Trimorphism of N-methylurea: crystal structures, phase transitions and thermodynamic stabilities

• Published in: CrystEngComm Vol. 18; no. 25; pp. 4772 - 4778
• Main Authors: Baaklini, G., Gbabode, G., Clevers, S., Négrier, P., Mondieig, D., Coquerel, G.
• Format: Journal Article
• Language: English
• Published: Royal Society of Chemistry 01.01.2016
• Subjects: Chemical Sciences; Cristallography; Crystal structure; Differential scanning calorimetry; Movements; Phase transformations; Quenching; Stability; Thermodynamics; X-rays
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/C6CE00652C

Melt crystallization of N-methylurea (NMU) was investigated by X-ray and differential scanning calorimetry (DSC) techniques. In addition to the known orthorhombic stable Form I, two new polymorphic forms (II and III) crystallizing in the centrosymmetric space group P2 sub(1)/cwere obtained at different quenching temperatures. The crystal structures of Forms II and III were determined from X-ray powder diffraction data. The crystallographic results for the monoclinic forms are presented in this study, as well as a detailed comparison of crystal structures of the three polymorphs. The nature of hydrogen bonds in Form II and Form III were found to be similar; hence, a first order mechanism governed by means of shear movements is suggested where a transition vianucleation and growth could occur at low temperatures. The relative thermodynamic stabilities of the three forms of NMU were investigated by DSC analyses. Form II and Form III exhibited a reversible solid-solid phase transition at circa-118 degree C. Both forms were found to be monotropically related to the stable Form I.