Crystal structure analysis of the adduct 1:1 hexamethylenetetramine and succinic acid (HMT−C4) as a function of the temperature

Abstract

Co−crystals of hexamethylenetetramine [N4(CH2)6, (HMT)] and dicarboxylic acids of general formula HOOC(CH2)n−2COOH with 4 ≤ n ≤ 14 (Cn), called HMT−Cn, exhibits structural properties strongly dependent on the length of the dicarboxylic acid chain, showing a wide variety of thermotropic phase transition sequence. This study focuses on describing the structural features of the HMT−C4 (1:1) adduct, a co-crystal of HMT and succinic acid (C4), exhibiting a reversible first-order phase transition at 222(4)K, as evidenced by differential scanning calorimetry (DSC) experiments. In contrast to a prior investigations , which did not identify this phase transition, our comprehensive temperature-dependent analysis, incorporating single crystal X-ray diffraction and DSC experiments, reveals a reversible structural change in the HMT−C4 (1:1) adduct structure upon cooling and heating. The crystal structures of the different observed phases are discussed in detail, emphasizing the role of O−H···N intermolecular hydrogen bonds in maintaining the connectivity between HMT and C4 molecules. The observed phase transition involves doubling the unit cell volume and significant reorientation of the −COOH groups at the end of the C4 chain at around 222(4)K. Our findings underscore the importance of additional temperature-dependent experiments, such as DSC, in avoiding erroneous structure modeling due to symmetry changes during temperature variations. The preservation of the acidic character of the C4 dicarboxylic chain throughout the temperature range adds additional insights to the understanding of the structural behavior of all the HMT−Cn (1:1) adducts.