Rotational spectrum and structure of the complex HCNCO₂

Radiofrequency and microwave rotational spectra of the complexes HCN CO₂ and DCN CO₂ have been obtained using molecular beam electric resonance spectroscopy. The spectra are characteristic of prolate asymmetric rotors. The spectroscopic constants obtained are HCNCO₂ DCNCO₂ [A-(B+C/2)](MHz) 9817(86) 9750(650) (B+C/2)(MHz) 2044.056(10) 1929.53(24) (B-C/2)(MHz) 182.42(60) 162(4) Δ_J (kHz) 0. 0070(6) δ_K (kHz) 137(50) 229(32) μ_a (D) 3.2067(32) 3.2279(29) eqQ_a^N (MHz) -4.075(14) ... These are shown to be consistent with a C_2ν structure in which the nitrogen of the HCN bonds to the carbon of the CO₂ with a weak bond length of 3.00 Å. The HCN subunit subtends an average angle of 17.4(2)° with the line joining the centers of mass of the two submolecules, and the average bending vibrational amplitude of the CO₂ is 11(1)°. The difference between the average amplitudes of the in-plane and out-of-plane bending vibrations of the HCN is less than 4°. The stretching force constant for the weak bond is 0.049 mdyn/Å in HCNCO₂, and the induced dipole moment is 0.361 D.