Anti-parallel dimer and tetramer formation of propylene carbonate

作者：Tagawa Ayana; Numata Tomoko; Shikata Toshiyuki^*

来源：AIP Advances, 2017, 7(9): 095103.

DOI：10.1063/1.5002118

Raman scattering and infrared (IR) absorption spectra of enantiopure (R)-propylene carbonate ((R)PC) and racemic propylene carbonate (PC) were recorded at room temperature, 25 degrees C, in benzene (Bz) solution and in the pure liquid state to investigate the presence of dimers and other higher order intermolecular associations. (R)PC and PC both demonstrated a strong C=O stretching vibrational band. The band exhibited changes in its shape and resonance wavenumber highly dependent on the concentrations of PCs, whereas a difference between the chirality of (R) PC and PC had little influence. In an extremely dilute condition, doubly split bands were observed at 1807 and 1820 cm(-1) in both Raman and IR spectra, which are assigned to the characteristic bands of isolated monomeric PCs. An additional band appeared at 1795 cm(-1) in a dilute to concentrated regime, and its magnitude strengthened with increasing concentrations accompanied with slight increasing in the magnitude of 1807 cm(-1) band in Raman spectra, while an increase in the magnitude of 1807 cm(-1) band was clearly greater than that of 1795 cm(-1) band in IR spectra. The spectrum changes at 1795 and 1807 cm(-1) were attributed to characteristics of anti-parallel dimer formation of PCs caused by strong dipole-dipole interactions between C=O groups. Moreover, another additional signal was clearly observed at 1780-1790 cm(-1) in a concentrated regime, and became the primary signal in the pure liquid state with slight increasing in the intensity of 1795 cm(-1) band in Raman spectra. On the other hand, in IR spectra the observed increasing of 1780-1790 cm(-1) band was much less than that of 1795 cm(-1) band. These newly found spectrum changes in the concentrated regime are attributed to the formation of anti-parallel tetramers of PCs based on the characteristics of band selection rule found in Raman and IR spectra. Equilibrium constants for the anti-parallel dimer (K-D) and tetramer formation (K-T) of PCs in Bz solution and in the pure liquid state were also determined using the Raman and IR data assuming chemical processes: 4PC <-> 2(PC)(2) <-> (PC)(4).

• 出版日期2017-9