Vibrational spectra of R and S methyl para tolyl sulfoxide and their racemic mixture in the solid, liquid state and in water solution.

The vibrational properties of the chiral sulfoxide methyl-p-tolyl-sulfoxide (Metoso) were investigated by infrared and Raman spectroscopy in the solid, liquid and aqueous solu-tion phases, for both the enantiopure compounds and their racemic mixture. Experi-mental data were complemented by DFT calculations on the isolated enantiomer and on the two RR and RS dimeric conformers to support spectral interpretation and mode as-signment. The IR and Raman spectra of the crystalline enantiomer and racemic mixture are similar, indicating comparable molecular organization and intermolecular interac-tions in the solid state. Upon melting, band broadening and frequency shifts are observed, consistent with molecular disorder and the breaking of weak intramolecular interactions, accompanied by changes in the S–O and S–CH₃ and C-H stretching frequencies. In aque-ous solution, further broadening and opposite shifts of these bands reflect the formation of Metoso–H₂O complexes through hydrogen bonds. Theoretical spectra reproduce the ob-served trends and confirm that either solvent or phase transitions control the balance be-tween intra- and intermolecular interactions thus influencing the vibrational degrees of freedom of the model chiral sulfoxide.