Endohedral fullerene

Endohedral fullerenes, often called endofullerenes, are unique molecular structures. They consist of a fullerene cage that encloses additional atoms, ions, or even small molecular clusters. A fullerene is a hollow sphere made of carbon atoms. When something is trapped inside this sphere, it becomes an endohedral complex. These structures are important because they act as tiny, stable containers. They allow scientists to study how individual atoms or molecules behave when they are isolated from their surroundings.

To describe these structures, scientists use a specific notation. In the past, a metal atom inside a C60 fullerene was simply written as MC60. This was not very clear because it did not show if the metal was inside or outside. In 1991, a better system was proposed using the "at" sign (@). In this notation, atoms to the left of the @ sign are inside the cage. The atoms to the right represent the carbon network. For example, M@C60 means the atom M is trapped inside. A more complex version is K2(K@C59B). This describes a 60-atom cage where one boron atom has replaced a carbon. It also contains one potassium atom inside and two potassium atoms on the outside.

There are two primary categories of these complexes. The first type is known as endohedral metallofullerenes. These cages contain electropositive metals. Scientists create them using a 1 kV direct current arc reactor or through laser evaporation. During this process, electrons transfer from the metal atom to the carbon cage. This transfer usually involves between 2 and 3 charge units. However, in the La2@C80 complex, about 6 electrons can transfer. This makes the carbon cage an anion, which is a negatively charged ion. These anionic cages are incredibly stable. They can withstand temperatures between 600 and 850 °C in the air.

Metallofullerenes can vary significantly in size and composition. While many contain only one metal atom, others are much more complex. Researchers have isolated di-metal complexes and tri-metal carbide fullerenes. One famous example is Sc3C2@C80. A major breakthrough occurred in 1999 when Harry Dorn and his coworkers synthesized Sc3N@C80. This was the first time a molecule fragment was successfully included inside a fullerene cage. To make this, they used arc-vaporization at temperatures up to 1100 °C. They used graphite rods packed with scandium(III) oxide, iron nitride, and graphite powder. This process took place in a nitrogen atmosphere at 300 Torr.

The second category is non-metal doped fullerenes. These include complexes containing noble gases like helium, neon, argon, krypton, or xenon. Creating these is quite difficult. For example, to make He@C60, scientists must pressurize C60 to about 3 bar in a noble-gas atmosphere. Even then, only about one out of every 650,000 cages successfully traps a helium atom. At much higher pressures of 3 kbars, the incorporation rate can reach 0.1%. Surprisingly, nitrogen and phosphorus can also form these complexes, such as N@C60 and P@C60. Unlike the metallofullerenes, these do not involve a charge transfer between the center atom and the cage.

These molecules can also trap entire small molecules. Scientists have synthesized endofullerenes containing dihydrogen (H2@C60), water (H2O@C60), hydrogen fluoride (HF@C60), and methane (CH4@C60). These encapsulated molecules can show unusual physical properties. Theoretically, if these cages are compressed, the molecules inside might break apart. Their fragments could then react with the inside of the fullerene cage to create new substances.

Endohedral fullerenes serve as highly efficient atomic traps. Most atomic traps require complex equipment like magnetic traps or laser cooling. In contrast, fullerene cages provide a trap that is stable at room temperature for a very long time. This stability allows for the study of unique quantum mechanical phenomena. For instance, scientists can use ENDOR spectroscopy to observe the compression of an atomic wave function. The cages also act like a Faraday cage. This means the carbon shell isolates the trapped atom from external electric fields. This isolation is vital for exploring how particles interact with their environment without interference.