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

The endohedral complexes of diatomic guest molecules H-2, N-2, O-2, F-2, HF, CO, LiH, LiF, BN, and BeO with C-60 have been characterized computationally by employing second-order Moller-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 C-60 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 C-60. When encapsulated, all guest molecules (except for BN) exhibit bond elongation (up to 0.07 angstrom) and, consequently, a red shift in vibrational stretching frequencies. In fact, the calculated vibrational properties of the H-2@C-60 complex agree well with those derived from experiment. The C-60 geometry is not perturbed significantly upon encapsulation, but a subtle tendency to decrease the carbon-carbon bond alternation is observed. Polar guest molecules inside C-60 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.