Squeezing formaldehyde into C60 fullerene

The cavity inside fullerene C60 provides a highly symmetric and inert environment for housing atoms and small molecules. Here we report the encapsulation of formaldehyde inside C60 by molecular surgery, yielding the supermolecular complex CH2O@C60, despite the 4.4 Å van der Waals length of CH2O exceeding the 3.7 Å internal diameter of C60. The presence of CH2O significantly reduces the cage HOMO-LUMO gap. Nuclear spin-spin couplings are observed between the fullerene host and the formaldehyde guest. The rapid spin-lattice relaxation of the formaldehyde 13C nuclei is attributed to a dominant spin-rotation mechanism. Despite being squeezed so tightly, the encapsulated formaldehyde molecules rotate freely about their long axes even at cryogenic temperatures, allowing observation of the ortho-to-para spin isomer conversion by infrared spectroscopy. The particle in a box nature of the system is demonstrated by the observation of two quantised translational modes in the cryogenic THz spectra.

Cl@Si20X20 cages: evaluation of encapsulation nature, structural rigidity, and 29Si-NMR patterns using relativistic DFT calculations

The experimental characterization of Cl@Si₂₀ endohedral clusters, featuring different ligands such as [Cl@Si₂₀H₂₀]^- (1) [Cl@Si₂₀H₁₂Cl₈]^- (2), and [Cl@Si₂₀Cl₂₀]^- (3), provides insight into the variable encapsulation environment for chloride anions. The favorable formation of such species enables the evaluation of the encapsulation nature and the role of the inner anion in the rigidity of the overall cluster. Our results show a sizable interaction which increases as -66.7, -100.8, and -130.3 kcal mol^-1 from 1 to 3, respectively, featuring electrostatic character. The orbital interaction involves 3p-Cl → Si₂₀X₂₀ and 3s-Cl → Si₂₀X₂₀ charge transfer channels and a slight contribution from London dispersion-type interactions. These results show that the inner bonding environment can be modified by the choice of exobonded ligands. Moreover, ²⁹Si-NMR parameters are depicted in terms of the chemical shift anisotropy (CSA), leading to a strong variation of the three principal tensor components (δ₁₁, δ₂₂, δ₃₃), unraveling the origin of the experimental ²⁹Si-NMR chemical shift (δ_iso) differences along the given series. Thus, the Si₂₀ cage is a useful template to further evaluate different environments for encapsulating atomic species.

C2-insertion into a fullerene orifice

The embedment of a C_n-unit into a carbon network constituting fullerene(s) potentially enables a cage-expansion. Herein, we report a C2-insertion into a fullerene orifice in which the mechanism was examined computationally. The C2-embedded open-[60]fullerene possesses an orifice enlarged from an octagon to a decagon, while the inner space was notably expanded as confirmed by the dynamic motion of the incarcerated H₂O molecule.

Synthesis of endohedral fullerenes by molecular surgery

Encapsulation of atoms or small molecules inside fullerenes provides a unique opportunity for study of the confined species in the isolated cavity, and the synthesis of closed C₆₀ or C₇₀ fullerenes with enclosed atoms or molecules has recently developed using the method of 'molecular surgery'; in which an open-cage intermediate fullerene is the host for encapsulation of a guest species, before repair of the cage opening. In this work we review the main methods for cage-opening and closure, and the achievements of molecular surgery to date.

Synthesis and 83Kr NMR spectroscopy of Kr@C60

Synthesis of Kr@C₆₀ is achieved by quantitative high-pressure encapsulation of the noble gas into an open-fullerene, and subsequent cage closure. Krypton is the largest noble gas entrapped in C₆₀ using 'molecular surgery' and Kr@C₆₀ is prepared with >99.4% incorporation of the endohedral atom, in ca. 4% yield from C₆₀. Encapsulation in C₆₀ causes a shift of the ⁸³Kr resonance by -39.5 ppm with respect to free ⁸³Kr in solution. The ⁸³Kr spin-lattice relaxation time T₁ is approximately 36 times longer for Kr encapsulated in C₆₀ than for free Kr in solution. This is the first characterisation of a stable Kr compound by ⁸³Kr NMR.

Reversible Diels–Alder Addition to Fullerenes: A Study of Dimethylanthracene with H2@C60

The study of isolated atoms or molecules inside a fullerene cavity provides a unique environment. It is likely to control the outer carbon cage and study the isolated species when molecules or atoms are trapped inside a fullerene. We report the Diels–Alder addition reaction of 9,10-dimethyl anthracene (DMA) to H2@C60 while 1H NMR spectroscopy is utilized to characterize the Diels–Alder reaction of the DMA with the fullerene. Through 1H NMR spectroscopy, a series of isomeric adducts are identified. The obtained peaks are sharp, precise, and straightforward. Moreover, in this paper, H2@C60 and its isomers are described for the first time.

Hydrogenation of cage-opened C60 derivatives mediated by frustrated Lewis pairs

Multiply-carbonylated fullerene derivatives were found to work as one component in frustrated Lewis pairs which caused an Si-H bond activation in the presence of B(C₆F₅)₃, leading to the carbonyl hydrogenation in up to 99% yield. The Lewis acid-mediated reductive arylation also took place to furnish a corresponding ketal derivative.

Reductive Decarbonylation of a Cage-Opened C60 Derivative

The decarbonylation of a cage-opened C₆₀ derivative was examined by employing single-electron reductants. During the reaction, an H₂O molecule was spontaneously encapsulated inside the cage (up to 78%) through the thus-formed 14-membered-ring orifice even though the H₂O encapsulation had long been considered to require an orifice consisting of at least 16 atoms. The crystallographic analysis revealed an orifice shape closer to a circle which significantly contributes to the decreased activation barrier for the H₂O encapsulation.

Water-Mediated Thermal Rearrangement of a Cage-Opened C60 Derivative

The partial zipping of a fullerene orifice was achieved by a water-mediated thermal rearrangement at 150 °C for one day while the orifice size changed from 16- to 14-membered ring with the generation of a fused pentagon. The addition of B(C6 F5 )3 was found to facilitate the reaction likely due to the coordination to carbonyl groups on the orifice. By extending the reaction time, the decarbonylation took place to give another 14-membered-ring orifice where the Michael addition of water occurred under acidic conditions. The computational study suggested that the formation of a carboxylic acid and Fischer-type carbene plays a key role in the C-C bond cleavage/reformation processes during the rearrangement.

Chemical shielding of H2O and HF encapsulated inside a C60 cage

Molecular surgery provides the opportunity to study relatively large molecules encapsulated within a fullerene cage. Here we determine the location of an H₂O molecule isolated within an adsorbed buckminsterfullerene cage, and compare this to the intrafullerene position of HF. Using normal incidence X-ray standing wave (NIXSW) analysis, coupled with density functional theory and molecular dynamics simulations, we show that both H₂O and HF are located at an off-centre position within the fullerene cage, caused by substantial intra-cage electrostatic fields generated by surface adsorption of the fullerene. The atomistic and electronic structure simulations also reveal significant internal rotational motion consistent with the NIXSW data. Despite this substantial intra-cage interaction, we find that neither HF or H₂O contribute to the endofullerene frontier orbitals, confirming the chemical isolation of the encapsulated molecules. We also show that our experimental NIXSW measurements and theoretical data are best described by a mixed adsorption site model.

Cage-Expansion of Fullerenes

Despite the first proposal on the cage inflation of fullerenes in 1991, the chemical expansion of fullerenes has been still a formidable challenge. Herein, we provide an efficient methodology to expand [60] and [70]fullerene cages by the inclusion of totally C5N unit, giving nitrogen-containing closed structures as C₆₅N and C₇₅N with double fused heptagons. This method consists of two steps commenced with the construction of an opening by the reaction with triazine as a C3N source, followed by the cage reformation using N-phenylmaleimide as a C2 source. We also synthesized endohedral cages, demonstrating that the encapsulated H₂O molecule inside the C₇₅N cage prefers the orientation which maximizes the intramolecular interaction with the carbon wall. Additionally, we revealed the existence of a through-space magnetic dipolar interaction between the encapsulated H₂ molecule and the embedded N atom.

Reactions on a 1,2-Dicarbonyl Moiety of a Fullerene Skeleton

A 1,2-dicarbonyl moiety on a cage-opened fullerene skeleton is one of suitable building blocks for the further derivatization. Herein, we discuss the chemical transformation of a 1,2-dicarbonyl compound into β-oxo-phosphorus ylide, acid anhydride, and α-methylene carbonyl derivatives. Despite possessing a sterically small methylene unit in the last one, the release of an encapsulated water molecule was significantly supressed whereas the β-oxo-phosphorus ylide bearing three bulky p-tolyl groups on the P-atom enabled the faster insertion/release dynamics, implying the flexibility of the phosphonium substituent. The replacement of the carbonyl group with phosphorus ylide and methylene units largely varied electrochemical properties of the fullerene skeleton, likely arising from the anionic charge delocalized over the entire molecule and removal of an electron-withdrawable carbonyl group, respectively.

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 >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.

Infrared spectroscopy of an endohedral water in fullerene

An infrared absorption spectroscopy study of the endohedral water molecule in a solid mixture of H₂O@C₆₀ and C₆₀ 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-H₂O and ortho-H₂O transitions. Eight vibrational transitions with rotational side peaks were observed in the mid-infrared: ω₁, ω₂, ω₃, 2ω₁, 2ω₂, ω₁ + ω₃, ω₂ + ω₃, and 2ω₂ + ω₃. The vibrational frequencies ω₂ and 2ω₂ are lower by 1.6% and the rest by 2.4%, as compared to those of free H₂O. A model consisting of a rovibrational Hamiltonian with the dipole and quadrupole moments of H₂O 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 v₁ and v₂ 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 H₂O 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 H₂O in the molecular cage of C₆₀ was observed at 110 cm^-1 (13.6 meV).

Nonclassical Abramov Products Formed on Orifices of Cage-Opened C60 Derivatives

By nucleophilic addition of phosphite P(OMe)₃ to a cage-opened C₆₀ derivative, α-hydrophosphate and enol phosphate were obtained as kinetic and thermodynamic products, respectively. Different from classical Abramov products bearing a phosphorus-carbon bond, these products have a phosphorus-oxygen bond. The observed anomaly originates from the fully conjugated π system, which significantly stabilizes zwitterionic intermediates bearing a phosphorus-oxygen bond. The thus formed enol phosphate was found to exhibit an intense absorption band that extended to 730 nm, reflecting the intramolecular charge-transfer transitions. We also report domino phosphorylation reactions, which gave a cage-opened C₆₀ derivative bearing a direct P-C bond.

A Solid-State Intramolecular Wittig Reaction Enables Efficient Synthesis of Endofullerenes Including Ne@C60 , 3 He@C60 , and HD@C60

An open‐cage fullerene incorporating phosphorous ylid and carbonyl group moieties on the rim of the orifice can be filled with gases (H₂, 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. Known transformations complete conversion of the product fullerene to C₆₀ containing the endohedral species. As well as providing an improved synthesis of large quantities of ⁴He@C₆₀, H₂@C₆₀, and D₂@C₆₀, the method allows the efficient incorporation of expensive gases such as HD and ³He, to prepare HD@C₆₀ and ³He@C₆₀. The method also enables the first synthesis of Ne@C₆₀ by molecular surgery, and its characterization by crystallography and ¹³C NMR spectroscopy.

An Androsterone-H2 @C60 hybrid: Synthesis, Properties and Molecular Docking Simulations with SARS-Cov-2

We report the synthesis and characterization of a fullerene‐steroid hybrid that contains H₂@C₆₀ and a dehydroepiandrosterone moiety synthesized by a cyclopropanation reaction with 76 % yield. Theoretical calculations at the DFT‐D3(BJ)/PBE 6‐311G(d,p) level predict the most stable conformation and that the saturation of a double bond is the main factor causing the upfield shielding of the signal appearing at −3.13 ppm, which corresponds to the H₂ located inside the fullerene cage. Relevant stereoelectronic parameters were also investigated and reinforce the idea that electronic interactions must be considered to develop studies on chemical‐biological interactions. A molecular docking simulation predicted that the binding energy values for the protease‐hybrid complexes were −9.9 kcal/mol and −13.5 kcal/mol for PL^pro and 3CL^pro respectively, indicating the potential use of the synthesized steroid‐H₂@C₆₀ as anti‐SARS‐Cov‐2 agent.

Pressure-induced annulative orifice closure of a cage-opened C60 derivative

A cage-opened C60 derivative was found to undergo an unusual annulative orifice-closure reaction under high-pressure conditions, in which the orifice size changed from a 16- to a 13-membered ring. The structure was different from that obtained by the reaction at 1 atm. The theoretical calculations suggested that the formation of the former one is thermodynamically favored.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Cation recognition on a fullerene-based macrocycle

Heterocyclic orifices in cage-opened fullerene derivatives are regarded as potential ligands toward metals or ions, being reminiscent of truncated fullerenes as a hypothetical class of macrocycles with spherical π-conjugation. Among a number of cage-opened examples reported thus far, the coordination ability and dynamic behavior in solution still remained unclear due to difficulties in structural determination with multiple coordination sites on the macrocycles. Herein, we present the detailed solution dynamics of a cage-opened C60 derivative bearing a diketo bis(hemiketal) moiety in the presence of alkali metal ions. The NMR spectroscopy disclosed the coordination behavior which is identified as a two-step process with a 1 : 2 stoichiometry. Upon coordination to the Li+ ion, the macrocycle largely varies its properties, i.e., increased absorption coefficients in the visible region due to weakly-allowed charge transfer transitions as well as the inner potential field from neutral to positive by the charge delocalization along with the spherical π-surface. The Li+-complexes formed in situ underwent unprecedented selective dehydroxyhydrogenation under high-pressure conditions. These findings would facilitate further studies on fullerene-based macrocycles as metal sensors, bulky ligands in organic reactions, and ion carriers in batteries and biosystems.

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 340 K 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.

Bond Dissociation and Reactivity of HF and H2O in a Nano Test Tube

Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning the properties of molecular materials. We entrapped HF and H₂O molecules within the fullerene C₆₀ cage, encapsulated within a single-walled carbon nanotube (X@C₆₀)@SWNT, where X = HF or H₂O. (X@C₆₀)@SWNT represents a class of molecular nanomaterial composed of a guest within a molecular host within a nanoscale host, enabling investigations of the interactions of isolated single di- or triatomic molecules with the electron beam. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-angstrom resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration-corrected high-resolution transmission electron microscopy imaging, or indirectly by vibrational electron energy loss spectroscopy in situ during scanning transmission electron microscopy experiments. Experimental measurements indicate that the electron beam triggers homolytic dissociation of the H-F or H-O bonds, respectively, causing the expulsion of the hydrogen atoms from the fullerene cage, leaving fluorine or oxygen behind. Because of a difference in the mechanisms of penetration through the carbon lattice available for F or O atoms, atomic fluorine inside the fullerene cage appears to be more stable than the atomic oxygen under the same conditions. The use of (X@C₆₀)@SWNT, where each molecule X is "packaged" separately from each other, in combination with the electron microscopy methods and density functional theory modeling in this work, enable bond dynamics and reactivity of individual atoms to be probed directly at the single-molecule level.

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.

An orifice design: water insertion into C60

Using an open-cage C₆₀ derivative possessing an orifice designed on the basis of computational studies, we have experimentally demonstrated the quantitative encapsulation of H₂O as well as effective conversion into H₂O@C₆₀ in an overall yield remarkably higher than the previously reported methods by ca. 2–5 times.

The designed orifice substructure enabled the efficient conversion into H₂O@C₆₀via a four-step reaction process in 70% isolated yield with an occupation level exceeding 98%.

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 synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C₆₀ 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 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. The ¹H spin‐lattice relaxation times (T₁) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C₆₀ cage. The synthesis of CH₄@C₆₀ opens a route to endofullerenes incorporating large guest molecules and atoms.

Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60- and H2O@C59N. Chem Sci 2018;9:5666-5671. [PMID: 30062000 PMCID: PMC6050629 DOI: 10.1039/c8sc01031e]

We report a high-resolution photoelectron imaging study of cryogenically-cooled H2O@C60- and H2O@C59N- endohedral fullerene anions. The electron affinity (EA) of H2O@C60 is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C60, while the EA of H2O@C59N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C59N. The opposite shifts are found to be due to the different electrostatic interactions between the encapsulated water molecule and the fullerene cages in the two systems. There is a net coulombic attraction between the guest and host in H2O@C60-, but a repulsive interaction in H2O@C59N-. We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H2O molecule, providing further insights into the guest-host interactions in H2O@C60- and H2O@C59N-.

Quantum-rotor-induced polarization

Quantum-rotor-induced polarization is closely related to para-hydrogen-induced polarization. In both cases, the hyperpolarized spin order derives from rotational interaction and the Pauli principle by which the symmetry of the rotational ground state dictates the symmetry of the associated nuclear spin state. In quantum-rotor-induced polarization, there may be several spin states associated with the rotational ground state, and the hyperpolarization is typically generated by hetero-nuclear cross-relaxation. This review discusses preconditions for quantum-rotor-induced polarization for both the 1-dimensional methyl rotor and the asymmetric rotor H₂¹⁷ O@C₆₀ , that is, a single water molecule encapsulated in fullerene C₆₀ . Experimental results are presented for both rotors.

Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene H_{2}O@C_{60}

Water exists in two forms, para and ortho, that have nuclear spin states with different symmetries. Here we report the conversion of fullerene-encapsulated para water to ortho water. The enrichment of para water at low temperatures is monitored via changes in the electrical polarizability of the material. Upon rapid dissolution of the material in toluene the excess para water converts to ortho water. In H_{2}^{16}O@C_{60} the conversion leads to a slow increase in the NMR signal. In H_{2}^{17}O@C_{60} the conversion gives rise to weak signal enhancements attributed to quantum-rotor-induced nuclear spin polarization. The time constants for the para-to-ortho conversion of fullerene-encapsulated water in ambient temperature solution are estimated as 30±4  s for the ^{16}O isotopolog of water, and 16±3  s for the ^{17}O isotopolog.

NMR Lineshapes and Scalar Relaxation of the Water-Endofullerene H217 O@C60

The ¹⁷ O isotopomer of the water-endofullerene H₂ O@C₆₀ displays a remarkable proton NMR spectrum, with six well resolved peaks. These peaks are due to the J-coupling between the water protons and the ¹⁷ O nucleus, which has spin-5/2. The resolution of these peaks is enabled by the suppression of water proton exchange by the fullerene cage. The six peaks display an unusual pattern of linewidths, which we model by a Liouville-space treatment of scalar relaxation due to quadrupolar relaxation of the ¹⁷ O nuclei. The data are consistent with rotational diffusion of the water molecules on the sub-picosecond timescale.

Synthesis and Properties of Open Fullerenes Encapsulating Ammonia and Methane

We describe the synthesis and characterisation of open fullerene (1) and its reduced form (2) in which CH₄ and NH₃ are encapsulated, respectively. The ¹H NMR resonance of endohedral NH₃ is broadened by scalar coupling to the quadrupolar ¹⁴n nucleus, which relaxes rapidly. This broadening is absent for small satellite peaks, which are attributed to natural abundance ¹⁵N. The influence of the scalar relaxation mechanism on the linewidth of the ¹H ammonia resonance is probed by variable temperature NMR. A rotational correlation time of τ_c=1.5 ps. is determined for endohedral NH₃, and of τ_c=57±5 ps. for the open fullerene, indicating free rotation of the encapsulated molecule. IR spectroscopy of NH₃@2 at 5 K identifies three vibrations of NH₃ (ν₁, ν₃ and ν₄) redshifted in comparison with free NH₃, and temperature dependence of the IR peak intensity indicates the presence of a large number of excited translational/ rotational states. Variable temperature ¹H NMR spectra indicate that endohedral CH₄ is also able to rotate freely at 223 K, on the NMR timescale. Inelastic neutron scattering (INS) spectra of CH₄@1 show both rotational and translational modes of CH₄. Energy of the first excited rotational state (J=1) of CH₄@1 is significantly lower than that of free CH₄.

Dipolar molecules inside C70: an electric field-driven room-temperature single-molecule switch

We propose a two-state electric field-driven room-temperature single-molecule switch based on a dipolar molecule enclosed inside ellipsoidal fullerene C₇₀. We show that the two low-energy minima of the molecular dipole inside the C₇₀ cage provide distinguishable molecular states of the system that can be switched by application of an external electric field.

Alignment of 17O-enriched water-endofullerene H2O@C60 in a liquid crystal matrix

We present a 17O and 1H NMR study of molecular endofullerene H2O@C60 dissolved in the nematic liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA). The 17O NMR peak is split into five components by the 17O residual quadrupolar coupling, each of which is split into a triplet by the 1H-17O residual dipolar coupling and scalar coupling. The splittings are analysed in terms of the partial alignment of the encapsulated water molecules. Order parameters describing the alignment are estimated. It is found that the preferential orientation of the endohedral water molecule has the molecular plane perpendicular to the liquid crystal director.

Structural modification of open-cage fullerene C60 derivatives having a small molecule inside their cavities

We studied the chemical modifications of open-cage fullerene C60 derivatives encapsulating a small molecule such as H2 and H2O. In these reactions, the entrapped molecules can be used as a probe for monitoring the reactions. The solubilizing group (methoxyethyl group) was successfully introduced on the bis(hemiketal) moiety in the open-cage C60 derivative. Further specific reactions were also investigated for 2-methoxyethoxymethyl (MEM)-substituted C60 derivatives.