Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60– and H2O@C59N–

The guest–host interactions in the H2O@C60 and H2O@C59N endohedral fullerenes are probed by high-resolution photoelectron imaging.


Introduction
Endohedral fullerenes with encapsulated atoms, molecules or clusters have attracted wide interest due to their unique electronic, magnetic, and optical properties. [1][2][3][4] Since the rst observation of the endohedral fullerene La@C 60 in a mass spectrum in 1985, 5 a variety of such novel guest-host complexes containing noble gas atoms, 6,7 the N atom and the N 2 molecule, 8 metal atoms and metal clusters, 4,9 have been synthesized using the arc discharge or ion bombardment methods. These harsh production conditions were unable to make endofullerenes containing light molecules. 10 A more rational synthetic approach, called molecular surgery on the fullerene surfaces, 11,12 was successfully applied to the macroscopic synthesis of H 2 @C 60 , 13 followed by the syntheses of H 2 O@C 60 , 14,15 H 2 O@C 59 N, 16 HF@C 60 , 17 (H 2 O) 2 @C 70 , 18 and very recently even (H 2 O$HF)@C 70 . 19 The H 2 O@C 60 and H 2 O@C 59 N species are of special interest because the water molecule is isolated without hydrogen bonds. Many experimental and theoretical studies have been carried out to elucidate the novel properties of H 2 O@C 60 , such as its polarity, 20-26 quantum dynamics, 27-30 magnetic, 16,31 mechanical, 32 thermal 33 and electric properties 25,26,34,35 as well as its chemical reactivity. 24,36,37 One of the most interesting questions about H 2 O@C 60 concerned the nature of the guest-host interactions of the water molecule trapped in the C 60 cage. No detectable difference was observed between the UV-Vis absorption spectra of the empty C 60 and H 2 O@C 60 , suggesting that the water molecule has very weak interactions with the cage. 14 This observation was further conrmed by studies of nuclear spin relaxation 31 and electric conductance. 35 However, theoretical calculations found strong dispersion interactions 22,23,[38][39][40] between the free rotating water molecule and C 60 . 14,23,27,40,41 The quantized rotational levels and the nuclear spin-isomerism of ortho-and para-water in H 2 O@C 60 were studied by inelastic neutron scattering, farinfrared spectroscopy, and nuclear magnetic resonance. [27][28][29] These studies revealed a splitting of the ground rotational state of ortho-H 2 O and a symmetry-breaking of the C 60 cage, indicating a quadrupolar interaction between H 2 O and C 60 . 30 In addition, the dipole moment of H 2 O@C 60 was measured to be around 0.5 D, 25,26 in good agreement with theoretical calculations. [20][21][22][23][24] The signicant reduction of the dipole moment of the encapsulated H 2 O is a result of the strong shielding effect by the nonpolar C 60 cage. A recent study reported that the rotation of the encapsulated water can be electrostatically perturbed by introducing polarized C(C 60 )-X (X: heteroatom) bonds. 42 Unlike the extensive studies on H 2 O@C 60 , the H 2 O@C 59 N endohedral azafullerene was only synthesized very recently in the dimer form, (H 2 O@C 59 N) 2 . 16 The presence of the N atom breaks the symmetry of the fullerene and introduces a polar center. Theoretical calculations suggested an attractive electrostatic interaction between the O atom of H 2 O and the N atom of C 59 N. 16,43,44 Comparison of the different guest-host interactions in H 2 O@C 60 and H 2 O@C 59 N would be very interesting. In particular, the electron affinity (EA) of the endohedral fullerenes can be a good probe of these guest-host interactions, because the extra electron in the C 60 À and C 59 N À anions is expected to be sensitive to the encapsulated H 2 O molecule.
Here, we present a high-resolution photoelectron (PE) imaging study of the H 2 O@C 60 À and H 2 O@C 59 N À anions cooled in a cryogenic ion trap. The EA of H 2 O@C 60 is accurately measured to be 2.6923 AE 0.0008 eV, which is 0.0088 eV higher than the EA of C 60 , 45 while the EA of H 2 O@C 59 N is measured to be 3.0058 eV AE 0.0007 eV, which is 0.0092 eV lower than the EA of C 59 N. 46 The opposite shis suggest different guest-host interactions between the encapsulated water molecule and the fullerene cages, which are understood by an electrostatic model. A net coulombic attraction between the water molecule and the HOMO electron in H 2 O@C 60 À is found to stabilize the anion and enhance the EA of H 2 O@C 60 compared to C 60 , while a repulsive interaction in H 2 O@C 59 N À destabilizes the anion and decreases the EA of H 2 O@C 59 N relative to C 59 N. In addition, low-frequency features in the PE spectra are observed and tentatively attributed to the hindered rotational excitations 30 of the encapsulated H 2 O molecule, providing further insights into the guest-host interactions in H 2 O@C 60 À and H 2 O@C 59 N À .

Experimental method
The experiment was carried out using our third-generation electrospray PE imaging apparatus, 47 equipped with a cryogenically-cooled Paul trap 48 and a high-resolution PE imaging lens. 49 The electrospray solutions were prepared by dissolving H 2 O@C 60 or (H 2 O@C 59 N) 2 samples in a mixed solvent of o-dichlorobenzene/CH 3 CN (1/3 ratio in volume), to which tetrakis(dimethylamino)ethylene 50,51 was added as a reducing agent. Anions from the electrospray source were guided into a cryogenically-controlled Paul trap operated at 4.5 K and thermally cooled via collisions with 1 mTorr He/H 2 (4/1 in volume) background gas. 48 The cold anions were pulsed out of the ion trap at a 10 Hz repetition rate into the extraction zone of a time-of-ight mass spectrometer. The desired anions, H 2 O@C 60 À or H 2 O@C 59 N À , were selected by a mass gate and photodetached by the third harmonic of a Nd:YAG laser (354.7 nm) and a tunable dye laser in the interaction zone of the imaging lens. 49 The PE images were inverse-Abel transformed and reconstructed using both pBasex and BASEX. 52,53 The PE spectra were calibrated with the known spectra of Au À at different photon energies. The kinetic energy (KE) resolution was 3.8 cm À1 for electrons with 55 cm À1 KE and about 1.5% (DKE/KE) for KE above 1 eV in the current experiment.

Results and discussions
3.1. The photoelectron images and spectra of H 2 O@C 60 À and H 2 O@C 59 N À at 354.7 nm Fig. 1 shows the PE images and spectra of H 2 O@C 60 À and H 2 O@C 59 N À at 354.7 nm. The rst intense peak in each spectrum, labeled as 0 0 0 , represents the 0-0 transition from the anion to the neutral and denes the EAs for H 2 O@C 60 and H 2 O@C 59 N, which are measured more accurately in the low photon energy spectra (vide infra). The peaks at higher binding energies represent transitions from the ground vibrational state of the anion to the excited vibrational levels of the neutral ground electronic state. They are better resolved in the highresolution PE images at lower photon energies near the detachment threshold to be discussed below. Fig. 1b also shows a weak peak (X 0 ) at $1.2 eV, which is derived from the parent dimer dianion, (H 2 O@C 59 N) 2 2À with the same m/z as the monoanion. A similar dimer dianion was also observed in the 354.7 nm PE spectrum of C 59 N À recently. 46 The low binding energy for the dianion was due to the strong intramolecular Coulomb repulsion. 47,54,55 Fig. 1 Photoelectron images and spectra of (a) H 2 O@C 60 À and (b) H 2 O@C 59 N À at 354.7 nm. The double arrow below the images indicates the direction of the laser polarization. Note the image corresponding to peak X 0 in (b) is cut off. The 354.7 nm spectra of the endohedral fullerenes appear to be nearly identical to those of their corresponding parent fullerenes, 45,46 as directly compared in Fig. 2. This observation suggests that indeed the encapsulated water molecule has little effect on the electronic and geometrical structures of the fullerene hosts. However, upon closer examination, a small spectral shi was revealed in each case, as shown in the expanded threshold region given in the respective inset of Fig. 2. Surprisingly, the two endohedral fullerenes exhibit opposite shis. The electron binding energy of H 2 O@C 60 À was observed to be shied slightly higher relative to that of C 60 À (Fig. 2a), whereas the electron binding energy of H 2 O@C 59 N À was shied slightly lower relative to that of C 59 N À . The opposite spectral shis suggest subtle differences in the guest-host interactions of the encapsulated water molecule with the fullerene or azafullerene cages.
3.2. The high-resolution photoelectron images and spectra of H 2 O@C 60 À and H 2 O@C 59 N À near detachment thresholds To measure the EAs more accurately and to resolve lowfrequency vibrations, we measured PE images for H 2 O@C 60 À and H 2 O@C 59 N À at lower photon energies near the detachment thresholds, as shown in Fig. 3. We found that the detachment cross sections for the endohedral fullerenes were weaker than those of the corresponding empty fullerenes, 45,46 in particular near the detachment thresholds. The spectra shown in Fig. 3 were averaged from 300 000 to 500 000 laser shots. At 456.60 nm (Fig. 3a), the 0 0 0 peak with a linewidth of 38 cm À1 at an electron kinetic energy of 186 cm À1 denes the most accurate value for the EA of H 2 O@C 60 as 2.6923 AE 0.0008 eV, which is 0.0088 eV higher than the EA of C 60 . 45 The detachment cross section at this wavelength for H 2 O@C 60 À was particularly poor. The features below the 0 0 0 peak in Fig. 3a were partly due to background noise and partly due to hot band transitions, which were amplied relative to the 0 0 0 transition. At 411.12 nm (Fig. 3d), the 0 0 0 peak with an electron kinetic energy of 80 cm À1 and linewidth of 15 cm À1 yields the most accurate EA for H 2 O@C 59 N to be 3.0058 eV AE 0.0007 eV, which is 0.0092 eV lower than the EA of C 59 N. 46 In addition to the near-threshold spectra, two more spectra were taken to resolve low-frequency vibrational features for H 2 O@C 60 and H 2 O@C 59 N, as shown in Fig. 3b, c, e and f, respectively. There are two types of vibrations for the endohedral fullerenes, one involving the fullerene cages and the other involving the encapsulated water molecules including the hindered rotations. The latter should be particularly sensitive to the guest-host interactions in the endohedral fullerenes. Fig. 3a-c resolve seven vibrational peaks, labeled as A-G for H 2 O@C 60 , while Fig. 3f resolves two peaks, A and B for H 2 O@C 59 N. The relative intensities of the low frequency peaks (A, B) are quite weak for both species, but they seem to be reproducible. The binding energies and shis relative to the 0 0 0 peak for all the vibrational features are summarized in Table 1.
Peaks C, D, F, G with shis of 271, 353, 531, and 710 cm À1 , are similar to those observed in the PE spectra of C 60 À and they should correspond to vibrational modes involving the C 60 cage. 45 The strong and highly non-Franck-Condon peak F observed in the 445.60 nm spectrum (Fig. 3c) is also observed for C 60 À , which was attributed to strong Hertzberg-Teller coupling. 45 The weak peak E with a shi of 406 cm À1 corresponds to a H u (1) vibrational mode of C 60 also observed by inelastic neutron scattering. 56 These observations suggest that the H 2 O molecule has little effect on the geometrical and electronic structure of the C 60 host. Additionally, two weak peaks A and B with small shis of 35 and 95 cm À1 , corresponding to very low-frequency transitions, are also tentatively identied. The lowest vibrational frequency of C 60 is around 260 cm À1 . 45,56 Hence, these features should correspond to the hindered rotational excitations of the encapsulated water molecule, as revealed by the rigorous full-dimensional quantum calculations of the coupled translation-rotation eigenstates of the water molecule in H 2 O@C 60 . 30 In Fig. 3f, similarly the two weak peaks A and B with shis of 74 and 128 cm À1 were tentatively iden-tied as the hindered rotational excitations of the encapsulated water molecule. The observation of hindered rotational transitions indicates weak interactions between the encapsulated water molecule and the fullerene cages. The relatively high frequencies observed for the hindered rotational transitions in H 2 O@C 59 N suggest stronger guest-host interactions in this system.
The PE images of H 2 O@C 60 À and H 2 O@C 59 N À in Fig. 3 all exhibit distinct p-wave character with the photoelectron angular distributions parallel to the direction of the laser polarization, similar to those for C 60 À and C 59 N À . 45,46 These observations indicate that the encapsulated water molecule does not affect the s-like HOMO of the fullerene cages. The p-wave nature of the outgoing electron is partly responsible for the low detachment cross sections near threshold according to the Wigner threshold law. 57 3.3. The opposite shis of the EAs in H 2 O@C 60 and H 2 O@C 59 N relative to the empty fullerenes: an electrostatic model The opposite shis of the EAs of H 2 O@C 60 and H 2 O@C 59 N relative to their corresponding empty cages are consistent with previous theoretical calculations. 24,43 The different effects of the encapsulated water on the EAs can be glimpsed from the electrostatic potential maps of the HOMO of the fullerene anions, as presented in Fig. 4. The extra charge in the half-lled HOMO of C 60 À is evenly distributed on the surface (Fig. 4a).
Even though the encapsulated water molecule was known to have no preferred directions, 14,23,27,40,41 it breaks the symmetry and dynamically induces a slightly higher charge density on the cage surface, where the H atoms point to (Fig. 4b). On the contrary, the HOMO of C 59 N À is partially localized on the N atom and the C atoms around the N atom (Fig. 4c). 46 The water molecule in H 2 O@C 59 N À has been shown to adopt a global minimal structure with the O atom pointing to the N atom of the cage due to a weak N/O attractive interaction. 43,44 Despite its orientation preference, the water encapsulation has relatively little effect on the HOMO of C 59 N À (Fig. 4d). However, this orientation of the water molecule brings the electronegative O atom closer to the extra charge, inducing a repulsive interaction. A simple electrostatic model is used to understand the interactions between the water molecule and the extra charge in the HOMO of the fullerene cages and to obtain insights about the observed different EA shis in the two systems. In the model, partial charges on the water molecule are represented by point charges with À2q located on the oxygen atom and +q on each H atom, where q is obtained from a Mulliken population analysis of the total wavefunction of the water molecule. The Coulomb interaction can be expressed as: where q i and r i represent the charge and position of each atom in the water molecule, 4 HOMO (r) is the Kohn-Sham Table 1 The observed vibrational peaks, their binding energies (BE) for H 2 O@C 60 À and H 2 O@C 59 N À from the photoelectron spectra in Fig. 3. Their shifts to peak 0 0 0 are compared with the vibrational frequencies of C 60

Anions
Peaks BE a (eV) Shis (cm À1 ) Vib. freq. a (cm À1 )  Fig. 4 The electrostatic potential maps for the HOMO of (a) C 60 À , (b) H 2 O@C 60 À , (c) C 59 N À , (d) H 2 O@C 59 N À , calculated at B3LYP/6-311++G(d,p) level using the GAUSSIAN 09 package. 58 wavefunction of the HOMO of H 2 O@C 60 À and H 2 O@C 59 N À extracted from DFT calculations. 59 The numerical integration is done with a ne grid converging to 1 meV accuracy. All the geometry optimization and electronic structure calculations were done using DFT at B3LYP/6-311++G(d,p) level of theory with the GAUSSIAN 09 package. 58 The Coulomb interaction in H 2 O@C 60 À was calculated to be À23 meV, indicating an attractive interaction between the encapsulated water molecule and the HOMO electron in H 2 O@C 60 À . This weak attraction, which is in good agreement with previous calculations, 22,[38][39][40] stabilizes the H 2 O@C 60 À anion and increases the EA of H 2 O@C 60 relative to C 60 . On the other hand, the simple electrostatic calculation on the H 2 O@C 59 N À anion yields a repulsive interaction of 64 meV. Hence, the water encapsulation destabilized the HOMO of the C 59 N À anion, reducing the EA of H 2 O@C 59 N relative to C 59 N. This repulsive interaction is expected from the orientation of the H 2 O molecule in C 59 N À and its asymmetric electron density distribution (Fig. 4d). Even though the electrostatic model is rather crude, it correctly predicts the directions of the EA shis in the two endohedral fullerenes. The interactions between the encapsulated water molecule and the fullerene cages are so weak that they were not detectable in the UV-Vis absorption spectra 14 or the electrical conductance experiment. 35

Conclusions
In conclusion, we report a high-resolution photoelectron imaging study of two endohedral fullerene anions, H 2 O@C 60 À and H 2 O@C 59 N À . Accurate electron affinities are obtained for H 2 O@C 60 (2.6923 AE 0.0008 eV) and H 2 O@C 59 N (3.0058 AE 0.0007 eV) for the rst time. The EA of H 2 O@C 60 is found to be higher than that of C 60 by 0.0088 eV, whereas the EA of H 2 O@C 59 N is found to be lower than that of C 59 N by 0.0092 eV. These small EA shis reect the weak guest-host interactions in the endohedral fullerenes and the opposite shis are understood using a simple electrostatic model between the encapsulated H 2 O molecule and the HOMO of the fullerene anions. Low-frequency features due to the hindered rotational transitions of the encapsulated water molecule are also tentatively identied, providing further insight into the weak guest-host interactions in the two endohedral fullerenes.

Conflicts of interest
The authors declare no competing nancial interests.