Raman and infrared spectroscopy of pyridine under high pressure

We report the structural transitions of pyridine as a function of pressure up to 26 GPa using in situ Raman spectroscopy and infrared absorption spectroscopy. By monitoring changes in the Raman shifts in the lattice region as well as the band profiles in both Raman and IR spectra, a liquid-to-solid transition at 1 GPa followed by solid-to-solid transitions at 2, 8, 11, and 16 GPa were observed upon compression. These transitions were found to be reversible upon decompression from 22 GPa. A further chemical transformation was observed when compressed beyond 22 GPa as evidenced by the substantial and irreversible changes in the Raman and infrared spectra, which could be attributed to the destruction of the ring structure. The observed transformations in pyridine were also compared to those for benzene. The similar transition sequence with well-aligned transition pressures suggests that these isoelectronic aromatics may have similar structures and stabilities under high pressure.

Figure 1

(Color online) Raman spectrum of pyridine (a) in comparison with the IR spectrum (b) in the internal mode region of $375–3250{\text{cm}}^{-1}$, both collected at near-ambient pressure. The left vertical axis denotes IR absorbance (with the absolute intensity labeled by the vertical bar) while the right axis refers to Raman intensity. The IR spectrum in the region of $2950–3200{\text{cm}}^{-1}$ has been scaled by a factor of 3. The region from 1300 to $1400{\text{cm}}^{-1}$ was truncated due to the strong diamond $T_{2\text{g}}$ mode in this region while the region of $1650–2950{\text{cm}}^{-1}$ was omitted due to the lack of spectroscopic features of interest. The assignments are labeled for most fundamental vibrational modes. The unlabeled bands in the IR spectrum are overtones or combinations. See text and Table .

Figure 2

(Color online) Selected Raman spectra of pyridine in the lattice mode region collected upon compression. The spectra are offset vertically for clarity. The pressures are labeled above each spectrum. The numbers 1–5 labels the five lattice modes observed during compression. See text.

Figure 3

(Color online) Raman spectra of pyridine on compression in the spectral region of (a) $400–1160{\text{cm}}^{-1}$ and (b) $1100–3300{\text{cm}}^{-1}$ at selected pressures. The ranges of $700–950{\text{cm}}^{-1}$ and $1700–2800{\text{cm}}^{-1}$ were omitted due to the lack of fundamental vibrational modes while the spectra were truncated in the region of $1300–1400{\text{cm}}^{-1}$ due to the strong diamond $T_{2\text{g}}$ mode in this region. The spectra are offset vertically for clarity.

Figure 4

(Color online) Raman shift of pyridine as a function of pressure on compression in the lattice mode region. Solid straight lines are linear fits to the data. Vertical dashed lines mark the suggested phase transition boundaries. Different symbols denote lattice modes with different origins with numbers labeled from low to high frequencies. See text and Fig. 2.

Figure 5

(Color online) Representative IR spectra of pyridine upon compression in the spectral region of $650–1750{\text{cm}}^{-1}$. Some intense IR absorption bands (e.g., at $\sim 700$ and $\sim 1600{\text{cm}}^{-1}$) display a truncated intensity (disconnected line) at the detector saturation level. The absolute absorbance intensity is labeled by the vertical bar at the top left corner. The pressures are labeled above each spectrum. The spectra are offset vertically for clarity.

Figure 6

(Color online) Raman spectra of pyridine upon decompression at selected pressures. The vertical arrow denotes the decompression sequence from the highest pressure of 21.4 GPa. The top spectrum was collected at 2.5 GPa by decompression from 26 GPa.

Figure 7

(Color online) IR spectra of pyridine upon decompression from (a) 20 GPa to (b) 0.4 GPa in comparison with another run with the highest compression pressure of (c) 26.1 GPa and recovered sample at (d) 0.2 GPa. The spectral region of $1900–2700{\text{cm}}^{-1}$ was truncated due to the diamond absorption and the lack of characteristic IR modes. The absolute absorbance intensity is labeled by the vertical bar at the top right corner. The spectra are offset vertically for clarity.