Solvation stages of HCl and HBr in crystalline phases with methanol and small ethers: Acid-ether cluster complexes in amorphous and crystal phases

Solid state solvation of HCl and HBr (i.e., HX acids) was investigated by FTIR spectroscopy, with methanol (MeOH), dimethyl ether (DME), and tetrahydrofuran (THF) acting as solvents. The study is an extension of previous investigations of acid hydrates. Effort was devoted to finding those composition ratios for which the acid and the solvent readily form mixed crystalline and amorphous solids and to the study of the extent of acid solvation in these solids. The results were interpreted with the help of density functional theory (DFT) calculations for (HX),(solvent),, clusters and for protonated solvent ions. A dramatic difference was observed between methanol and ether solvation. For methanol, crystal and amorphous ionic solids form with MeOH: HX ratios of 1: 1, 2: 1, and 3: 1, in analogy to the hydrate case. The parallel to the HX-hydrate series apparently extends to the formation of the methanol analogues of the hydronium and Zundel cations, for the corresponding solvent-acid ratios. In contrast, mixed acid-ether solids tend to be acid-rich; new crystal solids were discovered, with HX-ether ratios of 6:1, 4:1, 2:1, and 1:1. In the limit of high acid-ether ratio, the acid is clearly molecular. In the 1:1 crystals, very intense and broad bands in the 800-1500 cm(-1) range suggest proton sharing between the halide and the 0 atom of the ether; and a Zundel-like species [ether..H+..X-] is suggested as the basic unit. Such units polarize, mutually solvate, and stabilize each other. In the case of HCl, the Zundel unit appears to persist to intermediate acid:ether ratios, despite dilution by the extra acid. However, in the case of 2:1 and 4:1 HBr-THF, the acid is fully molecular, though perturbed significantly by solvation.