Rotationally induced vibrational mixing in formaldehyde

Almost-degenerate perturbation theory is used to derive an effective Hamiltonian describing the vibrational states of H2CO. Eigenvalues have been determined for energies up to 8600 cm−1 above the zero-point energy. Both curvilinear and rectilinear representations of the vibrational dynamics are presented and explored. Although differences are observed between the two effective Hamiltonian matrix elements, their eigenvalues generally agree to better than a wave number for the energies studied. Using the Watson Hamiltonian, the mechanism of rotationally induced vibrationally mixing is investigated as a function of K, the projection of the total angular momentum onto the body-fixed a axis. The combination of a-axis Coriolis coupling and Fermi couplings leads to extensive vibrational mixing between the rotational–vibrational states in this energy regime.