Infrared-induced reactivity of N2O on small gas-phase rhodium clusters.

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Disparate effects of Cu and V on structures of exohedral transition metal-doped silicon clusters: a combined far-infrared spectroscopic and computational study.

The growth mechanisms of small cationic silicon clusters containing up to 11 Si atoms, exohedrally doped by V and Cu atoms, are described. We find that as dopants, V and Cu follow two different paths: while V prefers substitution of a silicon atom in a highly coordinated position of the cationic bare silicon clusters, Cu favors adsorption to the neutral or cationic bare clusters in a lower coordination site. The different behavior of the two transition metals becomes evident in the structures of Si(n)M(+) (n = 4-11 for M = V, and n = 6-11 for M = Cu), which are investigated by density functional theory and, for several sizes, confirmed by comparison with their experimental vibrational spectra. The spectra are measured on the corresponding Si(n)M(+)·Ar complexes, which can be formed for the exohedrally doped silicon clusters. The comparison between experimental and calculated spectra indicates that the BP86 functional is suitable to predict far-infrared spectra of these clusters. In most cases, the calculated infrared spectrum of the lowest-lying isomer fits well with the experiment, even when various isomers and different electronic states are close in energy. However, in a few cases, namely Si(9)Cu(+), Si(11)Cu(+), and Si(10)V(+), the experimentally verified isomers are not the lowest in energy according to the density functional theory calculations, but their structures still follow the described growth mechanism. The different growth patterns of the two series of doped Si clusters reflect the role of the transition metal's 3d orbitals in the binding of the dopant atoms.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

Far-infrared spectra of yttrium-doped gold clusters Au(n)Y (n=1-9).

The geometric, spectroscopic, and electronic properties of neutral yttrium-doped gold clusters Au(n)Y (n=1-9) are studied by far-infrared multiple photon dissociation (FIR-MPD) spectroscopy and quantum chemical calculations. Comparison of the observed and calculated vibrational spectra allows the structures of the isomers present in the molecular beam to be determined. Most of the isomers for which the IR spectra agree best with experiment are calculated to be the energetically most stable ones. Attachment of xenon to the Au(n)Y cluster can cause changes in the IR spectra, which involve band shifts and band splittings. In some cases symmetry changes, as a result of the attachment of xenon atoms, were also observed. All the Au(n)Y clusters considered prefer a low spin state. In contrast to pure gold clusters, which exhibit exclusively planar lowest-energy structures for small sizes, several of the studied species are three-dimensional. This is particularly the case for Au(4)Y and Au(9)Y, while for some other sizes (n=5, 8) the 3D structures have an energy similar to that of their 2D counterparts. Several of the lowest-energy structures are quasi-2D, that is, slightly distorted from planar shapes. For all the studied species the Y atom prefers high coordination, which is different from other metal dopants in gold clusters.

Infrared spectroscopy and binding geometries of oxygen atoms bound to cationic tantalum clusters.

The binding of isolated oxygen atoms to small tantalum clusters is investigated using vibrational spectroscopy. Infrared spectra of Ta(n)O(0,1,2)(+) (n = 6-11) are reported in the range of 90-1100 cm(-1), comprising both the internal cluster modes and the adatom's vibrations. The vibrational spectra are compared to the results of DFT calculations for n = 6-8, which show that the oxygen atoms bind preferentially as 2-fold bridging adatoms. Addition of one or two O atoms induces only minor distortions of the underlying metal cluster core.

Structures of silicon cluster cations in the gas phase.

We present gas-phase infrared spectra for small silicon cluster cations possessing between 6 and 21 atoms. Infrared multiple photon dissociation (IR-MPD) of these clusters complexed with a xenon atom is employed to obtain their vibrational spectra. These vibrational spectra give for the first time experimental data capable of distinguishing the exact internal structures of the silicon cluster cations. By comparing the experimental spectra with theoretical predictions based on density functional theory (DFT), unambiguous structural assignments for most of the Si(n)(+) clusters in this size range have been made. In particular, for Si(8)(+) an edge-capped pentagonal bypriamid structure, hitherto not considered, was assigned. These structural assignments provide direct experimental evidence for a cluster growth motif starting with a pentagonal bipyramid building block and changing to a trigonal prism for larger clusters.

Nature of Ar bonding to small Co(n+) clusters and its effect on the structure determination by far-infrared absorption spectroscopy.

Far-infrared vibrational spectroscopy by multiple photon dissociation has proven to be a very useful technique for the structural fingerprinting of small metal clusters. Contrary to previous studies on cationic V, Nb, and Ta clusters, measured vibrational spectra of small cationic cobalt clusters show a strong dependence on the number of adsorbed Ar probe atoms, which increases with decreasing cluster size. Focusing on the series Co(4) (+) to Co(8) (+) we therefore use density-functional theory to analyze the nature of the Ar-Co(n) (+) bond and its role for the vibrational spectra. In a first step, energetically low-lying isomer structures are identified through first-principles basin-hopping sampling runs and their vibrational spectra are computed for a varying number of adsorbed Ar atoms. A comparison of these fingerprints with the experimental data enables in some cases a unique assignment of the cluster structure. Independent of the specific low-lying isomer, we obtain a pronounced increase in the Ar binding energy for the smallest cluster sizes, which correlates nicely with the observed increased influence of the Ar probe atoms on the IR spectra. Further analysis of the electronic structure motivates a simple electrostatic picture that not only explains this binding energy trend but also rationalizes the stronger influence of the rare-gas atom compared to the preceding studies by the small atomic radius of Co.

Probing C-O bond activation on gas-phase transition metal clusters: infrared multiple photon dissociation spectroscopy of Fe, Ru, Re, and W cluster CO complexes.

The binding of carbon monoxide to iron, ruthenium, rhenium, and tungsten clusters is studied by means of infrared multiple photon dissociation spectroscopy. The CO stretching mode is used to probe the interaction of the CO molecule with the metal clusters and thereby the activation of the C-O bond. CO is found to adsorb molecularly to atop positions on iron clusters. On ruthenium and rhenium clusters it also binds molecularly. In the case of ruthenium, binding is predominantly to atop sites, however higher coordinated CO binding is also observed for both metals and becomes prevalent for rhenium clusters containing more than nine atoms. Tungsten clusters exhibit a clear size dependence for molecular versus dissociative CO binding. This behavior denotes the crossover to the purely dissociative CO binding on the earlier transition metals such as tantalum.

The adsorption of CO on group 10 (Ni, Pd, Pt) transition-metal clusters.

The adsorption of a single CO molecule on clusters of the Group 10 transition metals is characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy. The cationic, neutral, and anionic carbonyl complexes contain between 3 and up to 25 metal atoms. The C-O stretching frequency nu(CO) shows that while both nickel and platinum clusters adsorb CO only in atop positions, palladium clusters exhibit a variety of binding sites. These findings can be rationalized by considering the increasing role relativistic effects play in the electronic structure of the cluster complexes going down the group. Conclusions for the cluster-support interactions for size-selected supported particles are drawn from the charge dependence of nu(CO) for the gas-phase species.

Generation and reactivity of enantiomeric (BINOLato)Ni+ complexes with chiral secondary alcohols in the gas phase.

In the presence of secondary alcohols, electrospray ionization of dilute methanolic solutions of nickel(II) salts and 1,1'-bis-2-naphthol (BINOL) leads to complexes of the formal composition [(BINOLato)Ni(CH3CH(OH)R)]+ (BINOLato refers to a singly deprotonated (R)- or (S)-1,1'-bis-2-naphthol ligand; R=CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, n-C6H13, c-C6H11, and C6H5). Upon collision-induced dissociation, each mass-selected nickel complex either loses the entire secondary alcohol ligand or undergoes bond activation followed by elimination of the corresponding ketone, as revealed by deuterium labeling. When enantiomeric BINOLato ligands (R or S) are combined with chiral secondary alcohols (R or S), differences in the branching ratios between these channels for the two stereoisomers of the secondary alcohols provide insight into the chiral discrimination operative in the C--H- and O--H-bond activation processes. For saturated alkan-2-ols, the chiral discrimination is low, and if any preference is observed at all, ketone elimination from the homochiral complexes (R,R and S,S) is slightly favored. In contrast, the diastereomeric (BINOLato)Ni+ complexes of 1-phenylethanol exhibit preferential ketone losses for the heterochiral systems (S,R and R,S).

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study.

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.

Molecular adsorption of H2 on small cationic nickel clusters.

An H2 molecule can bind molecularly on Ni4+ but it binds exclusively dissociatively on Ni5+ and Ni6+ and the adsorption behavior correlates with the relative reactivity of the metal clusters towards H2.

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Experimental vibrational spectra of gas-phase tantalum cluster cations.

We present gas-phase infrared spectra of tantalum cluster cations containing 6-20 atoms. Infrared multiple photon dissociation of their complexes with argon atoms is used to obtain vibrational spectra in the region between 90 and 305 cm(-1). Many spectra have features in common with the vibrational spectra of the lighter homologs, vanadium and niobium, pointing to a common cluster growth mechanism.

Unexpected gas-phase reactivity of the CH3OH adduct of Michler's hydrol blue: proton-shuttle catalysis and stepwise radical expulsions.

Electrospray ionization of a methanolic solution of Michler's hydrol blue, bis(4-(N,N-dimethylamino)phenyl)methyl tetrafluoroborate, 1+BF(4)-, produces the formal methanol adduct [1+.CH(3)OH], which shows an unusual gas-phase chemistry. Tandem mass-spectrometry experiments and complementary theoretical studies indicate that this adduct corresponds to the methyl ether of Michler's hydrol protonated at one of the dimethylamino groups, cation 3+. Collision-induced dissociation of mass-selected 3+ leads to two sequential expulsions of open-shell species, resulting in a formal loss of [C(2)H(6)O], whereas no expulsion of methanol is observed. In contrast, interaction of gaseous 3+ with a single molecule of a suitable base triggers an exoergic loss of methanol via proton-shuttle catalysis within the collision complex. The occurrence of this exothermic proton transfer also prevents the application of the otherwise successful kinetic method for the determination of the gas-phase proton affinity of the methyl ether of Michler's hydrol.

Argon physisorption as structural probe for endohedrally doped silicon clusters.

We report on an element-dependent critical size for argon physisorption at 80 K on transition-metal-doped silicon clusters. Argon does not attach to elemental silicon clusters but only to surface-located transition-metal atoms. Thus physisorption provides structural information. Specifically, the minimal cluster size for the formation of endohedral singly metal-doped silicon cages has been determined. The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.

Structural aspects of long-lived C7H8(2+) dications generated by the electron ionization of toluene.

The structure of the C7H8(2+) dication generated upon electron ionization of toluene is investigated by experimental and theoretical means. For the long-lived C7H8(2+) dication, the experimental findings obtained with a novel SIFT/GIB instrument suggest complete loss of structural integrity corresponding to the toluene structure. Instead, the manifold of C7H8(2+) dications most likely to be formed is assigned to a mixture of the cycloheptatriene dication and ring-protonated benzylium ions.