The dipolar endofullerene HF@C60

Abstract

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...

First Synthesis and Characterization of CH4@C60

“…TheI NEPT pulse sequence was used as defined by Morris and Freeman [48] with an interpulse delay of t = 1 4 J HC = 2.012 ms (J HC = 124.3 Hz). This is alarge deshielded shift of the cage 13 CNMR resonance in comparison with the effect of smaller molecular endohedral species (HF@C 60 , Dd =+0.04 ppm, [17] H 2 @C 60 , Dd =+ 0.08 ppm, [16] and H 2 O@C 60 , Dd =+0.11 ppm [15,16] ), consistent with the large size of methane.Correlation of Dd with the van der Waals radius of the enclosed species has been described for the inert gas@C 60 series. The 13 CNMR resonance for the cage in CH 4 @C 60 appears at d C = 143.20 ppm, shifted by Dd =+0.52 ppm relative to C 60 itself.…”

“…and is similar to that reported for the equivalent C 60 solvate, [45] with the exception of as pherically symmetrical electron density distribution located at the center of the fullerene,c orresponding to the endohedral methane molecule.T he electron density map shows af aint spherical shell around the main center of the endohedral electron density,ataradius of 1.03 ( Figure 3). [17,[20][21][22] Aclassical description in which the methane explores arandom set of orientations would give as imilar result. [17,[20][21][22] Aclassical description in which the methane explores arandom set of orientations would give as imilar result.…”

“…[17,18] Them acroscopic quantities of endohedral fullerenes provided by molecular surgery have allowed detailed investigation of physical properties,i ncluding by neutron scattering, infrared spectroscopy,a nd NMR spectroscopy. [19] These methods have shown that, as ar esult of the inert and highly symmetrical environment of the cavity,a ne ntrapped molecule behaves much as would be expected in the very lowpressure gas state, [17,[19][20][21][22][23] displaying free rotation at cryogenic temperatures. [19][20][21][22][23][24] The16-membered orifice of 2 is too small to allow entry of bigger guests,b ut these can be accommodated by the larger (17-membered) opening of fullerene 3.…”

Isolation of the simplest hydrated acid

“…In contrast, the “molecular surgery” approach can produce molecule-encapsulating fullerenes with almost-perfect selectivities under mild conditions in solution ( 15 ). Using this method, endohedral C 60 encapsulating a single molecule of H 2 ( 16 ), He ( 17 ), H 2 O ( 18 ), and HF ( 19 ) was synthesized.…”

“…S19). The infrared (IR) bands for the HO–H and F–H bonds were not observed, probably due to the shielding effects of the cage, which was the same for H 2 O@C 60 ( 18 ), HF@C 60 ( 19 ), and H 2 O@C 70 ( 23 ). However, interesting suppression of the characteristic IR bands of C 70 was observed for HF@C 70 and (H 2 O·HF)@C 70 (fig.…”