The Torsional Spectrum of CH3SiH3: The (v6 = 3 ← 1) Band

The far-infrared spectrum of gaseous CH3SiH3 has been measured with a Fourier transform spectrometer between 180 and 380 cm−1 under relatively large pressure-path length conditions. The absorption path length was 124 m and the pressure was 20 Torr. Measurements were made at room temperature with an unapodized resolution of 0.01 cm−1. A congested spectrum due to the overlap of several pure torsional bands has been observed. A total of 576 transitions from the (ν6 = 3 ← 1) torsional band have been identified, involving 15 different (K, σ) subbands, where σ = 0, +1, −1 labels the different torsional sublevels. The upper torsional state of this band (ν12 = 0, ν6 = 3) is significantly perturbed by the upper level (ν12 = 1, ν6 = 0) in the E1 vibrational fundamental (ν12 = 1 ← 0) reported earlier (Moazzen-Ahmadi et al., J. Mol. Spectrosc.137, 166, 1989). The (K = 6, σ = −1) vibrational subband for ΔK = −1 shows resonant perturbation. The identification of the PQ−16 subband has now been extended in J above the value where the interacting levels have their minimum separation. Two perturbation-allowed (ν6 = 3 ← 0) transitions have also been assigned. The measurements of the (ν6 = 3 ← 1) band and frequencies from previously reported experiments were fitted to within the experimental uncertainty by an effective Hamiltonian which included 22 parameters for the ground vibrational state, 10 parameters for the ν12 = 1 vibrational state, and 2 parameters which characterize the interactions between these two states. The global data set contained 2607 frequencies. The form of the effective Hamiltonian is severely constrained by the large number of precision data on various torsional levels in the ground vibrational state and the excited vibrational state (ν12 = 1, ν6 = 0). The effective Hamiltonian for a vibrational fundamental of E1 symmetry and the interactions between the torsional stack of this state and that of the ground vibrational state are discussed in detail.