Infrared studies of intramolecular interactions including ionic hydrogen bonding in tertiary and quaternary ammonium halides having a hydroxy- or acetoxy-group β to the ammonium function

Abstract

I.r. measurements of a series of tertiary and quaternary ammonium halides having a hydroxy- or acetoxy-group β to the ammonium function showed the following characteristic i.r. behaviour. (i) Compared with the corresponding free stretching vibration band of the OH or the N ⁺ H group, marked shifts to lower wavenumbers (Δν_OH 256–409 and Δν_{N ⁺ H} 567–860 cm^–1 in CHCl₃) accompanied by large increases of integrated intensities (10^–4A_OH 8.25–17.70 and 10^–4A_{N ⁺ H} 8.76–21.14 mol^–1 dm³ cm^–2 in CHCl₃) were found for ammonium halides. (ii) The shift (Δν_OH or Δν_{N ⁺ H}) and the intensity (A_OH or A_{N ⁺ H}) increase as the ionic radius of the halide ion decreases in each series of ammonium halides. (iii) The Δν_OH and Δν_{N ⁺ H} values are markedly influenced by the type of ammonium group and β-substituent, respectively. (iv) The Δν_OH value of quaternary ammonium halides described above is larger than that found for intermolecular ionic hydrogen bonding between alcohols and tetra-n-butylammonium halides and shows a dependence upon the torsion angle N ⁺ CCO, τ, in the N ⁺–C–C–OR system. (v) Compared with the corresponding v_CO found in the free amine, considerable shifts to higher wavenumber, Δν_CO, were found for the CO stretching band of the acetoxy-group in ammonium halides; in CHCl₃, Δν_CO, is 14–32 cm^–1. (vi) In CHCl₃ and in D₂O, the Δν_CO and ν_CO values also show dependence upon τ(viz. a dependence upon the spatial distance between acetoxy and ammonium groups). (vii) A fairly linear correlation exists between Δν_OH of alcohols and Δν_CO, of acetates. Points (i)–(iii) are attributed to intramolecular ionic hydrogen bonding between OH or N ⁺ H and halide ion. Points (iv)–(vii) can be understood by considering the factors influencing the polarization of the OH bond and the double-bond character of the acetoxy carbonyl, especially the through-space charge effect of the ammonium group.