title: Quartic force fields for excited electronic states: rovibronic reference data for the 1 (2)A' and 1 (2)A″ states of the isoformyl radical, HOC.
abstract: Quartic force fields (QFFs) have been shown to be an effective, accurate, and relatively compact means of computing rovibrational spectroscopic data for numerous molecules with numerous applications. However, excited states have been nearly excluded from the this approach since most accurate QFFs are based on the "gold standard" coupled cluster singles, doubles, and perturbative triples [CCSD(T)] method which is not readily extended to excited states. In this work, rovibronic spectroscopic data is provided for the isoformyl radical, a molecule of significance in combustion and astrochemistry, both through the traditional means of variational access to excited states with CCSD(T) and in the novel extension of QFFs routinely to treat electronically excited states through the standard coupled cluster excited state approach, equation of motion (EOM) CCSD. It is shown here that the new EOM-based QFF provides structural parameters and rotational constants that are quite close to those from a related CCSD(T)-based QFF for the 1 (2)A(″) excited state of HOC. The anharmonic vibrational frequency percent differences between the two QFFs are less than 0.4% for the O-H stretch, less than 1.9% for the C-O stretch, and around 3.0% for the bend. Even so, the pure excited state EOM-QFF anharmonic frequencies are still very good abinitio representations that may be applied to systems where electronically excited states are not variationally accessible. Additionally, rovibrational spectroscopic data is provided for the 1 (2)A(') ground state of HOC and for both the ground and excited state of DOC.
authors.affiliation: Georgia Southern University, Department of Chemistry, Statesboro, GA 30460, USA.
authors.forename: W James
authors.initials: WJ
authors.lastname: Morgan
authors.affiliation: Georgia Southern University, Department of Chemistry, Statesboro, GA 30460, USA. Electronic address: rfortenberry@georgiasouthern.edu.
authors.forename: Ryan C
authors.initials: RC
authors.lastname: Fortenberry
mesh_terms: D055598:Chemical Phenomena; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry*; D012015:Reference Standards; D012399:Rotation; D013057:Spectrum Analysis; D013816:Thermodynamics; D014732:Vibration
chemical_list: D005609:Free Radicals
keywords: Astrochemistry; Coupled cluster theory; EOM-CCSD; Quartic force fields; Rovibronic transitions; Spectroscopic data