Hydrogen bonding in cytosinium dihydrogen phosphite

In the title compound, C4H8N3O4P+·H2PO3 −, the cytosine molecule is monoprotonated and the phosphoric acid is in the monoionized state. Strong hydrogen bonds, dominated by N—H⋯O interactions, are responsible for cohesion between the organic and inorganic layers and maintain the stability of this structure.

In the title compound, C 4 H 8 N 3 O 4 P + ÁH 2 PO 3 À , the cytosine molecule is monoprotonated and the phosphoric acid is in the monoionized state. Strong hydrogen bonds, dominated by N-HÁ Á ÁO interactions, are responsible for cohesion between the organic and inorganic layers and maintain the stability of this structure.

Comment
Studies of metal ion-nucleic acid interactions are of great current interest, since metal ions play a crucial role in the structure and function of nucleic acid and genetic information transfer (Kabanos et al., 1992). Cytosine (6-aminopyrimidin-2-one) is one of the pyrimidines found in deoxyribonucleic acids. It has been the subject of several investigations with the aim of studying the electrostatic properties of its monohydrate form (Weber & Craven, 1990), the relative stabilities of its tautomeric forms and its hydration effects and hydrogen bonding (Sivanesan et al., 2000).
In several crystal structures of purines and pyrimidines with inorganic anions, the structural cohesion is assured by strong hydrogen bonds, as was observed in guaninium sulfate monohydrate  and adeninium perchlorate . The potential importance of hydrogen bonding in the structure and function of biomolecules has been well established (Jeffrey & Saenger, 1991); in particular, N-H···O hydrogen bonds are most predominant in determining the formation of secondary structure elements in proteins, base-pairing in nucleic acids and their biomolecular interactions. This structure analysis of cytosinium hydrogenphosphite (I) was undertaken as part of a more general investigation into the nature of hydrogen bonding between organic bases or amino acids and inorganic acids in their crystalline forms (Messai et al., 2009;Benali-Cherif, Abouimrane et al., 2002;Benali-Cherif, Benguedouar et al., 2002;Benali-Cherif et al., 2007).
The asymmetric unit contains one protonated cytosine rings and one hydrogenphosphite anion (Fig. 1). The main feature of the alkyl or aryl ammonium hydrogenphosphite is that the anionic subnetwork is built up through short strong hydrogen bonds (Blessing, 1986) and the organic cations are bonded to the phosphite layers by weaker hydrogen bonds (Masse & Levy, 1991) forming a two-dimensional network of hydrogen bonds (Fig. 2).
The inorganic moiety is a network of H 2 P O 3 tetrahedra, connected by short and strong hydrogen bonds. Inside these chains each H 2 P O 3 group is connected to its two adjacent neighbours by strong hydrogen bonds (O5-H2···O3) to build a two-dimensional network along the c direction. Some similarities may be observed between the present atomic arrangement and the corresponding hydrogenphosphites investigated earlier (Bendheif et al., 2003). cytosine is monoprotonated at atom N3. Some base stacking is retained and hydrogen bonding between cytosine rings, as found cytosinium nitrate (Cherouana, Bouchouit et al., 2003), and cytosinium oxalate monohydrate (Bouchouit et al., 2005)  The title compound (I) was crystallized from a 1:1 aqueous solution of cytosine [4-aminopyrimidine-2(1H)-one] and phosphorous acid. Yellow crystals grew after a few days, at room temperature and were manually separated for single-crystal X-ray analysis.

Refinement
The title compound crystallizes in the non centrosymmetric space group P 1 . All non-H atoms were refined with anisotropic atomic displacement parameters. All H atoms were located in Fourier maps; and treated as riding on their parent atoms, with    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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq