Thermal, spectroscopic and structural analysis of a thermosalient phase transformation in tapentadol hydrochloride

Presented herein are detailed optical, thermal, spectroscopic and structural analyses of the phase transformation occurring in tapentadol hydrochloride (C₁₄H₂₄NO⁺·Cl^-), a phenomenon already reported [Fischer et al. (2006); Patent: WO 2006000441 A2]. The thermal behaviour of the compound was studied using single-crystal X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The compound undergoes a first-order reversible phase transition at T_heat = 318.0 (1) K, T_cool = 300.0 (1) K, as assessed by the coexistence of both phases in the vicinity of the transition and the abrupt changes observed in the unit-cell parameters with temperature. The process is accompanied by clear thermosalient behaviour, with a conspicuous movement of the samples. On cooling, the transformation leads from a P2₁2₁2₁ symmetry (Z' = 1) to P2₁, with an abrupt change in β [90 ↔ 94.78 (1)°] and duplication of the asymmetric unit contents (Z' = 2). The main structural differences observed across the transition are extremely small, with almost no changes in the stronger, non-covalent interaction scheme involving the `conventional' (N-H...Cl, O-H...Cl) hydrogen bonds.