Vibrational intensities and anharmonicity in MIR, NIR and Raman spectra of liquid CHCl3, CDCl3, CHBr3 and CDBr3: Spectroscopic and theoretical study

Abstract

This work provides new quantitative information on vibrational spectra of liquid CHCl3, CDCl3, CHBr3 and CDBr3. Deconvolution of the mid-infrared (MIR), near-infrared (NIR) and Raman spectra permitted for determination of the parameters of fundamentals, overtones and combination bands. Simultaneously, the anharmonic vibrational spectra were calculated by MP2 and B3LYP methods. Using these data, for the first time, we estimated the experimental and theoretical contributions from the anharmonic vibrations (overtones and combination bands) to MIR spectra of all studied compounds. For chloroforms and CHBr3 these vibrations contribute to less than 5.5% (experimental) and 8% (theoretical) of MIR intensity. Only for CDBr3 this contribution is much higher (21% and 24%) due to strong Fermi resonance (FR) between ν4 and ν3 + ν5 vibrations. Besides, we estimated the contributions from the MIR and NIR ranges to the total vibrational intensity. As expected, the contributions from NIR range are small (<2.5%) for all studied samples. It is of note that the theoretical contributions from MIR and NIR regions are very close to the experimental ones. It results from accurate theoretical prediction of the intensities of the fundamental bands. On the other hand, the intensities of the overtones and combination bands are predicted less satisfactory, what is clearly seen in NIR spectra. The MIR spectra of all compounds reveal the presence of FR. The FR resonances increase the intensity of the non-fundamental transitions, for CDBr3 this increase is significant. Interestingly, Raman spectra of both chloroforms were also influenced by FR. As a result, the spectra of CHCl3 and CDCl3 develop two intense binary combinations of a total contribution ≈12%.Our results reveal that the overtones and combination bands are significantly weaker as compared with the fundamental bands. Some departures from this trend result from FR. We did not observe any correlation between the intensities of the fundamentals and the corresponding overtones and combination bands. The bands due to the first overtones are broader as compared with the fundamental bands, but we did not observe any regular relationship. Surprisingly, the more regular trend was observed for the second overtone of the ν1 band, which is four times broader than the corresponding fundamental band.