Hydrogen rich molecules are consistently observed in the atmospheres of planets. It is reasonable then to assume that they may also be stored deep in the planet interior at high pressure conditions. Therefore, understanding the properties of these molecules under extreme conditions is crucial for understanding planets as a whole. Advances in experimental techniques continue to push the upper limits of pressure which can be attained in the lab. While the standard crystallographic technique of X-ray diffraction is generally unable to isolate the precise location of hydrogen atoms within the crystal structures, infrared and Raman spectroscopy are highly sensitive to local chemical environments, including changes to covalent and hydrogen bonds. In this work, we use density functional theory (DFT) to expand the known structural candidate space of molecular mixtures to predict which compounds are thermodynamically favoured at planetary conditions. We identify new phases for direct comparison to experiment, matching theoretical vibrational spectra to the pressure evolution seen in experimental Raman and IR spectra. We show how phase transitions in molecular mixtures can be driven by a change in the hydrogen bond network arising due to molecular reorientation to enable more efficient packing at high pressure. Furthermore, we apply ab initio molecular dynamics to these mixtures to demonstrate both the dynamically stability and the range of states which exist at the high temperatures relevant to planet interiors.