Gas-phase spectroscopy of H2O@C60+ and H2O@C60H+ in the mid-infrared: the challenges of searching for endohedral fullerenes in space

Abstract

Molecular surgery allows the synthesis of a unique class of molecules known as endohedral fullerenes, where a small species is enclosed within the hollow cavity of a fullerene cage. In this work, mid-IR spectra are presented for the endohedral fullerenes H₂O@C₆₀⁺ and H₂O@C₆₀H⁺, obtained using helium tagging spectroscopy and IRMPD spectroscopy, respectively. These are the first gas-phase vibrational spectra of an endohedral fullerene containing an IR-active guest species. Both spectra look remarkably similar to those of the empty fullerenes, with many of the same features present at slightly blue-shifted frequencies. Interestingly, the water bending mode is not observed in either of the spectra. This has been attributed to a polarisation of the π-electrons of the fullerene cage, reducing the total dipole moment and the IR intensities. The similarity of the spectra with those of the empty fullerenes means that these species will be very hard to distinguish using only IR spectroscopy. This may have significant implications for the detection of H₂O@C₆₀⁺, H₂O@C₆₀H⁺, or indeed other endohedral fullerenes, in space. The IR emission features of the endohedral fullerene will overlap with those of the empty fullerene, so astronomical detection will need to rely on other spectroscopic techniques.