IR spectroscopy studies of molecular states of alkali-metal acetates in acetic acid solution

Abstract

This work presents the results of IR spectroscopic studies of the molecular states of alkali-metal (Li, Na, K, Cs) acetates in glacial acetic acid. The associates M(Ac ·nHAc)·pHAc (I) have been shown to form with n≈ 8–9 and varying number p of the outer-sphere HAc molecules, depending on the salt concentration. The anion Ac ·nHAc^– is symmetrical about the central fragment O—H—O with a very strong H-bond. The anion negative charge is located mainly on this fragment and on the two nearest O⋯H—O fragments. The first coordination sphere of M⁺ comprises only oxygen atoms from the O—H—O group and from the anion's terminal CO groups. Associates I form a microvolume of structurized liquid phase which can be considered as a prototype of liquid-crystalline lamellar or ribbon-like structures produced by alkali acid soaps. When water is added, H₂O molecules hydrate both anions and cations, M⁺, equalizing the polarizing influence of the latter on the anion. For hydrated salts the radius of the ordered liquid-phase microvolume around the cation M⁺ increases. On the whole, water addition produces a similar effect on the composition and structure of associates I as it does with liquid-crystalline water-free alkali acid soaps.