Millimeter-wave spectroscopy of CoNO Produced by UV laser photolysis of Co(CO)₃NO

The rotational spectrum of cobalt mononitrosyl (CoNO) produced by ultraviolet photolysis of Co(CO)₃NO was observed in the millimeter-wave region. Seven rotational transitions in the ground state ranging from J=6-5 to 12-11, with hyperfine splittings due to the Co nucleus (I=7/2), were detected in a supersonic jet environment, while higher-frequency transitions in the range from J=29-28 to 35-34 were measured in the ground, ν₁, ν₂, ν₃, and 2 ν₂ vibrational states using a free-space absorption cell. It was confirmed from the observed spectral pattern that the CoNO molecule has a linear structure with the electronic ground state of ¹∑⁺ symmetry. The rotational lines in the 2 ν₂(∑) and ν₃ states were observed to be perturbed by Fermi resonance. The equilibrium rotational constant B_e is determined to be 4682.207(15) MHz. The CoN bond length is derived to be 1.5842 Å assuming the NO bond length of 1.1823 Å. A large nuclear spin-rotation interaction constant, C_I =123.8 (11) kHz, was determined, suggesting a ¹Π electronic excited state lying close to the ground state.