Rotational spectra of vibrationally excited CCH and CCD.
The millimeter-wave rotational spectra of the lowest bending and stretching vibrational levels of CCH and CCD were observed in a low pressure discharge through acetylene and helium. The rotational, centrifugal distortion, and fine structure constants were determined for the (02(0)0) and (02(2)0) bending states, the (100) and (001) stretching levels, and the (011) combination level of CCH. The same pure bending and stretching levels, and the (110) combination level were observed in CCD. Apparent anomalies in the spectroscopic constants in the bending states were shown to be due to l-type resonances. Hyperfine constants, which in CCH are sensitive to the degree of admixture of the A 2Pi excited electronic state, were determined in the excited vibrational levels of both isotopic species. Theoretical Fermi contact and dipole-dipole hyperfine constants calculated by Peric et al. [J. Mol. Spectrosc. 150, 70 (1991)] were found to be in excellent agreement with the measured constants. In CCD, new rotational lines tentatively assigned to the (100) level largely on the basis of the observed hyperfine structure support the assignment of the C-H stretching fundamental (nu1) by Stephens et al. [J. Mol. Struct. 190, 41 (1988)]. Rotational lines in the excited vibrational levels of CCH are fairly intense in our discharge source because the vibrational excitation temperatures of the bending vibrational levels and the (110) and (011) combination levels are only about 100 K higher than the gas kinetic temperature, unlike the higher frequency stretching vibrations, where the excitation temperatures are five to ten times higher.
Theoretical Rovibronic Treatment of the X̃ (2)Σ(+) and Ã (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 Ã (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 Ã (2)Π electronically excited state, lowering the frequencies from 2 to 9%. The anharmonic fundamental vibrational frequencies of Ã (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 Ã (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 Ã (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, Ã (2)Π C2H, and X̃ (1)Σ(+) C2H(-).
Detection of the interstellar C6H radical
On a detecte trois doublets a 23,6-40,2 et 43,0 GHz, identifies aux transitions J=17/2-15/2; J=29/2-27/2 et J=31/2-29/2 d'un nouveau radical interstellaire dans l'etat 2 π 3/2