High resolution study of the ν 2 and ν 5 rovibrational fundamental bands of thionyl chloride: Interplay of an evolutionary algorithm and a line-by-line analysis.

The ν ₂ and ν ₅ fundamental bands of thionyl chloride (SOCl ₂ ) were measured in the 420 cm ^-1 -550 cm ^-1 region using the FT-far-IR spectrometer exploiting synchrotron radiation on the AILES beamline at SOLEIL. A straightforward line-by-line analysis is complicated by the high congestion of the spectrum due to both the high density of SOCl ₂ rovibrational bands and the presence of the ν ₂ fundamental band of sulfur dioxide produced by hydrolysis of SOCl ₂ with residual water. To overcome this difficulty, our assignment procedure for the main isotopologues ³² S ¹⁶ O ³⁵ Cl ₂ and ³² S ¹⁶ O ³⁵ Cl ³⁷ Cl alternates between a direct fit of the spectrum, via a global optimization technique, and a traditional line-by-line analysis. The global optimization, based on an evolutionary algorithm, produces rotational constants and band centers that serve as useful starting values for the subsequent spectroscopic analysis. This work helped to identify the pure rotational submillimeter spectrum of ³² S ¹⁶ O ³⁵ Cl ₂ in the v ₂ =1 and v ₅ =1 vibrational states of Martin-Drumel et al. [J. Chem. Phys. 144, 084305 (2016)]. As a by-product, the rotational transitions of the v ₄ =1 far-IR inactive state were identified in the submillimeter spectrum. A global fit gathering all the microwave, submillimeter, and far-IR data of thionyl chloride has been performed, showing that no major perturbation of rovibrational energy levels occurs for the main isotopologue of the molecule.