First order temperature dependent phase transition in a monoclinic polymorph crystal of 1,6-hexanedioic Acid: an interpretation based on the landau theory approach

• Published in: Molecules (Basel, Switzerland) Vol. 19; no. 7; pp. 10137 - 10149
• Main Authors: Fun, Hoong-Kun, Chantrapromma, Suchada, Ong, Lye-Hock
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 11.07.2014; MDPI
• Subjects: Acids; Adipates - chemistry; adipic acid; Asymmetry; Crystal structure; Crystals; Discontinuity; Hot Temperature; Hydrogen bonding; Hydrogen bonds; Landau theory; Models, Theoretical; Nylon; Phase transformations; Phase Transition; Phase transitions; Polymorphism; Temperature; Unit cell
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules190710137

Crystals of 1,6-hexanedioic acid (I) undergo a temperature-dependent reversible phase transition from monoclinic P21/c at a temperature higher than the critical temperature (Tc) 130 K to another monoclinic P21/c at temperature lower than Tc. The phase transition is of first order, involving a discontinuity and a tripling of the b-axis at Tc whereas the other unit cell parameters vary continuously. The transition is described by the phenomenological Landau theory. The crystal structure analyses for data collected at 297(2) K and 120.0(1) K show that there is half of a molecule of (I) in the asymmetric unit at 297(2) K whereas there are one and a half molecules of (I) in the asymmetric unit at 120.0(1) K. At both temperatures, 297(2) and 120.0(1) K, intermolecular O-H···O hydrogen bonds link the molecules of I into infinite 1D chains along [101] direction. However there are significantly more O-H···O hydrogen bonds presented in the 120.0(1) K polymorph, thereby indicating this phase transition is negotiated via hydrogen bonds. The relationship of the conformational changes and hydrogen bonding for these two polymorphs are explained in detail.