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.     

Structures and energetics of electrosprayed uracil(n)Ca2+ clusters (n = 14-4) in the gas phase

Clusters of uracil (U) about a calcium dication, U(n)Ca(2+) (n = 14-4), have been studied in the gas phase by both experimental and theoretical methods. Temperature dependent blackbody infrared radiative dissociation (BIRD) experiments were performed on U(n)Ca(2+) clusters with n = 14-5 and the observed Arrhenius parameters are reported here. Master equation modeling of the BIRD kinetics data was carried out to determine threshold dissociation energies. Initial geometry calculations were performed using the B3LYP density functional and 3-21G(d) basis set. A sample of ten conformations per cluster was obtained through a simulated annealing study. These structures were optimized using B3LYP/6-31G(d) level of theory. Fragment-based hybrid many body interaction (HMBI) MP2/6-311++G(2df,2p)/Amoeba calculations were performed on representative conformations to determine theoretical binding energies. Results were examined in relation to cluster size (n). A significant increase in the energy required to remove uracil from U(6)Ca(2+) when compared to larger clusters supports previous reports that the calcium ion is coordinated by six uracil molecules in the formation of an inner shell. For clusters larger than n = 6, an odd-even alternation in threshold dissociation energies was observed, suggesting that the outer shell uracil molecules bind as dimers to the inner core. Proposed binding schemes are presented. Multiple structures of U(5)Ca(2+) are suggested as being present in the gas phase where the fifth uracil may be either part of the first or second solvation shell.     

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.     

Molecular structures and energetics of the (TiO2)n (n = 1-4) clusters and their anions

The (TiO2)n clusters and their anions for n = 1-4 have been studied with coupled cluster theory [CCSD(T)] and density functional theory (DFT). For n > 1, numerous conformations are located for both the neutral and anionic clusters, and their relative energies are calculated at both the DFT and CCSD(T) levels. The CCSD(T) energies are extrapolated to the complete basis set limit for the monomer and dimer and calculated up to the triple-zeta level for the trimer and tetramer. The adiabatic and vertical electron detachment energies of the anionic clusters to the ground and first excited states of the neutral clusters are calculated at both levels and compared with the experimental results. The comparison allows for the definitive assignment of the ground-state structures of the anionic clusters. Anions of the dimer and tetramer are found to have very closely lying conformations within 2 kcal/mol at the CCSD(T) level, whereas that of the trimer does not. In addition, accurate clustering energies and heats of formation are calculated for the neutral clusters and compared with the available experimental data. Estimates of the titanium-oxygen bond energies show that they are stronger than the group VIB transition metal-oxygen bonds except for tungsten. The atomization energies of these clusters display much stronger basis set dependence than the clustering energies. This allows the calculation of more accurate heats of formation for larger clusters on the basis of calculated clustering energies.     

Study of the structure, energetics, and vibrational properties of small ammonia clusters (NH3)n (n = 2-5) using correlated ab initio methods

Equilibrium geometries, interaction energies, and harmonic frequencies of (NH3)n isomers (n = 2-5) have been computed using correlated calculations (MP2) in conjunction with Dunning's aug-cc-pVXZ (X = D, T, Q) basis sets and the Counterpoise procedure. Whenever available, literature values for the binding energy and geometry of dimers and trimers agree well with our data. Low lying isomers for (NH3)4 and (NH3)5 have been found to have similar binding energies (roughly 16 and 20 kcal/mol for the tetramer and pentamer, respectively), perhaps suggesting the presence of a very smooth energy landscape. Using BSSE corrected forces or freezing the monomer structure to its gas phase geometry have been found to have only a weak impact on the energetic and structural properties of the clusters. The effect of zero-point energy (ZPE) on the relative stability of the clusters has been estimated using harmonic frequencies. The latter also highlighted the presence of vibrational fingerprints for the presence of double acceptor ammonia molecules. Many-body effects for (NH3)n isomers (n = 2-4) have been investigated to explore the possibility of building a pairwise interaction model for ammonia. In the frame of the work presented, we have found the 3-body effect to account for 10-15% of the total interaction energy, whereas the 4-body effects may be neglected as first approximation.     

Fragmentation characteristics of b(n) (n=2-15) ions from protonated peptides

The fragmentation reactions of protonated oligoalanines (trialanine, tetraalanine and pentaalanine) and the fragments present in the electrospray ionization (ESI) mass spectrum of polyalanine have been studied by collisionally activated dissociation (CAD) mass spectrometry (MS(2) and MS(3) experiments). The MS(n) experiments provided strong evidence that the m/z 71n+1 ion series in the ESI mass spectrum of polyalanine is a b(n) series. These ions are formed via the b(n) -y(m) pathway of amide bond cleavage, which results in the formation of a proton-bound complex of an oxazolone and a peptide/amino acid. Also, the MS(2) spectra of the b(n) series from polyalanine revealed that the chain length of b(n) ions influences significantly the dissociations taking place. For example, b(n) ions start losing H(2)O at n ≥5 and the losses of CO and CO+NH(3) decrease in intensity from b(2) to b(15). The elimination of H(2)O+NH(3) and the elimination of 61 mass (HN=C=O+H(2)O) commence with b(6); their abundances initially increase up to ~ b(8)-b(9) and then gradually decrease until b(15) (largest fragment studied). The tandem mass spectrometry experiments help to elucidate the dissociation mechanisms of the observed structures of biopolymer fragments.     

Structure and stability for (AlN) n + and (AlN) n + (n=1–15) clusters

Using density functional theory (DFT) method with 6-31G* basis set, we have carried out the optimizing calculation of geometry, vibrational frequency and thermodynamical stability for (AlN) _n ⁺ and (AlN) _n ⁺ (n=1–15) clusters. Moreover, their ionic potential (IP) and electron affinity (EA) were discussed. The results show that the electrical charge condition of the cluster has a relatively great impact on the structure of the cluster and with the increase of n, this kind of impact is reduced gradually. There are no Al-Al and N-N bonds in the stable structure of (AlN) _n ⁺ or (AlN) _n ^-, and the Al-N bond is the sole bond type. The magic number regularity of (AlN) _n ⁺ and (AlN) _n ^- is consistent with that for (AlN) _n , indicating that the structure with even n such as 2, 4, 6, ... is more stable. In addition, (AlN₁₀ has the maximal ionization power (9.14 eV) and the minimal electron affinity energy (0.19 eV), which manifests that (AlN)₁₀ is more stable than other clusters.     

An ab initio study of the formation and structure of NO+-(N2)n (n = 1 and 2) clusters

The geometries and clustering energies of the NO⁺ (N₂)_n (n = 1 and 2) clusters have been determined by ab initio calculations at different levels. At the MP4SDQ/6-31G^* level, plus zero-point vibrational corrections making use of the HF/4-31G-optimized geometries, the energy differences between clusters and their relative fragments have been calculated to be −5.3 and −4.1 kcal/mol for n = 1 and n = 2, respectively. These values can be compared to the experimental enthalpies of clustering of −4.4 and −3.9 kcal/mol. The interaction between NO⁺ and N₂ and the structures of two stable cluster ions have also been examined.     

Structure and properties of Mn(n), Mn(n)-, and Mn(n)+ clusters (n = 3-10)

Electronic and geometrical structures of Mn(3)-Mn(10) together with their singly negatively and positively charged ions are computed using density functional theory with generalized gradient approximation. The ground-state spin multiplicities in the neutral series are 16, 21, 4, 9, 6, 5, 2, and 5, for Mn(3)-Mn(10), respectively. Thus, there is a transition from a ferromagnetic ground state to a ferrimagnetic ground state at Mn(5). The energy difference between ferrimagnetic and ferromagnetic states in Mn(n) grows rapidly with increasing n and exceeds 2 eV in Mn(10). The corresponding change from ferro- to ferrimagnetic ground state occurs at Mn(6)(-) and Mn(3)(+) in the anionic and cationic series, respectively. Beginning with Mn(6), the ion spin multiplicities differ from that of the neutral by +/-1 (i.e., they obey the empirical "+/-1 rule"). We found that the energy required to remove an Mn atom is nearly independent of the charge state of an Mn(n) cluster and the number of atoms in the cluster, except for Mn(3). The results of our calculations are in reasonable agreement with experiment, except for the experimental data on the magnetic moments per atom, where, in general, we predict smaller values than the experiment.     

Photoelectron spectroscopic study of the hydrated nucleoside anions: Uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1)

The hydrated nucleoside anions, uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1), have been prepared in beams and studied by anion photoelectron spectroscopy in order to investigate the effects of a microhydrated environment on parent nucleoside anions. Vertical detachment energies (VDEs) were measured for all eight anions, and from these, estimates were made for five sequential anion hydration energies. Excellent agreement was found between our measured VDE value for thymidine(-)(H(2)O)(1) and its calculated value in the companion article by S. Kim and H. F. Schaefer III.     

Structure and properties of the aluminum borates Al(BO2)n and Al(BO2)n(-), (n = 1-4)

The geometrical and electronic structures of Al(BO(2))(n) and Al(BO(2))(n)(-) (n = 1-4) clusters are computed at different levels of theory including density functional theory (DFT), hybrid DFT, double-hybrid DFT, and second-order perturbation theory. All aluminum borates are found to be quite stable toward the BO(2) and BO(2)(-) loss in the neutral and anion series, respectively. Al(BO(2))(4) belongs to the class of hyperhalogens composed of smaller superhalogens, and should possess a large adiabatic electron affinity (EA(ad)) larger than that of its superhalogen building block BO(2). Indeed, the aluminum tetraborate possesses the EA(ad) of 5.6 eV, which, however, is smaller than the EA(ad) of 7.8 eV of the AlF(4) supehalogen despite BO(2) is more electronegative than F. The EA(ad) decrease in Al(BO(2))(4) is due to the higher thermodynamic stability of Al(BO(2))(4) compared to that of AlF(4). Because of its high EA and thermodynamic stability, Al(BO(2))(4) should be capable of forming salts with electropositive counter ions. We optimized KAl(BO(2))(4) as corresponding to a unit cell of a hypothetical KAl(BO(2))(4) salt and found that specific energy and energy density of such a salt are competitive with those of trinitrotoluol (TNT).     

Valence-state alternatives in diastereoisomeric complexes [(acac)2Ru(mu-QL)Ru(acac)2]n (QL2- = 1,4-dioxido-9,10-anthraquinone,n = +2, +1, 0, -1, -2)

The title complexes were obtained in neutral form (n = 0) as rac (1) and meso isomers (2). 2 was crystallized for X-ray diffraction and its temperature-dependent magnetism studied. It contains two antiferromagnetically coupled ruthenium(III) ions, bridged by the quinizarine dianion QL(2-) (quinizarine = 1,4-dihydroxy-9,10-anthraquinone). The potential of both the ligand (QLo --> QL4-) and the metal complex fragment combination [(acac)2RuII]2 --> ([(acac)2RuIV]2)4+ to exist in five different redox states creates a large variety of combinations, which was assessed for the electrochemically reversibly accessible 2+, 1+, 0, 1-, 2- forms using cyclic voltammetry as well as EPR and UV-vis-NIR spectroelectrochemistry. The results for the two isomers are similar: Oxidation to 1+ or 2+ causes the emergence of a near-infrared band (1390 nm), without revealing an EPR response even at 4 K. Reduction to 1- or 2- produces an EPR signal, signifying metal-centered spin but no near-infrared absorption. Tentatively, we assume metal-based oxidation of [(acac)2RuIII(mu-QL2-)RuIII(acac)2] to a mixed-valent intermediate [(acac)2RuIII(mu-QL2-)RuIV(acac)2]+ and ligand-centered reduction to a radical complex [(acac)2RuIII(mu-QL.3-)RuIII(acac)2 (-) with antiferromagnetic three-spin interaction.     

LaC4^n（n=—2,—1,0,＋1,＋2）分子簇的结构与稳定性

本文用密度泛函方法研究了ＬａＣ４ｎ（ｎ＝－２,－１,０,＋１,＋２）分子簇的结构与稳定性。振动频率分析表明,在所提出的九个构型中,当ｎ＝－２,０,＋１,＋２时,稀土位于碳环上最稳定,而当ｎ＝－１时,尽管稀土位于碳环上能量最低,但没有找到稳定的构型,我们的结果还指出,稀土元素是分子簇中对外部环境最敏感的部位,即最具有反应活性     

Ion core structure in (C(2)n+ and (CS2)n- (n=3-10) studied by infrared photodissociation spectroscopy

Infrared photodissociation spectra of (CS(2))(n) (+) and (CS(2))(n) (-) with n=3-10 are measured in the 1100-2000 cm(-1) region. All the (CS(2))(n) (+) clusters exhibit three bands at approximately 1410, approximately 1490, and approximately 1540 cm(-1). The intensity of the 1540 cm(-1) band relative to those of the other bands increases with increasing the cluster size, indicating that the band at 1540 cm(-1) is assignable to the antisymmetric CS stretching vibration of solvent CS(2) molecules in the clusters. On the basis of density functional theory calculations, the 1410 and 1490 cm(-1) bands of (CS(2))(n) (+) are assigned to CS stretching vibrations of the C(2)S(4) (+) cation core with a C(2) form. The (CS(2))(n) (-) clusters show two bands at around 1215 and 1530 cm(-1). Similar to the case of cation clusters, the latter band is ascribed to the antisymmetric CS stretching vibration of solvent CS(2) molecules. Vibrational frequency analysis of CS(2) (-) and C(2)S(4) (-) suggests that the 1215 cm(-1) band is attributed to the antisymmetric CS stretching vibration of the CS(2) (-) anion core with a C(2v) structure.     

Transition state structure and energetics of the N(2)O + X (X = Cl,Br) reactions

The structural and vibrational properties of the transition state of the N(2)O + X (X = Cl,Br) reactions have been characterized by ab initio methods using density functional theory. We have employed Becke's hybrid functional (B3LYP), and transition state optimizations were performed with 6-31G(d), 6-311G(2d,2p), 6-311+G(3d,2p), and 6-311+G(3df,2p) basis sets. For the chlorine atom reaction the coupled-cluster method (CCSD(T)) with 6-31G(d) basis set was also used. All calculations resulted in transition state structures with a planar cis arrangement of atoms for both reactions. The geometrical parameters of transition states at B3LYP are very similar, and the reaction coordinates involve mainly the breaking of the N-O bond. At CCSD(T)/6-31G(d) level a contribution of the O-Cl forming bond is also observed in the reaction coordinate. In addition, several highly accurate ab initio composite methods of Gaussian-n (G1, G2, G3), their variations (G2(MP2), G3//B3LYP), and complete basis set (CBS-Q, CBS-Q//B3LYP) series of models were applied to compute reaction energetics. All model chemistries predict exothermic reactions. The G3 and G2 methods result in the smallest deviations from experiment, 1.8 and 0 kcal mol(-1), for the enthalpies of reaction for N(2)O reaction with chlorine and bromine, respectively. The G3//B3LYP and G1 methods perform best among the composite methods in predicting energies of the transition state, with a deviation of 1.9 and 3.0 kcal mol(-1), respectively, in the activation energies for the above processes. However, the B3LYP/6-311+G(3df,2p) method gives smaller deviations of 0.4 and -1.0 kcal mol(-1), respectively. The performance of the methodologies applied in predicting transition state energies was analyzed.     

Anion photoelectron spectroscopy of germanium and tin clusters containing a transition- or lanthanide-metal atom; MGe(n)- (n = 8-20) and MSn(n)- (n = 15-17) (M = Sc-V, Y-Nb, and Lu-Ta)

The electronic properties of germanium and tin clusters containing a transition- or lanthanide-metal atom from group 3, 4, or 5, MGe(n) (M = Sc, Ti, V, Y, Zr, Nb, Lu, Hf, and Ta) and MSn(n) (M = Sc, Ti, Y. Zr, and Hf), were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, and Lu, the threshold energy of electron detachment of MGe(n)(-) exhibits local maxima at n = 10 and 16, while in the case of the group 4 elements Ti, Zr, and Hf, it exhibits a local minimum only at n = 16, associated with the presence of a small bump in the spectrum. A similar behavior is observed for MSn(n)(-) around n = 16, and these electronic characteristics of MGe(n) and MSn(n) are closely related to those of MSi(n). Compared to MSi(n), however, the larger cavity size of a Ge(n) cage allows metal atom encapsulation at a smaller size n. A cooperative effect between the electronic and geometric structures of clusters with a large cavity of Ge(16) or Sn(16) is discussed together with the results of experiments that probe their geometric stability via their reactivity to H(2)O adsorption.     

Infrared spectra of the Li+-(H2)n (n=1-3) cation complexes

The Li+-(H2)n n=1-3 complexes are investigated through infrared spectra recorded in the H-H stretch region (3980-4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H-H stretch band of Li+-H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 A increasing by 0.004 A when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+-H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Spirko, Chem. Phys. 330, 190 (2006)]. The H-H stretch band of Li+-(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2-Li+-H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+-H2. The Li+-(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1.     

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.     

A comprehensive theoretical study on the reactions of Sc+ with CnH2n+2 (n=1-3): structure,mechanism, and potential-energy surface

The reactions of Sc(+)((3)D) with methane, ethane, and propane in the gas phase were studied theoretically by density functional theory. The potential energy surfaces corresponding to [Sc, C(n), H(2n+2)](+) (n=1-3) were examined in detail at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(d,p) level of theory. The performance of this theoretical method was calibrated with respect to the available thermochemical data. Calculations indicated that the reactions of Sc(+) with alkanes are multichannel processes which involve two general mechanisms: an addition-elimination mechanism, which is in good agreement with the general mechanism proposed from earlier experiments, and a concerted mechanism, which is presented for the first time in this work. The addition-elimination reactions are favorable at low energy, and the concerted reactions could be alternative pathways at high energy. In most cases, the energetic bottleneck in the addition-elimination mechanism is the initial C--C or C--H activation. The loss of CH(4) and/or C(2)H(6) from Sc(+)+C(n)H(2n+2) (n=2, 3) can proceed along both the initial C--C activation branch and the Cbond;H activation branch. The loss of H(2) from Sc(+)+C(n)H(2n+2) (n=2, 3) can proceed not only by 1,2-H(2) and/or 1,3-H(2) elimination, but also by 1,1-H(2) elimination. The reactivity of Sc(+) with alkanes is compared with those reported earlier for the reactions of the late first-row transition-metal ions with alkanes.