Conformational transformation coupled with the order–disorder phase transition in 2-methyl-1,3-cyclohexanedione crystals

Abstract

The half-chair conformation of the dynamically disordered molecular ring of 2-methyl-1,3-cyclohexanedione, C(7)H(10)O(2), transforms to a sofa below Tc = 244 K, when the crystal undergoes a continuous phase transition induced by the onset of halting large-amplitude vibrations of methylene groups C(4)H(2) and C(5)H(2). The temperature dependence of the crystal structure has been investigated by X-ray diffraction. The Ibam symmetry of the crystal reduces below Tc to space group Pccn. The mechanism of the phase transition and of the conversion of the ring conformation is discussed.

Mechanisms of Reversible Phase Transitions in Molecular Crystals: Case of Ciclopirox

“…58 Upon cooling from 25°to −35 °C, Figure 10 indicates that the c and β parameters of the CPX1 unit cell decrease progressively, while the a and b parameters increase. Although the expansion of unit cell parameters upon cooling is unusual among organic crystals, 29,30,44 such behavior can be understood by considering the [110] packing direction of cyclohexyl moieties in CPX1 (Figure 7a). Indeed, the centroid-to-centroid distance between two cyclohexyl substituents can be expressed as the length (a⃗ + b ⃗ )/4, and the representation of the van der Waals spheres (Figure 8) highlights that decreasing this distance upon cooling would necessarily induce unfavorable interpenetrations of the spheres.…”

“…A literature overview covering the two last decades (see e.g. refs − ) indicates however that such simple classification suffers insufficiencies, since the existence of complex behaviors cannot be readily addressed without extension or modification of the existing framework. Concepts such as cooperativity, packing frustration, concerted movements, microstructure and mosaicity, modulated structures, zip-like mechanisms, anomalous thermal expansion, lattice strains, internal molecular motions, mesoscopic effects, etc.…”

First Order Temperature Dependent Phase Transition in a Monoclinic Polymorph Crystal of 1,6-Hexanedioic Acid: An Interpretation Based on the Landau Theory Approach

“…Reports on several types of phase transitions due to hydrogen bonding [1,2] and their conformation changes [3] have been published. In our previous investigations, we have reported the studies of hydrogen bonding phase transitions in phenol/benzoic acid and amine adducts [4,5,6,7,8].…”

Temperature-induced dynamical conformational disorder in 4-vinyl benzoic acid molecular crystals: A molecular simulation study

“…Globular-shape molecules such as adamantane 5,6 and its substituted compounds 7,8 are known to show disorder associated with orientational degrees of freedom resulting in the well-known plastic crystalline phase. 8 In the case of crystals of 2-methyl-1,3-cyclohexane dione 9 the disorder is associated with the onset of large-amplitude vibrations of methylene groups. In the case of molecular crystals of stilbene and azobenzene [10][11][12][13] the disorder results in a minor conformer, which is related to the major conformer by a twofold rotation about the long molecular axis.…”