A new high-pressure phase of n-butanol and its structural relationship with the low-temperature structure

The structural behavior and crystallization of n-butanol under low-temperature and high-pressure conditions have been systematically investigated by single-crystal X-ray diffraction. At ambient pressure and low temperature, n-butanol crystallizes in triclinic space group P1, denoted as phase α, with two crystallographically independent molecules per asymmetric unit (Z' = 2). The structure of phase α is characterized by alternating polar and hydrophobic layers, stabilized by distinctive zig-zag hydrogen-bonded chains along [100], closely approximating twofold screw axis (2₁) symmetry. Under high-pressure, a new centrosymmetric phase β (space group P2₁/n, Z' = 3) emerges. Phase β have similar aggregation motif stabilized by hydrogen-bonded zig-zag chains as one observed in phase α, but with oppositely oriented intermolecular vectors: (i) one chain relate the molecules by the 2₁ screw axis; and (ii) the other by glide-plane symmetry. Intriguingly, the recurrence of closely related structural motifs in both phases strongly suggests a common crystallization mechanism rooted in the formation of similar pre-associated molecular aggregates. These findings illuminate the sophisticated interplay between thermodynamic parameters and structure, advancing the understanding of linear monoalcohols aggregation.