The balance of weak CH...N bonds involving the H 3C- and -CN groups has been related to the structural rearrangement between centrosymmetric and polar acetonitrile structures. The linear highly polar molecules arrange antiparallel in phase alpha below a freezing temperature of 225 K/0.1 MPa and above a freezing pressure of 0.38(5) GPa/296 K and in a polar mode in phase beta below 206 K/0.1 MPa and at pressures higher than 0.63(5) GPa/296 K. The alpha <--> beta phase transition has been considered as a supramolecular reaction between 2-dimensional and 3-dimensional hydrogen-bonding networks, the latter favoring the polar association. Acetonitrile has been in situ pressure-frozen, and its structure has been determined at room temperature by single-crystal X-ray diffraction at 0.57(5), 0.63(5), and 1.50(5)GPa. The crystallization pressure at 296 K has been determined as 0.38(5) GPa both by ruby fluorescence in a diamond anvil-cell and by the compressibility measurement in a cylinder-and-piston device. Acetonitrile mixed with methanol trimerized at 1.60 GPa and 473 K yielding 4-amino-2,6-dimethylpyrimidine: it was in situ crystallized, and the structure of a single crystal, recovered at ambient conditions, was determined.