C n carbon clusters (n=4−6, 60, 70): interesting cases of computation/observation comparisons

Abstract

Abstract. Recent theoretical studies proved some of the small carbon clusters C, (n ~ 1 -10) are represented by several isomeric structures. Evaluation of the isomer relative stabilities is necessary for comparison of theoretical data with observation, this being exemplified by the C4 and C6 cases. For the former cluster a new energy estimation has been suggested based on the isomerism reasoning. Relative-stability evaluation for clusters of different dimensions is discussed on the C6o/C7o system, pointing out a strong pressure-dependency of the picture. The scheme is matched to the recent observations of Bethune and Meijer. The vigorous interest in larger carbon species of the C6o type is connected with continuing studies in small carbon clusters C, (n ~ 1 -10)-for review, [1]. From the point of view of the stability evaluations the species could be treated in a unified way. However, with respect to technical demands the area of the small species (e.g., C4 [2-8], Cs [9, 10], or C 6 [11, t2]) is naturally understood in a more complete way than the larger species [13][14][15]. Several recent computational studies demonstrated isomerism for some of the small clusters. It requires a more complex scheme (cf. Table 1, exemplifying the C4 case) for comparison of theoretical data with observation than the isomerism-free situation (cf. Table 2 with the C 5 system). Another instructive example is represented by the C 6 species, which was predicted [11] to possess the D3h cyclic structure in its ground state, however, followed by linear triplet structure and other two species. However, a consistent thermodynamic treatment [16] shows that at the observational temperature (T = 2500 K) the actual stability order is substantially different from that found at 0 K: more than 95% of the mixture is formed by the linear structure (Table 3)! Systems with cluster isomerism can be classified according to the presence of the isomeric relative-stability interchange. Three evaluations of the energy differences AE between the linear and Dzh rhombic C 4 isomers are considered in Fig. t. In addition to the previous evaluations [5,6], we derived the third value from an isomeric

Is the C8ring nonplanar?

“…have received considerable research attention for more than two decades [1]. Nevertheless, the interest has recently become even stronger in both computations [2][3][4][5][6][7][8][9][10][11][12][13] and observations [14][15][16][17][18][19][20][21] in an apparent connection with the large carbon spheroidal molecules [22]. The computational studies [1] mostly predict linear arrangements for odd-numbered small carbon aggregates, while with the even species two isomers are suggested, planar cyclic and linear (triplet electronic state).…”

A nonplanar cyclic minimum-energy structure of singlet C9

“…In fact, the small clusters had been studied well before fullerene discovery [5], by both computational [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and experimental [-22-41] means. C 9 belongs to the species for which at least fragments of information are available from experiment [42].…”