Temperature-Induced Reversible First-Order Single Crystal to Single Crystal Phase Transition in Boc-γ⁴(R)Val-Val-OH: Interplay of Enthalpy and Entropy

Abstract

Crystals of Boc-γ(4)(R)Val-Val-OH undergo a reversible first-order single crystal to single crystal phase transition at Tc ≈ 205 K from the orthorhombic space group P22121 (Z' = 1) to the monoclinic space group P21 (Z' = 2) with a hysteresis of ∼2.1 K. The low-temperature monoclinic form is best described as a nonmerohedral twin with ∼50% contributions from its two components. The thermal behavior of the dipeptide crystals was characterized by differential scanning calorimetry experiments. Visual changes in birefringence of the sample during heating and cooling cycles on a hot-stage microscope with polarized light supported the phase transition. Variable-temperature unit cell check measurements from 300 to 100 K showed discontinuity in the volume and cell parameters near the transition temperature, supporting the first-order behavior. A detailed comparison of the room-temperature orthorhombic form with the low-temperature (100 K) monoclinic form revealed that the strong hydrogen-bonding motif is retained in both crystal systems, whereas the non-covalent interactions involving side chains of the dipeptide differ significantly, leading to a small change in molecular conformation in the monoclinic form as well as a small reorientation of the molecules along the ac plane. A rigid-body thermal motion analysis (translation, libration, screw; correlation of translation and libration) was performed to study the crystal entropy. The reversible nature of the phase transition is probably the result of an interplay between enthalpy and entropy: the low-temperature monoclinic form is enthalpically favored, whereas the room-temperature orthorhombic form is entropically favored.

Reversible Single-Crystal-to-Single-Crystal Phase Transition of Chiral Salicylidenephenylethylamine

“…The enthalpy at the exothermic and endothermic peaks was −0.69 and 0.67 kJ·mol −1 , respectively. This small thermal hysteresis might be a general feature of SCSC transition because small hysteresis was reported also in the previous papers [4,7,10]. DSC curves in wide temperature ranges indicated no existence of additional phase transition ( Figure S1).…”

“…Hence, the reversible SCSC phase transition is a fascinating phenomenon because of its potential applications in ferroelectric and switchable dielectric devices [1][2][3]. Several molecular crystals that undergo temperature-induced reversible SCSC phase transition have been reported [4][5][6][7][8][9][10].…”

“…Crystals 2017, 7, 7 8 of 10 The dielectric constant ε r increased from 2.636 to 2.671 (+1.33%) on cooling from 5.7 • C to 1.3 • C (Figure 7c). From the temperature dependence of the lattice constants in Figure 2, the surface area of the (010) face should decrease (−0.83%) and the thickness along the b-axis should also decrease (−0.09%) in the phase transition from the α-form (10 • C) to the β-form (0 • C).…”

A temperature-induced order–disorder phase transition in a 4-substituted 4,2′:6′,4′′-terpyridine

“…Considering these antecedents we decided to investigate the closely related species 4 0 -(isoquinolin-4-yl)-4,2 0 :6 0 ,4 00 -terpyridine (iqtp), which possesses a similar coordination capacity to ptp and where the only difference is the metal-binding fused-ring isoquinolin-4-yl instead of 4 0 -pyridyl. Unexpectedly, during the structural study by X-ray diffraction we observed a temperature-dependent single-crystal to single-crystal first-order phase transition, a phenomenon that, although not new in organic crystals (Pal et al, 2014;Drebushchak et al, 2011;Kubicki, 2004), to the best of our knowledge is novel in this type of terpyridine-based compound.…”

Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder

“…Conformational flexibility in the crystalline state may contribute to an order-disorder phase transition in which different conformers co-exist in the high-temperature form and are transformed into a single stable conformer in the low-temperature form upon cooling; this transformation may or may not be associated with a change in the space group (Quah et al, 2012). Temperatureinduced translationengleiche group-subgroup relations such as the monoclinic-to-orthorhombic phase transition observed in barbituric acid dihydrate (P2 1 /n $ Pnma) (Nichol & Clegg, 2005) and Boc-4 (R)Val-Val-OH crystals (P2 1 $ P22 1 2 1 ) (Pal et al, 2014) result in twinned crystals, whereas klassengleichetype phase transitions associated with loss of translational symmetry will produce antiphase domains (Mü ller, 2013). In these cases, as well as in the title compound, the lowtemperature and low-symmetry phases are monoclinic pseudomerohedral or nonmerohedral twins.…”