Vibrational and theoretical studies of urea and magnesium-urea complexes

Abstract

The molecular structure and IR spectra of urea, H 2 NCONH 2 , in gas phase and in acetonitrile solution, as well as of the two complexes [MgU 4 Cl 2 ] and [MgU 6 ]Cl 2 have been observed. The in¯uence of environmental changes to geometry and spectra are shown. Various basis sets have been employed to safeguard the validity of the reported ®ndings, using polarization functions for all calculations to get the correct pyramidal amide con®guration. The erroneous low energy of the C 2v symmetry group, after the addition of the ZPVE correction, is discussed. For the solvated urea molecule a reduction of the energy barrier, compared to the gas phase urea, between the two minimum con®gurations, C 2 and C s , and the planar geometry, is observed. The lowest energy minimum in acetonitrile is found to be the C 2 symmetry group, while for the two complexes, the local symmetry of urea is C s or C 2 depending on the complex, or even on the coordination position of urea in the complex. The wagging motion of the amide group is also discussed in all the studied urea species. The computed geometries and most of the spectroscopic results are in good agreement with the available experimental data.

Massive Intercalation of Urea in Montmorillonite

“…The successful intercalation was clearly shown by a significant expansion of thedggj .spacingfi-om 1.52 to 1.74 nm in the Ca-UCM (Fig. A gallery height of 0.84 nm estimated from the XRD pattern was in good agreement with the calculated diameter, approximately 0.84 nm for the Ca(urea)g^''" complex (Supplemental Table 1), strongly suggesting its presence within the interlayer space (Raptis et al, 2000). A gallery height of 0.84 nm estimated from the XRD pattern was in good agreement with the calculated diameter, approximately 0.84 nm for the Ca(urea)g^''" complex (Supplemental Table 1), strongly suggesting its presence within the interlayer space (Raptis et al, 2000).…”

“…Successftil complexation could be further confirmed by the unique thermal behavior ofthe coordinated urea molecules, which showed that their stepwise release depended on the ntimber of coordinated molecules (Keuleers et al, 2000;Raptis et al, 2000). In particular, the Ca-urea complex showed a thermal response similar to that ofthe Mn-(urea)g complex (Raptis et al, 2000), which strongly indicates the formation of a Ca(urea)g^"*" complex (Supplemental Fig.…”

“…Metal-urea complexes had been prepared exclusively by precipitation in an excess of solvent (Keuleers et al, 2000;Raptis et al, 2000). For the first time, to the best of our knowledge, we established the complexation of urea with various anhydrous Ca and Mg salts in a molten state, which would lead to optimization ofthe urea intercalation as well as to a systematic characterization ofthe Ca-UCM.…”

“…Recently, Park et al (2004) introduced a melt of a mixture of urea and metal salt to avoid the presence of solvents like water during the intercalation of urea. Coexistence of urea molecules and a metal salt results in ligand interaction between the metal cations and the urea molecules to weaken the H bond among the urea molecules (Keuleers et al, 2000;Park et al, 2004;Raptis et al, 2000). Consequently, the melting temperature could decrease enough to allow the formation of stable metal-urea complexes in a molten state without any thermal decomposition of the urea.…”

“…Consequently, the melting temperature could decrease enough to allow the formation of stable metal-urea complexes in a molten state without any thermal decomposition of the urea. Intercalation of metal-urea complexes into layered clay minerals has not been exploited, however, although the complexes themselves have been well characterized (Keuleers et al, 2000;Raptis et al, 2000). Intercalation of metal-urea complexes into layered clay minerals has not been exploited, however, although the complexes themselves have been well characterized (Keuleers et al, 2000;Raptis et al, 2000).…”