The vibrational dipole moment function of HCN

Dipole moment measurements for the 100 and 011 excited vibrational states of HCN are presented. These, and previously measured, dipole moments are combined with infrared intensity measurements to obtain dipole moment functions for HCN in both normal and internal coordinate systems. The dipole moment results are 〈μ〉₁₀₀=2.980 53(6), 〈μ〉 ₀₁₁=2.9769(7), an μ_e = 3.016 D. The first dipole derivatives with respect to normal coordinates, from IR intensities, are μ₁=+0.0029, μ₂=+0.178, and μ₃=+0.1111 D. These results plus the vibrational dependence of the dipole moments give the following second derivatives: μ₁₁=-0.003 38, μ₂₂=-0. 0251, μ₃₃=+0.0012 D. Dipole moments of several states of DCN are calculated and are in excellent agreement with experimental results.