Substitution structure of cyanogen, NCCN, from high-resolution far infrared spectra

The lowest lying vibrational bands of the gas-phase spectra of cyanogen, NCCN, and four of its isotopomers, , , , and , were recorded with a Fourier transform interferometer. The resolution was limited by the maximum optical path difference (MOPD) attainable with the interferometer to . Rovibrational transitions of the ν₅ () and also the ν₂-ν₅ () band systems were assigned for all five isotopomers. The use of an effective Hamiltonian for linear molecules to fit the data yielded precise spectroscopic vibrational and rotational constants for the vibrational states (v₁v₂v₃v₄v₅) or (v₄v₅)=(00), (01), (02), (03), and (01000). These data include the first rotationally resolved transitions involving (01000). Complete substitution (r_s) structures of cyanogen, based on both single and double isotopic substitution of the parent species, were calculated. The derived structure is r_CC=138.48(17) pm and r_CN=115.66(13) pm. The two r_s structures coincide within the errors due to remaining contributions of zero-point vibrations.