The high-resolution infrared analysis of the ν₃ and ν₆ bands of CD₃Br

The ν₃ parallel, and ν₆ perpendicular infrared-active vibrational bands have been analyzed for both CD₃⁷⁹Br and CD₃⁸¹Br at 0.0015 cm⁻¹ resolution. The band origins for the v₃ = 1 state are 577.28925(2) cm⁻¹ and 575.97685(2) cm⁻¹ for the CD₃⁷⁹Br and CD₃⁸¹Br isotopologues, respectively; for the v₆ = 1 state the band origins are 713.46997(1) cm⁻¹ and 713.38156(1) cm⁻¹, respectively. The v₃ = 1 and v₆ = 1 vibrational states are possibly the only unperturbed vibrational states in CD₃Br, and thus yield the most reliable infrared spectroscopy-based determination of the ground state rotational constants. The spectroscopic constants for both vibrational states, as well as the ground vibrational state were improved compared to previous studies through a direct fit of the vibrational bands, as well as a separate fit of the ground state combination differences. Ab initio calculations were performed to examine the efficacy of high-level theoretical calculations to accurately predict spectroscopic constants. Though not used in fitting the experimental data, nuclear quadrupole hyperfine splitting by the bromine atoms was observed in both vibrational states, an uncommon observation in the infrared spectra of polyatomic molecules.