Raman spectroscopy study of acetonitrile at low temperature

Highlights

• Raman spectra of liquid CH₃CN at low temperature were measured.

• All the vibrational modes of CH₃CN at different temperatures were analyzed in detail.

• Six Raman bands can be identified in the lattice mode.

• Phase transitions of CH₃CN were occurred at −50 and −60 °C, respectively.

• Fermi resonance parameters of CH₃CN at different temperatures were discussed.

Abstract

We report the low-temperature studies of liquid CH₃CN by Raman spectral measurements at ambient pressure with decreasing the temperature from 20 to −196 °C. Detailed internal modes especially the lattice modes analysis revealed that the structural phase transitions of acetonitrile from liquid to solid phase β and solid phase β to solid phase α were occurring at −50 and −60 °C, respectively. Further, the Fermi resonance parameters between the fundamental ν₂ and combination (ν₃ + ν₄) of CH₃CN at different temperatures were calculated based on the Bertran's equations. It is found that the Fermi resonance parameters as a function of temperature become discontinued at −50 and −60 °C, which coincides with discontinuities observed in the Raman shifts of CH₃CN at −50 and −60 °C. The results suggest that the Fermi resonance parameters could be used as an indicator to assess the structural phase transition for CH₃CN under low temperature.

Introduction

Acetonitrile (CH₃CN) is the simplest organic nitrile with excellent solvent properties; it is widely used for extracting fatty acids from vegetable and animal oil in the fatty acid industry, and used as the reaction medium of the recrystallization of steroidal drugs in pharmaceutical industry [1]. Moreover, it also has a lot of applications in fabric dyeing, light industry, spice manufacturing, photographic materials manufacturing and lithium batteries [[2], [3], [4], [5], [6], [7], [8], [9], [10]].

As an important basic solvent in organic and coordination chemistry, it has been extensively investigated by various methods, for example, infrared spectroscopy, X-ray diffraction, Raman spectroscopy, theoretical simulations [[11], [12], [13], [14]]. Previous spectroscopic studies of CH₃CN have been focused on the structural phase transitions induced by high pressure and low temperature. Studies of CH₃CN under high pressure using Raman spectroscopy were conducted up to 24.8 GPa by Ma et al. [11] and by Chen et al. [15] in the range of 1 atm to 20.18 GPa. Those studies demonstrated the structural phase transitions of CH₃CN from liquid to solid phase β, solid phase β to solid phase α and solid phase α to solid phase γ occurred under 20 GPa. On the other hand, since Putanm et al. [16] determined that solid CH₃CN existed in two forms (solid phases β and α) with the structural phase transitions occurred at about 217 and 229 K respectively, a number of spectroscopic studies confirmed the existence of the two solid phases [14,17,18]. Previously, Renée et al. [19] have investigated the structure features of acetonitrile at low temperature by X-ray diffraction, and proved in solid acetonitrile, the existence of a high-temperature solid phase β (206 K) and a low temperature solid phase α (201 K). Besides, a polarized Raman study in the 77–293 K range at the atmospheric pressure reported that the vibrational frequency shifts of the internal vibrational modes and the external modes of CH₃CN displayed apparently discontinuous changes with decreasing the temperature. The results concluded that CH₃CN underwent a liquid-solid phase β structural phase transition between 220 and 229 K and a solid β-solid α structural phase transition between 208 and 212 K [18].

Although quite a number of works have investigated the structural changes of CH₃CN in solid by Raman and infrared spectroscopy at low temperature or high pressure, the vibrational properties of the internal modes especially the lattice modes of CH₃CN at solid phases β and α lack detailed discussion. Previous Raman study have checked for the further solid-state phase transitions at the lower temperature of 20 K, however, the observed Raman spectra for solid phase α were not consistent with the proposed larger unit cell [20]. Recent polarized Raman study was limited to develop a correlation between the structural phase transitions and the vibrational properties of CH₃CN, without analyzed the temperature dependence of the Raman spectra in the lattice mode region [18]. Due to its industrial and pharmaceutical applications, a better comprehension of vibrational properties of CH₃CN under low temperature condition is desirable. Moreover, studies on the behaviors of the internal modes especially the lattice modes of CH₃CN under low-temperature condition by Raman spectroscopy were not reported detailed in literature yet.

In this paper, we provide direct experimental evidence for the structural phase transitions in the low-temperature phase of CH₃CN and study the evolution of the internal modes and lattice modes of CH₃CN across the temperature-induced structural phase transition by using Raman spectroscopy. Our Raman measurements reveal the emergence of six Raman-active modes in the lattice region of solid CH₃CN in the low-temperature solid phases β and α, and they are in good agreement with the previous results obtained by Anderson et al. [20]. Our results provide clear evidence for the lack of lattice modes in the low-temperature solid phases β and α of CH₃CN. In addition, the Fermi resonance parameters of CH₃CN at different temperatures have been calculated based on the Bertran's equations and the effect of the low temperature on the Fermi resonance parameters has been analyzed. The results suggest that the Fermi resonance parameters are robust and convenient indicators of structural phase transition for CH₃CN under low temperature.