Reactions of nitric oxide on Rh6+ clusters: abundant chemistry and evidence of structural isomers

Chemistry of (and on) transition metal clusters: a Fourier transform ion cyclotron resonance study of the reaction of niobium cluster cations with nitric oxide

The reactions of niobium cluster cations, Nb(+)(n) (n = 2-19), with nitric oxide have been investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR). The overall reaction rate constants are found to be in reasonable agreement with collision rates calculated using the surface charge capture model. The dominant reaction for small clusters (n <9) involves reaction-induced fragmentation resulting in the loss of either NbO or NbN. By contrast, the main reaction observed for the larger clusters (n> 11) is sequential NO chemisorption. Clusters n = 9, 10 exhibit both extremes of behaviour and are the only clusters upon which there is evidence of NO decomposition with N(2) loss observed whenever multiple NO molecules are co-adsorbed. The rate constants for each process have been determined as a function of cluster size.

Structural isomers and reactivity for Rh6 and Rh6+

The structure, energetics, and interconversion of isomers of Rh(6) and Rh(6)(+) are studied by using density functional theory with Gaussian basis sets, using guess structures derived from basin-hopping simulations, and obtained by using the Sutton-Chen potential. A large range of spin multiplicities is considered for each isomer. Our calculations suggest two low-lying structures as possible structural isomers: a square bipyramid and a trigonal prism. The reactivity of these two candidate structural isomers with respect to adsorption of nitric oxide is studied via location of reaction transition states and calculation of reaction barriers. Similarities and differences with surface reaction studies are highlighted. These data provide powerful evidence that structural isomerism, and not different spin states, is responsible for the observed biexponential reaction kinetics.

Dramatic size effects and evidence of structural isomers in the reactions of rhodium clusters, Rhn±, with nitrous oxide

The reactions of gas phase rhodium clusters, Rhn+/- (n<30), with nitrous oxide, N2O, have been investigated under single collision conditions by Fourier transform ion cyclotron resonance mass spectrometry. The only significant reaction observed is the sequential generation of oxides. Absolute rate constants for the reactions of all clusters have been determined and, in the case of the cationic clusters especially, they exhibit large fluctuations as a function of cluster size with local minima observed for n=5, 19, 28. Striking similarities are observed with the variation in rate constants for these clusters in reactions with small hydrocarbons (C. Adlhart and E. Uggerud, J. Chem. Phys., 2005, 123, 214709). Corresponding size effects are also observed but are less marked in the reactions of the anionic clusters. The reactions of several clusters exhibit marked deviations from simple pseudo-first-order kinetics suggesting the presence of multiple isomeric forms: Rh11+, Rh12+ and Rh8- exhibit characteristic biexponential decays which are interpreted in terms of the existence of different structural forms of the cluster which have markedly different reactivity. By contrast, Rh6+, Rh7+ and Rh8+ show rates which apparently increase with time, probably due to collisional activation. Thermalisation of the clusters prior to reaction by exposure to pulses of argon results in changes to the kinetics of these anomalous systems which can be explained in terms of collision induced isomerisation.

Reactions of Nitrogen Monoxide on Cobalt Cluster Ions: Reaction Enhancement by Introduction of Hydrogen

Absolute cross sections for NO chemisorption, NO decomposition, and cluster dissociation in the collision of a nitrogen monoxide molecule, NO, with cluster ions Con+ and ConH+ (n=2-5) were measured as a function of the cluster size, n, in a beam-gas geometry in a tandem mass spectrometer. Size dependency of the cross sections and the change of the cross sections by introduction of H to Con+ (effect of H-introduction) are explained by a statistical model based on the RRK theory, with the aid of the energetics obtained by a DFT calculation. It was found that the reactions are governed by the energetics rather than dynamics. For instance, Co3+ does not react appreciably with NO because the reactions are endothermic, while Co3H+ does because the reaction becomes exothermic by the H-introduction.

Dissociative and associative attachment of NO to iron clusters

Electronic and geometrical structures of iron clusters with associative (FeNO, Fe2NO, Fe3NO, Fe4NO, Fe5NO, and Fe6NO) and dissociative (OFeN, OFe2N, OFe3N, OFe4N, OFe5N, and OFe6N) attachments of NO, as well as the corresponding singly negatively and positively charged ions, are computed using density functional theory with generalized gradient corrections. Both types of isomers are found to be stable and no spontaneous dissociation was observed during the geometry optimizations. The ground states correspond to dissociative attachment of NO for all iron clusters Fe(n), except for Fe and Fe+. All of the OFe(n)N clusters have ferrimagnetic ground states, except for OFe2N, OFe2N-, OFe4N, and OFe4N-, which prefer the ferromagnetic coupling. In the ferrimagnetic states, the excess spin density at one iron atom couples antiferromagnetically to the excess spin densities of all other iron atoms. Relative to the high-spin Fe(n) ground state, the lowest energy ferrimagnetic state quenches the total magnetic moments of iron clusters by 7, which is to be compared with a reduction in the magnetic moment of one in the lowest energy ferromagnetic states. Dissociation of NO on the iron clusters has a pronounced impact on the energetics of reactions; the Fe(n)NO+CO-->Fe(n)N+CO2 channels are exothermic while the OFe6N+CO--> Fe6N+CO2 channels are nearly thermoneutral.

Nitric Oxide Decomposition on Small Rhodium Clusters, Rhn+/-

The decomposition of nitric oxide on small charged rhodium clusters Rh(n)(+/-) (6 < n < 30) has been investigated by Fourier transform ion cyclotron resonance mass spectrometry. For both cationic and anionic naked clusters, the rates of reaction with NO increase smoothly with cluster size in the range studied without the dramatic size-dependent fluctuations often associated with the reactions of transition-metal clusters. The cationic clusters react significantly faster than the anions and both exhibit rate constants exceeding collision rates calculated by average dipole orientation theory. Both the approximate magnitude and the trends in reactivity are modeled well by the surface charge capture model recently proposed by Kummerlöwe and Beyer. All clusters studied here exhibit pseudo-first-order kinetics with no sign of biexponential kinetics often interpreted as evidence for multiple isomeric structures. Experiments involving prolonged exposure to NO have revealed interesting size-dependent trends in the mechanism and efficiency of NO decomposition: For most small clusters (n < 17), once two NO molecules are coadsorbed on a cluster, N(2) is evolved, generating the corresponding dioxide cluster. By analogy with experiments on extended surfaces, this observation is interpreted in terms of the dissociative adsorption of NO in the early stages of reaction, generating N atoms that are mobile on the surface of the cluster. For clusters where n < 13, this chemistry, which occurs independently of the cluster charge, repeats until a size-dependent, limiting oxygen coverage is achieved. Following this, NO is observed to adsorb on the oxide cluster without further N(2) evolution. For n = 14-16 no single end-point is observed and reaction products are based on a small range of oxide structures. By contrast, no evidence for N(2) production is observed for clusters n = 13 and n > 16, for which simple sequential NO adsorption dominates the chemistry. Interestingly, there is no evidence for the production of N(2)O or NO(2) on any of the clusters studied. A simple general mechanism is proposed that accounts for all observations. The detailed decomposition mechanisms for each cluster exhibit size (and, by implication, structure) dependent features with Rh(13)(+/-) particularly anomalous by comparison with neighboring clusters.

Relaxation dynamics and structural isomerism in Nb10 and Nb10+

The structure, energetics, and interconversion of isomers of Nb10 and Nb10+ are studied using density functional theory with Gaussian basis sets, using guess structures derived from basin-hopping simulations with the Finnis-Sinclair [Philos. Mag. A 50, 45 (1984)] potential. These results are used as input to a master equation approach to model the relaxation of these clusters. Ionization potentials are calculated for all relevant minima, as are the infrared spectra. On the basis of these data, and known experimental results, plausible explanations are given for the biexponential reaction kinetics observed for Nb10 and Nb10+ with respect to small molecule adsorbates. In principle, this approach could be extended to investigate any midsized transition metal cluster that exhibits structural isomerism.

C–H activation of alkanes on Rhn+ (n=1–30) clusters: Size effects on dehydrogenation

The rate coefficients for the dehydrogenation of ethane, propane, and isobutane with cationic rhodium atoms Rh+ and clusters Rh+ n of up to 30 atoms were measured under single-collision conditions in a Fourier-transform ion cyclotron resonance mass spectrometer. The reaction rates are cluster size dependent and parallel for all the three alkanes. While the reactions proceed close to the theoretical collision rates for a large number of clusters, characteristic minima are observed for Rh+ (5/6/9/19/28). The degree of dehydrogenation varies with the cluster size with maxima for 10< or =n< or =15 for the three alkanes and for n=3 and 2-4 in the cases of ethane and propane, respectively. However, complete dehydrogenation is only observed for the reaction of Rh+ 11 with propane. Dehydrogenation is remarkably selective and no other neutral products than H2 are observed. The results are interpreted in terms of likely cluster geometries.

Preparing transition-metal clusters in known structural forms: The mass-analyzed threshold ionization spectrum of V3

The first results are presented of a new experiment designed both to generate and characterize spectroscopically individual isomers of transition-metal cluster cations. As a proof of concept the one-photon mass-analyzed threshold ionization (MATI) spectrum of V3 has been recorded in the region of 44,000-45,000 cm-1. This study extends the range of a previous zero-kinetic-energy (ZEKE) photoelectron study of Yang et al. [Chem. Phys. Lett. 231, 177 (1994)] with which the current results are compared. The MATI spectra reported here exhibit surprisingly high resolution (0.2 cm-1) for this technique despite the use of large discrimination and extraction fields. Analysis of the rotational profile of the origin band allows assignment of the V3 ground state as and the V3+ ground state as , both with D3h geometry, in agreement with the density-functional theory study of the V3 ZEKE spectrum by Calaminici et al. [J. Chem. Phys. 114, 4036 (2001)]. There is also some evidence in the spectrum of transitions to the low-lying excited state of the ion. The vibrational structure observed in the MATI spectrum is, however, significantly different to and less extensive than that predicted in the density-functional theory study. Possible reasons for the discrepancies are discussed and an alternative assignment is proposed which results in revised values for the vibrational wave numbers of both the neutral and ionic states. These studies demonstrate the efficient generation of cluster ions in known structural (isomeric) forms and pave the way for the study of cluster reactivity as a function of geometrical structure.