Atomic and Molecular Endofullerenes: Spins in a box

富勒烯是由碳原子组成的足球形状的笼子，英国科学家哈里·克罗托（Harry Kroto）因发现富勒烯而获得1996年诺贝尔奖。笼子里是一片空白。化学家和物理学家已经找到了许多巧妙的方法，将原子或分子困在微小的富勒烯笼子里。这些被包裹的化合物被称为内富勒烯，记为A@C60。一种引人注目的方法被称为“分子外科手术”，它通过一系列化学反应在富勒烯上开一个洞，将一个小分子或原子插入每个富勒烯笼中，然后再通过一系列化学反应将这些洞“缝合”起来，使里面的原子或分子重新形成原始的笼。最初的例子是氢（H2@C60）和水（H2O@C60）。我们团队对报告方法进行了极大的改进，并将其扩展到HF@C60。我们的团队最近在封装甲烷以获得CH4@C60方面取得了突破，这是第一次将有机分子放入C60中。该方法使用了一个比以前更大的孔，为包裹氨（NH3）、氧（O2）和甲醛（CH2O）等其他有趣的分子开辟了道路。在气相中，氨（NH3）在电磁波谱的微波区表现出不寻常的共振。这种共振与金字塔形氨分子的“反转”有关，类似于一把伞在强风中被反转。这种氨共振具有重要的历史意义，因为它被用于第一个MASER实验（受激辐射的微波放大），这是激光的前身。在普通的实验条件下，由于氨与邻近分子的相互作用，这种微波激射共振被猝灭。然而，它可能存在于被困在C60分子封闭腔内的氨中。我们打算找出答案。许多小的对称分子表现出一种叫做自旋异构的现象。这意味着它们以几种形式存在，以其磁性原子核的结构来区分，并且它们相互转化的速度很慢。我们将利用核磁共振（NMR）等技术研究甲烷、氨和甲醛等受限分子的自旋同分异构体，核磁共振可以检测强磁场中原子核的射频发射。在某些情况下，可以利用自旋同分异构体来增强核磁共振信号。这是潜在的重要，因为核磁共振在整个科学中广泛用于检查物质的结构和运动-最著名的例子是MRI（磁共振成像）。任何增加核磁共振信号强度的技术都具有潜在的重要意义。氧（O2）是一种不寻常的分子，因为它在基态有两个不成对的电子自旋。因此，氧具有轻微的磁性。我们将使用一种叫做电子顺磁共振（EPR）的技术来研究富勒烯封装的氧中未成对电子自旋的行为，在这种技术中，未成对电子在强磁场中被监测微波发射。我们有理由相信，一个氧原子质量数为16，另一个氧原子质量数为18的氧分子，在低温下会有非常不寻常和有用的EPR性质。元素氦（He）有两种稳定的同位素，分别是氦-3和氦-4。氦-3 （3He）是一种非常适合核磁共振的原子核，它能发出强而窄的信号。然而，它是一种非常稀有和昂贵的气体。我们将3He封装在富勒烯笼中，通过将固体材料暴露在激光使3He进入强极化状态的3He气体中，大大增强了氦-内富勒烯的3He核磁共振信号。极化3He-endofullerene固体可以作为示踪物质应用，例如在磁共振成像中。

Fullerenes are football-shaped cages of carbon atoms, for the discovery of which the British scientist Harry Kroto won the Nobel prize in 1996. Inside the cage is an empty space. Chemists and physicists have found many ingenious ways of trapping atoms or molecules inside the tiny fullerene cages. These encapsulated compounds are called endofullerenes and denoted A@C60. A remarkable method is called "molecular surgery" in which a series of chemical reactions is used to open a hole in the fullerene, a small molecule or atom is inserted into each fullerene cage, and a further series of chemical reactions is used to "sew" the holes back up again to reform the pristine cage with the atom or molecule inside. Initial examples were hydrogen (H2@C60) and water (H2O@C60). Our team greatly improved the reported method and extended it to HF@C60. Our team recently achieved a breakthrough in encapsulating methane to give CH4@C60 - the first time an organic molecule has been put inside C60. The route developed, using a larger hole than before, opens the way to encapsulating other interesting molecules such as ammonia (NH3), oxygen (O2) and formaldehyde (CH2O).In the gas phase, ammonia (NH3) displays an unusual resonance in the microwave region of the electromagnetic spectrum. This resonance is associated with the "inversion" of the pyramid-shaped ammonia molecule, similar to an umbrella being inverted in a strong wind. This ammonia resonance is of great historical significance, since it was used for the very first MASER experiment (microwave amplification by stimulated emission of radiation), which was the precursor of the laser. This MASER resonance is quenched for ammonia in ordinary experimental conditions, by the interaction of the ammonia with neighbouring molecules. However it may exist for ammonia trapped inside the closed cavity of a C60 molecule. We intend to find out. Many small symmetrical molecules display a phenomenon called spin-isomerism. This means that they exist in several forms distinguished by the configurations of their magnetic atomic nuclei, and which convert only slowly into each other. We will study the spin-isomerism of confined molecules such as methane, ammonia, and formaldehyde by using techniques such as nuclear magnetic resonance (NMR), which detects radio frequency emissions from the atomic nuclei in a strong magnetic field. In some circumstances, spin-isomerism may be exploited to give strongly enhanced NMR signals. This is potentially important since NMR is widely used throughout science for examining the structure and motion of matter - the most famous example being MRI (magnetic resonance imaging). Any technique that increases the strength of NMR signals is potentially of great importance. Oxygen (O2) is an unusual molecule since it has two unpaired electron spins in the ground state. For this reason, oxygen is slightly magnetic. We will study the behaviour of the unpaired electron spins in fullerene-encapsulated oxygen by using a technique called electron paramagnetic resonance (EPR) in which the unpaired electrons are monitored for microwave emission in a strong magnetic field. We have reason to believe that oxygen molecules in which one of the oxygen atoms has atomic mass number 16, and the other one has atomic mass number 18, will have very unusual and useful EPR properties at low temperature. The element Helium (He) has two stable isotopes, called helium-3 and helium-4. Helium-3 (3He) is a very favourable nucleus for NMR, giving a strong, narrow signal. However it is a very rare and expensive gas. We will encapsulate 3He inside fullerene cages and greatly enhance the 3He NMR signals of the helium-endofullerene by exposing the solid material to 3He gas which has been brought into a strongly polarized state by using lasers. The polarized 3He-endofullerene solid may have applications as a tracer substance, for example in magnetic resonance imaging.