Probing the interaction between the encapsulated water molecule and the fullerene cages in H(2)O@C(60)(–) and H(2)O@C(59)N(–)

We report a high-resolution photoelectron imaging study of cryogenically-cooled H(2)O@C(60)(–) and H(2)O@C(59)N(–) endohedral fullerene anions. The electron affinity (EA) of H(2)O@C(60) is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C(60), while the EA of H(2)O@C(59)N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C(59)N. 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 H(2)O@C(60)(–), but a repulsive interaction in H(2)O@C(59)N(–). We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H(2)O molecule, providing further insights into the guest–host interactions in H(2)O@C(60)(–) and H(2)O@C(59)N(–).