VIBRATION AND ROTATION OF CO IN C(60) AND PREDICTED INFRARED-SPECTRUM

We present the Hamiltonian for the vibrations and rotations of CO insi de a freely rotating or fixed C-60 molecule and we calculate its eigen states from an atom-atom model potential. The ensuing level structure can be understood in terms of three basic characteristics. (i) Simulta neous rotations of CO and its position vector R, which give rise to a rotational structure similar to that of free CO. The effective rotatio nal constants differ considerably, however. (ii) Splittings of the lev els by the icosahedral field of C-60 which perturb the regular rotatio nal structure, because they are of the same order of magnitude as the rotational spacings. (iii) Large frequencies associated with the (near ly harmonic) vibrations of CO against the hard walls of the C-60 cage: 209 cm(-1) for the radial excitation and 162 cm(-1) for the twofold d egenerate libration. These vibrations give a rovibrational level struc ture similar to that of a linear triatomic molecule, the radial excita tion resembles a bond stretch (Sigma) state, the libration a II-bendin g state. From the eigenstates we calculate the line strengths of the e lectric dipole transitions allowed by the icosahedral symmetry. Additi onal (approximate) selection rules are found, and the infrared spectru m of CO@C-60 is predicted. (C) 1996 American Institute of Physics.