Abstract:The cavity inside fullerenes provides a unique environment for the study of isolated atoms and molecules. We report encapsulation of hydrogen fluoride inside C 60 using molecular surgery to give the endohedral fullerene HF@C 60 . The key synthetic step is the closure of the open fullerene cage while minimizing escape of HF. The encapsulated HF molecule moves freely inside the cage and exhibits quantization of its translational and rotational degrees of freedom, as revealed by inelastic neutron scattering and infrared spectroscopy. The rotational and vibrational constants of the encapsulated HF molecules were found to be redshifted relative to free HF. The NMR spectra display a large 1 H-19 F Jcoupling typical of an isolated species. The dipole moment of HF@C 60 was estimated from the temperature-dependence of the dielectric constant at cryogenic temperatures and showed that the cage shields around 75% of the HF dipole.Molecular endofullerenes consist of fullerene cages encapsulating small molecules, which are free to rotate and translate inside the cage. 1 The dihydrogen and water endofullerenes H 2 @C 60 , H 2 O@C 60 , and their isotopologues, have been synthesized by the procedure known as 'molecular surgery', in which synthetic operations are used to open a hole in the fullerene allowing encapsulation of the guest, followed by a suturing technique to reform the pristine fullerene shell. [2][3][4] Recently the approach has been extended to C 70 and C 59 N. [5][6][7] The confined molecules display quantization of their coupled translational and rotational degrees of freedom, and exhibit phenomena such as nuclear spin isomerism and orthopara conversion. [8][9][10][11][12] Recently it was shown that nuclear spin conversion of the encapsulated water molecules in H 2 O@C 60 leads to a change in the dielectric constant of the material. 13 One system of great interest is HF@C 60 , in which each fullerene cage contains a single hydrogen fluoride (HF) molecule. This material offers the possibility to study the spectroscopic properties of nearisolated and freely rotating HF molecules under a wide range of conditions, free from the complications of dimerization and hydrogen bonding. Predictions of the properties of HF@C 60 have been made using classical, 14 semiempirical 15,16 and quantum chemistry techniques. [17][18][19][20] Furthermore it has been postulated that endofullerenes containing freely rotating electric dipoles could exhibit ferroelectricity, due to cooperative alignment of the interacting electric dipole moments. 21 2The first examples of open-cage endofullerenes encapsulating a hydrogen fluoride molecule have recently appeared, including HF@1. 22,23 Herein we report the successful suturing of HF@1 to give the closed-cage species HF@C 60 . We present NMR, infrared, and neutron scattering data on HF@C 60 which show that the translational and rotational motions of the HF molecule inside the cage are quantized. Interactions with the cage modify the rotational and vibrational constants of the encapsula...
The endohedral fullerene CH 4 @C 60 , in which each C 60 fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C 60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C 60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The 1 H spin‐lattice relaxation times ( T 1 ) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C 60 cage. The synthesis of CH 4 @C 60 opens a route to endofullerenes incorporating large guest molecules and atoms.
An infrared absorption spectroscopy study of the endohedral water molecule in a solid mixture of H 2 O@C 60 and C 60 was carried out at liquid helium temperature. From the evolution of the spectra during the ortho-para conversion process, the spectral lines were identified as para-H 2 O and ortho-H 2 O transitions. Eight vibrational transitions with rotational side peaks were observed in the mid-infrared: ω 1 , ω 2 , ω 3 , 2ω 1 , 2ω 2 , ω 1 + ω 3 , ω 2 + ω 3 , and 2ω 2 + ω 3 . The vibrational frequencies ω 2 and 2ω 2 are lower by 1.6% and the rest by 2.4%, as compared to those of free H 2 O. A model consisting of a rovibrational Hamiltonian with the dipole and quadrupole moments of H 2 O interacting with the crystal field was used to fit the infrared absorption spectra. The electric quadrupole interaction with the crystal field lifts the degeneracy of the rotational levels. The finite amplitudes of the pure v 1 and v 2 vibrational transitions are consistent with the interaction of the water molecule dipole moment with a lattice-induced electric field. The permanent dipole moment of encapsulated H 2 O is found to be 0.50 ± 0.05 D as determined from the far-infrared rotational line intensities. The translational mode of the quantized center-of-mass motion of H 2 O in the molecular cage of C 60 was observed at 110 cm −1 (13.6 meV).
It has recently emerged that the succinimide linkage of a maleimide thiol addition product is fragile, which is a major issue in fields where thiol functionalisation needs to be robust. Herein we deliver a strategy that generates selective cysteine thiol labelling reagents, which are stable to hydrolysis and thiol exchange.
An open-cage fullerene incorporating phosphorous ylid and carbonyl group moieties on the rim of the orifice can be filled with gases (H 2 ,He, Ne) in the solid state,and the cage opening then contracted in situ by raising the temperature to complete an intramolecular Wittig reaction, trapping the atom or molecule inside.K nown transformations complete conversion of the product fullerene to C 60 containing the endohedral species.A sw ell as providing an improved synthesis of large quantities of 4 He@C 60 ,H 2 @C 60 ,and D 2 @C 60 ,the method allows the efficient incorporation of expensive gases such as HD and 3 He,t op repare HD@C 60 and 3 He@C 60 .T he method also enables the first synthesis of Ne@C 60 by molecular surgery,and its characterization by crystallography and 13 CNMR spectroscopy.
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