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The crystal structure of a new high-temperature phase of nitric acid dihydrate, HNO3·2H2O, has been determined at 225 K by single-crystal X-ray diffraction. The H atom of the nitric acid is delocalized to one water mol­ecule, leading to an association of equimolar NO3- and H5O2+ ionic groups. The asymmetric unit contains two mol­ecules of HNO3·2H2O. The two independent mol­ecules are related by a pseudo-twofold c axis, by a translation of 0.54 (approximately ½) along b, with a mean atomic distance difference of 0.3 Å, except for one H atom of the water mol­ecules (1.5 Å), because of their different orientations in the two mol­ecules. The two independent mol­ecules, linked by strong hydrogen bonds, are arranged in layers. These layers are linked by weaker hydrogen bonds oriented approximately along the c axis. A three-dimensional hydrogen-bond network is observed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101010101/sk1488sup1.cif
Contains datablocks global, NAD

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101010101/sk1488NADsup2.hkl
Contains datablock NAD

Comment top

It is known that polar stratospheric aerosols play a crucial role in ozone depletion in Arctic and Antarctic regions (Salomon, 1988). Heterogeneous processes which occur at the surface of these aerosols lead to the easy formation of chlorine radicals (Cl, ClO), resulting in catalytic ozone destruction. The reaction mechanism remains controversial, due to the lack of information on the physical state of some hydrates of nitric and sulfuric acids. Previous experimental reports indicated the possible crystallization of the dihydrate of nitric acid (Ji & Petit, 1992, 1993), denoted NAD. Recent powder and single-crystal X-ray diffraction experiments (Lebrun et al., 2001) confirmed the formation of NAD and identifed one stable crystalline phase, denoted NAD(I), at low temperature. Under particular thermal conditions, the crystallization of a new polymorphic phase, denoted NAD(II) and never previously observed, was revealed (Mahé, 1999). NAD(II) transforms at 235 K, on heating, into nitric acid trihydrate, denoted NAT, via a peritectic reaction: NAD(II) liquid + NAT (Mahé et al., 2000). \sch

As for NAD(I), the solid phase of NAD(II) crystallizes in space group P21/n with two independent molecules, denoted A and B, in the asymmetric unit. These molecules are arranged in planes linked together by hydrogen bonds. One layer of molecules is shown in Fig. 1 and selected geometric parameters are given in Table 1.

The delocalization of the nitric acid H atom to one water molecule leads to the formation of a nitrate ion, NO3-, an oxonium ion, H3O+, and a water molecule linked together by strong hydrogen bonds. In each independent molecule, the oxonium ion is strongly linked, via one H atom (H2A or H2B), to the water molecule involving the acceptor O atom (O5A or O5B), leading to an unsymmetrical H5O2+ ion. The mean values of these `intramolecular' hydrogen bonds are O···O 2.51 and O···H 1.63 Å, and O—H···O 176°. The oxonium ion is also linked to the nitrate ion via weaker hydrogen bonds (O2A···H3A—O4A and O2B···H3B—O4B), with mean values of O···O 2.64 and O···H 1.87 Å, and O—H···O 175°. The hydrogen-bonding geometry is reported in Table 2. Only two O atoms of the NO3- ions are engaged in three acceptor hydrogen bonds (see Fig. 1), explaining the significantly shorter length observed for one N—O bond (NA—O3A and NB—O3B).

The two independent molecules, A and B, are related by a pseudo twofold c axis with a translation of 0.54 (approximately 1/2) along the b axis. Applying this pseudo-symmetry to molecule B, the mean distance between the corresponding atoms is low (0.3 Å), except for the water molecules (1.5 Å between H5A and H5B), because of their different orientations, as shown in Fig. 1. Molecules A and B are linked together by weak O1A···H4B—O5B and O1B···H4A—O5A hydrogen bonds. The mean values of these hydrogen bonds are O···O 2.830 and O···H 2.02 Å, and O—H···O 160.5°.

In one plane, the molecular groups formed by molecules A and B are related by a diagonal symmetry plane. Each group is linked to six other neighbouring groups via three different hydrogen bonds, denoted (1) (O4A—H1A···O1Bii), (2) (O4B—H1B···O1Ai) and (3) (O5B—H5B···O5Aiii), leading to a complicated two-dimensional hydrogen-bond network, as shown in Fig. 1 [symmetry codes: (i) -1/2 - x, y - 1/2, 1/2 - z; (ii) 1/2 - x, 1/2 + y, 1/2 - z; (iii) x - 1, y, z]. Query.

A perspective view of the structure, nearly parallel to the b axis, is shown in Fig. 2. The structure may be described by two different planes, L1 and L2, spaced by c/2 and parallel to the (a,b) plane. L2 is deduced from L1 by inversion symmetry. These layers are weakly linked by one type of hydrogen bond, denoted (4) in Fig. 2, involving the `out of layer' H atom, H5A, of the water molecule in layer L1 as donor to the nitrate ion of molecule A located in layer L2. The structure may be described by the translation of this group of two linked layers along the [001] direction, leading to a complex three-dimensional hydrogen-bond network.

Experimental top

Despite numerous attempts with the NAD composition X(HNO3) = 1/3, no single-crystal of NAD(II) was obtained in situ on the four-circle diffractometer. The adapted Bridgman method used at this composition did not succeed, since NAD(II) always transformed at 235 K into NAT + liquid. According to the phase diagram established by Ji & Petit (1992, 1993) and Mahé (1999), a composition near the eutectic liquid NAM + NAD [X(HNO3) = 0.38] (NAM is nitric acid monohydrate) phase was chosen, since the kinetics of crystallization of NAM and NAD are slower than those observed at X(HNO3) = 0.33. The liquid was enclosed in a sharpened Lindemann tube. Thermal treatment was performed using a low-temperature nitrogen gas flow device. The liquid [X(HNO3) = 0.38] was rapidly cooled from 293 K to 183 K and crystallized into the NAD(I) form, with liquid remaining. After a rapid increase of the temperature up to 200 K, the solid was slowly heated up to 225 K and the transition NAD(I) NAD(II) was observed at 210 K. At 225 K, a mixture of the two solids was obtained: large single-crystal domains of NAD(II) surrounded by crystalline solid of NAM, as provided by the binary phase diagram for HNO3·H2O (Ji & Petit, 1992, 1993). A large single-crystal domain of NAD(II) of sufficiently good quality was selected and data collection was possible, in the range θ = 2–20°. The low-temperature nitrogen gas flow failed before the end of the data collection, so that only 467 unique reflections were measured instead of the 756 expected. Other attempts at single-crystal growth were carried out without success. During the data collection, three reflections were measured every 60 min. These reference intensities increased linearly (13.2, 9.7 and 12.1%, respectively), probably due to some molecular rearrangement of the single-crystal. An averaged correction was applied to the diffracted intensities.

Refinement top

Despite the low number of unique measured reflections and the poor quality of the crystal, the refinement led to a good reliability factor (3.08%) with 140 refined parameters. The H atoms were located on a difference Fourier map. During the refinement procedures, short hydrogen bond lengths were found for the oxonium ion of molecule A. Consequently, the structure was refined with the O4B—H1B, O4B—H2B and O4B—H3B hydrogen bond lengths constrained between 0.8 and 1.0 Å.

Computing details top

Data collection: PW1100 software (Philips, 19??); cell refinement: PW1100 software; data reduction: PW1100 software; program(s) used to solve structure: SHELXS86 (Sheldrick, 1985); program(s) used to refine structure: SHELXL93 (Sheldrick, 1993); molecular graphics: ORTEPIII (Burnett & Johnson, 1996).

Figures top
[Figure 1] Fig. 1. The projection along [001] of the two independent molecules of NAD, A and B, showing their neighbourhood in one layer (L1 plane) and the atom-numbering scheme. Hydrogen bonds are indicated by thin broken lines and displacement ellipsoids are drawn at the 50% probability level.
[Figure 2] Fig. 2. The projection of NAD along the [010] direction, showing the two types of layers (L1 and L2) and the crystal packing.
Nitric acid dihydrate top
Crystal data top
HNO3·2H2OF(000) = 416
Mr = 99.05Dx = 1.638 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ynCell parameters from 25 reflections
a = 9.674 (3) Åθ = 3–8°
b = 12.920 (4) ŵ = 0.19 mm1
c = 6.484 (3) ÅT = 225 K
β = 97.71 (3)°Cylindrical, colourless
V = 803 (1) Å3unknown mm (radius)
Z = 8
Data collection top
Philips PW1100 four-circle
diffractometer
Rint = 0
Radiation source: fine-focus sealed tubeθmax = 20°, θmin = 2°
Graphite monochromatorh = 99
ω/2θ scansk = 012
503 measured reflectionsl = 06
467 independent reflections3 standard reflections every 60 min
414 reflections with I > 3σ(I) intensity decay: 11.5%
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: FullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.031All H-atom parameters refined
wR(F2) = 0.083 w = 1/[σ2(Fo2) + (0.0617P)2]
where P = (Fo2 + 2Fc2)/3
S = 1.13(Δ/σ)max = 0.001
414 reflectionsΔρmax = 0.14 e Å3
140 parametersΔρmin = 0.15 e Å3
3 restraintsExtinction correction: SHELXL93 (Sheldrick, 1993)
0 constraintsExtinction coefficient: 0.009 (3)
Crystal data top
HNO3·2H2OV = 803 (1) Å3
Mr = 99.05Z = 8
Monoclinic, P21/nMo Kα radiation
a = 9.674 (3) ŵ = 0.19 mm1
b = 12.920 (4) ÅT = 225 K
c = 6.484 (3) Åunknown mm (radius)
β = 97.71 (3)°
Data collection top
Philips PW1100 four-circle
diffractometer
Rint = 0
503 measured reflectionsθmax = 20°
467 independent reflections3 standard reflections every 60 min
414 reflections with I > 3σ(I) intensity decay: 11.5%
Refinement top
R[F2 > 2σ(F2)] = 0.0313 restraints
wR(F2) = 0.083All H-atom parameters refined
S = 1.13Δρmax = 0.14 e Å3
414 reflectionsΔρmin = 0.15 e Å3
140 parameters
Special details top

Experimental. The data collection recorded between 15 and 20 ° (-9 < h < 9; 0 < k < 12; 0 < l < 6) is not complete since the nitrogen gas flow equipement failed before the end of the experiment. Only 193 intensities have been collected. Other attempts on single-crystal growth were carried out without success.

Geometry. During the refinement procedure O4B—H1B, O4B—H2B and O4B—H3B were constrained between 0.8 and 1.0 Å.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
NA0.1046 (4)0.4615 (3)0.2011 (6)0.0240 (13)
O1A0.2333 (3)0.4701 (2)0.1371 (5)0.0304 (12)
O2A0.0548 (3)0.3712 (2)0.2277 (5)0.0314 (12)
O3A0.0312 (3)0.5375 (2)0.2344 (6)0.0401 (12)
O4A0.2080 (3)0.3880 (3)0.3896 (6)0.0327 (13)
O5A0.3732 (3)0.2385 (2)0.3409 (6)0.0261 (13)
NB0.0952 (4)0.0813 (3)0.2027 (7)0.0232 (11)
O1B0.2157 (3)0.0731 (2)0.1442 (5)0.0289 (12)
O2B0.0487 (3)0.1701 (2)0.2239 (5)0.0352 (13)
O3B0.0301 (3)0.0035 (3)0.2393 (5)0.0350 (12)
O4B0.1882 (3)0.1659 (2)0.3909 (6)0.0303 (12)
O5B0.3676 (3)0.2819 (2)0.2136 (6)0.0331 (13)
H1A0.245 (4)0.459 (4)0.369 (7)0.051 (10)*
H2A0.265 (4)0.338 (3)0.366 (8)0.053 (10)*
H3A0.135 (5)0.384 (4)0.338 (9)0.062 (11)*
H4A0.332 (6)0.194 (4)0.257 (10)0.074 (11)*
H5A0.395 (5)0.217 (4)0.457 (9)0.042 (9)*
H1B0.218 (5)0.101 (3)0.381 (9)0.069 (12)*
H2B0.249 (4)0.207 (3)0.328 (7)0.050 (10)*
H3B0.114 (4)0.170 (4)0.344 (8)0.072 (12)*
H4B0.343 (5)0.343 (4)0.210 (8)0.046 (7)*
H5B0.440 (5)0.270 (4)0.262 (9)0.068 (11)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
NA0.027 (2)0.018 (2)0.029 (3)0.003 (2)0.007 (2)0.007 (3)
O1A0.0186 (19)0.0252 (18)0.055 (3)0.0038 (12)0.0033 (18)0.002 (2)
O2A0.0277 (17)0.0149 (17)0.051 (3)0.0069 (13)0.0023 (17)0.002 (2)
O3A0.0270 (18)0.0238 (19)0.069 (3)0.0053 (15)0.007 (2)0.008 (2)
O4A0.0221 (16)0.0243 (19)0.052 (4)0.0017 (16)0.005 (2)0.006 (2)
O5A0.0274 (18)0.023 (2)0.026 (3)0.0021 (13)0.004 (2)0.002 (2)
NB0.022 (2)0.023 (3)0.024 (4)0.001 (2)0.001 (2)0.001 (2)
O1B0.0211 (18)0.0259 (18)0.042 (3)0.0020 (13)0.0119 (18)0.0020 (18)
O2B0.0283 (17)0.023 (2)0.054 (3)0.0092 (14)0.0075 (18)0.003 (2)
O3B0.0299 (18)0.029 (2)0.047 (3)0.0066 (15)0.0065 (19)0.002 (2)
O4B0.0291 (18)0.0262 (19)0.035 (3)0.0013 (14)0.001 (2)0.002 (2)
O5B0.030 (2)0.0273 (18)0.044 (3)0.0039 (15)0.012 (2)0.001 (2)
Geometric parameters (Å, º) top
NA—O1A1.263 (4)NB—O1B1.278 (4)
NA—O2A1.265 (4)NB—O2B1.246 (4)
NA—O3A1.214 (4)NB—O3B1.227 (4)
O4A—H1A1.00 (5)O4B—H1B0.89 (3)
O4A—H2A0.88 (5)O4B—H2B0.89 (3)
O4A—H3A0.75 (5)O4B—H3B0.82 (3)
O5A—H4A0.86 (4)O5B—H4B0.83 (3)
O5A—H5A0.80 (3)O5B—H5B0.82 (3)
O2A—NA—O3A121.2 (3)O2B—NB—O3B122.0 (3)
O2A—NA—O1A117.9 (3)O2B—NB—O1B117.8 (3)
O3A—NA—O1A120.9 (3)O3B—NB—O1B120.1 (3)
H3A—O4A—H1A111 (3)H3B—O4B—H2B114 (3)
H3A—O4A—H2A117 (3)H3B—O4B—H1B109 (3)
H1A—O4A—H2A114 (3)H2B—O4B—H1B109 (3)
H5A—O5A—H4A114 (3)H4B—O5B—H5B117 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O5B—H4B···O1A0.83 (3)2.04 (5)2.832 (4)159 (5)
O5A—H4A···O1B0.86 (4)2.00 (6)2.827 (4)162 (7)
O4A—H3A···O2A0.75 (5)1.88 (5)2.626 (4)176 (4)
O4B—H3B···O2B0.82 (3)1.85 (4)2.663 (4)175 (5)
O4B—H2B···O5B0.89 (3)1.57 (4)2.459 (4)178 (5)
O4A—H2A···O5A0.88 (5)1.68 (4)2.553 (5)175 (4)
O4B—H1B···O1Ai0.89 (3)1.75 (4)2.640 (4)177 (4)
O4A—H1A···O1Bii1.00 (5)1.53 (5)2.522 (5)171 (5)
O5B—H5B···O5Aiii0.82 (3)1.98 (5)2.800 (4)173 (6)
O5Aii—H5Aii···O2Aiv0.80 (3)2.10 (5)2.884 (5)167 (5)
Symmetry codes: (i) x1/2, y1/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x1, y, z; (iv) x, y+1, z.

Experimental details

Crystal data
Chemical formulaHNO3·2H2O
Mr99.05
Crystal system, space groupMonoclinic, P21/n
Temperature (K)225
a, b, c (Å)9.674 (3), 12.920 (4), 6.484 (3)
β (°) 97.71 (3)
V3)803 (1)
Z8
Radiation typeMo Kα
µ (mm1)0.19
Crystal size (mm)unknown (radius)
Data collection
DiffractometerPhilips PW1100 four-circle
diffractometer
Absorption correction
No. of measured, independent and
observed [I > 3σ(I)] reflections
503, 467, 414
Rint0
θmax (°)20
(sin θ/λ)max1)0.481
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.031, 0.083, 1.13
No. of reflections414
No. of parameters140
No. of restraints3
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)0.14, 0.15

Computer programs: PW1100 software (Philips, 19??), PW1100 software, SHELXS86 (Sheldrick, 1985), SHELXL93 (Sheldrick, 1993), ORTEPIII (Burnett & Johnson, 1996).

Selected geometric parameters (Å, º) top
NA—O1A1.263 (4)NB—O1B1.278 (4)
NA—O2A1.265 (4)NB—O2B1.246 (4)
NA—O3A1.214 (4)NB—O3B1.227 (4)
O4A—H1A1.00 (5)O4B—H1B0.89 (3)
O4A—H2A0.88 (5)O4B—H2B0.89 (3)
O4A—H3A0.75 (5)O4B—H3B0.82 (3)
O5A—H4A0.86 (4)O5B—H4B0.83 (3)
O5A—H5A0.80 (3)O5B—H5B0.82 (3)
O2A—NA—O3A121.2 (3)O2B—NB—O3B122.0 (3)
O2A—NA—O1A117.9 (3)O2B—NB—O1B117.8 (3)
O3A—NA—O1A120.9 (3)O3B—NB—O1B120.1 (3)
H3A—O4A—H1A111 (3)H3B—O4B—H2B114 (3)
H3A—O4A—H2A117 (3)H3B—O4B—H1B109 (3)
H1A—O4A—H2A114 (3)H2B—O4B—H1B109 (3)
H5A—O5A—H4A114 (3)H4B—O5B—H5B117 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O5B—H4B···O1A0.83 (3)2.04 (5)2.832 (4)159 (5)
O5A—H4A···O1B0.86 (4)2.00 (6)2.827 (4)162 (7)
O4A—H3A···O2A0.75 (5)1.88 (5)2.626 (4)176 (4)
O4B—H3B···O2B0.82 (3)1.85 (4)2.663 (4)175 (5)
O4B—H2B···O5B0.89 (3)1.57 (4)2.459 (4)178 (5)
O4A—H2A···O5A0.88 (5)1.68 (4)2.553 (5)175 (4)
O4B—H1B···O1Ai0.89 (3)1.75 (4)2.640 (4)177 (4)
O4A—H1A···O1Bii1.00 (5)1.53 (5)2.522 (5)171 (5)
O5B—H5B···O5Aiii0.82 (3)1.98 (5)2.800 (4)173 (6)
O5Aii—H5Aii···O2Aiv0.80 (3)2.10 (5)2.884 (5)167 (5)
Symmetry codes: (i) x1/2, y1/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x1, y, z; (iv) x, y+1, z.
 

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