High-resolution FTIR spectroscopy of ethylene-d3 (C2HD3): rovibrational constants for the ground state and the v8 = 1 state

The Fourier transform infrared (FTIR) spectrum of the v8 band of ethylene-d3 (C2HD3) was recorded with a unapodized resolution of 0.0063 cm−1 in the wavenumber region of 830–1000 cm−1. Improved rovibrational constants for the upper state (v8 = 1) up to five quartic centrifugal distortion terms were derived by assigning and fitting a total of 1249 infrared transitions using Watson’s A-reduced Hamiltonian in the Ir representation. The root-mean-square (rms) deviation of the fit was 0.00062 cm−1, close to the accuracy (1/10th of a linewidth) of the measurements of the line positions of ±0.0007 cm−1. The ground state rovibrational constants of C2HD3 were also determined with higher accuracy than previously reported from a fit of 568 combination-differences from the present infrared measurements with an rms deviation of 0.00038 cm−1, using Watson’s A-reduced Hamiltonian. The calculated inertial defect is 0.069489(69) μÅ2. The C-type v8 band of C2HD3 was determined to be centered at 918.731925(40) cm−1. The ground state constants of C2HD3 derived from the experimental GSCD fits were in reasonable agreement with the equilibrium state constants derived from theoretical calculations using the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD(T)/cc-pVTZ levels of theory.