Reduced effective vibrational-rotational Hamiltonian for bending vibrational levels of acetylene molecule

Using symmetry arguments effective vibrational-rotational Hamiltonians for bending vibrational levels of C 2 H 2 and C 2 D 2 have been written up to the fourth order in the Amat-Nielsen ordering scheme. These Hamiltonians describe all bending energy levels of molecules in ground electronic state. Hamiltonian for C 2 D 2 contains Darling- Dennison resonance interaction terms (2(omega) 4 is congruent to 2(omega) 5 ). The latter differs slightly from that used by T.R. Huet, M. Herman, and J.W.C. Johns. It contains additional terms. It has been shown that these effective Hamiltonian are ambiguous. The transformations to the reduced form have been suggested.