A study on the Fermi resonance of phenol under the effects of pressure and temperature by Raman spectroscopy

• Raman spectra of phenol at different pressures and temperatures were measured.
• Fermi coupling coefficients were calculated based on the FR theory.
• Fermi coupling coefficient turnover between 1.912 and 2.244 GPa was observed.
• Fermi coupling coefficient exhibited monotonic reduction as the temperature decreased.
• A conformation evolving induced by pressure and temperature on FR was analyzed.

The ν1–ν18a Fermi resonance (FR) of phenol were investigated by pressure-dependent Raman spectroscopy from atmospheric up to P = 15.2 GPa and temperature-dependent Raman spectroscopy from 40 down to T = −180 °C, respectively. In the case of pressure, we found the Fermi coupling coefficient W, which were calculated based on the FR theory, exposed a value turnover between 1.912 and 2.244 GPa in the process of increasing the pressure. This turnover phenomenon of the Fermi coupling coefficient W has been ascribed to the crystal structure of phenol evolving towards a more symmetric structure with pressure, from a structure like a pseudo-threefold helical chain at ambient pressure to like a ribbon arrangement at 1.912 GPa, then to adopt a herringbone arrangement at much higher pressure. On the other hand, we also found the Fermi coupling coefficient W exhibited monotonic reduction without turnover points appearing by decreasing the temperature. The tendencies of the Fermi coupling coefficient W with temperature were in good agreement with the pressure dependence of the Fermi coupling coefficient W in the region of ambient to 1.912 GPa, indicating that the effect of pressure and temperature on the FR of phenol in this region might be the same. A conformation evolving induced by pressure and temperature on the ν1 and ν18a FR of phenol have been analyzed.

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