Structure and phase transition in 1-(diaminomethylene)thiouron-1-ium m-nitrobenzoate monohydrate and the structure of its anhydrous deuterated analogue

Abstract

The single crystals of 1-(diaminomethylene)thiouron-1-ium 3-nitrobenzoate monohydrate, [C3H7N6][C6H4(NO2)(COO)].H2O, (1) suitable for the X-ray analysis were grown using a solution growth technique at room temperature. The first order phase transition of (1) was detected by differential scanning calorimetry (DSC) measurements. The room temperature (RT) and low temperature (LT) phases are triclinic. Attempts to obtain a deuterated analogue 1 by usual reaction with heavy water lead to anhydrous monoclinic crystals of deuterated d7 1-(diaminomethylene)thiouron-1-ium 3-nitrobenzoate, [C3D7N6][C6H4(NO2)(COO)], (1D). RT and LT phases (1RT and 1LT) as well as anhydrous deuterated d7 (1D) were characterized by X-ray single crystal analysis, vibrational spectroscopy and DFT calculations. In the crystals the 1-(diaminomethylene)-thiouron-1-ium cations are not strictly planar, but twisted. The oppositely charged components, i.e. 1-(diaminomethylene)-thiouron-1-ium cation and 3-nitrobenzoate, in both RT and LT phases interact each other by the N—H…O hydrogen bonds with R22(8) and R21(6) in a similar way, while different in 1D. Hirshfeld surface and the analysis of the 2D-fingerprint plots are illustrating both qualitatively and quantitatively interactions governing the formation of the supramolecular architecture of RT and LT phases of 1 and deuterated d71D. Assignment of the vibrational bands have been supported by the DFT calculations and the isotropic frequency shift.