X-ray diffraction and vibrational spectra of the guanidine:maleic acid (1:1) complex—two crystal forms

Abstract

From an equimolar mixture of guanidine and maleic acid, two different crystal modifications of guanidinium hydrogen maleate, CH₆N ⁺₃ ·C₄H₃O ⁻₄ , were isolated and their crystal structures determined using three-dimensional diffractometer data. Crystals of both forms are orthorhombic:Pca2₁,a=19.012(8)b=3.758(1),c=11.119(5) Å,Z=4 for form I;Fdd2,a=18.505(2),b=20.007(3),c=8.406(2) Å,Z=16 for form II. The structures were solved by direct methods and refined by full-matrix least squares to conventionalR values of 0.064 for form I and 0.043 for form II, with 488 and 605 reflections respectively. The structures consist of planar guanidinium cations C(NH₂) ⁺₃ and hydrogen-maleate residues C₄H₃O ⁻₄ . There are no significant differences in distances and angles within the guanidinium and hydrogen-maleate units. However, a substantial difference appears in the length and geometry of the intramolecular hydrogen bond O⋯H⋯O. The packing of cations and anions is also completely different in the two forms. Infrared and Raman spectra of the title compound are presented, and salient differences between them and the spectrum of the symmetrically bonded potassium hydrogen maleate are discussed briefly.