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.

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.

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.

Dihydrogen, H 2 , exists as two allotropes or nuclear spin isomers: a nuclear singlet state with antiparallel (vV) nuclear spins, termed parahydrogen (pH 2 ), and a nuclear triplet state with parallel (vv) nuclear spins, termed ortho-hydrogen (oH 2 ). 1According to the Pauli Principle, singlet pH 2 is required to occupy even rotational states (J ) 0, 2, 4, ...) and triplet oH 2 odd rotational states (J ) 1, 3, 5, ...). Since J ) 0 is the lowest rotational level of H 2 , pH 2 is the ground rotational state and oH 2 a rotationally excited state. The energy gap between the J ) 0 and J ) 1 rotational states is 120 cm -1 (0.343 kcal mol). The equilibrium mixture at any given temperature is termed eH 2 . At room temperature (RT), triplet oH 2 dominates at equilibrium (eH 2 ) 75% oH 2 /25% pH 2 ) because of its higher statistical weight and the small energy gap between the two rotational levels.The two allotropes of hydrogen, oH 2 and pH 2 , may be quantitatively incarcerated into C 60 to form the endofullerene guest@host complexes [2][3][4] oH 2 @C 60 and pH 2 @C 60 , respectively. We have developed 5 the following procedure for producing samples of pH 2 -enriched H 2 @C 60 in which the incarcerated H 2 spin isomers are not in spin equilibrium at RT: (1) adsorbing a sample of eH 2 @C 60 on the external surface of NaY zeolite at RT; (2) cooling the sample to 77 K; (3) immersing the sample in liquid oxygen at 77 K, thereby converting the sample to a 50% oH 2 /50% pH 2 mixture (which is eH 2 at 77 K); (4) removing the spin catalyst (O 2 ) by applying a vacuum; (5) rapidly bringing the sample to RT before the sample can back-convert to equilibrium at RT; and (6) extracting the sample into a solvent at RT. The sample at RT is enriched in pH 2 (nonequilibrium 50% pH 2 rather than the equilibrium 25% pH 2 ) and is termed *pH 2 @C 60 .At RT, the back conversion of *pH 2 @C 60 to eH 2 @C 60 takes days in the absence of an added spin catalyst. The conversion of *pH 2 @C 60 to eH 2 @C 60 (eq 1) is followed conveniently by 1 H NMR spectroscopy using a sample that contains HD@C 60 , which does not have spinrotational isomers, as an internal standard.At RT, nitroxides serve as paramagnetic spin catalysts for the back conversion of *pH 2 @C 60 to eH 2 @C 60 . 5 We reasoned that if a derivative of H 2 @C 60 could be rapidly switched from a diamagnetic nitroxide precursor to a paramagnetic nitroxide, a "magnetic switch" for forward conversion (eH 2 @C 60 f *pH 2 @C 60 ) at 77 K and back conversion (*pH 2 @C 60 f eH 2 @C 60 ) at RT would be available. We describe the design and demonstration of such a magnetic switch based on the reversible nitroxide/hydroxylamine system 7 The red curve corresponds to the 1 H NMR signals after conversion. Clearly, the proportion of oH 2 @1 relative to HD@1 decreased as expected: the signal intensities are consistent with a 50%/50% mixture of oH 2 @1/pH 2 @1, corresponding to *pH 2 @1. At RT, the back conversion rate of *pH 2 @1 to eH 2 @1 without a spin catalyst is similar to that of *pH 2 @C 60 to eH ...