Experimental determination of the interaction potential between a helium atom and the interior surface of a C60 fullerene molecule

The interactions between atoms and molecules may be described by a potential energy function of the nuclear coordinates. Non-bonded interactions are dominated by repulsive forces at short range and attractive dispersion forces at long range. Experimental data on the detailed interaction potentials for non-bonded interatomic and intermolecular forces is scarce. Here we use terahertz spectroscopy and inelastic neutron scattering to determine the potential energy function for the non-bonded interaction between single He atoms and encapsulating C60 fullerene cages, in the helium endofullerenes 3He and 4He, synthesised by molecular surgery techniques. The experimentally derived potential is compared to estimates from quantum chemistry calculations, and from sums of empirical two-body potentials.Comment: 25 pages, 14 figures, submitted to Journal of Chemical Physic

Characterisation of the confining potential for He atoms in the endofullerenes 3He@C60 and 4He@C60 by INS over an extended energy range

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Fine structure in the solution state 13C-NMR spectrum of C60 and its endofullerene derivatives

The 13C NMR spectrum of fullerene C60 in solution displays two small “side peaks" on the shielding side of the main 13C peak, with integrated intensities of 1.63% and 0.81% of the main peak. The two side peaks are shifted by -12.6 ppb and -20.0 ppb with respect to the main peak. The side peaks are also observed in the 13C NMR spectra of endofullerenes, but with slightly different shifts relative to the main peak. We ascribe the small additional peaks to minor isotopomers of C60 containing two adjacent 13C nuclei. The shifts of the additional peaks are due to a secondary isotope shift of the 13C resonance caused by the substitution of a 12C neighbour by 13C. Two peaks are observed since the C60 structure contains two different classes of carbon-carbon bonds with different vibrational characteristics. The 2:1 ratio of the side peak intensities is consistent with the known structure of C60. The origin and intensities of the 13C side peaks are discussed, together with an analysis of the 13C solution NMR spectrum of a 13C-enriched sample of C60, which displays a relatively broad 13C NMR peak due to a statistical distribution of 13C isotopes. The spectrum of 13C-enriched C60 is analyzed by a Monte Carlo simulation technique, using a theorem for the second moment of the NMR spectrum generated by J-coupled spin clusters.<br/

Synthesis of Ar@C₆₀ using molecular surgery

Synthesis of Ar@C60 is described, using a route in which high-pressure argon filling of an open-fullerene and photochemical desulfinylation are the key steps for &gt;95% encapsulation of the noble gas. Enrichment by recycling HPLC leads to quantitative incorporation of argon in the product endofullerene, with a mass recovery of tens of milligrams, allowing the first characterisation of fine structure in the solution 13C NMR spectrum

Quantised translational states of atomic He confined inside a nearly spherical cage: the endofullerene He@C60

Highly innovative &#39;molecular surgery&#39; techniques have been developed in recent years to synthesise small molecule endofullerenes in which the molecular cage of C60 completely encloses and entraps a quantum rotor such as H2, H2O or HF. The physical entrapment provides a nanolaboratory environment in which to study the isolated molecule and to exploit its physical properties. In this new proposal we are shifting emphasis to the study of entrapped atomic species. In particular 3He is highly receptive to NMR with the capacity to be prepared in a hyperpolarised nuclear spin state. Yet to develop laser-pumping hyperpolarisation protocols and exploit the possibilities that such a spin system far from equilibrium would provide, a detailed characterisation of the translational energy levels of the He atom inside its cage is required. This is uniquely available using neutron scattering.</span

An internuclear J-coupling of ³He induced by molecular confinement

The solution-state 13C NMR spectrum of the endofullerene 3He@C60 displays a doublet structure due to a J-coupling of magnitude 77.5 ± 0.2 mHz at 340K between the 3He nucleus and a 13C nucleus of the enclosing carbon surface. The J-coupling increases in magnitude with increasing temperature. Quantum chemistry calculations successfully predict the approximate magnitude of the coupling. This observation shows that the mutual proximity of molecular or atomic species is sufficient to induce a finite scalar nuclear spin-spin coupling, providing that translational motion is restricted by confinement. The phenomenon may have applications to the study of surface interactions and to mechanically bound species

Probing the interaction of noble gases with the inside of a C60 carbon cage, INS of 3He@C60 and Ne@C60 endofullerene

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Terahertz spectroscopy of the helium endofullerene He@C₆₀

We studied the quantized translational motion of single He atoms encapsulated in molecular cages by terahertz absorption. The temperature dependence of the THz absorption spectra of 3He@C60 and 4He@C60 crystal powder samples was measured between 5 and 220 K. At 5 K there is an absorption line at 96.8 cm−1 (2.90 THz) in 3He@C60 and at 81.4 cm (2.44 THz) in 4He@C60, while additional absorption lines appear at higher temperature. An anharmonic spherical oscillator model with a displacement-induced dipole moment was used to model the absorption spectra. Potential energy terms with powers of two, four and six and induced dipole moment terms with powers one and three in the helium atom displacement from the fullerene cage center were sufficient to describe the experimental results. Excellent agreement is found between potential energy functions derived from measurements on the 3He and 4He isotopes. One absorption line corresponds to a three-quantum transition in 4He@C60, allowed by the anharmonicity of the potential function and by the non-linearity of the dipole moment in He atom displacement. The potential energy function of icosahedral symmetry does not explain the fine structure observed in the low temperature spectra

First synthesis and characterization of CH4@C60

The endohedral fullerene CH4@C60, in which each C60 fullerene cage encapsulates a single methane molecule, has been synthesised for the first time. CH4 is the largest molecule, with the greatest number of atoms, to have been encapsulated in C60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was successfully completed, even though it is significantly inhibited by the presence of the endohedral molecule. The 13C NMR resonance for the cage nuclei in CH4@C60 is deshielded by Dd = +0.52 ppm relative to C60. The crystal structure of the nickel(II) octaethylporphyrin / benzene solvate shows no significant distortion of the carbon cage, relative to the C60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane, existing in a set of quantised rotational-translational states at 100 K. The 1H and 13C spin-lattice relaxation times (T1) for endohedral methane havebeen measured. The 1H T1 values are similar to those observed in the gas phase, also indicating that methane is freely rotating inside the C60 cage. The 1H relaxation rate constant T1 -1 increases with increasing temperature and suggests a significant spin-rotation contribution to the relaxation. The successful synthesis of CH4@C60 opens a route to endofullerenes incorporating larger guest molecules than those encapsulated previously

First Synthesis and Characterisation of CH4@C60

Raw data supporting journal article: Bloodworth, S. et al (2019). First Synthesis and Characterisation of CH4@C60. Angewandte Chemie International Edition. DOI: 10.1002/anie.201900983 and 10.1002/ange.201900983 </span