The ro-vibrational spectroscopy of H2NCO (2A′) and HNCOH and the peculiar case of their isomers <i>cis</i> - and <i>trans</i> -HNCHO

Abstract

While quantum chemical treatment of the prebiotic, astrochemically relevant H2NCO molecule’s rotational constants and fundamental vibrational frequencies produces exceptionally accurate values, the cis-HNCHO isomer is more of a challenge to standard electronic structure methods. The trans-HNCHO conformer is well-behaved like H2NCO due to the nature of the local potential energy surface, but the zero-point vibrational energy of the torsional motion is significantly greater than the barrier, implying that this isomer cannot be observed. The 2.6 kcal mol−1 lower-energy cis-HNCHO conformer showcases a small pseudo-Jahn–Teller distortion about the HNCO torsion coordinate at planarity. This produces a challenging environment for the quartic force field (QFF) treatment of the rotational and vibrational spectroscopic data. Even so, the accuracy of the H2NCO spectroscopic constants from this explicitly correlated coupled cluster theory QFF implies that similar accuracy should be present for the mathematically well-behaved trans-HNCHO, which exists in a shallow but true minimum. The constants computed herein may allow for a more detailed search of experimental rotation spectra for cis-HNCHO, which would provide reference data necessary for potential searches for this molecule in space. Rotational constants are also provided for the four conformers of HNCOH, and the fundamental vibrational frequencies in the range of James Webb Space Telescope observation are provided for all isomers in order to allow for the possibility of multiple wavelength observation of this family of molecules, which may play a role in the molecular origins of life.