Propane-1,3-diaminium hydrogenarsenate monohydrate

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, (C3H12N2)[HAsO4] H2O, contains a network of propane-1,3-diaminium cations, hydrogenarsenate anions [mean As—O = 1.687 (2) Å] and water molecules. The crystal packing involves anion-to-anion and water-to-anion O—H O hydrogen bonds, resulting in infinite chains containing the unusual R3 (10) graph-set motif. Cation-toanion and cation-to-water N—H O hydrogen bonds generate a three-dimensional overall structure.

The organic species interacts with the hydrogenarsenate/ water chains by way of six N-HÁ Á ÁO hydrogen bonds [mean HÁ Á ÁO = 1.89 Å , mean N-HÁ Á ÁO = 171 and mean NÁ Á ÁO = 2.793 (2) Å ]. One of the acceptor O atoms is part of a water molecule, and the other five are parts of hydrogenarsenate groups. This hydrogen-bonding scheme results in a threedimensional network (Fig. 3).
Aqueous ammonia was added to this solution to raise the pH to about 12, which is beyond the second end-point for H 3 AsO 4 (i.e. the predominant species is [HAsO 4 ] 2À ). Platy crystals of (I) grew as the water evaporated over the course of a few days.

Figure 3
The crystal packing of (I). Dashed lines indicate hydrogen bonds.
The O-bound H atoms were found in difference maps and refined as riding on their carrier O atoms in their as-found relative positions. H atoms bonded to C and N atoms were placed in idealized positions (C-H = 0.99 Å and N-H = 0.91 Å ) and refined as riding, allowing for free rotation of the -NH 3 groups. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases.
We thank the EPSRC National Crystallography Service (University of Southampton, England) for the data collection. Special details 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. 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. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.