Theoretical Rovibronic Treatment of the X̃ (2)Σ(+) and à (2)Π States of C2H and the X̃ (1)Σ(+) 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 X̃ (2)Σ(+) ground state of the ethynyl radical as well as the X̃ (1)Σ(+) 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 à (2)Π 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 à (2)Π electronically excited state, lowering the frequencies from 2 to 9%. The anharmonic fundamental vibrational frequencies of à (2)Π C2H are reported to be ν2 = 538.0 cm(-1) (bend), ν3 = 1832.2 cm(-1) (C-C stretch), and ν1 = 3008.1 cm(-1) (C-H stretch). These à (2)Π frequencies match closely to predicted transitions from previous theoretical work as well as observed transitions from matrix isolation results. Electron binding energies for the X̃ (2)Σ(+) C2H ← X̃ (1)Σ(+) C2H(-) and à (2)Π C2H ← X̃ (1)Σ(+) C2H(-) transitions are found to be 2.975 and 3.4 eV, respectively. Spectroscopic data are also provided for the (13)C and deuterated isotopologues of X̃ (2)Σ(+) C2H, à (2)Π C2H, and X̃ (1)Σ(+) C2H(-).