Infrared transitions of C2D6 from v 4 to the interacting system v 4+ v 6, v 4+ v8, v 3+ 2v 4: rotational analysis, and torsional splitting in the v 3+ 2v 4 and v 3+ v 4 states

Abstract

The hot infrared transitions of C2D6 from the υ4(A1u ) to the υ4 + υ6(A2g ) and υ4 + υ8(E g ) vibrational states, observed from 960 to 1180 cm−1, have been rotationally analysed on a high-resolution Fourier transform spectrum (full width at half-maximum about 0·0030 cm−1). The vibration-rotation interactions affecting the upper vibrational states are very similar to those of the corresponding cold system. A strong x,y Coriolis interaction between υ4 + υ6 and υ4 + υ8, with K-level crossing, generates large displacements of the rotational components of both vibrational states, tuning them to additional local resonances in several spectral regions. Thus l resonances with Δl = ±2, Δk = ±1 occur within υ4 + υ8. A x,y Coriolis-type resonance between υ4 + υ8(−l,K − 1) and υ3 + 2υ4(K) occurs at K = 11,12,13, and a further coupling of υ4 + υ8(+l,K + 1) and υ3 + 2υ4(K + 3) is most effective at K = 11 and 12. These resonances induce torsional splittings on the perturbed levels of υ4 + υ8 and allow us to determine the torsional splittings in the υ3 + 2υ4 state. The vibration-rotation constants of υ4 + υ6, υ4 + υ8 and υ3 + 2υ4, several interaction parameters and the torsional splitting of υ3 + 2υ4 have been determined by least-squares fit of 1391 observed transition wavenumbers, with an overall standard deviation σ = 0·75 × 10−3 cm−1. The vibrational wavenumbers found for the four torsional components of (υ3 + 2υ4)− υ4 are υ(E3d) = 1040·961 82(809)cm−1, υ(A3d) = 1041·218 27(865)cm−1, υ(E3s) = 1041·225 23(662)cm−1 and υ(A1s) = 1041·407 77(633)cm−1. These are anomalous for both the order of the torsional components and the magnitudes of their separations. We believe that this is mainly due to the interactions of υ3 + 2υ4 with the torsional manifolds with υ3 = 0 and υ3 = 2, through the vibration-torsion Hamiltonian term (∂V 6/∂q 3)q 3cos (6γ)]/2. The further observation of a few doublets of υ8 and υ3 + υ4 at resonance provides information on the torsional splitting of the latter state.