Study on the Rovibrational Interactions and a₁/a₂ Splittings in the v₃/v₅/v₆ Triad of CH₃D

High-resolution Fourier transform infrared spectrum of CH₃D has been recorded in the region of the fundamental bands v₃, v₅, and v₆ between 900 and 1700 cm^-1. High sensitivity of the equipment used as well as high accuracy of the recorded line positions gave the possibility of assigning the first-time transitions with the upper state J quantum number up to 23. In the analysis the new ground vibrational state information [O. N. Ulenikov, G. A. Onopenko, N. E. Tyabaeva, J. Schroderus, and S. Alanko, J. Mol. Spectrosc. 193, 249-259 (1999)] were used. In addition, modification of the Hamiltonian model of the interacting vibrational states (v₃ = 1), (v₅ = 1), and (v₆ = 1) allowed the theoretical description of numerous effects and peculiarities in the spectrum. In particular, sets of a₁/a₂ splittings have been assigned and explained, not only for upper state K = 1 and 2 but also for K = 4 and 5. Moreover, unusually giant exotic splittings have been found for K = 7. The spectroscopic parameters reproducing the initial experimental energies with accuracy close to the experimental uncertainty have been determined.