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Phys. Chem. Chem. Phys., 2009, 11, 10385 - 10390, DOI: 10.1039/b912803d

Infrared signatures of the NCCO radical

Peter R. Schreiner, Hans Peter Reisenauer, Edit Mátyus, Attila G. Császár, Ali Siddiqi, Andrew C. Simmonett and Wesley D. Allen

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 2(CO str.) = 1889.2 cm-1 and 3(CC str.) = 782.0 cm-1. Our variational vibrational computations reveal strong mixing of the 3 stretching fundamental and the 4 + 5 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.

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