Carbon clusters systems

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

This chapter has shown how hypercarbon atoms and metal atoms can form mixed metal-carbon cluster systems. Our concern has been mainly with... 

If the focus of interest is on the carbon clusters themselves, then of course no substitute system can be used. However, for studying the convergence of properties towards bulk values one can minimize the termination effects by saturating the dangling bonds in the simplest possible way, i.e. with hydrogen. By that approach one can both avoid the problem of handling an excessive number of open shells, and obtain a series of molecules that converge towards bulk properties more smoothly than the bare carbon clusters. 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Since two mechanisms are possible for the competition between association and reaction, detailed ab initio calculations of the potential surface are even more necessary in theoretical determinations of the rates of association channels. More experimental work is also needed it is possible that as a larger number of competitive systems is studied, our understanding of the competition will increase. Critical systems for interstellar modeling include the association/reactive channels for C+ and bare carbon clusters, as well as for hydrocarbon ions and H2. 

In the simple steady-state model of Thaddeus,117 bare carbon cluster seed molecules with 12 carbon atoms are used with reaction 28 to produce large linear carbon clusters with sizeable abundances since it is assumed that the C +l ions produced in reaction 28 do not dissociate when they recombine with electrons if n >12. Rather, neutral Cn+1 clusters are formed which either photodissociate (slowly) or recombine further with C+. In this limited system, cluster growth would be catastrophic were it not for photodissociation. The large abundances of carbon clusters with 20 < n < 40 suggests that such molecules may well be the carriers of the well-known DIBs.118... 

A search for alternative energy supplies has triggered efforts to develop efficient homogeneous catalysts for Fischer-Tropsch-type syntheses via hydrogenation of carbon monoxide, a likely future key material available, for example, through oxidation of coal (33, 327, 328, 417, 418). Metal cluster systems have been used in attempts to emulate the presently used heterogeneous catalysts. The important reactions are methanation,... 

Information about internuclear distances in organic compounds has led to the view that the effective radius of an atom varies directly with bond order. This is understandable for elements like carbon, with a limited range of hybridized states, but less so for metallic (cluster) systems. The problem is threefold ... 

In all the above examples, the carbonyl group donates two electrons to the metal cluster unit. Four-electron donation by the carbonyl group has recently been observed although this appears to be a much less frequent mode of bonding in cluster systems, it has often been invoked to explain the properties of absorbed carbon monoxide on a metal surface. 

As has been pointed out in the past (e.g. concerning the linear-cyclic equilibrium in Ceand Cio carbon clusters (40)), Hartree-Fock underestimates the resonance stabilization of aromatic relative to non-aromatic systems (in the case at hand, between the N- and / -protonated isomers) and MP2 overcorrects. The structures are found to be nearly isoenergetic at the CCSD level inclusion of connected triple excitations favors the N-protonated ion. The direction of the effect of connected quadruples is somewhat unclear, and a CCSD(TQ) or CCSDT(Q) calculation impossible on systems this size, but the contribution will anyhow be much smaller in absolute magnitude than that of connected triple excitations, particularly for systems like these which are dominated by a single reference determinant. We may therefore infer that at the full Cl limit, the N-protonated species will be slightly more stable than its / -protonated counterpart. 

IMS-MS applications published in the literature can be grouped into two categories. The first contains studies of conformations of flexible molecules. Such flexible molecules include synthetic polymers and biopolymers such as peptides, proteins, and oligonucleotides. The studies of the second category deal with the geometry of cluster ions such as carbon clusters, semiconductor clusters, metal clusters, salt clusters, ion-ligand clusters. The major conclusions regarding structure of these systems are reviewed in Sect. 5. 

It is useful to consider this idea in the context of earlier experimental observations of carbon clusters. [Ro84] In a similar experiment, that group published, but did not remark on, a factor of two enhancement of the Ceo cluster relative to its immediate (even) neighbors. One should bear in mind that cluster abundance variations on that order are seen in many systems, but they are not thought to come from a uniquely stable cluster like the hypothesized buckminsterfullerene. 

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

---