High resolution spectroscopic studies of sulfur-32 trioxide (O-16) and sulfur-34 trioxide (O-16)

The two sulfur isotopomers of sulfur trioxide, 32S 16O₃ and 34S16O₃, have been studied via high resolution infrared and Coherent anti-Stokes Raman (CARS) spectroscopy. The ν₁ symmetric SO stretching Q-branches observed by the latter are quite different, being especially complex for 32S16O₃. Understanding this band structure required analysis of infrared-active hot bands originating from the ν₂ and ν₄ bending mode levels to the states 2ν₂ (l = 0), ν₂ + ν ₄ (l= ±1), and 2ν₄ (l = 0, ±2). These upper states are strongly coupled to the ν ₁ symmetric stretching mode levels via Fermi resonance and indirect Coriolis interactions, causing the CARS spectra of these two molecules to be highly perturbed and quite different in appearance.; From the analysis of 11000 32S16O ₃ and 7000 34S16O₃ infrared transitions, accurate rovibrational constants have been determined for these mixed states, leading to respective values for ν₁, $aB1$, and $aC1$ of 1064.924(11), 0.000 840 93(64), and 0.000 418 19(58) cm −1 for 32S16O₃ and 1064.920(84), 0.000 834 5(54), and 0.000 410(11) cm−1 for 34S16O₃. These parameters reproduce the respective CARS spectra very well and permit determination of accurate rotational constants for the equilibrium structure of SO₃. The results are B_e values of 0.349 763 96(64) and 0.349 760 61(334) cm−1 and r_e values of 141.734 027(130) and 141.734 704(677) pm for 32S 16O₃ and 34S16O₃ respectively, values that agree within the uncertainties of the measurements. The structure of SO₃ is thus one of the best determined configurations of any nonlinear polyatomic molecules.; As part of the analysis, the infrared-active ν₂ and ν ₄ bending modes were found to be strongly coupled via Coriolis and l-resonance interactions. These interactions cause noticeable wavenumber and intensity perturbations in the ν₂, ν₄ spectra. Although the emphasis of this work was to fit the transitions, an accompanying analysis of the intensities yielded an average value of ±0.62(8) for the dipole derivative ratio (∂μx/∂Q _4x)/(∂μz/∂Q₂) and a positive sign for the product of this ratio with the ζy ₂₄ Coriolis constant, for which experiment gives ±0.5940(15). The sign of individual terms are not determinable from experiment but ab initio calculations were done which indicate that the signs of ∂μ x/∂Q_4x and ∂μz/∂ Q₂ are both positive and hence ζy ₂₄ is also positive.