Encapsulation of diatomic molecules in fullerene C60: implications for their main properties

Abstract

The endohedral complexes of diatomic guest molecules H₂, N₂, O₂, F₂, HF, CO, LiH, LiF, BN, and BeO with C₆₀ have been characterized computationally by employing second-order Møller–Plesset (MP2) theory and its density-fitting local (DF-LMP2) variant. The interaction energies, equilibrium geometries, dipole moments and harmonic vibrational frequencies of these complexes have been systematically calculated. It was found that all guest molecules are stabilized inside the C₆₀ cage, with the most pronounced stabilization effect (of about 50 kcal mol⁻¹) observed for the polar covalent BeO and BN molecules. It is noteworthy that the normally short-lived BN molecule is the only guest molecule that was found to chemisorb on the inner surface of C₆₀. When encapsulated, all guest molecules (except for BN) exhibit bond elongation (up to 0.07 Å) and, consequently, a red shift in vibrational stretching frequencies. In fact, the calculated vibrational properties of the H₂@C₆₀ complex agree well with those derived from experiment. The C₆₀ geometry is not perturbed significantly upon encapsulation, but a subtle tendency to decrease the carbon–carbon bond alternation is observed. Polar guest molecules inside C₆₀ are located at an off-center position and a significant decrease in their dipole moments upon encapsulation is observed. The importance of explicitly taking into account electron correlation effects, as well as full geometry relaxation, to yield a correct description of the complexes investigated is clearly demonstrated. The present results may serve as a guide for future attempts to synthesize such complexes employing the “molecular surgery” approach.