The ν7 vibrational states of C 3O 2. A least-squares fit of the rigid-bender model to the v7ν7l7 energies and rotational constants
The rigid-bender model is used to treat the large-amplitude, low-frequency, bending vibration ν7 of C 3O 2. Different parameterizations of the bending potential function are considered, and a simple two-term power series is found to give the best fit. With this parameterization, using a least-squares fit to energies and B values, the ν7 potential function is determined for the ground state as well as for the states in which ν2, ν3, ν4, ν6, 2 ν6, ν1 + ν3, ν1 + ν4, ν2 + ν3, and 2 ν2 + ν4 are excited. The excitation of other vibrations has in some cases a drastic effect on the ν7 potential. In the ground state the potential has a 29 cm -1 barrier at the linear position, in ν1 + ν3 the barrier increases to 79 cm -1, while in 2 ν2 + ν4 the barrier vanishes. An equilibrium potential is determined by correcting the ground state potential for the effects of zero-point motion of the normal vibrations ν1, …, ν6. This potential has a 35.6-cm -1 barrier with a minimum at α = 11.14°, where 2α is the angular deviation from linearity. The model accurately predicts the quartic and sextic centrifugal distortion terms for the low-lying v7ν7l7 states. Second-order l-type coupling is included in the calculations of the quartic terms. The effects of this coupling, which are most pronounced for the ν7 ≥ 2 states, adequately explain the negative D term recently reported for the ν2 + 4 ν70 state.