Crystal Structures and 35Cl NQR Spectra of the Metastable and the Stable Phase of Guanidinium bis-Monochloroacetate, [C(NH2)3 ]⊕[(ClH2C)COOH ••• OOC(CH2Cl)l⊖

Abstract

Abstract From acid aqueous solutions of guanidinium carbonate and monochloroacetic acid, (ClH2C)COOH :H2 NC( = NH)NH2 ≧ 2.5, the compound [C(NH2)3]⊕ [(ClH2C)COOH • • • OOC(CH2Cl)]⊖ crystallizes with the space group Ci 1P1̄,Z = 4, a = 1147.4 (5) pm, b= 1113.2 (5) pm, c = 876.5 (4) pm, α = 88.66 (2)°, β = 80.31 (2)°, γ = 84.41 (2)° (metastable phase I). Cooled to 77 K once, phase I transforms at room temperature slowly into the stable phase II, orthorhombic, D2h 15 - Pbca, Z = 8, a= 1299.2 (4) pm, b= 1533.7 (4) pm, c= 1073.9 (3) pm. The crystal structure determinations show for both phases an ionic lattice with guanidinium cations [C(NH2)3]⊕ and acid bis-(monochloroacetate) anions [(ClH2C)COOH • • • OOC(CH2Cl)]⊖ in which a monochloroacetic acid molecule and a monochloroacetate ion are bound to the dimer anion by an asymmetric hydrogen bond O-H • • • O. A strong crystal field effect, much dependent on temperature, is observed in the 35Cl NQR quadruplet spectrum of phase I, with a frequency spread of ≈4 MHz at 300 K, whereas in phase II the frequency splitting of the observed 35Cl NQR doublet is almost constant between 77 K and 310 K, about 700 kHz. The phase transition I → II is very sluggish and unidirectional. The transition II → I needs the recrystallization of II from water. Structure and dynamics of the two solid phases are discussed.