies released following formation of cytosine, uracil, and thymine from FM via the main radical routes amount to -59, -81, and -104 kcal/mol, respectively. Increasing temperature induces unfavorable changes in both kinetic and thermodynamic aspects of the suggested routes. However, the main routes are still more favored than the alternative pathways at the temperature up to the boiling point of FM.
Prebiotic building blocks for the formation of biomolecules are important in understanding the abiotic origin of biomolecules. However, there is a limited choice of the building blocks as precursors for the biomolecules. Acetylene (HCCH) is found in Titan's atmosphere and is an abiotic-precursor of pyrimidine bases. HCCH reacts with urea to form both cytosine and uracil. The mechanisms for the formation of both cytosine and uracil were studied by density functional theory at B3LYP/6-311G(d,p) level. Ethynyl radicals (˙CCH) are relevant for the chemistry of Titan's atmosphere therefore both HCCH and ˙CCH were evaluated as carbon sources. The pathways, for both HCCH and ˙CCH, lead to intermediates with an unsaturated-group that facilitate the formation of the six-membered ring of the pyrimidine bases. The predicted structures for cytosine and uracil were compared with labeled cytosine and uracil that were formed from the reaction of DCCD with urea. The results suggest that cytosine is formed from HCCH while uracil is formed from ˙CCH. The mechanisms are energetically feasible and there is no conclusive evidence for the preferred pathway (HCCH or ˙CCH). The pathways were further extended for the formation of both uric acid and 8-oxoguanine from HCCH and urea, and demonstrate the utility of HCCH as a carbon source for diverse biomolecules. Biuret is identified as a precursor for the pyridimine bases, and it unifies the free radical pathways for the pyrimidine bases with those of triazines. The pathways are appropriate for the reducing atmosphere that creates both radicals and electrons due to ionizing radiation on Titan. The mechanisms are feasible for the extraterrestrial formation of the pyrimidine bases.
(Abstract) The global minima of both neutral and anionic clusters of VGe3-/0 were determined using different quantum chemical methods (DFT, RCCSD(T), CASSCF/CASPT2). On the basis of the ground states identified, most excited bands in the anion photoelectron spectrum of VGe3- were assigned. The tetrahedral isomers of both charged states are the most stable ones. A singlet state (Cs , 1A') of the tetrahedral isomer has the globally lowest energy on the potential hypersurface of VGe3-. Two states 12A' and 12A" of the neutral tetrahedral isomer are nearly degenerate and identified as the competing ground state of VGe3. From the anionic ground state, four of five bands in the anion photoelectron spectrum of VGe3- were determined to be the consequences of one-electron transitions starting from the anionic ground state 1A'. Both nearly degenerate neutral ground states are responsible for generation of the first band. Two different transitions from the anionic ground state 1A' to the first two nearly degenerate excited states (22A' and 22A") of the neutral underlie the second lowest ionization band. Two higher levels of ionization recorded in the spectrum were assigned to the two higher excited states 42A' and 52A' of the neutral. Franck-Condon factor simulations of the first band were performed to obtain more insights into experimental bands of the spectrum.
An extensive replica exchange molecular dynamics (REMD) simulation was performed to investigate the progress patterns of the inhibition of (−)-epigallocatechin-3-gallate (EGCG) on the Aβ16-22 hexamer. Structural variations of the oligomers without and with EGCG were monitored and analyzed in detail. It has been found that EGCG prevents the formation of Aβ oligomer through two different ways by either accelerating the Aβ oligomerization or reducing the β-content of the hexamer. It also decreases the potential “highly toxic” conformations of Aβ oligomer, which is related to the conformations having high order β-sheet sizes. Both electrostatic and van der Waals interaction energies are found to be involved to the binding process. Computed results using quantum chemical methods show that the π-π stacking is a critical factor of the interaction between EGCG and the peptides. As a result, the binding free energy of the EGCG to the Aβ peptides is slightly larger than that of the curcumin.
Partial electron localization functions ELF(σ_loca), ELF(π) and ELF(σ_delo) of boron Bn and silicon MSi12 double ring (DR) clusters were analyzed. In a DR, separated basins are localized within peripheral bonds (σ), delocalized outside inner bonds (π), or delocalized above and below peripheral bonds (σ). MO spectrum of skeleton D6h Si12 DR follows delightfully the hollow cylinder model. A mixture of different sets of MOs makes the D6h Si12 structure highly unstable. Upon interacting with 3d orbitals of Cr dopant, such a mixed behavior of MO sets is removed and the Cr@Si12 DR becomes a global minimum structure.
The structural, electronic and mechanical properties of monoclinic Li2Si2O5 are explored using density functional theory. Different exchange–correlation functionals are considered and the results are correlated to experimental data. The calculated electronic band structure and density of states indicate that monoclinic Li2Si2O5 has an insulating character with an indirect band gap of 4.98eV. Elastic stiffness coefﬁcients and the bulk, shear and Young’s moduli are also calculated. Our calculations predict that Li2Si2O5 is a ductile compound. We show that monoclinic Li2Si2O5 behaves as a specially orthotropic material, meaning that the structure can be masked by the orthorhombic form.
High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si2M with M = H, Li, Na, Cu, and Ag toward the first step of methanol activation reaction. Standard heats of formation of these small triatomic Si clusters were determined. Potential-energy profiles were constructed using the coupled-cluster theory with extrapolation to complete basis set CCSD(T)/CBS, and CCSD(T)/aug-cc-pVTZ-PP for Si2Cu and Si2Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si2M possess low-spin states and mainly stem from an M–O connection in preference to Si–O interaction, except for the Si2H case. In two competitive pathways including O–H and C–H bond breakings, the cleavage of the O–H bond in the presence of all clusters studied becomes predominant. Of the mixed clusters Si2M considered, the dissociation pathways of both O–H and C–H bonds with Si2Li turns out to have the lowest energy barriers. The most remarkable finding is the absence of the overall energy barrier for the O–H cleavage with the assistance of Si2Li. The breaking of O–H and C–H bonds with the assistance of Si2H, Si2Li, and Si2Na is kinetically preferred with respect to the Si2Cu and Si2Ag cases, apart from the case of Si2Na for O–H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu3, Pt3, and PtAu2, the energy barriers for the O–H bond activation in the presence of small Si species, especially Si2H and Si2Li, are found to be lower. Consequently, these small mixed silicon clusters can be regarded as promising alternatives for the expensive metal-based catalysts currently used for methanol activation particularly and other dehydrogenation processes of organic compounds. The present study also suggests a further extensive search for other doped silicon clusters as efficient and more realistic gas-phase catalysts for important dehydrogenation processes in such a way that they can be experimentally prepared and implemented.
The B44 cluster has a cage-like structure containing two hexagonal, two heptagonal and two nonagonal holes. The presence of nonagonal holes is a new and remarkable finding since they have never been reported before in clusters. The present work does not only identify the new chiral members, but also provide more insight into the growth motif of large-sized boron clusters.
(Abstract) Recently, metallic hetero-fullerenes were experimentally prepared from mixed Ge-As clusters and heavier elements of groups 14 and 15. We found that shape of these hetero-fullerenes doped by transition metal appears to be a general structural motif for both silicon and germanium clusters when mixing with phosphorus and arsenic atoms. Structural identifications for MSi8P6, MSi8As6, MGe8P6 and MGe8As6 clusters, with M being a transition metal of group 6 (Cr, Mo and W), showed that most MA8E6 clusters, except for Cr-doped derivatives CrSi8As6, CrGe8P6 and CrGe8As6, exhibit a high symmetry fullerene shape in which metal dopant is centered in D3h A8E6 hetero-cage consisting of six A3E2 pentagonal faces and three A2E2 rhombus faces. The stability of MA8E6 metallic hetero-fullerene is significantly enhanced by formation an electron configuration of [1S2 1P6 1D10 1F14 1G18 2S2 2P6 2D10] enclosing 68 electrons. The A8E6 hetero-cages give a great charge transfer (~4 electrons) to centered dopant, establishing subsequently a d10 configuration for metal, and as a consequence it induces an additional stabilization of the resulting ME8P6 fullerene in a high symmetry D3h shape, and completely quenches the high spin of the metal atom finally yielding a singlet spin ground state.
The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B4(2-), B5(+/-), B6, B7(-), B8(2-), B9(-), B10(2-), B11(-), B12, B13(+), B14(2-) and B16(2-) are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B12 and B13(+) is confirmed. The π electrons of B18(2-), B19(-) and B20(2-) obey the disk aromaticity rule with an electronic configuration of [1σ(2)1π(4)1δ(4)2σ(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5(+), Li7B5H5 and M@BnHn(q) clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B7C(-), B8C, B6C2, B9C(-), B8C2 and B7C3(-) is in conflict with the Hückel electron count. This is also the case for the ions B11C5(+/-) whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.
The cage-like structures containing octagonal holes are located as the lowest-lying isomers for the B. The presence of octagonal holes, which have been found for the first time, not only gives us new insight into the bonding motif, but also marks a breakthrough in the structural characteristics of boron clusters since they were never expected to be stable units for elemental clusters. These cages are composed of both delocalized σ and π electron systems that consequently make them aromatic and thermodynamically stable.
Interactions of ethylene and its 1,2-dihalogenated derivatives with CO2 induce the formation of twenty four molecular complexes with stabilization energies in the range of 1.1 to 7.5 kJ mol−1 as computed at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level. The stability of the parent C2H4⋯CO2 complex is due to a π⋯π* interaction which has not yet been reported in the complexes of CO2-philic compounds and CO2. The cis-XCHCHX⋯CO2 complexes are found to be slightly more stable than the trans-XCHCHX⋯CO2, with X = F, Cl and Br. Generally, the overall stabilization energy of each complex is determined by the C–H⋯O hydrogen bond and the C–X⋯C Lewis acid–base interaction, in which the latter plays a larger role. Substitution of two H atoms in CH2CH2 by the same halogen atoms stabilizes the complexes XCHCHX⋯CO2, and for the same dihalogenated derivatives, the stability of XCHCHX⋯CO2 tends to increase from X = F via Cl and to Br. The obtained results suggest that the contraction of the C–H bond involved in the C–H⋯O hydrogen bond and the blue-shift of its stretching frequency depend not only on a polarization of the C–H bond in the isolated monomer but also on the geometric shape of the complex formed.
The methanol activation pathways occurring on small pure and mixed silicon clusters Sim-nMn with M = Be, Mg, Ca and m = 3-4, n = 0-1 were investigated using quantum chemical computations (density functional theory B3LYP/aug-cc-pVTZ and coupled-cluster theory CCSD(T)/CBS extrapolated from energies with the aug-cc-pVnZ basis sets, n = D, T, Q) to examine their thermodynamic and kinetic feasibilities. In all cases considered, the cleavage of the O-H bond is favored over that of the C-H bond. The O-H bond cleavage in the presence of the singlet Si3 cluster is thermodynamically less preferred than on mixed Si2M clusters, even though it becomes more kinetically favored. Most importantly, the energy barriers for the O-H bond breaking on the singlet Si3, Si2Ca, and Si3Ca clusters are found to be lower than the previously reported results for metal clusters, catalytic metal surfaces, metal oxides, etc. The small mixed Si clusters thus appear to be good catalysts for methanol activation and most probably in other dehydrogenation processes from the X-H bonds of organic compounds. These findings suggest further extensive searches for doped silicon clusters as realistic catalysts that can experimentally be prepared, for methanol activation particularly and dehydrogenation processes generally.
Abstract: Geometric and electronic structures of the boron cluster B14 and its silicon derivatives B13Si+, B13Si-, and B12Si2 were determined using DFT calculations (TPSSh/6-311+G(d)). The B12Si2 fullerene which is formed by substituting two B atoms at two apex positions of the B14 fullerene by two Si atoms, was also found as the global minimum structure. We demonstrated that the electronic structure and orbital configuration of these small fullerenes can be predicted by the wavefunctions of a particle on a cylinder. The early appearance of high angular node MOs in B14 and B12Si2 can be understood by this simple model. Replacement of one B atom at a top position of B14 by one Si atom, followed by addition or removal of one electron does not lead to a global minimum fullerene structure for the anion B13Si- and cation B13Si+. The early appearance of the 5ζ1 orbital in B13Si+ causes a lower stability for the fullerene-type structure.
Optical spectra in the UV-VIS region of the hydrate d doubly charged tetramer Ag42+ and hydrated multiply charged hexamer Ag6p+ silver clusters encapsulated inside the sodalite cavity of LTA-type zeolite have been systematically predicted using DFT, TD-DFT and CASSCF/CASPT2 methods. The optical behaviour of the model hydrated clusters [Ag6(H2O)8(Si24H24O36)]p+ is very sensitive with respect to their charge. Among the cations [Ag6(H2O)8(Si24H24O36)]p+ , only the embedded hydrated quadruply charged silver hexamer [Ag6(2O)8(Si24H24O36)]4+ show a strong absorption band at ~420 nm (blue light) and emits light in red color. The absorption spectrum of the hydrated doubly charged silver tetramer cluster [Ag4(H 2O)m(Si24H24O36)]2+ which shifts slightly and steadily with the increasing amount of interacting water molecules to longer wavelengths, has strong peak in blue region. The water environment forces t he silver tetramer to relocate in one side of the cavity instead of at its center as in the case of non-hydrated [Ag4(Si24H24O36)]2 cluster. Water molecules act as ligands significantly splitting the energy levels of excited states of the Ag42+ and Ag64+clusters. This causes the absorption spectra of the clusters to broaden and the emission to shift to the green-yellow and red part of the visible region.
We investigated the structures of the singly and doubly magnesium-doped silicon clusters in both neutral and cationic states, SinMgm0/+, with n = 1-10 and m = 1 and 2. Total atomization energies (TAEs), heats of formation (ΔHf), and binding energies (Ebs) were determined using the composite G4 method. The Ebs of the Mg-doped clusters are decreased with respect to those of the pure Si counterparts, irrespective of the charge state. As no experimental values are actually available for these systems, the predicted thermochemical values can be used with an expected error margin of ±3 kcal/mol (±0.15 eV or ±12 kJ/mol), due to the uncertainty on the experimental heat of formation of the silicon atom and of the method used. The growth sequence of the singly doped neutral SinMg is similar to that of the singly doped neutral SinLi clusters. In SinMg structures, the Mg atom tends to favor addition on either an edge or a face of the anionic ground-state structure Sin⁻ framework. Only in Si8Mg, Mg substitutes a Si atom in the Si9 framework. For the cations SinMg⁺, the behavior of Mg differs from that of Li. The Mg atom seems to cap one edge or face of the cationic Sin⁺ instead of the neutral bare Sin as in the case of Li. The doubly Mg-doped neutral SinMg2 clusters grow basically following a method comparable to that of the doubly doped neutral SinX2 with X = Li and Al reported in previous studies. In their growth pattern, one Mg atom substitutes into a position of Sin+1, whereas the other Mg atom is usually added on an edge, or a face, of the existing cluster. There are however a few exceptions to this observation, such as Si10Mg2. In this size, the cyclic framework Si5-Mg-Si5-Mg turns out to be more stable than the cage-type Si10-Mg2. The SinMg2⁺ cations contain the cationic Sin+1⁺ frameworks in which one Mg atom actually substitutes into a Si position and the remaining Mg atom caps on an edge, or a face. Again, Si10Mg2⁺ appears as an exception to this trend. The most interesting result of this study is that the Mg dopant, due to its large electron transfer capacity, behaves as a cation, Mgδ+, and thereby induces an ionic entity with the Sinδ anionic partner. The resulting Mg cation can serve as a linker between Sikδ blocks, leading to stabilized linear and cyclic [(Sik)Mg]l structures. In the systems with k = 3, 5, 7, 8, and 10, the linear frameworks can be regarded as possible starting blocks for silicon assemblies, giving rise to potential 1D nanowire materials.
A doping of small boron clusters by silicon atoms leads to formation of stable boron nanoribbon structure. We present an analysis on the geometric and electronic structure, using MOs and electron localization function (ELF) maps, of boron ribbons represented by the dianions B10Si2 2-and B12Si2 2-. Effect of Si dopants and origin of the underlying electron count […π 2(n+1) σ 2n ] are analyzed. Interaction between both systems of delocalized π and σ electrons creating alternant B-B bonds along the perimeter of a ribbon induces its high thermodynamic stability. The enhanced stability is related to the self-locked phenomenon. Small boron-based clusters (Bn and BnXm) are known to exist in different forms including the planar, quasi-planar, tubular and cage-like structures. 1-9 Of the planar or quasi-planar forms, the ribbons are of particular interest as they formally have pseudo one-dimensional structures and could thus be used as building blocks for assembling low dimension materials. Boron ribbons have been found to be stabilized upon doping of boron clusters by some main group elements. The elongated double-chain (DC) planar ribbon structures BnH2 have been found as the global equilibrium forms up to n = 12 by quantum chemical calculations. 10, 11 Following addition of negative charge to a DC, which gives rise to a dianionic state, the B22H2 2-dianion with a length of 17.0 Å has been shown as the energetically lowest-lying isomer among the series BnH2 2-with n = 6-22. 12 A variety of elongated DC planar boron ribbon structures such as the B7Au2-, 13 B2nC2H2 (n = 2-9), 14 BnH2 m (m =-2,-1 and 0), 10-12 Li2BnH2 (n = 6-22) 12 were identified. Li et al. reported that boron dihydrides B8H2 and B12H2 are ribbons including three and five adjacent B4 rhombuses, respectively, and the diagonal B-B bond of these B4 rhombuses are alternant. 10 Such an alternant character of the diagonal B-B bond of the B4 rhombuses is linked to an alternation of 10.1 (C2h, 3 Bg) 0.0 10.2 (C2h, 1 Ag) 0.7 12.1 (C2h, 1 Ag) 0.0 12.2 (CS, 1 A') 10.7 Fig 1. Geometries of two lowest-lying isomers of B10Si2 2-10.1 and 10.2 and B12Si2 2-12.1 and 12.2. Some characteristics include point group and electronic state (in parentheses) and relative energies (ΔE, kcal/mol, computed using DFT with TPSSh functional and 6-311+G(d) basis set and ZPE corrections.
A doping of small boron clusters by silicon atoms leads to formation of stable boron nanoribbon structure. We present an analysis on the geometric and electronic structure, using MOs and electron localization function (ELF) maps, of boron ribbons represented by the dianions B10Si22- and B12Si22-. Effect of Si dopants and origin of the underlying electron count […π2(n+1) σ2n] are analyzed. Interaction between both systems of delocalized π and σ electrons creating alternant B-B bonds along the perimeter of a ribbon induces its high thermodynamic stability. The enhanced stability is related to the self-locked phenomenon.
A B3N3Si8 cage is formed upon substitution of Si sites of rhombus faces of the pure Si14 cluster by B and N atoms. Doping by the ion Mn+ leads to the hetero-silicon fullerene B3N3Si8Mn+ which comprises three rhombus BNBN, Si3B and Si3N, and four pentagons (two Si2B2N and two Si2BN2). Hetero-atoms form polarized Si-N and Si-B bonds as indicated by electron localization function (ELF) maps and NBO charges. The Mn center connects the B3N3Si8 cage by ionic interactions. Valence electrons of B3N3Si8Mn+ occupy a shell configuration of [1S2 1P6 1D10 1F14 1G12 2S2 2P6 2D10] and induce a certain thermodynamic stability. The high spin of the Mn+ metal cation is completely quenched within the hetero-Si fullerene.
The absolute binding free energy of an inhibitor to HIV-1 Protease (PR) was determined throughout evaluation of the non-bonded interaction energy difference between the two bound and unbound states of the inhibitor and surrounding molecules by the fast pulling of ligand (FPL) process using non-equilibrium molecular dynamics (NEMD) simulations. The calculated free energy difference terms help clarifying the nature of the binding. Theoretical binding affinities are in good correlation with experimental data, with . The paradigm used is able to rank two inhibitors having the maximum difference of ∼1.5 kcal/mol in absolute binding free energies.
Electronic structures of both the anionic and neutral triatomic species TiGe2-/0 were theoretically studied employing single-reference (DFT and RCCSD(T)) and multiconfigurational (CASSCF/CASPT2 and CASSCF/NEVPT2) methods with large basis sets. The ground state of TiGe2- (C2v) was identified to be 4B1 but the 2A1 state is nearly degenerate, whereas the 3B1 is clearly the ground state of the neutral TiGe2 (C2v). On the basis of the computed ground and excited states of both neutral and anionic structures, all electronic transitions giving rise to experimental anion photoelectron bands in the spectrum of TiGe2- can now be assigned. The X band of the anion photoelectron spectrum is attributed to a one-electron transition between two ground states 4B1 → 3B1. Three neutral excited states 23A2, 25B1 and 35B1 are energetically responsible for the B band upon one-electron photodetachement from the anionic ground state 4B1. The C band is assigned to the transition 4B1 → 25A1. A transition from the nearly-degenerate ground state 2A1 of the anion to the low-spin 1A1 of the final neutral state can be ascribed to the A band. Furthermore, the first two bands progressions whose normal vibrational modes were accessible from CASSCF/CASPT2 calculations, were also simulated by determination of multi-dimensional Franck-Condon factors.
We revisited the aromaticity of silicon tetramer Si4, Si42+ and isoelectronic four-membered rings without and with a planar tetracoordinate carbon dopant (Si4C2+). Electron localizability indicators and magnetic ring currents were used to probe bonding patterns. Comparison with Al42�, Si2Al2, Al4C2� and Si2Al2C was performed. The 14 (Si42+), 16 (Si4) and 18 (Si4C2+) valence electrons systems exhibit diatropic ring current,but this is determined by r electrons. Electrons in p orbitals do not significantly take part in the diatropic magnetic response which determines aromaticity. These clusters can thus be regarded as r-aromatic species and do not follow the classical Hückel rule.
Stabilized fullerene and tubular forms can be produced in boron clusters Bn in small sizes from n ~ 14 upon doping by transition metal atoms. B14Fe and B16Fe are stable tubes whereas B18Fe and B¬20Fe are stable fullerenes. The magnetic moment in B18Fe is completely quenched. Their formation and stability suggest the use of dopants to induce different growth paths leading to larger cages, fullerenes and tubes of boron.
This chapter consists of a review on the geometric, electronic structure and chemical bonding in a number of small boron clusters doped by two transition metal elements. First-row transition metals introduce not only new class of boron clusters but also particular growth patterns. In the bimetallic cyclic motif, the two metals are vertically coordinated to the planar B n strings along the symmetry axis. The same M–M axis persist in double ring tubular forms. A bimetallic configuration model has been used to rationalize the electronic structure and stability for both bimetallic cyclic and tubular boron clusters. The anti-bonding π* and δ* MOs of dimeric metals enjoy stabilizing interactions with the B 7 , B 8 strings and B 14 double ring, thus inducing an enhanced stability for the doped cluster. Formation of bimetallic tube requires the occupancy of the molecular orbital configuration of [(σ 4s) 2 (π) 4 (π*) 4 (δ) 4 (σ 3d) 2 (δ*) 4 ]. At least 20 electrons are thus needed to populate the electron shell. However, there is no fixed electron count, but this rather depends on the nature of the metallic dopants.
A comprehensive review on geometric and electronic structures, spectroscopic and energetic properties of small niobium clusters in the range from two to twenty atoms, Nbn, n = 2–20, in three different charged states is presented including a systematic comparison of quantum chemical results with available experimental data to assign the lowest-lying structures of Nbn clusters and their IR spectra and some basic thermochemical parameters including total atomization (TAE) and dissociation (D e) energies based on DFT and CCSD(T) results. Basic energetic properties including electron affinities, ionization energies, binding energies per atom, and stepwise dissociation energies are further discussed. Energetic parameters of small sizes often exhibit odd–even oscillations. Of the clusters considered, Nb2, Nb4, Nb8 and Nb10 were found to be magic as they hold the numbers of valence electrons corresponding to the closed-shell in the electron shells [1S/1P/2S/1D/1F…..]. Nb10 has a spherically aromatic character , high chemical hrT high chemical hardness and large HOMO–LUMO gap. The open-shell Nb15 system is also particularly stable and can form a highly symmetric structure in all charged states. For species with an encapsulated Nb atom, an electron density flow is present from the cage skeleton to the central atom, and the greater the charge involved the more stabilized the cluster is.