Dipole moments of highly excited vibrational states of HCN

Measurement of the Stark effect by intracavity photoacoustic spectroscopy of weak visible vibrational overtone transitions of HCN has produced dipole moments for several excited vibrational levels: μ(005) (15 552 cm−1)=3.142(3) D; μ(105) (17 550 cm−1)=3.157(7) D; μ(402) (14 654 cm−1)=3.075(1) D; μ(104) (14 670 cm−1)=3.095(7) D. The resultant dipole moments are compared with both ab initio theory and linear extrapolation from lower states. The agreement between observation and these predictions is fair. No evidence is seen for asymptotic behavior in the region of CH stretching investigated. The use of additive bond dipole moments to describe molecular dipole moments in HCN is problematic at best.