Question

(a) Describe briefly how you would synthesize the solid state compound \(\mathrm{Na}_{3} \mathrm{C}_{60}\) (b) The \({ }^{13} \mathrm{C}\) MAS NMR spectrum of \(\mathrm{C}_{\omega}\) recorded at room temperature shows a single narrow resonance, but as the temperature falls the spectrum broadens to a feature covering several \(\mathrm{kHz}\). Describe the origin of the differences in the spectra. (c) Reaction of \(\mathrm{C}_{60}\) with rubidium and caesium produces an intercalation compound with the formula, \(\mathrm{Rb}_{2} \mathrm{CsC}_{60}\). Describe, with the aid of a diagram, why you would expect this compound to be a type II superconductor. (d) Explain what is meant by the underlined terms in part (c).

Short answer

(a) The solid state compound Na3C60 can be synthesized by preparing a stoichiometric mixture of elemental sodium (Na) and C60, initiating a reaction under high heat and pressure, and then extracting the product, Na3C60. (b) The difference in the 13C MAS NMR spectra of Cω at varying temperatures is due to the change in the motion of C60 molecules. At room temperature, this motion is fast, creating a single narrow resonance. When the temperature decreases, this motion slows, and the resultant spectrum broadens due to increased anisotropic chemical shift. (c) The intercalated compound Rb2CsC60 is expected to be a type II superconductor due to its unique structure. The presence of Rb and Cs cations between the layers of C60 molecules facilitates charge-transfer doping and enhances electron-phonon coupling, promoting superconductivity. The admittance of magnetic flux vortices between the layers is a characteristic feature of type II superconductors. (d) Intercalation compounds are materials in which guest atoms or ions are incorporated between the layers of host lattices, enhancing their electronic and magnetic properties. Type II superconductors exhibit superconductivity while allowing partial magnetic flux penetration between their two critical magnetic fields, distinguishing them from type I superconductors.

Step-by-step solution

(a) Synthesis of Na3C60

For synthesizing the solid state compound Na3C60, first, prepare a stoichiometric mixture of elemental sodium (Na) and C60. Place the mixture in a closed container (e.g. a stainless steel ball mill) and apply high heat and pressure to initiate a chemical reaction. Maintain this condition for a sufficient amount of time to ensure that the reaction goes to completion. Finally, the product, Na3C60, can be obtained by extracting the excess unreacted sodium using an appropriate solvent and isolating the target compound after solvent removal.

(b) 13C MAS NMR Spectra of Cω and Temperature Effect

The 13C MAS NMR spectrum of Cω at room temperature shows a single narrow resonance because the motion of C60 molecules is fast, averaging the magnetic environment around the 13C nucleus. As the temperature decreases, the motion of the C60 molecules slows down, leading to a broader spectrum due to the frozen-in orientational disorder. This results in a higher anisotropic chemical shift, causing the spectrum to cover several kHz.

(c) Rb2CsC60 Type II Superconductivity

Type II superconductors have two critical magnetic fields: the lower critical field (Hc1) and the upper critical field (Hc2). Between these two fields, they allow magnetic flux penetration in the form of flux vortices. Rb2CsC60 is an intercalation compound, with Rb and Cs cations inserted between the layers of C60 molecules. These cations induce charge-transfer doping and enhance the electron-phonon coupling, promoting superconductivity. Due to the layered structure and the introduction of magnetic ions (Rb and Cs), Rb2CsC60 is expected to be a type II superconductor, allowing magnetic flux vortices to form and penetrate between the layers.

(d) Explanation of Underlined Terms

Intercalation compounds are a class of materials formed by the insertion of guest atoms, ions, or molecules between the layers of host lattices. This process enhances the electronic and magnetic properties of the compounds. Type II superconductors are materials that exhibit superconductivity but allow some magnetic flux penetration between their two critical magnetic fields, Hc1 and Hc2. This ability of partial flux penetration distinguishes them from type I superconductors, which exhibit a complete Meissner effect and perfect diamagnetism below their critical magnetic field.