Theoretical Rovibronic Treatment of the (X)over-tilde 2Σ+ and (A)over-tilde 2Π States of C2H and the (X)over-tilde 1Σ+ State of C2H- from Quartic Force Fields

Quartic force fields (QFFs) have been shown to be an effective ab initio tool for the generation of highly accurate anharmonic spectroscopic data but have only recently been extended to the description of electronically excited states using equation-of-motion (EOM) coupled cluster theory. In this study, rovibrational data are provided for the (X) over tilde (2)Sigma(+) ground state of the ethynyl radical as well as the (X) over tilde (1)Sigma(+) ground state of the acetylide anion using traditional QFF methods in conjunction with the new EOM-based QFF method to describe the ethynyl radical's (A) over tilde (2)Pi excited state. These data sets include fundamental vibrational frequencies, rotational constants, and vibrationally averaged structures in addition to other spectroscopic constants. An anticipated theoretical correction (ATC) is shown to be an effective way of improving the accuracy of the fundamental modes of the ethynyl radical's (A) over tilde (2)Pi electronically excited state, lowering the frequencies from 2 to 9%. The anharmonic fundamental vibrational frequencies of (A) over tilde (2)Pi C2H are reported to be nu(2) = 538.0 cm(-1) (bend), nu(3) = 1832.2 cm(-1) (C-C stretch), and nu(1) = 3008.1 cm(-1) (C-H stretch). These (A) over tilde (2)Pi frequencies match closely to predicted transitions from previous theoretical work as well as observed transitions from matrix isolation results. Electron binding energies for the (X) over tilde (2)Sigma(+) C2H <- (X) over tilde (1)Sigma(+) C2H- and (A) over tilde (2)Pi C2H <- (X) over tilde (1)Sigma(+) C2H- transitions are found to be 2.975 and 3.4 eV, respectively. Spectroscopic data are also provided for the C-13 and deuterated isotopologues of (X) over tilde (2)Sigma(+) C2H, (A) over tilde (2)Pi C2H, and (X) over tilde (1)Sigma(+) C2H-.