Ammonium salicylate: a synchrotron study

The structure of the title salt, NH4 +·C7H5O3 −, is stabilized by substantial hydrogen bonding between ammonium cations and salicylate anions that links the components into a two-dimensional array.

The structure of the title salt, NH 4 + ÁC 7 H 5 O 3 À , is stabilized by substantial hydrogen bonding between ammonium cations and salicylate anions that links the components into a twodimensional array.

Comment
There is an increasing demand for new materials that can be used for efficient, readily available, low-cost, high-energy neutron detection devices in the presence of a strong γ-ray background. The need for inexpensive neutron scintillators with reasonable optical transparency and fast response time led us to focus on growing, developing and characterizing single crystals of candidate materials. In this regard, materials based on organic scintillators are of particular interest because of their potential for low-level neutron detection via pulse shape discrimination (PSD) (Brooks, 1979;Kaschuck et al., 2002;Kachuk & Esposito, 2005).
In search of new neutron detecting materials with enhanced performance (efficiency and cost), we considered materials that duplicate the structural features of commonly used materials, for example, salicylic acid is a common component in liquid scintillation systems. Salts of salicylic acid are good candidates for dry solid scintillators. Knowledge of these structural data is important to the development of a fundamental understanding of its scintillating properties, and more generally a predictive capability for tailoring materials to achieve desired scintillation properties. To address these challenges, we report here our measurements of the crystal structure of ammonium salicylate (I).
The asymmetric unit of (I) comprises a salicylate cation and an ammonium anion (Fig. 1). The projection of the unit cell contents on the bc plane is shown in Fig. 2. The hydrogen bonding between the carboxylate group and ammonium ions contributes to the stabilization of this crystal packing. This bonding allows two ammonium ions to connect two salicylate ions by forming alternating eight-and twelve-membered rings (Fig. 3). These alternating rings run as strips parallel to c axis and phenyl rings are outwardly attached to them in zigzag patterns. The phenyl rings are stacked along a axis. The oxygens of the hydroxyl groups form weak hydrogen bonding to the ammonium ions (see Table 1). Salicylate salts are not rare. Salicylic acid makes salts with not only ammonium but also alkali metals (Wiesbrock & Schmidbaur, 2003a, b;Dinnebier et al., 2002).
Alkali salicylates are repoted as monohydrates (Li, Cs) (Wiesbrock & Schmidbaur, 2003a, b) or anhydrous forms (K, Rb) (Dinnebier et al., 2002). Similar to (I), alkali salicylates also form double helix type ribbons with phenyl rings attached in zipper shapes. However, the difference lies in the linkage of the ribbons. Carboxylate groups, water molecules and metal ions form the ribbons of hydrated alkali salicylates whereas carboxylate groups, hydroxyl group and metal ions do those of anhydrous alkali salicylates. The connectivity forming ribbons in (I) is mainly through O···H-N hydrogen bonding between the carboxylate groups and ammonium ions (Table 1 and Fig. 3). The hydrogen bonding is comparable to that seen in other salicylate compounds (Gellert & Hsu, 1983;Drake et al., 1993). In the case of Li, the phenyl rings are perpendicular to the ribbons. With larger alkali metals, the phenyl rings tilt toward the ribbons. (I) has weak hydrogen bonding between oxygens of hyddoxyl groups and ammonium ions, which favors tilt of the phenyl rings. In this type of structure, π-π stacking or hydrogen bonding between salicylate anions may not exist due to the large interplanar distance or co-planar distance between phenyl rings. supplementary materials sup-2 Experimental A repeated recrystallization process was applied. The crystals of (I) with high purity were obtained (1) from saturated commercial product (99%, Sigma-Aldrich) from methanol solution or (2) by precipitation of a solution of salicylic acid (99% Sigma-Aldrich) and ammonium water. The single crystals were coated with paratone oil and mounted onto a cryo-loop pin.