Infrared Signatures of the NCCO Radical.

The first definitive infrared signatures of the elusive NCCO radical have been measured using a microwave discharge technique combined with low-temperature matrix-isolation spectroscopy, resulting in a consistent set of vibrational assignments for six isotopologues.  The infrared spectra of these NCCO isotopologues were concomitantly established by rigorous variational nuclear-motion computations based on a high-level coupled-cluster quartic vibrational force field [ROCCSD(T)/cc-pCVQZ] and cubic dipole field [ROCCSD/cc-pCVTZ]. Our experimental and theoretical results for NCCO overturn the vibrational assignments in a NIST-JANAF compilation and those from a recent two-dimensional cross-spectral correlation analysis. For the parent isotopologue at 11 K in a nitrogen matrix, we find the signature bands n₂(CO str.) = 1889.2 cm^-1 and n₃(CC str.) = 782.0 cm^-1. Our variational vibrational computations reveal strong mixing of the n₃ stretching fundamental and the n₄+n₅ bending combination level for all isotopologues.  These Fermi resonances manifest a clear breakdown of the simple normal-mode picture of molecular vibrations at low energies.