Synthesis, Purification, and Rotational Spectroscopy of 1-Cyanocyclobutene (C 5 H 5 N).

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 (ν 27 and ν 17 ) 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 ν 17 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 a , G a J , G a K , G a JJ , F bc , F bc K , G b , G b J , and F ac ). 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 (Δ E 27,17 = 14.0588093 (43) cm -1 ) between ν 27 and ν 17 . The spectroscopic constants presented herein provide a starting point for future astronomical searches for 1-cyanocyclobutene.