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

化学的一个中心探索是构建新的化合物，并使用已知元素作为构建块来合成原子之间的全新连接。在我们的宇宙中，只有两种长寿元素没有在任何已知的化合物中找到，那就是氦和霓虹灯。因此，化学科学中剩余的最大挑战之一是将这些元素纳入合成化学。然而，目前的想法是，这是不可能的，因为氦和霓离子被认为几乎没有形成化学键的倾向。形成含氦和霓原子的化合物所面临的困难来自于它们完整而紧凑的电子壳层。例如，氦有一个完整的1s轨道，因此对共价键的形成是有阻力的。它也是中性原子中第一电离能最高的，因此在能量上不愿形成离子键。这些都是任何尝试诱导氦化学的巨大障碍，而霓虹灯在很大程度上也是如此。然而，令人鼓舞的迹象来自最近对另一种惰性气体--氩气的化学研究。在过去的几年中，已有研究表明，在极低的温度下，可以利用固体Ar中诱导的化学作用来制备含Ar的化合物。这项工作最初是由纯理论预测推动的，但后来被证明在实验上是可行的。特别是，插入化合物的形成，如HARF，是通过在低温Ar基质中对HF进行光化学刺激而实现的。然而，这些化合物是亚稳态的，即被困在高于解离极限的势能中，该离解极限将重新生成裸露的Ar原子。因此，这些化合物有可能迅速分解。最近的理论预测表明，含有氦或霓原子的稳定的给体-受体化合物是可能的。理想的受体分子包含一个可获得能量的轨道空位和一个相当大的偶极矩，这有助于稳定与氦或霓虹原子的任何相互作用。具有正确性质的分子包括过渡金属卤化物，如AGF和CuF。计算表明，He-CuF等加合物可以自发形成，He-CuF的结合能约为30kJ/mol。如果形成所谓的偶极封装物种，如NaF-He-CuF，则有望实现更强的键，接近100kJ/mol。因此，我们有了一条通往氦和霓虹化学的潜在途径，但接下来的挑战是如何将其付诸实践。在这里，我们提出了一种新的战略来访问这种新的和深刻的化学。对于氦化合物，我们建议利用氦纳米液滴提供的独特环境来合成加合物和偶极封装的化合物。分子可以通过吸收气体添加到氦纳米液滴中，对于金属氟化物，这些分子可以通过烘箱蒸发形成。在这一点上，氦纳米液滴的独特性质开始发挥作用，包括超低温(0.4K)和快速冷却，所有这些都是在液体环境中进行的。因此，使用这种方法可以使金属氟化物与氦温和接触。此外，低温液体环境提供了一种使用长程偶极作用力来引导两个金属氟化物进入正确方向的方法，以实现氦原子的偶极封装。一旦化合物形成，氦液滴就提供了另一个好处：一种使用红外耗尽光谱检测新化合物的便捷手段。对于霓虹灯化合物，我们将采用一种不同的实验方法，利用低温固体霓虹灯基质形成加合物和偶极封装的化合物。我们相信，这里提出的工作是国际领先的，将带来化学范式的改变。

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.