B. C. Guo

Metallo-Carbohedrenes [M8C12+ (M = V, Zr, Hf, and Ti)]: A Class of Stable Molecular Cluster Ions

Findings of magic peaks corresponding to M8C12+ (M = V, Zr, and Hf) formed from reactions of the respective metals with various small hydrocarbons, in conjunction with recent findings for the titanium system, establish metallo-carbohedrenes as a stable general class of molecular cluster ions. A dodecahedral structure of Th point symmetry accounts for the stability of these ionic clusters.

The clustering reactions of benzene with sodium and lead ions

Abstract Thermochemical properties are determined for the association reactions of benzene with Na + and Pb + ions. The values obtained are -28.0 kcal/mol and -31.2 cal/mol K for the enthalpy and entropy changes of the sodium ion, and -26.2 kcal/mol and -21.6 cal/mol K for Pb + , respectively. An ab initio calculation was conducted to determine the bonding and structure of Na + -benzene. The C 6v symmetry structure of Na + -benzene, and the small charge transfer from benzene to Na + suggest that the bonding of the ion complex is largely electrostatic in nature. Like other lead ion complexes, Pb + -benzene displays an unusually strong interaction. Possible bonding of Pb + -benzene is also discussed.

Metallo-carbohedrenes : formation of multicage structures

An unusual structural growth pattern has been found in the system of ZrmCn, in which multicage structures are formed. The experimental evidence shows that the first cage closes at Zr8C12. Surprisingly, subsequent cluster growth does not lead to the enlargement of the cage size as it usually does in the case of pure carbon clusters and water clusters, for example. Rather, multicage structures are developed, that is, a double cage at Zr13C22 and Zr14C21/23, a triple cage at Zr18C29, and a quadruple cage at Zr22C35. This feature distinguishes the class of metallo-carbohedrenes from the regular doped fullerenes.

The bonding strength of Ag+ (C2H4) and Ag+ (C2H4)2 complexes

Abstract The association reactions Ag+(C2H4)n−1+C2H4Ag+(C2H4)n(n= 1 or 2) were studied using high pressure mass spectrometry (HPMS). The enthalpy and entropy changes determined are −ΔH032.4 kcal/mol, and −ΔS0 = 30.2 cal/mol K for the n2 reaction. In the case of the n1 reaction, the Gibbs free energy change at 750 K is measured to be −ΔG017.1 kcal/mol. Using the free energy change and the calculated entropy change −ΔS022.1 cal/mol K, we derived an enthalpy change −ΔH033.7 kcal/mol for the n1 reaction. The bonding in both Ag+ (C2H4) and Ag+ (C2H4)2 complexes is extremely strong, and much higher than expected based on the simple electrostatic-interaction considerations. The strong bonding suggests that covalent interaction or a two-way donor—acceptor interaction dominates in the complexes.

Chemistry and kinetics of size‐selected cobalt cluster cations at thermal energies. I. Reactions with CO

The chemistry and kinetics of size‐selected Co+n cluster‐ion (n=2–8) reactions with CO are studied using a selected ion drift tube affixed with a laser vaporization source operated under well‐defined thermal conditions. All reactions studied in the present work are found to be association reactions. Their absolute rate constants, which are determined quantitatively, are found to have a strong dependence on cluster size. Similar to the cases of reactions with many other reactants such as H2 and CH4, Co+4 and Co+5 display a higher reactivity toward the CO molecule than do clusters of neighboring size. The multiple‐collision conditions employed in the present work have enabled a determination of the maximum coordination number of CO molecules bound onto each Co+n cluster. It is found that the tetramer tends to bond 12 CO molecules, the pentamer 14 CO, hexamer 16 CO, and so on. The results are interpreted in terms of Lauher’s calculation and the polyhedral skeletal electron pair theory. All the measured maxim...

Generation of metal–carbon and metal–nitrogen clusters with a laser induced plasma technique

During the course of investigating dehydrogenation reactions induced by transition metals, we find that using a carrier gas containing hydrocarbons and ammonia instead of pure helium, in conjunction with a laser vaporization device, enables the facile production of metal–carbon and metal–nitrogen clusters in both the neutral and ionic forms. With only a change in the nature of the carrier gas, a variety of new classes of clusters can be produced.

Studies of reactions of small titanium oxide cluster cations toward oxygen at thermal energies

Abstract Reactions of small titanium oxide cluster cations with oxygen are studied using a selected-ion drift tube operated at thermal energies, in conjunction with a laser vaporization source. Clusters of most compositions are observed to react rapidly with oxygen in the drift tube reactor, but a few of them display very inert behavior. The studies of size-selected oxide clusters indicate that the inert clusters belong to two series, namely, (TiO 2 ) + n (n ⩾ 3) and TiO 3 (TiO 2 ) + m (m ⩾ 1). Interestingly, reactions of the oxygen-poor clusters with the oxygen molecule are seen to yield products containing fewer titanium atoms, while the clusters with higher oxygen-to-titanium ratios undergo attachment of one or two oxygen atoms. The reaction patterns are interpreted in terms of the difference in the bonding structure of the clusters.

Reaction channels in a plasma reactor-laser vaporization source. Formation of carbon clusters and metal-carbon clusters

Two reaction channels have been found when light hydrocarbon gases are introduced into the carrier gas of a plasma reactor coupled with a laser vaporization source employing a metal rod assembly. These involve dehydrogenation to produce pure carbon clusters, and formation of metal-carbon bonds to produce metal-carbon clusters, MmCn−. Using a time-of-flight mass spectrometer system coupled with a laser vaporization plasma source, experiments are conducted to investigate the optimum conditions for selecting the desired reaction channels. Varying the conditions of the plasma reactor results in a striking difference in the species produced as evidenced in the recorded mass spectra. That is, either a pure carbon cluster series or a metal-carbon cluster series MmCn+ (with a magic peak at MgC12+) is obtained. These studies demonstrate an alternative way to produce beams of carbon clusters, and also have led to the further development of a method useful for investigating the properties of metal-carbon clusters.

Collision induced dissociation of titanium–carbon cluster cations

Titanium–carbon clusters are investigated by collision induced dissociation (CID) using our newly designed triple quadrupole mass spectrometer coupled with a laser vaporization source. Their fragmentation patterns are determined under various collision conditions. It is observed that the Met–Car Ti8C+12 mainly loses a neutral metal atom in the primary dissociation step and several metal atoms in sequential dissociation processes. The dissociation threshold of the Met–Car Ti8C+12 is estimated to be about 9 eV. In addition, several of the larger TixC+y cluster ions, including those containing nine or ten metal atoms, are found to fragment directly to Ti8C+12 during single step dissociation. Product channels for dissociation of Ti9C+12 establishes the ionization potential for Ti8C12 to be equal to, or less than the IP of the titanium atom.

Formation and stability of metallocarbohedrenes: TixMyC12(x + y = 8, M = Nb, Ta, Y, and Si)

Abstract Binary element metallocarbohedrenes, Ti x M y C + 12 (M = Nb, Ta, Y, and Si, x + y = 8) are produced using a laser-induced plasma source. The experimental findings enable the assessment of the results of several theoretical studies, providing indications of some shortcomings and pointingg out useful directions for future experimental work. Moreover, the findings provide further insight into the mechanisms responsible for the formation of metallocarbohedrenes. They show that the formation of σ-bonds between metal atoms and C 2 units are the key to the stability of metallocarbohedrenes, suggesting that late transitional metals should not lead to the formation of stable members of this class of clusters due to their rich d-electron character. The stability of these binary metallocarbohedrenes provides further evidence that these species do constitute another form of cluster, which have the potential of being the building blocks for useful new optical and catalytic materials.