Anharmonic coupling between fundamental modes in tetramethylurea

Pressure-induced hydrogen bonding modulating Fermi resonance between fundamental modes in xylitol molecule

Xylitol, as a typical polyol, has a broad range of application prospects. However, the molecular states of xylitol under different environments are rarely reported until now. In this work, the state changes of xylitol molecules under high pressure were analyzed by Raman spectra. A Fermi resonance phenomenon in the fundamental mode of xylitol at 2945 (±0.06) cm-1 and 2955 (±0.41) cm-1 was observed at 0.99 GPa. The Fermi doublets possess the same symmetry and close energy levels, which had not been changed by pressures. However, the high pressure shortened the atomic distances and applied the extra disturbance, providing the necessary conditions for energy transfer. Besides, the Fermi doublets decoupling happened at 4 GPa due to the breaking of hydrogen bonding. This work provides an important reference for studying molecular states and weak interactions of polyols under high pressures.

Pressure-induced Fermi resonance between fundamental modes in α-Hydroquinone

Fermi resonance (FR), a prevalent phenomenon in molecules, has an important effect in spectrum analysis. As an effective way to change the molecular structure and tune symmetry, high-pressure techniques can often induce FR. Hydroquinone (HQ) is a hydrogen-bonded crystal that tends to form a solid inclusion compound with a suitable guest and has wide applications. Inthiswork, a high-pressure technique was used to investigate α-HQ using high pressure to tune the symmetry to produce FR. Raman and infrared spectra of α-HQ were investigated at ambient pressure, and then Raman spectra under high pressure of α-HQ were investigated up to 19.64 GPa. Results indicated that there were two phase transitions found at about 3.61 and 12.46 GPa. Fundamental FR was not present in α-HQ molecules at ambient pressure. At 3.61 GPa, the first-order phase transition occurred due to the pressure-induced symmetry change, resulting in two Raman modes at 831 cm^-1 and 854 cm^-1 with the same symmetry, thereby providing evidence that the fundamental FR phenomenon occurred. Furthermore, the pressure-induced changes of the FR parameters were elucidated. Thus pressure provided an effective way to study FR between two asymmetric species.

Tuning the Fermi resonance of pyridine using ethanol molecules

The Fermi resonance (FR) phenomenon is prevalent in infrared and Raman spectroscopy, and it can be observed in a variety of molecules. In particular, pyridine is a compound that has two Fermi doublets: ν₁ ∼ ν₁₂ and ν₁ + ν₆ ∼ ν₈. To analyze the effect of environmental changes on the FR, this study first investigated the Raman spectra of pyridine mixed with ethanol at different concentrations. Results indicated that the FR parameters exhibited a nonlinear dependence on the pyridine concentration fractions, and changing the concentration fraction of pyridine led to different hydrogen bond strengths. Second, the interaction mechanism of pyridine-ethanol binary solutions was analyzed by two-dimensional correlation Raman spectroscopy (2DCRS). In addition, high-pressure Raman spectra of a 50% pyridine-ethanol binary solution were measured up to a pressure of 19.65 GPa by a diamond anvil cell technique, and the phase transition of the binary solution occurred at 6.35 GPa. Finally, the impact of ethanol on the FR of pyridine was determined by deducing the FR parameters at different pressures. The turning point at 6.35 GPa was consistent with the Raman frequency-pressure relationships. The results demonstrated that changes in the intensity of ν₁ did not affect the FR of ν₁ + ν₆ ∼ ν₈, whereas the undisturbed frequency ν₁ still played a role in the FR. When the pressure was compressed to 13.36 GPa, the disappearance of the Raman peaks (ν₁ and ν₁') was attributed to the tuning of the molecular symmetry by pressure during the phase transition.

Pressure-induced Fermi resonance between fundamental modes in phthalic anhydride

In situhigh-pressure Raman spectra of phthalic anhydride (PA) have been measured up to 16 GPa through diamond anvil cell technique. The results show that all the Raman bands are blue-shifted with the increase of pressure, accompanied by appearance of some new bands. A Fermi resonance phenomenon of the two Raman fundamental modes of PA at 773 cm^-1and 801 cm^-1is proposed at pressures above 6.6 GPa, where a possible first-order phase transition occurs. The pressure-induced changes of Fermi resonance parameters, e.g., intensity ratio, coupling coefficient and frequency gap of unperturbed transition, are discussed.

High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4'-Bipyridine Co-crystal

In attempt to the obtain detailed geometric information of proton transfer compound (subsequently denote as SQBP) formed between squaric acid (SQ)and 4,4′-bipyridine(BP), and to investigate the mechanisms of pressure-induced double proton transfer and related structural phase transition, we carried out in-situ high pressure Raman spectroscopy of SQBP up to 20 GPa. A solid-solid phase transition together with double proton transfer phenomenon was confirmed by Raman spectroscopy at about 1.5 GPa, and the activation of C = O stretching mode in Raman spectra indicates a square-ring structure of SQ with four symmetric C = O bond formation. These results are further supported by first-principals calculations and in-situ high pressure infrared absorption spectroscopy. Additionally, Raman intensity analysis suggests that a higher-order phase transition with planar BP molecular structure occurred in the pressure range of 3~6 GPa. As a result, the π electron delocalization effect in BP dominated the intensity enhancement of C = O stretching mode in SQ. To the best of our knowledge, this is the first time observation of the intensity enhancement of proton donor’s normal modes induced by proton acceptor’s π electron delocalization.