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.

Regio-, Stereo-, and Atropselective Synthesis of C60 Fullerene Bisadducts by Supramolecular-Directed Functionalization.

The regio- and stereocontrolled synthesis of fullerene bisadducts is a topic of increasing interest in fullerene chemistry and a key point for the full exploitation of these derivatives in materials science. In this context, while the tether-directed remote functionalization strategy offers a valid approach to this synthetic challenge, no examples of such control have yet been reported using nontethered species. Presented here is a conceptually novel, supramolecular-directed functionalization approach in which noncovalent interactions between untethered residues have been used, for the first time, to amplify (>2800-fold) the regio-, stereo-, and atropselective formation of a C60 fullerene bisadduct racemate from a complex mixture of 130 bisadducts. Remarkably, both enantiomers, which present a sterically demanding cis-1 C60 addition pattern, represent the first examples of fullerene derivatives which combine central, axial, and helical chirality.

Nuclear spin isomers of guest molecules in H2@C60, H2O@C60 and other endofullerenes.

Spectroscopic studies of recently synthesized endofullerenes, in which H2, H2O and other atoms and small molecules are trapped in cages of carbon atoms, have shown that although the trapped molecules interact relatively weakly with the internal environment they are nevertheless susceptible to appropriately applied external perturbations. These properties have been exploited to isolate and study samples of H2 in C60 and other fullerenes that are highly enriched in the para spin isomer. Several strategies for spin-isomer enrichment, potential extensions to other endofullerenes and possible applications of these materials are discussed.

Quantum rotation of ortho and para-water encapsulated in a fullerene cage.

Inelastic neutron scattering, far-infrared spectroscopy, and cryogenic nuclear magnetic resonance are used to investigate the quantized rotation and ortho-para conversion of single water molecules trapped inside closed fullerene cages. The existence of metastable ortho-water molecules is demonstrated, and the interconversion of ortho-and para-water spin isomers is tracked in real time. Our investigation reveals that the ground state of encapsulated ortho water has a lifted degeneracy, associated with symmetry-breaking of the water environment.

A photochemical on-off switch for tuning the equilibrium mixture of H2 nuclear spin isomers as a function of temperature.

The photochemical interconversion of the two allotropes of the hydrogen molecule [para-H(2) (pH(2)) and ortho-H(2) (oH(2))] incarcerated inside the fullerene C(70) (pH(2)@C(70) and oH(2)@C(70), respectively) is reported. Photoexcitation of H(2)@C(70) generates a fullerene triplet state that serves as a spin catalyst for pH(2)/oH(2) conversion. This method provides a means of changing the pH(2)/oH(2) ratio inside C(70) by simply irradiating H(2)@C(70) at different temperatures, since the equilibrium ratio is temperature-dependent and the electronic triplet state of the fullerene produced by absorption of the photon serves as an "on-off" spin catalyst. However, under comparable conditions, no photolytic pH(2)/oH(2) interconversion was observed for H(2)@C(60), which was rationalized by the significantly shorter triplet lifetime of H(2)@C(60) relative to H(2)@C(70).

Kinetics and solvent-dependent thermodynamics of water capture by a fullerene-based hydrophobic nanocavity.

Kinetic and thermodynamic properties of water encapsulation from organic solution by an open-cage [60]fullerene derivative have been investigated. 2D exchange NMR spectroscopy (EXSY) measurements were employed to determine the association and dissociation constants at 300-330 K (k(a) = 4.3 M(-1) × s(-1) and k(d) = 0.42 s(-1) at 300 K) in 1,1,2,2-tetrachloroethane-d(2) as well as the activation energies (E(a,ass) = 27 kJ mol(-1), E(a,diss) = 50 kJ mol(-1)). The equilibrium constants and thermodynamic parameters in various solvents (benzene-d(6), 1,2-dichlorobenzene-d(4), and dimethylsulfoxide-d(6)) were estimated using 1D-(1)H NMR spectroscopy. The parameters were dependent on the polarity of the solvent; ΔH depended linearly on the solvent polarity, becoming increasingly unfavorable as polarity increased. Mixtures of polar dimethylsulfoxide-d(6) in less polar 1,1,2,2-tetrachloroethane-d(2) showed a similar trend.

A magnetic switch for spin-catalyzed interconversion of nuclear spin isomers.

The interconversion of ortho-hydrogen (oH(2)) and para-hydrogen (pH(2)), the two nuclear spin isomers of dihydrogen, requires a paramagnetic spin catalyst such as a nitroxide. We report the design and demonstration of spin catalysis of the interconversion of oH(2) and pH(2) incarcerated in an endofullerene based on a reversible nitroxide/hydroxylamine system. The system is an example of a reversible magnetic spin catalysis switch that can increase the rate of interconversion of the nuclear spin isomers of H(2) by a factor of approximately 10(4).

NMR temperature-jump method for measuring reaction rates: reaction of dimethylanthracene with H2@C60.

We show a simple variant on Eigen's familiar temperature-jump method to measure rate constants. The sample is prepared in a sealed NMR tube, which is heated and then abruptly cooled. The NMR spectrum is then taken repeatedly until equilibrium is reestablished at the new temperature. The sample can be used over and over again. We demonstrate the technique on the reversible addition of 9,10-dimethylantracene to C(60). The C(60) contains H(2), and this provides an NMR signal upfield from TMS, well away from the rest of the spectrum. We show that the equilibrium constant for H(2)@C(60) is the same as that for (3)He@C(60).

Putting ammonia into a chemically opened fullerene.

We put ammonia into an open-cage fullerene with a 20-membered ring ( 1) as the orifice and examined the properties of the complex using NMR and MALDI-TOF mass spectroscopy. The proton NMR shows a broad resonance corresponding to endohedral NH 3 at delta H = -12.3 ppm relative to TMS. This resonance was seen to narrow when a (14)N decoupling frequency was applied. MALDI spectroscopy confirmed the presence of both 1 ( m/ z = 1172) and 1 + NH 3 ( m/ z = 1189), and integrated intensities of MALDI peak trains and NMR resonances indicate an incorporation fraction of 35-50% under our experimental conditions. NMR observations showed a diminished incorporation fraction after 6 months of storage at -10 degrees C, which indicates that ammonia slowly escapes from the open-cage fullerene.

Effect of xenon on fullerene reactions.

Solutions containing 3He@C60, 129Xe@C60, and varying amounts of 9,10-dimethylanthracene (DMA) were allowed to reach equilibrium, and the 3He and 129Xe NMR spectra were taken at the same temperature. Each spectrum showed peaks for the unreacted X@C60 and for the monoadduct. The ratios of the peak heights show that the included xenon atom substantially changes the equilibrium constant. This change is temperature dependent, meaning that the xenon atom changes both DeltaH and DeltaS for the reaction. DMA is more reactive with He@C60 at low temperatures and with Xe@C60 at higher temperatures. The difference in chemical shift between the monoadduct and the unreacted X@C60 is more than twice as large for Xe than for He and in the opposite direction. Calculations show that the electron density in Xe@C60 is higher than that in empty C60 on the outside of the cage.