Hydrogen Bonding Interactions in Amorphous Salicyl Salicylate

Abstract

The macroscopic physical properties of a chemical substance are determined by the intra-and intermolecular interactions of each individual molecule comprising it. Presently, reliable prediction of the three-dimensional structure of a multiple-atom chemical species from only its structural formula is not possible, [1] thus the ab initio prediction of bulk physical properties is likewise not currently attainable. To circumvent this obstacle, empirical studies of systematic chemical series have been pursued, and experimentally derived properties, such as crystal structure [2] and melting point, [3] have been correlated with structural formula.Our research is concerned with the production of supramolecular materials, a field that has promised much. [4] Recent work in the field of supramolecular materials assembly has focused upon the synthesis of molecules containing multiple hydrogen bonding motifs at chain termini [5] and inner-chain sites. [6] We wished to make a departure from this approach to understand the macroscopic physical effect of systematic changes in the chemical structure of simple species. We aimed to maximize the observed change in physical attributes while minimizing the structural change at the molecular level. We studied the molecular series: phenyl benzoate (1), phenyl salicylate (2), and salicyl salicylate (3).Each compound was melted and allowed to cool at ambient temperature. Compound 1 melted at 342 K and recrystallized at this temperature upon cooling; 2 melted at 317 K and undercooled to ambient temperature without crystallization, the low viscosity undercooled liquid could be maintained in this state for several weeks without difficulty; 3 melted over the range 412 ± 424 K and undercooled to a high viscosity liquid that could be moulded and stress-fractured: a potentially useful material (crystallization in 3 could not be induced by any means, including crystal seeding and holding at the crystallization temperature for several hours. Recrystallization could be achieved only by dissolution and reprecipitation.). Thus we observed gross physical changes with small molecular alterations in the series 1 ± 3.To elucidate the molecular interactions giving rise to these physical manifestations we reviewed the single-crystal structures of 1 [7] and 2, [8] and determined the structure of 3. In addition, 3 was studied extensively by NMR and IR spectroscopy.The low viscosity of the amorphous 3 at temperatures in the range 373 ± 423 K allowed study by 13 C NMR spectroscopy. The spectrum recorded at 423 K is shown in Figure 1, and was assigned by 2D NMR experiments, [9] and comparison with reference spectra. [10] Multiple resonances were observed for many peaks, most notably those carbon atoms lying along the functional backbone: C14, C9, C1, C3, and C4. The acid carbonyl, C2, gave a single resonance signal. The ester carbonyl resonance, C1, was composed of at least five discrete signals, indicating that at least this many environments were stable on the NMR time scale. This multiple-resonance effect pe...

Amorphism and Thermal Decomposition of Salicylsalicylic Acid—A Cautionary Tale

“…Its properties are that it is a clear glass that can be moulded and that stress fractures at room temperature. [5,7] There has also been a report of evidence for a nematic phase close to the melting point, [7] and it has recently been the subject of computational studies looking at the amorphous form to evaluate the possible utility of Crystal Structure Prediction (CSP) in relation to such materials. [8] In view of the relative ease with which the glassy amorphous phase can be produced and its reluctance to subsequently recrystallize without dissolution and re-precipitation, it has been considered by a number of workers as a model compound for a stable amorphous phase.…”

“…The Cambridge Structural Database [4] contains two reports of structures for salsalate [5,6] that are essentially in agreement. They both report a structure that is disordered between two conformers in a 72:28 ratio ( Figure 1).…”