title: Novel organic NLO material bis(N-phenylbiguanidium(1+)) oxalate - A combined X-ray diffraction, DSC and vibrational spectroscopic study of its unique polymorphism.
abstract: Three polymorphic modifications of bis(N-phenylbiguanidium(1+)) oxalate are reported, and their characterization is discussed in this paper. The non-centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (I), which was obtained from an aqueous solution at 313K, belongs to the monoclinic space group Cc (a=6.2560(2)Å, b=18.6920(3)Å, c=18.2980(5)Å, β=96.249(1)°, V=2127.0(1)Å(3), Z=4, R=0.0314 for 4738 observed reflections). The centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (II) was obtained from an aqueous solution at 298K and belongs to the monoclinic space group P21/n (a=6.1335(3)Å, b=11.7862(6)Å, c=14.5962(8)Å, β=95.728(2)°, V=1049.90(9)Å(3), Z=4, R=0.0420 for 2396 observed reflections). The cooling of the centrosymmetric phase (II) leads to the formation of bis(N-phenylbiguanidium(1+)) oxalate (III) (a=6.1083(2)Å, b=11.3178(5)Å, c=14.9947(5)Å, β=93.151(2)°, V=1035.05(8)Å(3), Z=4, R=0.0345 for 2367 observed reflections and a temperature of 110K), which also belongs to the monoclinic space group P21/n. The crystal structures of the three characterized phases are generally based on layers of isolated N-phenylbiguanidium(1+) cations separated by oxalate anions and interconnected with them by several types of N-H(...)O hydrogen bonds. The observed phases generally differ not only in their crystal packing but also in the lengths and characteristics of their hydrogen bonds. The thermal behaviour of the prepared compounds was studied using the DSC method in the temperature range from 90K up to a temperature near the melting point of each crystal. The bis(N-phenylbiguanidium(1+)) oxalate (II) crystals exhibit weak reversible thermal effects on the DSC curve at 147K (heating run). Further investigation of this effect, which was assigned to the isostructural phase transformation, was performed using FTIR, Raman spectroscopy and X-ray diffraction analysis in a wide temperature range.
authors.affiliation: Faculty of Science, Department of Inorganic Chemistry, Charles University in Prague, Hlavova 8, 128 40 Prague 2, Czech Republic.
authors.forename: Irena
authors.initials: I
authors.lastname: Matulková
authors.affiliation: Faculty of Science, Department of Inorganic Chemistry, Charles University in Prague, Hlavova 8, 128 40 Prague 2, Czech Republic.
authors.forename: Ivana
authors.initials: I
authors.lastname: Císařová
authors.affiliation: Department of Dielectrics, Institute of Physics ASCR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic.
authors.forename: Přemysl
authors.initials: P
authors.lastname: Vaněk
authors.affiliation: Faculty of Mathematics and Physics, Department of Chemical Physics and Optics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague 2, Czech Republic.
authors.forename: Petr
authors.initials: P
authors.lastname: Němec
authors.affiliation: Faculty of Science, Department of Inorganic Chemistry, Charles University in Prague, Hlavova 8, 128 40 Prague 2, Czech Republic. Electronic address: agnemec@natur.cuni.cz.
authors.forename: Ivan
authors.initials: I
authors.lastname: Němec
keywords: N-Phenylbiguanide salts; Polymorphism; Second harmonic generation; Thermal behaviour; Vibrational spectroscopy; X-ray diffraction