M. R. Zakin

Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2

The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo‐first‐order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state...

Electron binding and chemical inertness of specific Nbx clusters

Photoionization threshold measurements on niobium clusters Nbx x=4–29, reveal an unprecedentedly strong dependence on x, the number of atoms in the cluster. Major maxima in threshold energies occurring at x=8, 10, and 16 correspond to those clusters recently shown to be unreactive toward molecular deuterium.

Dependence of metal cluster reaction kinetics on charge state. II. Chemisorption of hydrogen by neutral and positively charged iron clusters

The kinetics of D2 chemisorption by gas‐phase neutral (Fex ) and positively charged (Fe+x) iron clusters has been studied under identical reactor conditions. Similar to the neutrals, the reaction rate for the ions displays a nonmonotonic dependence on x, varying by more than four orders of magnitude between x = 1 and x = 31. The presence of the positive charge is found to have a substantial influence on reaction rate for the majority of clusters in the 1–31 atom size range. Clusters containing 4–6 and >17 atoms experience an enhancement in rate while in contrast those containing 3 and 9–14 atoms experience a decrease in rate. Further, studies of H2 chemisorption onto Fe+x (x = 4–22) indicate an enhanced, cluster size‐dependent isotope effect for the cluster ions which is quite similar to that previously observed for Fex. The effect of the positive charge on D2 chemisorption reactivity is explained within the framework of the frontier orbital model of activated chemisorption by invoking an ‘‘effective’’ ac...

Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Abstract Large increases in the photoionization threshold energies of small V x , NB x , and Fe x clusters ( x = 3–25) induced by chemisorption of H 2 have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H 2 chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H 2 and NH 3 as adsorbates.

The Chemistry and Physics of Molecular Surfaces

This article reviews the results of several recent experiments performed in our laboratory designed to elucidate the fundamental chemical and physical properties of clusters of both transition metals and other refractory elements containing from one to several hundred atoms. The gas-phase reactivity of clusters towards a variety of reagents is explored using a fast-flow reactor system. Strong cluster size-dependent variations in reactivity are observed, especially for the case of hydrogen chemisorption. Measurement of cluster photoionization thresholds (IPs) provides a sensitive probe of the evolution of cluster electronic structure as a function of the number of constituent atoms.Cluster ionization potentials are observed to exhibit fluctuations about the smooth global falloff predicted by the classical drop model, indicating the non-bulk-like behavior of small clusters. Measurement of shifts in IP induced by chemisorption of different reagents provides insight into the nature of adsorbate-cluster bonding. The formation and properties of bare and metal-doped carbon clusters are explored, with particular emphasis on elucidating the photophysics and photochemistry of the postulated ultrastable larger clusters. The results suggest that further work is required to prove soccer ball-like structures for C50, C60, etc. Finally, infrared multiple-photon dissociation (IR-MPD) is demonstrated to be a viable technique for obtaining infrared spectra of absorbate-cluster complexes. This technique is an important new tool for obtaining information about the molecularity of gas-phase reactions beyond that currently available from mass spectrometric analysis. As an illustration of the method, IR-MPD spectra of methanol chemisorbed on small iron clusters are obtained.

Size‐selective dehydrogenation of benzene by gas‐phase niobium cluster ions (Nb+x)

The gas‐phase reaction of positively charged niobium cluster ions Nb+x with normal and perdeuterobenzene has been studied using a fast‐flow reactor and direct ion sampling techniques. Chemisorption of benzene onto the ion is facile, with the rate being essentially independent of x. However, both the resulting product distributions, consisting of species NbxC6H+m and NbxC6D+m with m=0,2,4,6, and the relative product yields not only display a dramatic dependence on cluster size, but are also remarkably similar to those previously observed for neutral Nbx using gentle, low‐fluence, single‐photon 6.42 eV photoionization. Direct observation of products with m≤6 indicates that both chemical dehydrogenation of benzene and desorption of hydrogen from the cluster ions must occur in the fast‐flow reactor. Further, the relative yield of dehydrogenated products is found to increase substantially with increase in cluster ‘‘temperature.’’ Complete dehydrogenation of benzene to m=0 is also found to occur for niobium clu...

Density dependence of attractive forces for hydrogen stretching vibrations of molecules in compressed liquids

An empirical analysis of pressure‐induced shifts for quasidiatomic C–H or O–H stretching vibrations in solution shows that the portion attributable to attractive forces exhibits an approximately quadratic dependence on the solvent density, with exponents in the range sA=2.1±0.3 for ten systems. This is much stronger than the linear density dependence (sA=1) postulated for attractive forces by Schweizer and Chandler. Theoretical estimates of the attractive part of the frequency shifts are derived by averaging attractive R−n potentials (n≥6) using the hard‐sphere radial distribution function. This yields a simple, explicit formula with an exponential dependence on density. We find attractive exponents of sA∼2.0±0.1 for the experimental density regime, whereas sA→1 for the zero‐density limit. The sA values at high density are insensitive to the potential exponent n and comparable to the repulsive exponents sR∼3.5 obtained from the hard‐sphere model. The density dependence for attractive as well as repulsive ...

Infrared spectroscopy of unsupported metal cluster complexes using multiple photon dissociation

Premiere observation de la dissociation induite par un laser IR des complexes agregat moleculaire ― adsorbat dans un faisceau moleculaire. Cas du systeme Fe x (CH 3 OH) y

High pressure study of associated media: Raman scattering of pyridine complexes in aqueous solution

The ν1 and ν12 ring breathing vibrational modes of pyridine in aqueous solution were studied as functions of concentration (up to 12 M) and pressure (up to 35 kbar) in a diamond anvil cell. The pyridine isotopes ‐h5 and ‐d5 exhibited marked differences. The pressure dependence of ν1 is quite linear for pyr‐h5 but has pronounced curvature for pyr‐d5 and this curvature increases as the concentration decreases. This unusual isotope effect is attributed to pyridine–water complexes involving both O–H ⋅ ⋅ ⋅N and C–H ⋅ ⋅ ⋅O hydrogen bonds. For ν12, the pressure dependence does not change with concentration, but it is at least five times more steep for pyr‐h5 than pyr‐d5. This may occur because the mass at all six ring vertices becomes equal for the ‐d5 isotope, making the volume change during vibration very small. At high concentrations, ν1 appears as two distinct peaks with different pressure dependence, indicative of uncomplexed and complexed pyridine. For certain combinations of solvent and concentration, a s...

Ionization threshold energies for metal clusters

We have measured the ionization threshold energies as a function of cluster size for V, Nb, and Fe clusters. The metal clusters are produced by laser vaporization of a metal substrate inside the throat of a pulsed nozzle. The clusters are detected by photoionization TOF mass spectrometry. Using tunable UV lasers, the ionization thresholds are measured as a function of cluster size. In addition, ionization thresholds for clusters with dissociatively chemisorbed hydrogen are found to increase significantly over that of the corresponding bare cluster.