Theoretical Rovibronic Treatment of the X̃ ²Σ⁺ and à ²Π States of C₂H and the X̃ ¹Σ⁺ State of C₂H^– from Quartic Force Fields

Abstract

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(-).

Quartic force field-derived vibrational frequencies and spectroscopic constants for the isomeric pair SNO and OSN and isotopologues

“…Small positive anharmonicities are well-known for other low-frequency modes of similar molecules. 16 Additionally, comparison between the theoretical gas phase values and the Ar-matrix condensed phase values also compares expectedly 24,25,48 with the CcCR ν 1 at 1566.4 cm −1 and the Ar-matrix ν 1 at 1527.2 cm −1 (Ref. 5).…”

Quantum astrochemical spectroscopy

“…Hence, if energies can be computed, a QFF can be made and analyzed vibrationally, rotationally, and rovibrationally. Equation-of-motion coupled cluster theory [93,94] has been employed recently to produce a QFF for excited states of HOC and C 2 H. [95,96] This work suggests that the experimentally unobserved CAH stretch of the low-lying 2 P state of C 2 H may actually be in the vicinity of 3008.1 cm 21 , lower than some previous estimates owing to its lack of observation. [96] This approach also produced a transition energy of 0.484 eV, quite close to the experimental value of 0.446 eV.…”

The performance of explicitly correlated methods for the computation of anharmonic vibrational frequencies

“…These accurate composite energy schema use complete basis set limit extrapolations (“C”), core correlation corrections (“cC”), scalar relativity (“R”), and often higher‐order electron correlation (“E”) and are called in our group CR, CcCR, and CcCRE QFFs depending upon which corrections are included . Such methods often achieve accuracies compared to gas‐phase experiment of better than 5.0 cm −1 for fundamental vibrational frequencies and 30 MHz for B and C rotational constants …”