This Theo Murphy Meeting Issue contains papers presented at a Discussion Meeting held at the Kavli Centre of the Royal Society in March 2012. The meeting brought together a wide variety of scientists working on different aspects of small-molecule endofullerenes—those intriguing chemical systems in which small molecules such as H2 or H2O are encapsulated in tiny carbon cages.
The development of this field was enabled by the breakthrough synthetic concept of ‘molecular surgery’, originally proposed by Rubin and co-workers in 2001 . This idea was brought to a spectacular conclusion by the groups of Komatsu and Murata, who achieved the complete molecular surgical synthesis of the closed H2@C60  and H2O@C60  systems. These impressive synthetic advances have made available small-molecule endofullerenes in high chemical purity and sufficient quantities for spectroscopic investigation, even by relatively insensitive methods such as neutron scattering. The encapsulated small molecules are closely confined but free to rotate, leading to quantization of the translational and rotational energy levels for the encapsulated molecules. In addition, many confined molecules are symmetrical quantum rotors and display the phenomenon of spin isomerism, implying an entanglement of spin and spatial quantum states. The carbon cages act as ‘nanolaboratories’, which protect the small molecules from their immediate environment, enabling detailed and quantitative study of their quantum behaviour, and high-level comparisons with theoretical expectations.
So far, most investigations have concentrated on the physical characterization and theoretical interpretation of the quantum behaviour of the confined molecules, and investigations of how the endohedral species couple to the world outside the cage. This research is driven by the intrinsic interest of these very unusual physical systems, and also by the possibility of finding new material or spectroscopic phenomena. For example, there are speculations that electric dipole couplings between quantum rotors in neighbouring cages could lead to phenomena such as ferroelectricity , or that the close coupling between the space and spin degrees of freedom might lead to new means of inducing nuclear spin hyperpolarization and the strong enhancement of NMR signals in the solid state. Although neither of these phenomena have been observed, at the time of writing, the articles in this issue show that a great deal of the groundwork that is required to underpin such developments has now been accomplished.
One of the authors (M.H.L.) became interested in this subject around 2002, stimulated by the fine articles by Tomaselli and co-workers [5–7] on the low-temperature solid-state NMR of interstitial complexes of H2 and C60. The entanglement of spin and space degrees of freedom for the interstitial hydrogen molecules is very intriguing, and resembles the case of freely rotating methyl groups that were studied extensively over many years by the Nottingham group of Professor Stan Clough and the other author of this article (A.J.H.). Compounds containing rotating methyl groups sometimes display a Haupt effect (an increase in nuclear polarization induced by a temperature jump) . M.H.L. wondered whether three-dimensional rotors such as H2 could exhibit a Haupt-like effect, but was not attracted to the practical challenges of studying the interstitial H2/C60 complexes, which are only stable in a hydrogen atmosphere. Through a chance acquaintance, M.H.L. was alerted to the recently published work of Yves Rubin in Los Angeles , and by good fortune was able to visit his laboratory soon after and strike up a collaboration. Shortly afterwards, Rubin alerted M.H.L. to the remarkable synthetic advances in Japan, and generously recommended a collaboration with that group instead. That was the start of what has been a fulfilling and instructive scientific journey, which has led M.H.L. far from his initial comfort zone of NMR methodology into quite different scientific areas such as neutron scattering and infrared spectroscopy.
At the same time, Nick Turro and his co-workers in New York had come to similar conclusions about the intrinsic interest and potential of these beautiful molecular systems and had initiated a very fast-moving research programme, which rapidly led to many beautiful synthetic and spectroscopic results. With typical generosity of spirit, Nick suggested that all groups join forces in investigating these materials and proposed sharing globally the synthetic output of the collaborating Kyoto and New York teams. In a very short space of time, the immense enthusiasm and organizing skill of Nick led to the assembly of an impressive international consortium comprising synthetic chemists, nuclear and electromagnetic resonance spectroscopists, infrared spectroscopists, neutron scattering specialists, photochemists and quantum theoreticians, many of whom have contributed to research articles in the current issue. A key event was the endofullerene symposium organized by Nick in New York in 2008, at which many of the consortium were brought together for the first time and which firmly established the free exchange of ideas and materials among its members across the globe.
It is appropriate to start this issue with a short account by Prof. Komatsu of the first synthesis of H2@C60 by complete molecular surgery , since this achievement underpins most of the field. It is illuminating to read how sulfur was inserted into the cage: this crucial step widens the orifice, allowing the insertion of small molecules, and also provides a relatively easy route for the resealing of the cage afterwards. This elegant procedure is a fine example of nano-engineering—a discipline practiced for decades with great skill by synthetic chemists, since long before the term was invented.
The second article is by M.H.L. , which provides a review of the quantum energy level structure and spectroscopy of light-molecule endofullerenes.
The article by Turro and co-workers  ranges widely over the many different spectroscopic phenomena expected and observed for these molecular systems, with special attention paid to the observation of the spin isomers and their interconversion. Since the para spin isomer of H2 has no net nuclear spin it does not provide an NMR signal. The conversion of ortho-H2 into para-H2, and vice versa, must therefore be deduced by relatively subtle changes in the intensity of the ortho-H2 NMR signal. The Turro group developed a clever method in which the signals from endohedral HD act as an intensity reference. This works because HD does not display spin isomers, and because its signals are spectrally resolved from those of ortho-H2 through the proton–deuteron spin–spin coupling and an isotope shift. In this way, it was possible to monitor spin-isomer conversion induced by radical moieties attached to the outside of the cage, and by photo-excited triplet excitations on the cage itself. This article also contains some preliminary NMR data on 17O-enriched H2O@C60.
The following articles consider the physical properties of H2 and its isotopologues when encapsulated in fullerenes. Murata and co-workers report on kinetic and thermodynamic differences between H2, HD and D2 when incorporated into an open-cage fullerene . Rõõm and co-workers  provide a detailed exposition of the infrared spectroscopy of H2, HD and D2 encapsulated in C60. The theoretical treatment of this problem requires solution of the quantum dynamics for a vibrating quantum rotor moving in a three-dimensional spherical potential. Experimental infrared data and an initial theoretical interpretation are presented for the case of H2@C70, in which the confining potential is elongated along one axis. The article by Horsewill and co-workers  discusses inelastic neutron scattering (INS) of H2@C60, with particular attention paid to the temperature dependence of the spectra, which enables the assignment of the transitions. Xu et al.  present a theoretical study of the quantum dynamics of HD encapsulated in C60, in particular the temperature dependence of the INS spectra and associated selection rules.
The coupling of electron spins to endohedral species is the topic of the next set of articles. Filidou et al.  present electron–nuclear double resonance data on the photo-excited triplet states of fullerenes with attached exohedral groups and on the photo-excited triplet state of H2@C60. Hyperfine couplings are observed between the electron spin and atomic nuclei in the exohedral group (in the first case) and the proton nuclei of the endohedral H2 molecule (in the second case). Density functional theory (DFT) calculations are presented for the excited triplet state of H2@C60.
The effect of unpaired electrons on the nuclear relaxation of endohedral H2 in solution is investigated by Rastrelli et al. . DFT calculations are conducted on endofullerenes with attached nitroxide moieties. Guldi and co-workers  consider the electronic properties and energetics of fullerenes and how these are modified by endohedral metals.
The successful synthesis of H2O@C60 by molecular surgery  has provided systems that are potentially even more exciting than H2@C60. This Meeting Issue concludes with an article by Concistrè et al.  on the solid-state NMR of H2O@C60. The experimental data are interpreted using a model of cage distortion by the endohedral molecule, which possesses an electric dipole moment.
It is very sad that this Meeting Issue will not be read by Nick Turro, who passed away at the end of 2012. His multidisciplinary vision has shaped this field more than anyone else.
One contribution of 13 to a Theo Murphy Meeting Issue ‘Nanolaboratories: physics and chemistry of small-molecule endofullerenes’.
- © 2013 The Author(s) Published by the Royal Society. All rights reserved.