The Fullerene Neighbours
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aDepartment of Chemistry, National Chung-Cheng University, Ming-Hsiung, Chia-Yi 621, Taiwan bDepartnment of Chemistry, The University of Arizona, Tucson, AZ 85721, USA
ABSTRACT Semiempirical quantum-chemical calculations are reported for the fullerenic structures C6 0 , Si 6 0, Ge6o, N60 ; B36 N24 , B36 P24 , A136N24 , A1 36 P 2 4 ; and various BNn A new route towards B/N clusters is considered, being based on squares and hexagons. The pattern always requires six squares. The route can produce species of similar or even higher stability comparing to the conventional pentagon/hexagon pattern. Four particular stoichiometries emerge from the available AM1 computations: B 12 N 12 , B 2 sN 2 8 , B3 6 N3 6 , and B 36 N2 4 . INTRODUCTION During formation of the C6 0 research [1-31, silicon aggregates have become [4-16] a natural subject of study, and there are mass-spectrometric evidences [10,17] of large charged silicon and even germanium clusters. Nitrogen aggregate N20 [18,19] has also attracted attention. The doped fullerenes containing boron, nitrogen, and other atoms [2034] represent another interesting class. The non-carbon species BnNm can be considered as a limiting case of the B/N doped fullerenes. In fact, three B 3 0 N30 isomers have been computed [24,25], but it is of course only a small fraction of all possibilities. COMPUTATIONS The computations were performed with the standard parametrizations of the AM1 semiempirical quantum-chemical method [35] available in the SPARTAN program package [36]. The main part of the computations was done with this program implementation at an Iris/Silicon Graphics XZ4000 workstation (though some computations were also carried out with the MOPAC 5.0 [37] and GAUSSIAN 92 [38] program packages). The Ge 6 0 cage was computed at the MNDO level. The full geometry optimizations were carried out using the analytical energy gradient, and vibrational analysis was performed numerically for selected cases. Only singlet electronic states were considered. RESULTS AND DISCUSSION Table I deals with the C 60 , Si6 0 , Ge,0, and N60 cages (Geo0 has a distorted structure, possibly owing to a (pseudo) Jahn-Teller effect, but it is not very different from the exact Ih, symmetry - Figure 1). As different thermochemical reference states are involved, heats of formation cannot be directly applied and we use related heats of atomization instead. *
On a leave of absence from the Academy of Sciences of the Czech Republic, Prague. 175 Mat. Res. Soc. Symp. Proc. Vol. 359 ©1995Materials Research Society
Table I. Formation and atomization heatsa (kcal/mol) of C 6 0 , Si 6 0 , Ge 6 o, and N6 0
Species
tH}, 5
298 .8. 1
Ht,298.15,rel
-9280
0
Si 60 , AMi Ge 6o, MNDO
1296 2543
-5207 -2827
4073 6453
N 60 , AM1
3243
-3537
5743
C6o, AM1
973.3
"aThe formation
and atomization heat is denoted by the index f and at, respectively, both types of terms refer to the room temperature.
Figure 1. The MNDO optimized (near 176
Ih) structure
of Ge 6 o.
The difference between Si 60 and C 60 in AHt, 2 9
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