Kinetic behaviour investigations and crystal structure of nitric acid dihydrate

Abstract

X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO(3).2H(2)O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal. It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment. One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure. The intensities of diffracted lines with h odd are all very weak. The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO(3)(-)) and an H(5)O(2)(+) ionic group. The asymmetric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 A mean atomic distance difference), except for one H atom of the water molecules (0.86 A) because of their different orientations in the two molecules. The two molecules, linked by very strong hydrogen bonds, are arranged in layers. Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis. The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons. These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively.

Theoretical investigation of the coexistence of α and β-nitric acid trihydrates (NAT) molecular conformations

“…Experimental studies using the X-ray diffraction or infrared spectroscopy have identified the nitric acid monohydrates (NAM) [10], nitric acid dihydrates (NAD) [11,12] and the nitric acid trihydrates (NAT) [9,10].…”

Theoretical Study on Hydrogen‐Bond Effects in IR Spectra of High‐ and Low‐Temperature Phases of Nitric Acid Dihydrate

“…The results with two GGA parameterizations, namely, PBE and BLYP, show some small differences: the cell volume is increased by 2 % versus experiment with BLYP but reduced by almost the same amount with PBE, which overestimates the HB strength. Interlayer HB distances are underestimated in [13,14] [c] Previous DFT calculations performed with LCAO basis sets by Fernµndez-Torre et al [24] [d] Molecular dynamics simulations with plane-wave basis sets, performed by Walker et al [25] ChemPhysChem 2009, 10, 3229 -3238…”

“…[18] X-ray diffraction investigations led to determination of two different modifications of NAD at ambient pressure, namely, the high-(b) and low-temperature (a) phases, both of which crystallize in the monoclinic system in space group P2 1 /n. [13,14] A high-pressure modification was recently found [25] but is not likely to exist under stratospheric conditions. In both crystals the molecules are arranged in layers that are approximately perpendicular to the c axis.…”

“…The identification of NAD in the stratosphere may be hindered by the existence of two metastable phases, designated as a or NAD-I (lower temperature) and b or NAD-II (higher temperature), [13][14][15] whose vibrational spectroscopic features are difficult to distinguish due to the similar short-range organization inside their unit cells and their broad absorption bands.…”

“…They form two different types of aggregates within each unit cell, labelled A and B by Lebrun et al [13,14] The main characteristic of type B complexes is that they are linked by HBs to molecules within the same layer only, while those of type A are also responsible for the hydrogen bonding between layers. Type B complexes have a similar geometry in both a and b phases of NAD, while type A complexes have a slightly different orientation in each of them.…”