An ab initio study on the equilibrium structure and CCC bending energy levels of carbon suboxide

The molecular parameters of carbon suboxide, C3O2, have been determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double- through quadruple-zeta quality. The potential energy function for the large-amplitude CCC bending motion (the v(7) mode) was determined to be strongly anharmonic. The equilibrium structure of the molecule was found to be bent, with a barrier to linearity of only 18 cm(-1). The rotation-bending energy levels were then calculated using a semirigid-bender Hamiltonian. The vibrational energy levels and effective rotational constants determined for various v(7) states were found to be in good agreement with the experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.