High Pressure Raman Spectroscopy Investigation On Acetonitrile And Acetonitrile–water Mixture

Acetonitrile（CH₃CN）-water mixtures are extensively used in many fields such as organic synthesis, chromatography, electrochemistry and solvent extraction, because its peculiar properties different from other traditional organic solvents.[1-3] Acetonitrile is miscible with water or alcohol in any proportion at ambient pressure and room temperature. Acetonitrile are particularly favorable for study by vibrational spectroscopy and material science because the C ≡ N group of the acetonitrile and the O-H group of the water are good sensors of their environment.Both theoretical calculation and experimental research demonstrated that after mixed with water, the structure of the mixture can be divided into three types according to the concentration of acetonitrile as the addition of hydrogen bond in the water molecule. Previous investigations are focused on the structural changes and laws in liquid mixture by Raman spectra and X- ray diffraction spectra. In contrast, the pressure induced variation of acetonitrile water mixture is still unknown. So the investigation of acetonitrile-water mixtures under the confluence of pressure and concentration is very necessary to study the crystal structure and phase transition mechanism of the mixture.The main work of the article is the use of in situ high-pressure Raman scattering spectroscopy together with synchrotron radiation X-ray diffraction experiment method to analyze the transformation mechanism of acetonitrile and the high pressure research on three different mole ratio of acetonitrile- water binary mixed solution system.High pressure Raman scattering studies have been carried out on acetonitrile in a DAC at room temperature. The structural transitions of pure acetonitrile from liquid to α phase and α to β phase were detected from Raman spectra variations at 0.2 GPa and 0.8 GPa, respectively. The third structural transitions of β-γ phase is suggested to start from 4.95 GPa to 12.46 GPa, between this range the two phases coexist.High pressure Raman scattering studies have been carried out on acetonitrile–water mixture with a molar ratio of n（CH₃CN）:n（H₂O）=1:1, 1:3 and 1:7.25 in a diamond anvil cell（DAC）. The main vibrations of acetonitrile can be found in the mixture sample, but all of their peak intensities are weaker than the pure acetonitrile, and the relative Raman shifts of peak shows lower than in pure acetonitrile too. With the increase of water content, the characteristic vibrational modes of acetonitrile in the mixture were reduce, and the characteristic vibration of water can be observed in the mixed solution of 1:7.25 and 1:3. Moreover, compared with in pure acetonitrile, the phase transition of acetonitrile in the mixture is delayed. High pressure X-ray diffraction studies have been carried out on acetonitrile–water mixture with a molar ratio of 1:7.25 in a DAC at room temperature. The result shows that acetonitrile-water mixture is forming some new substance after crystallization under high pressure. Two possible structures of the acetonitrile–water mixture are proposed by indexing the unknown peaks observed in the X-ray diffraction pattern.The effect of compression rate on the crystal morphology of acetonitrile-water mixed solution in different ratios（1:1, 1:3, 1:7.25） were compared. Under low pressurization rate, it shows that acetonitrile is preferentially precipitated from the mixed sample, and the precipitation pressure of the three different molar ratios mixtures is 0.49 GPa, 1.04 GPa and 1.21 GPa. After the crystal precipitated, the liquid part to spare is acetonitrile-water binary solutions with a small amount of acetonitrile. Moreover, with further compression, the rest of liquid of three samples solidified at 0.99 GPa, 1.33 GP and 1.37 GPa, respectively. As for high pressurization rate which means the pressure increase rapidly until the hole sample solidified with fine grains suddenly, the crystallization pressure of three different samples under rapid compression is 0.62, 1.08 and 1.25 GPa, respectively. The grain size is inversely proportional to water content.We investigate the concentrations changes of the acetonitrile-water mixture in different ratios（1:1, 1:3, 1:7.25） before and after the acetonitrile precipitated by the means of Raman spectra under high pressure. It turned out that the solubility of the mixed solution has great fluctuations in the neighbourhood of the crystal precipitation point. And the concentrations of the three kinds of mixtures are constant and the value is close after the acetonitrile precipitated. It is indicate that the concentration of acetonitrile-water solution under high-pressure is close to the mixture in the mole ratio of 1:7.25.