Hydrogen-Bonding Modification in Biuret Under Pressure

Abstract

Biuret (CHNO) has been studied to 30 GPa by Raman spectroscopy and 50 GPa by X-ray diffraction. Raman peaks exhibit shoulders and splitting that suggests that the molecules undergo reorientation in response to compression. These are observed in three pressure ranges: the first from 3-5 GPa, the second from 8-12 GPa, and finally from 16-20 GPa. The particular modes in the sample that are observed to change in the Raman are strongly linked to the molecular vibrations involving the N-H and the C═O bond, which are most strongly coupled to the hydrogen-bonded lattice structure. The X-ray diffraction suggests that the crystal maintains a monoclinic structure to the highest pressures studied. Although there was a considerable degree of hysteresis observed in some X-ray runs, all the changes observed under pressure are reversible.

The role of H-bond in the high-pressure chemistry of model molecules

“…The strengthening of the H bond with compression has been evidenced in all the simplest amides: formamide [30,31], acetamide [32], oxamide [33] and malonamide [34] but in all these cases the explored pressure range was not large enough to observe a chemical instability, although in some cases it was close to 20 GPa. Much higher pressures have been reached in urea [35] and biuret [36] which show an astonishing stability under room temperature compression, but no chemical transformation are detected up to 50 GPa, the maximum pressure investigated. All these systems are characterized by strong directional 3D intermolecular interactions realized through a large number of H-bonds.…”

“…Similarly to many other aromatic and heteroaromatic systems [4] also pyridine is reported to transform into an amorphous solid when phase II is compressed above 20 GPa [47,48]. However, there are several peculiar aspects characterizing the pyridine reactivity: a very limited extent of the reaction, a reduction of the reactivity with increasing the crystal quality, and the missed observation of the propagation during Pressure shift of some Raman (malonamide [34] and biuret [36]) and infrared (urea [38]) bands relative to vibrational modes involving the amino and carbonyl groups. The softening shown by all the bands in some pressure intervals is ascribable to the strengthening of H-bonds.…”

“…The slope inversions and discontinuities observed in the case of urea and malonamide, respectively, coincide with crystalline phase transitions and are due to the reconstruction of the H-bonds arrangement. In the case of biuret the frequencies of the NH 2 stretching modes are limited to a maximum pressure of 13 GPa because the Raman intensities of these modes vanish for higher pressures [36]. [35].…”

“…Figure 1.Pressure shift of some Raman (malonamide[34] and biuret[36]) and infrared (urea[38]) bands relative to vibrational modes involving the amino and carbonyl groups. The softening shown by all the bands in some pressure intervals is ascribable to the strengthening of H-bonds.…”

Estudio de interacciones no covalentes en diferentes fases cristalinas de la urea

“…Los resultados obtenidos coinciden en que la participación del reordenamiento de los puentes de hidrógeno NH...O y NH...N están relacionados con las diferentes estructuras cristalinas formadas. [6,15] Miao y col. [16] fueron los primeros en aplicar la Teoría de Funcionales de la Densidad (TFD) bajo el contexto de Kohn-Sham al cristal de urea a diferentes presiones. Estos autores utilizaron un funcional de intercambio y correlación de Ceperley-Alder parametrizado por Perdew-Zunger y ondas planas como funciones de base.…”