Rovibrational constants of the ground and ν12 = 1 states of C2D4 by high-resolution synchrotron FTIR spectroscopy

Abstract

The Fourier transform infrared (FTIR) absorption spectrum of the ν12 fundamental band of ethylene-d4 (C2D4) was recorded in the 1000–1150cm−1 region with a resolution of 0.00096cm−1 using the THz/far-infrared beamline of the Australian Synchrotron. Upper state (ν12=1) rovibrational constants consisting of three rotational constants and up to five quartic constants were improved by assigning and fitting 3950 rovibrational transitions using Watson’s A-reduced and S-reduced Hamiltonians in the Ir representation. The band centres of the unperturbed A-type ν12 band are found to be 1076.984958(14) cm−1 and 1076.984813(14) cm−1 for A-reduced and S-reduced Hamiltonians respectively. The present analysis, covering a wider wavenumber range and higher J and Kc values (up to 58) than previous studies, yielded upper state constants including the band centre which are more accurate than previously reported. The rms deviation of the upper state (ν12=1) fit is 0.00040cm−1 in the A-reduction and 0.00041cm−1 in the S-reduction. Improved ground state rovibrational constants were also determined from the fit of 3151 ground state combination differences (GSCD) from the presently-assigned transitions of the ν12 band of C2D4 using Watson’s A-reduced and S-reduced Hamiltonians in the Ir representation. The rms deviation of the GSCD fit is 0.00036cm−1 in the A-reduction and 0.00035cm−1 in the S-reduction. The ground state constants of C2D4 derived from the experimental GSCD fit are in good agreement with those from theoretical calculations using the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CSSD/cc-pVTZ levels, up to five quartic constants.