Monodromy in the water molecule

The change in the energy level structure of a bent molecule as it starts to sample linear geometries has been discussed in terms of monodromy found in simple two-dimensional model problems. Infrared spectra of hot (T ∼ 3000 K) water are analysed for transitions involving states with high bending excitation which can sample linear HOH geometries. One hundred and thirty four new experimental energy levels are determined for bending states with 5 ⩽ v2⩽9 including band origins for 7ν2, 8ν2 and 9ν2. Plots of those levels with J = Ka show the characteristic rearrangement of energy levels predicted for a system displaying quantum monodromy. Quantum monodromy for water occurs about the ν2 = 7, J = 0 level. The barrier to linearity in H216O is estimated to lie at 11 114 ± 5 cm−1.