High resolution spectroscopic studies of ³²S¹⁶O₃ and ³⁴S¹⁶O₃ Public Deposited

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  • The two sulfur isotopomers of sulfur trioxide, ³²S¹⁶O₃ and ³⁴S¹⁶O₃, have been studied via high resolution infrared and Coherent anti-Stokes Raman (CARS) spectroscopy. The v₁ 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 v₂ and v₄ bending mode levels to the states 2v₂ (1=0), V₂ + V₄ (1=±l), and 2v₄ (1=0, ±2). These upper states are strongly coupled to the v₁ 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 11 000 ³²S¹⁶O₃ and 7000 ³⁴S¹⁶O₃ infrared transitions, accurate rovibrational constants have been determined for these mixed states, leading to respective values for v₁, a₁B, and a₁C 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 Bₑ values of 0.349 763 96(64) and 0.349 760 61(334) cm⁻¹ and rₑ values of 141.734 027(130) and 141.734 704(677) pm for ³²S¹⁶O₃ 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 v₂ and v₄ bending modes were found to be strongly coupled via Coriolis and 1-resonance interactions. These interactions cause noticeable wavenumber and intensity perturbations in the v₂, v₄ 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 (.fˣ/aQ₄ₓ)/(fᶻ/aQ₂) and a positive sign for the product of this ratio with the Cʸ₂₄ 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 fˣ/aQ₄ₓ and fᶻ/aQ₂ are both positive and hence Cʸ₂₄ is also positive.
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