The Raman spectrum of NF3: Coriolis coupling and transition dipole interaction in the liquid

First order Coriolis coupling in an e-symmetry mode of a symmetric top and its consequences on the associated Raman line in the liquid are investigated for classical rotations and a classical harmonic vibration. Except for quantum asymmetries, the overall features of the Raman line are reproduced. The second and fourth classical moments of the e-symmetry Raman line are given together with an expression of the linewidth for Debye diffusion. The relevant autocorrelation function <Trβ (0) β (t) ≳ is obtained for free rotation and J rotational diffusion. The Raman spectrum of gaseous NF3 was recorded and the band contours of ν3(e) and ν4(e) were analyzed to determine the intensity ratio between the contributions of γ±1 and γ±2. The Raman profiles were studied in the plastic crystal and in the liquid between 60 and 185 K. The intense infrared lines ν1(a1) and ν3(e) show complex structures made up of two peaks at low temperature, similar to the LO-TO structures of the intense infrared lines of crystals and which are due to a particularly strong transition dipole coupling. The separation between peaks, which is proportional to the integrated infrared intensity of the lines, can be reasonably well predicted by the theory of cubic crystals. The isotropic part of ν1 looses its pseudo-LO-TO structure above 100 K and narrows as the temperature increases, due to rotational averaging of transition dipole coupling. The weak infrared lines ν2(a1) and ν4(e) have temperature-sensitive, symmetrical Raman spectra which were analyzed in terms of molecular rotations. The NF3 molecules rotate more easily about the main axis with a ratio between the mean angles Φ∥ and Φ⊥ of rotation about the main axis and the other axis, respectively, equal to ∼3 at 78 K. The Coriolis perturbation is found to reduce the contribution of the spinning motion to the linewidth of ν4, by a factor of 3 owing to the large value of the Coriolis constant ζ4=−0.895.