A chemical holy grail: the synthesis of helium and neon-containing compounds

A central quest of chemistry is to construct new compounds and synthesise entirely new links between atoms using the known elements as the building blocks. The only two long-lived elements in our universe that are not found in any known chemical compounds are helium and neon. Consequently, one of the greatest remaining challenges in the chemical sciences is to incorporate these elements into synthetic chemistry. However, current thinking is that this is impossible, since helium and neon are thought to have almost no propensity to form chemical bonds.

The difficulties faced in forming compounds containing helium and neon atoms derives from their full and compact electronic shells. For example, helium has a full 1s orbital and is therefore resistive to covalent bond formation. It also has the highest first ionization energy of any neutral atom, and so is energetically unwilling to form ionic bonds. These are huge obstacles to any attempt to induce chemistry for helium, and they are largely shared by neon. Nevertheless, encouraging signs are derived from recent work on the chemistry of argon, another of the noble gases. In the past few years it has been shown that argon-containing compounds can be made using chemistry induced in solid argon matrices at very low temperatures. This work was driven initially by purely theoretical predictions but was shown subsequently to be experimentally viable. In particular, the formation of insertion compounds, such as HArF, was achieved using photochemical stimulation of HF in a low-temperature argon matrix. However, these compounds are metastable, i.e. are trapped in a potential energy well which lies above the dissociation limit that would regenerate bare argon atoms. It is therefore possible for these compounds to decompose rapidly.

Recent theoretical predictions suggest that stable donor-acceptor compounds containing helium or neon atoms are possible. The ideal acceptor molecule contains an energetically accessible orbital vacancy combined with a substantial dipole moment, which serves to stabilise any interaction with the helium or neon atom. Molecules with the right properties include transition metal halides such as AgF and CuF. Calculations suggest that adduct compounds, such He-CuF, can form spontaneously, with He-Cu binding energies of approximately 30 kJ/mol. Even stronger bonds, approaching 100 kJ/mol, are predicted to be achievable if so-called dipole-encapsulated species, such as NaF-He-CuF are formed. Thus we have a potential route to helium and neon chemistry, but the challenge then becomes how to put this into practice.

Here we propose a novel strategy to access this new and profound chemistry. In the case of helium compounds, we propose to synthesise both adduct and dipole-encapsulated compounds using the unique environment provided by helium nanodroplets. Molecules can be added to helium nanodroplets by pick-up of gases, and in the case of metal fluorides these can be formed by oven evaporation. At that point the unique properties of helium nanodroplets kick-in, which include the ultra-low temperature (0.4 K) and rapid cooling, all within a liquid environment. It is therefore possible to bring the metal fluorides into gentle contact with helium using this approach. Furthermore, the low temperature liquid environment provides a means of using long-range dipole forces to steer two metal fluorides into the correct orientation for dipole-encapsulation of a helium atom. Once the compounds have formed, the helium droplets provide another benefit: a convenient means for detecting the new compounds using IR depletion spectroscopy. In the case of neon compounds we will adopt a different experimental approach which exploits low temperature solid neon matrices to form both adduct and dipole-encapsulated compounds.

We believe that the work proposed here is internationally-leading and will deliver a paradigm change in chemistry.

This is a research programme with the potential for immediate and widespread impact across the whole of the chemical sciences. It is also a project with a simple but profound message that can be readily transferred into the public sphere, since it will seek to identify chemistry of the only two long-lived elements that were long thought to be chemically inert, helium and neon. Impact will be delivered in a number of ways, each of which is outline below:

PUBLICATIONS AND PROFESSIONAL DISSEMINATION: The proposed work is highly innovative and addresses a topic of broad familiarity to all chemists. It will therefore be credible to seek publication in the very highest impact journals such as Science, Nature, JACS, and Angewandte Chemie. Equally, the work will be presented at a wide range of conferences beyond the usual specialist subject meetings.

PUBLIC ENGAGEMENT: The basic concept of this research is very approachable and contains a simple underlying message. It therefore highly amenable for public engagement and we will address this in a number of ways including (1) public lectures, (2) through schools outreach and (3) through the production of educational materials (high quality web-based and via educational publications, such as Education in Chemistry). Engagement with the wider public is a particular aim and we will seek to maximise the profile and impact of this work through press releases at appropriate stages of the work.

EXPLOITATION: the techniques proposed here for the formation of helium- and neon-containing compounds could have significant long-term implications in synthetic chemistry. It is unclear whether or not such compounds will have commercial applications, but this will be discussed with the Enterprise and Business Development Office at the University of Leicester. We liaise regularly with EBDO to discuss IPR implications of our work and such meetings will be formally scheduled during the project proposed here.

TRAINING IMPACT: A researcher will be trained at the PDRA level. The opportunity arises to train someone in a wide range of cutting-edge experimental skills, which include the design and operation of a variety of advanced instrumentation. In addition, he/she would gain experience in the use of advanced scientific software, the analysis of spectroscopic data, and the practice and applications of ab initio quantum chemistry. Furthermore he/she will be expected to convey the work not only to specialist scientific audiences, but also to the wider public. Thus we expect to produce a well-rounded and highly trained individual who will be highly competitive in the UK jobs market.