Theoretical study on (Al2O3)n (n = 1-10 and 30) fullerenes and H2 adsorption properties

The structures and stabilities of (Al2O3)n (n = 1-10 and 30) clusters were studied by means of first principles calculations. The calculated results reveal that the global minima of small (Al2O3)n (n = 1-5) clusters are cage structures with high symmetries, in which Al and O atoms are three- and two-coordinated, respectively, and are linked to neighbors via single bonds. Beyond (Al2O3)5, we calculated both cage and cage-dimer structures for (Al2O3)n (n = 6-10), and the results show that, at this size range, cage-dimer structures are more stable than cage structures. Furthermore, an onion-like motif for (Al2O3)10 was studied, and it is interesting to find that, at this size, the onion structure is more favorable than cage and cage-dimer structures. For large clusters, a shell structure of Al60O90 is suggested. Electronic properties and calculations on hydrogen adsorption of these aluminum oxide structures are reported, and we discuss their possible use as hydrogen storage materials.     

Theoretical study on AlnO2 (n = 1–10) clusters and O2 adsorption on the Al(111) surface

The structural properties of neutral and ionic Al_nO₂ (n = 1–10) clusters have been systematically investigated using the density functional method B3LYP with a standard 6‐311+G(d) basis set. The calculated results show that in the Al_nO, Al_nO₂, and Al_nO (n ≥ 3) clusters, O atoms tend to penetrate into the aluminum clusters with some Al atoms moving outward. The binding energies and natural charges populations indicate that the oxygen‐etching is generally stronger in the order Al < Al_n < Al for n < 3, and Al > Al_n > Al for n ≥ 3. To further understand the mechanism of interaction between Al and O₂, the adsorption of O₂ on the Al(111) surface was studied using the density functional theory with plane wave pseudopotential method. The calculated results are consistent with the experimental observation that the O₂ molecule would dissociate on the Al(111) surface and be adsorbed in adjacent hollow sites, forming a local structure of Al₃O–Al₃O. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical study of the adsorption of H on Si n clusters, (n=3-10)

A recently proposed local Fukui function is used to predict the binding site of atomic hydrogen on silicon clusters. To validate the predictions, an extensive search for the more stable SinH (n=3-10) clusters has been done using a modified genetic algorithm. In all cases, the isomer predicted by the Fukui function is found by the search, but it is not always the most stable one. It is discussed that in the cases where the geometrical structure of the bare silicon cluster suffers a considerable change due to the addition of one hydrogen atom, the situation is more complicated and the relaxation effects should be considered.     

Theoretical study of the geometries and dissociation energies of molecular water on neutral aluminum clusters Al(n) (n = 2-25)

Geometries and dissociation energies of water molecules on Al(n) (n = 2-25) clusters were investigated using density functional theory with all electron relativistic spin-polarized calculations under the generalized gradient approximation. An extensive structure search was performed to identify the low-energy conformations of Al(n)H(2)O complexes for each size. Optimal adsorption sites were assigned for low-energy isomers of the clusters. Size and site specific dependences were studied for the Al(n)H(2)O complexes in stabilities, geometries, adsorption energies, dissociation energies, Al-O bond lengths, and other characteristic quantities. The stabilities and geometries revealed that H atom in H(2)O is not inclined to bond with Al atoms. The most stable Al(n)H(2)O configurations for each size tend to correspond to the most stable bare Al(n) cluster except of Al(6) and Al(24) clusters. The HO bond lengths increase generally 0.01 ? with respect to the isolated H(2)O in all of the adsorption complexes. The dissociation energy of an isolated H(2)O into HO and H was 5.39 eV, which decreased about two-thirds to the energy range of 0.83-2.12 eV with the help of Al(n) clusters. In spite of the fluctuations, the dissociation energies of Al(n)H(2)O complexes rise with the size increasing as a whole. In addition, we also found that the bare Al(n) clusters with high vertical ionization potentials usually have high dissociation energies of H(2)O in the corresponding adsorption models. The energetically preferred spin-multiplicity of all the odd-n Al(n)H(2)O complexes is doublet, and it is singlet for all the even-n complexes with exception of Al(2)H(2)O which is triplet.     

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.     

Theoretical study of mixed silicon-lithium clusters Si(n)Li(p)(+) (n=1-6, p=1-2)

Theoretical study on the structures of neutral and singly charged Si(n)Li(p)((+)) (n=1-6, p=1-2) clusters have been carried out in the framework of the density functional theory (DFT) with the B3LYP functional. The structures of the neutral Si(n)Li(p) and cationic Si(n)Li(p)(+) clusters are found to keep the frame of the corresponding Si(n), Li species being adsorbed at the surface. The localization of the lithium cation is not the same one as that of the neutral atom. The Li(+) ion is preferentially located on a Si atom, while the Li atom is preferentially attached at a bridge site. A clear parallelism between the structures of Si(n)Na(p) and those of Si(n)Li(p) appears. The population analysis show that the electronic structure of Si(n)Li(p) can be described as Si(n)(p)(-)+pLi(+) for the small sizes considered. Vertical and adiabatic ionization potentials, adsorption energies, as well as electric dipole moments and static dipolar polarizabilities, are calculated for each considered isomer of neutral species.     

TinC2n(n=1-6)分子簇的量子化学从头算研究

用量子化学从头计算方法研究了TinC2n(n=1-6)分子簇的几何构型和电子结构。这些TinC2n分子簇以TiC2为结构单元, 通过C-C或Ti-C键进一步连接而逐渐长大。研究结果可以较好地解释实验现象, 并说明Metcars的形成机理。     

Comparative ab initio study of CO adsorption on Sc(n) and Sc(n)O (n = 2-13) clusters

Using a cluster model, we investigated the similarities and differences in chemical activity and the magnetic properties of Sc(n) clusters (n = 2-13) and their oxides, Sc(n)O, toward CO molecule adsorption via a spin-polarized density functional theory approach. The Sc(n) and Sc(n)O clusters have similar chemical activity at small sizes of n = 2-10, whereas remarkable differences are observed at large sizes of n = 11-13. More interestingly, different magnetic responses are found in the Sc(n) and Sc(n)O clusters with the presence of CO molecule: The magnetic moment is attenuated significantly for Sc(n) with n = 2, 4, 12, and 13, whereas for Sc(n)O, it is enhanced at n = 4 and 13 and is reduced for n = 7, 8, 10, and 11. In particular, the magnetic moment remarkably increases from 7 μ(B) of Sc(13)O to 13 μ(B) of Sc(13)OCO, whereas it reduces from 19 μ(B) of Sc(13) to 5 μ(B) of Sc(13)CO.     

Electron scattering on OH-(H2O)n clusters (n = 0-4)

The cross sections for electron scattering on OH-(H2O)n for n = 0-4 were measured from threshold to approximately 50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV +/- 0.2 eV for OH- to 12.10 eV +/- 0.5 eV for OH-(H2O)4. For n > or = 1, we found that approximately 80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH-(H2O)3 and for OH-(H2O)4. The resonances were located at approximately 15 eV and were ascribed to the formation of dianions in excited states.     

Microhydration of Cs+ ion: a density functional theory study on Cs+-(H2O)n clusters (n=1-10)

Structure, energy enthalpy, and IR frequency of hydrated cesium ion clusters, Cs+-(H2O)n (n=1-10), are reported based on all electron calculations. Calculations have been carried out with a hybrid density functional, namely, Becke's three-parameter nonlocal hybrid exchange-correlation functional B3LYP applying cc-PVDZ correlated basis function for H and O atoms and a split valence 3-21G basis function for Cs atom. Geometry optimizations for all the cesium ion-water clusters have been carried out with several possible initial guess structures following Newton-Raphson procedure leading to many conformers close in energy. The calculated values of binding enthalpy obtained from present density functional based all electron calculations are in good agreement with the available measured data. Binding enthalpy profile of the hydrated clusters shows a saturation behavior indicating geometrical shell closing in hydrated structure. Significant shifts of O-H stretching bands with respect to free water molecule in IR spectra of hydrated clusters are observed in all the hydrated clusters.     

Structure and vibrations of Ga(n)N(n) (n = 3-10) clusters

The structure and harmonic vibrations of Ga(n)N(n) (n = 3-10) clusters have been investigated using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out of plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1029 cm(-1) for n = 10. Comparisons with C2n, B(n)N(n), and Al(n)N(n) clusters, the structure and bonding type for the Ga(n)N(n) (n=3-10) clusters are consistent with those of the C2n (n = 3, 5, 7, ...) clusters, the B(n)N(n) (n = 3-10), and Al(n)N(n) (n = 3-9) clusters.     

Density-functional study of structural and electronic properties of Si(n)C(n) (n=1-10) clusters

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the structural and electronic structure of Si(n)C(n) (n=1-10) clusters. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster size n equals 4. Cagelike structures are favored as the cluster size increases. A distinct segregation between the silicon and carbon atoms is observed for these clusters. It is found that the C atoms favor to form five-membered rings as the cluster size n increases. However, the growth motif for Si atoms is not observed. The Si(n)C(n) clusters at n=2, 6, and 9 are found to possess relatively higher stability. On the basis of the lowest-energy geometries obtained, the size dependence of cluster properties such as binding energy, HOMO-LUMO gap, Mulliken charge, vibrational spectrum, and ionization potential has been computed and analyzed. The bonding characteristics of the clusters are discussed.     

Computational study on the negative electron affinities of NO2 -.(H2O)n clusters (n=0-30)

Here we report negative electron affinities of NO(2)(-).(H2O)n clusters (n=0-30) obtained from density functional theory calculations and a simple correction to Koopmans' theorem. The method relies on the calculation of the detachment energy of the monoanion and its highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and explicit calculations on the dianion itself are avoided. A good agreement with resonances in the cross section for neutral production in electron scattering experiments is found for n=0, 1, and 2. We find several isomeric structures of NO(2)(-).(H2O)2 of similar energy that elucidate the interplay between water-water and ion-water interactions. The topology is predicted to influence the electron affinity by 0.5 and 0.4 eV for NO(2)(-).(H2O) and NO(2)(-).(H2O)2, respectively. The electron affinity of larger clusters is shown to follow a (n+delta)-1/3 dependence, where delta=3 represents the number of water molecules that in volume, could replace NO(2) (-).     

Structure and vibrations of BenOn (n=3-10) clusters

The structure and harmonic vibrations of Be(n)O(n) (n=3-10) clusters have been investigated using density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out of plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1597 cm(-1) for n=10. Comparisons with C(2n) clusters and B(n)N(n) clusters, the structure and bonding type for the Be(n)O(n) clusters are consistent with those of the C(2n) (n=3, 5, 7, ...) clusters and the B(n)N(n) clusters.     

Collision-induced dissociation studies of protonated ether--(H2O)n (n = 1-3) clusters

We have studied the protonated ether-(H2O)n (n = 1-3) complexes containing tetrahydrofuran, dimethyl, diethyl, dibutyl, and butylmethyl ethers using a flowing afterglow triple-quadrupole mass spectrometer. Collision-induced dissociation, CID, of all clusters with n = 1, 2 shows sequential water loss. The n = 3 cluster of dimethyl ether shows sequential water loss, while all other ether clusters display selective product formation. The CID spectra are interpreted based on known energetics, and theoretical studies of the dimethyl and diethyl ether systems.     

Electronic structure calculations of small Al n (n=2–8) clusters

The electronic states of small Al _n (n=2–8) clusters have been calculated with a relativistic ab-initio MO-LCAO Dirac-Fock-Slater method using numerical atomic DFS wave-functions. The excitation energies were obtained from a ground state calculation of neutral clusters, and in addition from negative clusters charged by half an electron in order to account for part of the relaxation. These energies are compared with experimental photo-electron spectra.     

Theoretical study of the electronic structure and stability of titanium dioxide clusters (TiO2)n with n = 1-9

The electronic structure and the stability of both neutral and singly charged (TiO2)n clusters with n = 1-9 have been investigated using the density functional B3LYP/LANL2DZ method. The lowest-lying singlet clusters tend to form some compact structures with one or two terminal Ti-O bonds, which are about 1.4-2.5 eV more stable than the corresponding triplet structures. For the lowest-lying structures, strong infrared absorption lines at 988-1020 cm(-1) due to terminal Ti-O bonds and below 930 cm(-1) due to Ti-O-Ti bridging bonds may be observed, with some characteristic lines at 530-760 cm(-1) due to 3-fold coordinated O-atoms that are comparable with the spectra of rutile and anatase bulk. The holes and excited electrons within triplet structures tend to be localized on the least coordinated O- and Ti-atoms, respectively, with some exceptions possibly due to the electron-hole interaction. The extra electrons within (TiO2)n- clusters and the holes within (TiO2)n+ clusters show a clearer preference of location on the least coordinated Ti- and O-atoms, respectively. For the lowest-lying (TiO2)n clusters, the cluster formation energy per TiO2 unit and the electron affinity tend to increase whereas the ionization potential tends to decrease with the cluster size n. On the other hand, the singlet-triplet and HOMO-LUMO gaps represent the lower and upper limits of the TiO2 bulk band gaps, respectively. The theoretical results agree well with the available experimental data and may be helpful for understanding the chemistry of small (TiO2)n clusters.     

Optical properties of (GaAs)n clusters (n = 2-16)

The electronic and geometrical structures of the lowest triplet states of (GaAs) n clusters ( n = 2-16) are studied using density functional theory with generalized gradient approximation (DFT-GGA). It is found that the triplet-state geometries are different from the corresponding singlet-state geometries; for n = 2-8, 10, and 11, the triplets and singlets have different topologies, while the (GaAs) 9, (GaAs) 12, (GaAs) 15, and (GaAs) 16 triplets possess a reduced symmetry, due to Jahn-Teller distortions. Except for GaAs, the singlet states are the ground states. Excitation energies and oscillator strengths are computed for excitations from the ground state to ten singlet states of all (GaAs) n clusters using time-dependent density functional theory. The adiabatic singlet-triplet gap is compared to the vertical gap, and the difference in the eigenvalues of the highest-occupied and lowest-unoccupied molecular orbitals (the HOMO-LUMO gap). While these three values show large oscillations for small n, they approach each other as the cluster size grows. Thus, the HOMO-LUMO gap computed using the DFT-GGA approach presents a rather reliable estimate of the adiabatic singlet-triplet gap.     

Geometrical and electronic properties of neutral and charged cesium clusters Cs(n) (n=2-10): a theoretical study

We have investigated the structure and electronic properties of cesium clusters following all electron ab initio theoretical methods based on configuration interaction, second-order Moller-Plesset (MP2) perturbation theory, and density-functional theory. Becke's three-parameter nonlocal hybrid exchange-correlation functional (B3LYP) is found to perform best on the present systems with a split valence 3-21G basis function. We have calculated the optimized geometries of neutral and singly charged cesium clusters having up to ten atoms, their binding energy per atom, ionization potentials (IPs), and adiabatic electron affinity (EA). Geometry optimizations for all the clusters are carried out without imposing any symmetry restriction. The neutral clusters having up to six atoms prefer planar structure and three-dimensional structure is preferred only when the number of atoms in a cluster is more than six. There is a good agreement between the present theoretical and reported experimental IP values for the neutral clusters with cluster size n相似文献   

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.