Fullerene ion chemistry: a journey of discovery and achievement

Abstract

An account is provided of the extraordinary features of buckminster fullerene cations and their chemistry that we discovered in our Ion Chemistry Laboratory at York University (Canada) during a 'golden' period of research in the early 1990s, just after C60 powder became available. We identified new chemical ways of C60 ionization and tracked novel chemistry of C60 (n+) as a function of charge state (n=1-3) with some 50 different reagent molecules. We found that multiple charges enhance reaction rates and diversify reaction products and mechanisms. Strong electrostatic interactions with reagent molecules were seen to reduce barriers to carbon surface bonding and charge-separation reactions, while intramolecular Coulomb repulsion appeared to localize charge on the surface or the substituent and so influence higher order chemistry, including 'spindle', 'star', 'fuzzy ball', 'ball-and-chain' and dimer ion formation. We introduced the notion of 'apparent' gas-phase acidity with measurements of proton-transfer reactions of multiply charged fullerene cations. We also explored the attachment of atomic metal cations to C60 and their subsequent reactions. All these findings were applied to the possible chemistry of fullerene cations in the interstellar medium with a focus on multiply charged fullerene ion formation and the intervention of fullerene cations in fullerene derivatization and molecular synthesis, with a view to their possible future detection.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Keywords: buckminster fullerene cations; charge state chemistry; electron transfer; extraterrestrial chemistry.

Figure 1.

Qualitative overview of the observed trend in the reactivity of C₆₀ⁿ⁺ cations with charge state n. The predominant reaction channels that were observed are listed in the order of general importance. RE is the electron RE of the cation in eV.

Figure 2.

The competition between electron transfer (dotted line) and adduct formation (solid line) observed for reactions of C₆₀²⁺ with ammonia and amines in our SIFT apparatus at room temperature at a helium bath pressure of 0.35 Torr.

Figure 3.

Overview of derivatization reactions of C₆₀⁺ observed at 298 K in helium at 0.35 Torr. The assigned structures are speculative.

Figure 4.

A correlation between reaction efficiency, k_obs/k_c, with the square of the pi orbital axis vector angle and the strain energy for addition reactions with cyclopentadiene at room temperature and a helium pressure of 0.35 Torr. k_obs is the measured rate coefficient and k_c is the collision rate coefficient, which is estimated to be 10⁻⁹ cm³ molecule⁻¹ s⁻¹. The dotted line defines the dependence of k_obs on the strain energy, E_strain.

Figure 5.

Overview of derivatization reactions of C₆₀²⁺ observed at 298 K in helium at 0.35 Torr. The assigned structures are speculative.

Figure 6.

Data recorded for the sequential addition of methyl isocyanide to C₆₀³⁺ in helium buffer gas at 0.35 Torr and 294 K.

Figure 7.

Isomers of C₆₀(C₅H₅N)₃³⁺ identified with CID. (a) ‘Star’ structure; (c) ‘ball-and-chain’ structure.

Figure 8.

Semi-empirical MP3 structure calculated for C₆₀(allene)₇²⁺.

Figure 9.

‘Molecular dock mechanism’ proposed for the 2+2 cyclization of cyanoacetylene assisted by C₆₀²⁺.

Figure 10.

Observed variation of the rate coefficient for proton transfer with the gas-phase basicity of X, GB(X), for proton-transfer reactions of the type indicated. at the onset of proton transfer.

Figure 11.

Proposed structure for M⁺(C₆₀)₄. Bonding is characterized by η⁶ interaction of the metal with the C₆₀ ligands (see lower left) and by η²-to-η² bonding of the ligands to one another (see left-most pair of ligands).

Figure 12.

Schematic of chemical pathways that we have proposed that can be initiated by doubly charged fullerenes in interstellar or circumstellar environments.