Pyridinium nitrate at 120 K

Cooling of (I) from 290 to 120 K resulted in an approximately 4% decrease of the unit-cell volume, which was shown (by fast determinations at 160, 200 and 250 K) to be practically linear in this range. It is noteworthy that, whilst at room temperature > 90 , on cooling it decreases, passing through 90 at 250 K. Therefore, in the present report, the non-standard cell setting with < 90 is used, for compatibility with the room-temperature data (Serewicz et al., 1965; Batsanov, 2004). The structure at 120 K (Fig. 1) is similar to that at room temperature (see Table 1), with the anisotropic displacement parameters approximately three times lower [Ueq of non-H atoms averaging 0.08 (1) AÊ 2 at 290 K versus 0.027 (5) AÊ 2 at 120 K]. The asymmetric unit comprises one pyridinium cation and one nitrate anion, the ionic nature of which is proven by the location of H atoms. Both ions are planar, and the dihedral angle between them increases from 13.7 (1) at 290 K to 21.1 (1) at 120 K. This change can be best approximated as a rotation of the anion around atom O1, which is hydrogen bonded to the cation (Table 2). The deviations of the nitrate Received 15 November 2004 Accepted 19 November 2004 Online 27 November 2004

# 2004 International Union of Crystallography Printed in Great Britain ± all rights reserved The structural unit of pyridinium nitrate, C 5 H 6 N + ÁNO 3 À , is a pyridinium±nitrate ion pair, held together by a strong NÐ HÁ Á ÁO hydrogen bond.

Comment
The present paper reports the low-temperature study of the title compound, (I). For the introduction and the roomtemperature results, see the preceeding paper (Batsanov, 2004).
Cooling of (I) from 290 to 120 K resulted in an approximately 4% decrease of the unit-cell volume, which was shown (by fast determinations at 160, 200 and 250 K) to be practically linear in this range. It is noteworthy that, whilst at room temperature > 90 , on cooling it decreases, passing through 90 at 250 K. Therefore, in the present report, the non-standard cell setting with < 90 is used, for compatibility with the room-temperature data (Serewicz et al., 1965;Batsanov, 2004).
The structure at 120 K ( Fig. 1) is similar to that at room temperature (see Table 1), with the anisotropic displacement parameters approximately three times lower [U eq of non-H atoms averaging 0.08 (1) A Ê 2 at 290 K versus 0.027 (5) A Ê 2 at 120 K]. The asymmetric unit comprises one pyridinium cation and one nitrate anion, the ionic nature of which is proven by the location of H atoms. Both ions are planar, and the dihedral angle between them increases from 13.7 (1) at 290 K to 21.1 (1) at 120 K. This change can be best approximated as a rotation of the anion around atom O1, which is hydrogen bonded to the cation ( Table 2). The deviations of the nitrate anion atoms from the pyridine ring plane illustrate this point, viz. O1 À0.302 (4), O2 0.081 (6), O3 0.174 (5) and N2 À0.006 (5) A Ê at 290 K versus O1 À0.265 (4), O2 0.274 (5), O3 0.496 (4) and N2 0.173 (4) A Ê at 120 K.
In fact, all H atoms in (I) participate in inter-pair CÐHÁ Á ÁO contacts which are shorter than the sum of van der Waals radii (Rowland & Taylor, 1996), correspond to the stabilizing part of the potential curve (Desiraju & Steiner, 1999) and can be interpreted as weak hydrogen bonds (Table 2).

Crystal data
All H atoms were re®ned in an isotropic approximation without constraints.

Special details
Experimental. The data collection nominally covered over a hemisphere of reciprocal space, by a combination of 3 sets of ω scans; each set at different φ and/or 2θ angles and each scan (5 sec/frame exposure) covering 0.3° in ω. Crystal to detector distance 4.42 cm. Crystals are stable in dry air but deteriorate in atmospheric air. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

sup-2
Acta Cryst. (2004). E60, o2426-o2428 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.