Synthesis, Purification, and Rotational Spectroscopy of 1-Cyanocyclobutene (C₅H₅N)

The rotational spectrum of 1-cyanocyclobutene from 130 to 360 GHz has been observed, assigned, and least-squares fit for the ground state and the two lowest-energy vibrationally excited states. Synthesis by UV photochemical dimerization of acrylonitrile and subsequent base-catalyzed dehydrocyanation affords a highly pure sample, yielding several thousand observable rotational transitions for this small organic nitrile. Over 2500 a-type, R-branch transitions of the ground state have been least-squares fit to low error with partial-octic A- and S-reduced Hamiltonians, providing precise determinations of the corresponding spectroscopic constants. In both reductions, computed spectroscopic constants are in close agreement with their experimentally determined counterparts. Two vibrationally excited states (ν₂₇ and ν₁₇) form a Coriolis-coupled dyad, displaying many a-type and b-type local resonances and related nominal interstate transitions. Somewhat unexpectedly, despite the very small permanent b-axis dipole moment, a number of b-type transitions could be observed for the ν₁₇ state; this is explained in terms of state mixing by the Coriolis perturbations. Over 2200 transitions for each of these states have been least-squares fit to a low-error, two-state, partial-octic, A-reduced Hamiltonian with nine Coriolis-coupling terms (G , G, G, G, F , F, G , G, and F). The availability of so many observed rotational transitions, including resonant transitions and nominal interstate transitions, enables a very accurate and precise determination of the energy difference (ΔE_27,17 = 14.0588093 (43) cm^-1) between ν₂₇ and ν₁₇. The spectroscopic constants presented herein provide a starting point for future astronomical searches for 1-cyanocyclobutene.