Microwave and radiofrequency Stark spectrum of ArHCN: A highly nonrigid molecule

Abstract

The rotational spectrum of the weakly bound complex ArHCN has been observed using molecular beam electric resonance spectroscopy. The spectrum is superficially characteristic of that of a linear molecule with both unusually large centrifugal distortion (requiring a J6 dependent distortion term to fit the data) and an unexpectedly large bending amplitude. The spectroscopic constants are The centrifugal distortion constant DJ is remarkably large and abnormally sensitive to isotopic substitution. Using the usual model, the stretching and bending force constants obtained from these data are an order of magnitude smaller than those similarly computed for the hydrogen halide complexes of argon. The calculated stretching and bending frequencies are 10 cm−1, predicting that excited vibrational levels should be populated in the beam. Three transitions have been observed which appear to correspond to an excited vibrational level of ArDCN, but poor signal-to-noise has prohibited their unambiguous assignment. While seemingly, the data support a hydrogen bonded structure for this complex, they require an anomalously short bond length of 2.7 A, despite the apparent weakness of the bond. The vibrationally averaged molecular geometry and force constants are seen to be unexpectedly sensitive to isotopic substitution. Our conclusion is that the observed behavior of this system is best interpreted in terms of strong coupling between the radial and angular degrees of freedom associated with the weak bond. The amplitude of oscillation in the two coordinates is large, making the ‘‘structure’’ of this complex nebulous.