High resolution analysis of the CD4 deuterated methane: Extended investigation of the pentad region

A highly accurate rotational–vibrational analysis of Fourier transform infrared spectra of the ${}^{12}$CD${}_4$ molecule is presented. The high resolution infrared spectra were measured with a IFS125 HR Fourier transform interferometer from Bruker at an optical resolution of 0.003 cm⁻¹ and analyzed in the 1750–2400 cm⁻¹ region. Here the $2{\nu }_2$, ${\nu }_2+{\nu }_4$, $2{\nu }_4$, ${\nu }_1$ and ${\nu }_3$ bands (altogether, nine sub-bands of different symmetry) of the pentad are located. The number of 1213/1993/1576/77/1582 transitions with the $J^{\text{max}}$ = 23/23/23/14/32 were assigned to the $2{\nu }_2$, ${\nu }_2+{\nu }_4$, $2{\nu }_4$, ${\nu }_1$ and ${\nu }_3$ bands of ${}^{12}$CD${}_4$. The obtained experimental data were used for the determination of the upper ro-vibrational energy values. To provide more correct values of the upper energies, more than 7800 highly accurate “hot” transitions from the dyad region were additionally processed. In general, 4088 upper ro-vibrational energies of the pentad (for comparison, 2525 upper ro-vibrational energies with the value of $J^{\text{max}}=20$ are known in the modern literature up to now) were determined, which were used then in the weighted fit procedure with a goal to determine the spectroscopic parameters (band centers, rotational, centrifugal distortion, tetrahedral splitting and resonance interaction parameters) of the effective Hamiltonian. The obtained $d_{\text{rms}}$ value is $5.5\times 10^{-4}$ cm⁻¹ which is almost one hundred times better than the reproduction of the same set of experimental data by the parameters known in the earlier literature.