Nigel G. Gotts

Isomers of small carbon cluster anions : linear chains with up to 20 atoms

The structure of small carbon cluster anions, Cn- (4 ≤ n ≤ 20), was investigated with the use of ion chromatography. With this technique, both the existence and the relative amounts of possible structural isomers can be determined. More than 99% of the ions C5- to C9- were found to be linear under these experimental conditions. Starting with C10-, a monocyclic isomer was observed, and linear and moncyclic structures coexisted from C10- through at least C20-. This result is in contrast to previous work on positive ions, which showed the existence of linear isomers from C5+ to C10+, with linear and cyclic isomers coexisting only from C7+ to C10+. Above C10+, no linear clusters were observed.

Do small fullerenes exist only on the computer? Experimental results on C=/−20 and C+/−24

Abstract Experimental results on the structure of carbon cluster anions and cations with 20 and 24 atoms are presented. Recent quantum chemical calculations predict either cage or graphitic structures to be lowest in energy for C20 and C24. However experimentally we find C+20 and C−20 are dominated by monocyclic ring structures, with C−20 exhibiting minor amounts of planar bicyclic structures and a linear structure. For C+24 and C−24 both monocyclic rings and planar bicycle rings are observed. No fullerene or “graphitic” structures are observed for these clusters. A number of possible bicyclic structures for C24 are discussed.

Carbon cluster anions: structure and growth from C5− to C62−

Abstract Carbon cluster anions are generated by laser desorption in a Smalley-type cluster source over a wide size range. The ions are mass selected and their isomer distributions and structures determined as a function of size, from C5− to C62−, using ion chromatography. The families of structures found were identical to those previously found for cation clusters, although the ranges of stability are quite different. For example, linear Cn− ions are detected to n = 30, while they are not detected above n = 10 for cations. The quantitative growth sequence is the same as for cations; linear at small sizes, transforming to a series of planar ring systems at intermediate values of n, followed by a very slow emergence of fullerenes at the largest values of n sampled. For n near 60, fullerenes comprise less than 20% of the isomers and planar rings more than 80%. For cations, fullerenes comprise more than 95% of the isomer distribution for even n in this size range. A number of possible explanations for this behavior are put forward. Annealing studies are performed for 10 ⩽ n ⩽ 30. At the low end of this size range annealing leads to pure linear isomeric distributions, but above n = 15 monocyclic rings begin to dominate. Between n = 20 and 30 annealing yields complete conversion of bicyclic planar rings to monocyclic rings.

The structures of small iron-carbon cluster anions. Linear to planar to three-dimensional

Abstract The structures of small mixed iron carbon cluster anions, Fe n C − m ( n =1, 2, 3 and m =2–8) have been studied using gas phase ion chromatography. In the one iron atom case, results show a competition between linear structures and two-dimensional (planar) clusters. The two iron atom clusters, on the other hand, are pure monocyclic rings with the two iron atoms nearest neighbors. The three iron species are three-dimensional, and several possible structures are evaluated. In all cases the carbon atoms maintain a continuous chain and no carbon dimers are observed that would be characteristic of MetCar precursors.

Evidence from Ion Chromatography Experiments That Met-Cars Are Hollow Cage Clusters

Ion chromatography studies were performed to assess various models proposed for the structure of M8C12 species, the met-cars. A laser desorption source was used to make a sequence of titanium-carbon clusters centered around Ti8C12+. The Ti8C12+ was determined to be a hollow cage cluster, with the dodechadron structure originally propposed termined to be a hollow cage cluster, with the dodecahedron structure originally proposed giving the best fit to experiment; cubic structures could be excluded. Collisional breakup of Ti8C12+ yielded only Ti7C12+ under the experimental conditions described herein, and modeling indicated that the cage structure was retained. Both Ti8C11+ and Ti8C13+ were made by the cluster source, and again, dodecahedral-type cage structures were consistent with experiment. The extra carbon atom in Ti8C13+ was attached exohedrally to a single titanium atom. No evidence for an endohedral species was found.

C+7 is cyclic: experimental evidence

Abstract Gas-phase ion chromatography experiments with variable injection energies were performed on C + 7 made by laser vaporization of graphite and by collision-induced dissociation of C + 10 and C + 12 . In all cases, the C + 7 cyclic isomer dominates over the linear isomer at lowest injection energies, indicating the cyclic isomer is the most stable form. This result is consistent with ab initio calculations and a kinetic model developed here.

Structures and energies of small carbon clusters: what experiment and theory have to say about C+8, C+9 and C+10

Abstract The structure and energies of linear and monocyclic carbon cluster cations C + n (8 ⩽ n ⩽ 10) have been investigated using gas phase ion chromatography experiments and ab initio calculations. Cluster ions are either produced directly in the ion source by laser ablation of graphite or by collision induced dissociation of larger clusters. The relative amount of linear and cyclic isomer was determined as a function of injection energy in the drift cell. Both linear and cyclic clusters are observed at all injection energies. Using simple kinetic arguments, the cyclic isomer is shown to be lower in energy than the linear cluster ion for n = 8,9 and 10. The stability of the cyclic ion relative to the linear isomer is estimated to be 23 ± 8 kcal mol −1 for C + 8 , 17 ± 8 kcal mol −1 for C + 9 and 47 ± 8 kcal mol −1 for C + 10 . Restricted open shell ab initio calculations are in very good agreement with these numbers.