Mixing of a Carboxylate Stretching Vibration and Methylene Scissoring in Monosubstituted Acetate Ions. Major Effects of Counter Cation and Environment

Abstract

Infrared absorption spectra in the range 1500-1350 cm^-1 (or lower) are reported for solid salts (XCH₂CO₂)_nM where M is an alkali (n = 1) or alkaline earth metal (n = 2), and X = CH₃, CN, F, Cl , Br or I, and also for aqueous XCH₂CO₂^-. With X = CN, Cl , Br or I, when a doubly charged cation is replaced by a singly charged one of nearly the same mass, or a smaller one by a larger one in the same group in the periodic table, infrared intensity is redistributed from bands at higher frequencies to bands at lower frequencies. The carboxylate stretching vibration has a high infrared intensity, and normally its (intrinsic) frequency is lowered systematically by counter- cations in the above order, but in these ions it mixes strongly with methylene scissoring, the intrinsic frequency of which is only weakly environment-dependent. The intensity redistribution reflects a redistribution in carboxylate stretching character among the resultant composite vibrations.

In propionate salts the situation is probably similar, but band overlaps often introduce uncertainty.

In aqueous solution the lower of the two frequencies involved has the higher infrared intensity when X = CN, Cl , Br, I or CH₃. For fluoroacetates, by contrast, the more intense band is that at the higher frequency, in water and in most solid salts; there is no systematic intensity redistribution as the cation is changed; the mixing of vibrations is more extensive and involved there.