New High Pressure Crystal Structures of 2,2-Dimethylbutane and 2,3-Dimethylaniline: Combined X‑ray, Raman, and Theoretical Studies

Abstract

Crystals of 2,2-dimethylbutane (DMB) and 2,3-dimethylaniline (DMA) have been crystallized from liquid under high-pressure in a diamond anvil cell. Single crystals of both compounds were obtained at 2.7 and 0.3 GPa, respectively. High-pressure X-ray measurements were performed, and new high pressure crystal structures of DMA and DMB have been refined. The freezing critical pressure for DMA (0.185 GPa) was measured with a piston–cylinder apparatus. The phase transition is accompanied by a considerable rise in molecular volume of 7.3%. For both compounds, experimental Raman spectra measured for the liquid and crystal forms have been compared to quantum chemically predicted spectra calculated for the isolated molecules and applying periodic computations. All the computational results obtained for the high frequency modes were shifted to higher values and needed to be scaled to reproduce experimental findings. The differences in the frequency values due to phase transitions of DMA and DMB were reproduced by theoretical calculations of the isolated molecules and periodic systems. The frequency values derived from periodic theoretical calculations agree better with the experimental frequencies measured for a given crystal structure (RMSDs equal to 16 and 31 cm–1 for DMA and DMB, respectively), whereas the frequency values obtained for the gas phase calculations agree better with experimental results collected for the liquid state (RMSDs equal to 9 and 17 cm–1 for DMA and DMB, respectively). The influence of the “empirical dispersion correction” (when unit cell parameters were constrained during optimization to the experimental values) on frequency values obtained from periodic calculations was also investigated. Computed theoretical frequencies obtained with the use of the correction agree better with experimental results