Theoretical prediction of the vibrational spectra of group IB trimers.

The molecular structures of the group IB trimers, Cu(3), Ag(3), and Au(3), have been determined by using the semi-empirical diatomics-in-molecules theory. The trimers are found to have C(2v) symmetry with bond angles between 65 degrees and 80 degrees . The trimers are bound with respect to dissociation to the asymptotic limit of an atom plus a diatom. The binding energies per atom for Cu(3), Ag(3), and Au(3) are 1.08, 0.75, and 1.16 eV, respectively. The vibrational frequencies of the trimers have been determined for comparison with experimental results. The vibrational frequencies are characterized by low values for the bending and asymmetric stretch modes. The frequency of the symmetric stretch of the trimer is higher than the stretching frequency of the corresponding diatomic. A detailed comparison of the theoretical results with the previously measured Raman spectra of matrix isolated Ag(3) is presented.