Rovibrational constants of the ground state and v₉ = 1 state of ¹³C₂D₄ by high-resolution Fourier transform infrared spectroscopy

The Fourier transform infrared (FTIR) rovibrational spectrum of the b-type ν₉ band of ¹³C₂D₄ was recorded at a unapodized resolution of 0.0063 cm^-1 in the 2230-2450 cm^-1 region. By assigning and fitting a total of 1171 rotationally resolved infrared transitions of the ν₉ band and using the Watson's A-reduced Hamiltonian in the I^r representation, rovibrational constants for the upper state (v₉ = 1) up to five quartic centrifugal distortion terms were derived for the first time. The root-mean-square (rms) deviation of the infrared fit was 0.00043 cm^-1. The ground state constants of ¹³C₂D₄ were determined with higher accuracy than previously by fitting 1485 ground state combination-differences (GSCDs) from the present and previous infrared measurements, with rms deviation of 0.00034 cm^-1. The band center of ν₉ band of ¹³C₂D₄ was determined to be at 2324.3593 cm^-1. The equilibrium state rovibrational constants up to all 5 quartic terms were derived from theoretical harmonic calculations at three levels of theory: B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD/cc-pVTZ using the principal axis coordinate system. These constants agreed reasonably well with the ground state constants of ¹³C₂D₄ derived from the present experimental GSCD fit. Furthermore, all 3 rotational constants of the upper state (v₉ = 1) and of the ground state of ¹³C₂D₄ were obtained from anharmonic calculations using B3LYP and MP2 levels with the cc-pVTZ basis set. The calculated rotational constants were found to agree with those derived experimentally within 0.40%.