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The semi-rigid bender $Hamiltonian1$ has been used to fit the rotation-vibration energy level separations in the fulminic acid (HCNO) molecule. The allowance made in the model for the variation of the CH and CN bond lengths with HCN bending angle proves to be very important, and as well as achieving a good fit we are able to make a detailed investigation of the shape of the HCN bending potential function. From the results we conclude that the equilibrium structure of HCNO is linear but that excitation of the $\nu 1$ or $\nu 2$ stretching vibrations gives rise to an effective HCN bending potential function having its minimum at a non-linear configuration.