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 (ΔνOH 256–409 and ΔνN + H 567–860 cm–1 in CHCl3) accompanied by large increases of integrated intensities (10–4AOH 8.25–17.70 and 10–4AN + H 8.76–21.14 mol–1 dm3 cm–2 in CHCl3) were found for ammonium halides. (ii) The shift (ΔνOH or ΔνN + H) and the intensity (AOH or AN + H) increase as the ionic radius of the halide ion decreases in each series of ammonium halides. (iii) The ΔνOH and ΔνN + H values are markedly influenced by the type of ammonium group and β-substituent, respectively. (iv) The ΔνOH 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 vCO found in the free amine, considerable shifts to higher wavenumber, ΔνCO, were found for the CO stretching band of the acetoxy-group in ammonium halides; in CHCl3, ΔνCO, is 14–32 cm–1. (vi) In CHCl3 and in D2O, the ΔνCO and νCO 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 ΔνOH of alcohols and ΔνCO, 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.