FTIR spectroscopic study of thioanisole and its two halogenated derivatives

The rovibrational spectra of thioanisole (TA) and its halogenated derivatives, 3-fluorothioanisole (3FTA) and 3-chlorothioanisole (3ClTA), were mea-sured using synchrotron-based Fourier transform infrared spectroscopy (FTIR) at the Canadian Light Source. Combined with density functional theory calculations, the stable structures and vibrational modes of TA, 3FTA, and 3ClTA in their vibrational states were analyzed. The theoretical vibrational mode frequencies were corrected by simulating the rotational structure of a vibrational band. The contributions of the cis- and trans-isomers of 3FTA and 3ClTA to the FTIR spectra at 298 K were estimated using the Boltzmann distribution, revealing their coexistence in the experimental spectra. The results indicate that both fluorine and chlorine substitution significantly affect the vibrational modes, particularly in the benzene ring. Compared to TA, the FTIR spectra of 3FTA and 3ClTA show changes in the frequencies and intensities of some vibrational modes, with halogen substitution causing specific modes to shift to higher wavenumbers. A comparison of the FTIR spectra of TA, 3FTA, and 3ClTA highlights the influence of halogen substitution on vibrational properties, emphasizing how the type and position of the substituent affect frequency shifts and spectral intensities. These findings provide deeper insights into how halogenation alters vibrational spectra, which is crucial for further spectral analysis and molecular structure determination.