Size- and shape-dependent polarizabilities of sandwich and rice-ball Co(n)Bz(m) clusters from density functional theory

The dipole polarizabilities of Co(n)Bz(m), (n, m = 1-4, m = n, n + 1) clusters are studied by means of an all-electron gradient-corrected density functional theory and finite field method. The dipole moments are relatively large for most of the clusters, implying their asymmetric structures. The total polarizability increases rapidly as cluster size, whereas the average polarizability shows "odd-even" oscillation with relatively large values at (n, n + 1). The polarizabilities exhibit clear shape-dependent variation, and the sandwich structures have systematically larger polarizability and anisotropy than the rice-ball isomers. The dipole polarizabilities are further analyzed in terms of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, ionization potential, and electron delocalization volume. We conclude that the polarizability variations are determined by the interplay between the geometrical and electronic properties of the clusters.     

Computational study of multiple-decker sandwich and rice-ball structures of neutral titanium-benzene clusters

Density functional theory calculations were applied to systematically and directly compare the relative energetic stability of multiple-decker sandwich and rice-ball structures for a variety of neutral Ti(m)Bz(n) clusters (m = 1-4, n = 1-5). Almost all structures favored the multiple-decker sandwich structure, as observed experimentally for early transition metals. The strength of each metal-benzene interaction averages 37 kcal/mol and remains relatively constant for sandwiches with three or more Ti atoms. The most stable smaller rice-ball structures did not have eta(6)-Bz bound to a single metal atom. Instead, the preferred coordination was having the plane of the benzene molecule parallel to a Ti(2) bond or a Ti(3) face, leading to some distortion of the benzene ring. The larger rice-ball structures, on the other hand, preferred to weaken the metal-metal bonds and retain eta(6)-Bz bound to a single metal atom, a structural motif shared with sandwiches.     

Photoelectron spectroscopy of nickel-benzene cluster anions

(Nickel)(n)(benzene)(m) (-) cluster anions were studied by both mass spectrometry and anion photoelectron spectroscopy. Only Ni(n)(Bz)(m) (-) species for which n > or =m were observed in the mass spectra. No single-nickel Ni(1)(Bz)(m) (-) species were seen. Adiabatic electron affinities, vertical detachment energies, and second transition energies were determined for (n,m)=(2,1), (2,2), (3,1), and (3,2). For the most part, calculations on Ni(n)(Bz)(m) (-) species by B. K. Rao and P. Jena [J. Chem. Phys. 117, 5234 (2002)] were found to be consistent with our results. The synergy between their calculations and our experiment provided enhanced confidence in the theoretically implied magnetic moments of several nickel-benzene complexes. The magnetic moments of small nickel clusters were seen to be extremely sensitive to immediate molecular environmental effects.     

Con(n=2～10)团簇的结构和磁性

采用密度泛函理论中的局域自旋密度近似(LSDA)和广义梯度近似(GGA)对Con(n=2～10)团簇的几何构型进行优化，并对能量、频率和磁性进行了计算,两种方法确定的基态构型完全一致,并从平均键长、平均配位数和对称性对磁性的影响进行了理论探讨.研究表明, Con(n=2～10)基态团簇的磁性在n=2～4时主要受平均键长的影响,在n=5～9时主要受平均配位数的影响,在n=10时受原子间距和平均配位数的相互影响,最终导致与Co8基态团簇具有相同的磁性.基态团簇在Co5和Co9出现了磁性局域最小点.     

Stern-Gerlach experiments of one-dimensional metal-benzene sandwich clusters: Mn(C6H6)m (M = Al, Sc, Ti, and V)

A molecular beam of multilayer metal-benzene organometallic clusters Mn(C6H6)m (M = Al, Sc, Ti, and V) was produced by a laser vaporization synthesis method, and their magnetic deflections were measured. Multidecker sandwich clusters of transition-metal atoms and benzene Scn(C6H6)n+1 (n = 1, 2) and Vn(C6H6)n+1 (n = 1-4) possess magnetic moments that increase monotonously with n. The magnetic moments of Al(C6H6), Scn(C6H6)n+1, and Vn(C6H6)n+1 are smaller than that of their spin-only values as a result of intracluster spin relaxation, an effect that depends on the orbital angular momenta and bonding characters of the orbitals containing electron spin. While Ti(C6H6)2 was found to be nonmagnetic, Tin(C6H6)n+1 (n = 2, 3) possess nonzero magnetic moments. The mechanism of ferromagnetic spin ordering in M2(C6H6)3 (M = Sc, Ti, V) is discussed qualitatively in terms of molecular orbital analysis. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition-metal atoms are formally zerovalent.     

(CoMn)n(n=1～5)合金团簇的结构和磁性

All geometry structures of (CoMn)n (n=1-5) clusters were optimized, and the energy, frequence and magnetism of (CoMn)n (n=1-5) clusters were calculated by using the local spin density approximation and generalized gradient approximation of density functional theory. The same ground state structures of CoMn alloy clusters were confirmed in two methods, and magnetism of CoMn alloy ground state clusters was studied systemically. In order to understand structure and magnetism of CoMn alloy clusters better, Co2n (n=1-5) and Mn2n (n=1-5) clusters were calculated by the same method as alloy clusters, whose ground state structure and magnetism were confirmed. Moreover, the ground state structure and magnetism of clusters with the corresponding CoMn alloy clusters was compared. Results indicated that for (CoMn)n (n=1-4) clusters, geometry structures of CoMn alloy clusters are the same as the corresponding pure clusters still, (CoMn)3 and (CoMn)4 exhibit magnetic bistability, show ferromagnetic and anti-ferromagnetic coupling, local magnetic moment of Co, Mn atoms in CoMn alloy clusters almost preserves magnetism of pure clusters still.     

Geometry and electronic structure of Vn(Bz)m complexes

First-principles calculations based on the generalized gradient approximation to the density functional theory are performed to explore the global geometries, ground-state spin multiplicities, relative stabilities, and energetics of neutral and anionic V(n)(Bz)(m) (n=1-3, m=1-4, with n相似文献   

Structure, bonding, and magnetism in manganese clusters

We investigate the structures and magnetic properties of small Mn(n) clusters in the size range of 2-13 atoms using first-principles density functional theory. We arrive at the lowest energy structures for clusters in this size range by simultaneously optimizing the cluster geometries, total spins, and relative orientations of individual atomic moments. The results for the net magnetic moments for the optimal clusters are in good agreement with experiment. The magnetic behavior of Mn(n) clusters in the size range studied in this work ranges from ferromagnetic ordering (large net cluster moment) for the smallest (n=2, 3, and 4) clusters to a near degeneracy between ferromagnetic and antiferromagnetic solutions in the vicinity of n=5 and 6 to a clear preference for antiferromagnetic (small net cluster moment) ordering at n=7 and beyond. We study the details of this evolution and present a picture in which bonding in these clusters predominantly occurs due to a transfer of electrons from antibonding 4s levels to minority 3d levels.     

Stern-gerlach study of multidecker lanthanide-cyclooctatetraene sandwich clusters

Multilayer lanthanide-cyclooctatetraene organometallic clusters, Lnn(C8H8)m (Ln = Eu, Tb, Ho, Tm; n = 1-7; m = n - 1, n, n + 1) were produced by a laser vaporization synthesis method. The magnetic deflections of these organometallic sandwich clusters were measured by a molecular beam magnetic deflection technique. Most of the sandwich species displayed one-sided deflection, while some of smaller Ln-C8H8 clusters showed symmetric broadening without or with only very small (or absent) net high-field deflection. In general, the total magnetic moments, calculated from the magnitude of the beams deflections, increase with the number of lanthanide atoms (i.e., with increasing sandwich layers); however for Tb-, Ho-, and Tm-C8H8 clusters with n > 3, the suppression of the magnetic moments was observed, possibly through antiferromagnetic interactions. For Eu-C8H8 clusters, we observe a linear increase of the magnetic moments with the number of Eu atoms up to n = 7, with average magnetic moment per Eu atom around 7 muB--similar to that displayed by conventionally synthesized mononuclear EuIIC8H8 complexes, indicating that Eu atoms exist as Eu2+ ions in the full sandwich Eun(C8H8)n+1 clusters. These results suggest that Eun(C8H8)n+1 is a promising candidate for a high-spin, one-dimensional building block in organometallic magnetic materials.     

Probing the structural and magnetic properties of transition metal-benzene anion complexes

Two types of transition metal-benzene anion complexes, (titanium)(n)(benzene)(m)? and (cobalt)(n)(benzene)(m)? (n ≤ 2, m ≤ 3) have been determined using density functional theory. The photoelectron spectra of Ti(n)Bz(m)? and Co(n)Bz(m)? (n ≤ 2, m ≤ 3) were discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of several low-energy isomers obtained by the structural optimization procedure. The binding of Ti and Co atoms to benzene molecules is accounted by 3d-π bonds, as revealed by the molecular orbitals. The topology of the electronic density has been analyzed, suggesting that the C-C bonds were weakened in the transition metal-benzene complexes in comparison to those in free benzene. Spin density distribution results show the spin densities for Ti(n)Bz(m)? and Co(n)Bz(m)? (n ≤ 2, m ≤ 3) reside mainly on the metal Ti and Co centers (70%-90%). A shift to lower magnetic moment with respect to the pure titanium/cobalt cluster anions indicates the solvent benzene molecule acts to demagnetize the bare titanium/cobalt cluster anions.     

Electronic properties of hydrogen-bonded complexes of benzene(HCN)(1-4): comparison with benzene(H2O)(1-4)

The electronic properties, specifically, the dipole and quadrupole moments and the ionization energies of benzene (Bz) and hydrogen cyanide (HCN), and the respective binding energies, of complexes of Bz(HCN)(1-4), have been studied through MP2 and OVGF calculations. The results are compared with the properties of benzene-water complexes, Bz(H(2)O)(1-4), with the purpose of analyzing the electronic properties of microsolvated benzene, with respect to the strength of the CH/π and OH/π hydrogen-bond (H-bond) interactions. The linear HCN chains have the singular ability to interact with the aromatic ring, preserving the symmetry of the latter. A blue shift of the first vertical ionization energies (IEs) of benzene is observed for the linear Bz(HCN)(1-4) clusters, which increases with the length of the chain. NBO analysis indicates that the increase of the IE with the number of HCN molecules is related to a strengthening of the CH/π H-bond, driven by cooperative effects, increasing the acidity of the hydrogen cyanide H atom involved in the π H-bond. The longer HCN chains (n ≥ 3), however, can bend to form CH/N H-bonds with the Bz H atoms. These cyclic structures are found to be slightly more stable than their linear counterparts. For the nonlinear Bz(HCN)(3-4) and Bz(H(2)O)(2-4) complexes, an increase of the binding energy with the number of solvent molecules and a decrease of the IE of benzene, relative to the values for the Bz(HCN) and Bz(H(2)O) complexes, respectively, are observed. Although a strengthening of the CH/π and OH/π H-bonds, with increasing n, also takes place for the Bz(H(2)O)(2-4) and Bz(HCN)(3-4) nonlinear complexes, Bz proton donor, CH/O, and CH/N interactions are at the origin of this decrease. Thus CH/π and OH/π H-bonds lead to higher IEs of Bz, whereas the weaker CH/N and CH/O H-bond interactions have the opposite effect. The present results emphasize the importance of both aromatic XH/π (X = C, O) and CH/X (X = N, O) interactions for understanding the structure and electronic properties of Bz(HCN)(n) and Bz(H(2)O)(n) complexes.     

Low-energy isomer identification, structural evolution, and magnetic properties in manganese-doped gold clusters MnAu(n) (n = 1-16)

The size-dependent electronic, structural, and magnetic properties of Mn-doped gold clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. A number of new isomers are obtained for neutral MnAu(n) (n = 1-16) clusters to probe the structural evolution. The two-dimensional (2D) to three-dimensional (3D) transition occurs in the size range n = 7-10 with manifest structure competitions. From size n = 13 to n = 16, the MnAu(n) prefers a gold cage structure with Mn atom locating at the center. The relative stabilities of the ground-state MnAu(n) clusters show a pronounced odd-even oscillation with the number of Au atoms. The magnetic moments of MnAu(n) clusters vary from 3 μ(B) to 6 μ(B) with the different cluster size, suggesting that nonmagnetic Au(n) clusters can serve as a flexible host to tailor the dopant's magnetism, which has potential applications in new nanomaterials with tunable magnetic properties.     

Strong spin-orbit effects in small Pt clusters: geometric structure, magnetic isomers and anisotropy

Ab initio density functional calculations including spin-orbit coupling (SOC) have been performed for Pt(n), n = 2-6 clusters. The strong SOC tends to stabilize planar structures for n = 2-5, whereas for clusters consisting of six atoms, three-dimensional structures remain preferred. SOC leads to the formation of large orbital magnetic moments and to a mixing of different spin states. Due to the spin-mixing the total magnetic moment may be larger or smaller than the spin moment in the absence of SOC. Both spin and orbital moments are found to be anisotropic. Because of the strong SOC the energy differences between coexisting magnetic isomers can be comparable to or even smaller than their magnetic anisotropy energies. In this case the lowest barrier for magnetization reversal can be determined by a magnetic isomer which is different from the ground state configuration.     

Stepwise hydration of ionized aromatics. Energies, structures of the hydrated benzene cation, and the mechanism of deprotonation reactions

The stepwise binding energies (DeltaHdegree(n-1,n)) of 1-8 water molecules to benzene(.+) [Bz(.+)(H2O)n] were determined by equilibrium measurements using an ion mobility cell. The stepwise hydration energies, DeltaHdegree(n-1,n), are nearly constant at 8.5 +/- 1 kcal mol-1 from n = 1-6. Calculations show that in the n = 1-4 clusters, the benzene(.+) ion retains over 90% of the charge, and it is extremely solvated, that is, hydrogen bonded to an (H2O)n cluster. The binding energies and entropies are larger in the n = 7 and 8 clusters, suggesting cyclic or cage-like water structures. The concentration of the n = 3 cluster is always small, suggesting that deprotonation depletes this ion, consistent with the thermochemistry since associative deprotonation Bz(.+)(H2O)(n-1) + H2O-->C6H5. + (H2O)nH+ is thermoneutral or exothermic for n > or = 4. Associative intracluster proton transfer Bz(.+)(H2O)(n+1) + H2O-->C6H5.(H2O)nH+ would also be exothermic for n > or = 4, but lack of H/D exchange with D2O shows that the proton remains on C6H6(.+) in the observed Bz(.+)(H2O)n clusters. This suggests a barrier to intracluster proton transfer, and as a result, the [Bz(.+)(H2O)n]* activated complexes either undergo dissociative proton transfer, resulting in deprotonation and generation of (H2O)nH+, or become stabilized. The rate constant for the deprotonation reaction shows a uniquely large negative temperature coefficient of K = cT(-67+/-4) (or activation energy of -34+/- 1 kcal mol-1), caused by a multibody mechanism in which five or more components need to be assembled for the reaction.     

Joint photoelectron and theoretical study of (Rh(m)Co(n))⁻ (m=1-5, n=1-2) cluster anions and their neutral counterparts

Anion photoelectron spectroscopic experiments and calculations based on density functional theory have been used to investigate and uniquely identify the structural, electronic, and magnetic properties of both neutral and anionic (Rh(m)Co(n)) and (Rh(m)Co(n))(-) (m=1-5, n=1-2) clusters, respectively. Negative ion photoelectron spectra are presented for electron binding energies up to 3.493 eV. The calculated electron affinities and vertical detachment energies are in good agreement with the measured values. Computational results for geometric structures and magnetic moments of both cluster anions and their neutrals are presented.     

Electronic and magnetic properties of manganese and iron-doped Ga(n)As(n) nanocages (n=7-12)

The electronic and magnetic properties of Mn- or Fe-doped Ga(n)As(n) (n=7-12) nanocages were studied using gradient-corrected density-functional theory considering doping at substitutional, endohedral, and exohedral sites. When doped with one atom, the most energetically favorable site gradually moves from surface (n=7-11) to interior (n=12) sites for the Mn atom, while the most preferred doping site of the Fe atom alternates between the surface (n=7,9,11) and interior (n=8,10,12) sites. All of the ground-state structures of Mn@Ga(n)As(n) have the atomlike magnetic moment of 5mu(B), while the total magnetic moments of the most stable Fe@Ga(n)As(n) cages for each size are about 2mu(B) except for the 4mu(B) magnetic moment of Fe@Ga(12)As(12). Charge transfer and hybridization between the 4s and 3d states of Mn or Fe and the 4s and 4p states of As were found. The antiferromagnetic (AFM) state of Mn(2)@Ga(n)As(n) is more energetically favorable than the ferromagnetic (FM) state. However, for Fe(2)@Ga(n)As(n) the FM state is more stable than the AFM state. The local magnetic moments of Mn and Fe atoms in the Ga(n)As(n) cages are about 4mu(B) and 3mu(B) in the FM and AFM states, respectively. For both Mn and Fe bidoping, the most energetically favorable doping sites of the transition metal atoms are located on the surface of the Ga(n)As(n) cages. The computed magnetic moments of the doped Fe and Mn atoms agree excellently with the theoretical and experimental values in the Fe(Mn)GaAs interface as well as (Ga, Mn)As dilute magnetic semiconductors.     

Magnetic structure variation in manganese-oxide clusters

Negative-ion photoelectron spectroscopy and ab initio simulations are used to study the variation in magnetic structure in Mn(x)O(y) (x = 3, 4[semicolon] y = 1, 2) clusters. The ferrimagnetic and antiferromagnetic ground-state structures of Mn(x)O(y) are 0.16-1.20 eV lower in energy than their ferromagnetic isomers. The presence of oxygen thus stabilizes low-spin isomers relative to the preferred high-spin ordering of bare Mn(3) and Mn(4). Each cluster has a preferred overall magnetic moment, and no evidence is seen of competing states with different spin multiplicities. However, non-degenerate isomags, which possess the same spin multiplicity but different arrangements of local moments, do contribute additional features and peak broadening in the photoelectron spectra. Proper accounting for all possible isomags is shown to be critical for accurate computational prediction of the spectra.     

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.     

原子簇FenB2(n=1～6)结构和性质的DFT研究

本文对上百个Fe_nB₂和Fe_n(n=1～6)原子簇模型进行密度泛函理论计算，用来模拟非晶态合金Fe-B体系的局域结构，并考察类金属元素硼的引入对体系性质的影响。将优化构型的键长和电荷分布与实验数值进行比较，发现本文所使用的原子簇模型在一定程度上可以真实、准确地反映非晶态合金Fe-B体系的局域结构。利用这些构型，我们对其键级、电子、催化以及磁学性质进行了讨论。结果表明：原子簇中均存在着强烈的Fe-B键作用，其中在高铁含量原子簇中Fe-Fe键的作用也较为明显；综合热力学、费米能级及态密度的研究结果，发现原子簇Fe₄B₂在合成氨和固氮过程中有可能表现出更为优越的催化活性。结合对原子簇Fe_n和Fe_nB₂(n=1～6)平均3d轨道布居数的分析，发现原子簇Fe_nB₂(n=1～6)的平均磁矩均小于相应原子簇Fe_n(n=1～6)的理论数值和纯金属铁的实验数值(5.7～6.0 BM)，也就是说原子簇Fe_nB₂(n=1～6)均表现出软磁性。     

FMBen（FM=Fe,Co,Ni;n=1-12）团簇结构和电子性质

采用密度泛函理论中的广义梯度近似对FMBen(FM=Fe,Co,Ni;n=1-12)团簇的几何构型进行优化,并对能量、频率和磁性进行了计算,同时考虑了电子的自旋多重度.结果表明,纯铍团簇的幻数是由电子的壳层模型确定,而FMBen团簇的幻数主要由几何效应来解释;掺杂铁磁性的过渡金属(Fe,Co,Ni)提高了纯团簇的稳定性.二阶能量差分表明FMBen(FM=Fe,Co,Ni)的幻数分别为5,10;5,10;4,10.通过对磁性质的研究发现掺杂不同的过渡金属时,磁矩出现了不同的变化规律.