Hexamer formation in tertiary butyl alcohol (2-methyl-2-propanol, C4H10O)

The crystal structure of phase II of tertiary butyl alcohol (2-methyl-2-propanol, C(4)H(10)O) has been solved using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. This trigonal P3 phase, which is stable at both low temperature and high pressure, and the triclinic P1 phase (phase IV) have very similar enthalpies, the calculations revealing only a 3.859 kJ mol(-1) enthalpy difference at ambient pressure, despite the substantial change of the intermolecular bonding motif from helical catemer to hexamer with an increase in pressure or reduction in temperature. The hexamers in the trigonal phase adopt a chair conformation. There are two unique hexamers: at low temperature these are centred at (0, 0, 1/2) and (2/3, 1/3, 0.961 (13)), and at high pressure the centres are (0, 0, 1/2) and (2/3, 1/3, 0.958 (14)). A slight flattening of the hexamers is observed at high pressure and the calculations confirm that phase II becomes more stable relative to phase IV on pressure increase.