C₆₀ Framework Response to Halogen Addition: the Stable Isomers of C₆₀Br_2m

Abstract

The isolation and spectroscopic characterization of halogenated fullerene-60 compounds has not advanced greatly during the 2 years of effort in this area. While the fully fluorinated C₆₀F₆₀ has been studied in some detail, other halogen addition processes have indicated chlorination up to C₆₀Cl₂₄ and bromination up to C₆₀Br₂₄. However, definitive structural information has to date only been provided for three compounds, namely C₆₀Br₆, C₆₀Br₆ and C₆₀Br₂₄. Iodine does not appear to form genuine addition compounds. In the work reported here semiempirical calculations using the AM1 approximation with the MOPAC molecular orbital program have been directed to comparing the possible stable isomers of the 1:1 addition compounds C₆₀X₂ for X = F, Cl and Br. The favoured isomers can be described as 1,2-additions (to a double bond at a hexagon-hexagon fusion) and 1,4-additions (to the terminal carbons of a butadiene moiety) with higher-energy isomers resulting from 1,6- and 1,8-additions. The other isomers represented by 1,3- 1,5- and 1,7-additions are only stable relative to dissociation in the case of the fluorine addition compounds. By contrast for Br₂ addition only the 1,2- and 1,4-isomers are stable toward dissociation. The calculations show that, at and near the addition site carbons, X₂ addition is adequately described in terms of local distortion of the C₆₀ sphere. The elementary model of C₆₀ as comprising formal single and double bonds is relevant since C₆₀ behaves as a 'poly- alkene ', with sp³ carbons replacing sp² carbons at the addition sites. This model offers an explanation for the unique structures observed for C⁶⁰Br⁶ and C⁶⁰Br²⁴ which the AM1calculations show to be very stable toward dissociation. However, the experimental C₆₀Br₈ structure is found to be relatively less stable than another isomer. Also high-stability isomers of C₆₀Br₄, C₆₀Br₁₀, C₆₀Br₁₂ and C₆₀Br₁₈ are predicted.