Rotational spectrum and structure of KCN

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Abstract
The spectrum of gaseous KCN was measured in the frequency range between 2 and 39 GHz by microwaveabsorption and by molecular‐beam electric‐resonance spectroscopy. Combination of the new results with earlier microwave data of KCN in the 100 GHz range made it possible to assign 64 transitions to the ground vibrational state and to fit them to the asymmetric rotor model. The three rotational constants, the five quartic distortion constants, and two sextic distortion coefficients could be determined. Assuming a CN distance of 1.162(10) Å we find r KC=2.6(1) Å and uKCN=76 °(10). The molecules thus have a nonlinear, T‐shaped structure. The inertial defect gives an estimated value of the lowest vibrational frequency of KCN ω2=157 cm−1, which is in reasonable agreement with ω2=139 cm−1 from matrix‐isolation studies.