Intramolecular hydrogen bonds with large proton polarizability in semisalts of mono- and di-N-oxides of N,N′-tetraalkyl-o-xylyldiamines

Heteroconjugated NO⁺H … N NO … H⁺N and homoconjugated NO⁺H … ON NO … H⁺ON intramolecular hydrogen bonds formed in semisalts of mono- and di-N-oxides of N,N′-tetraalkyl-o-xylyldiamines were studied by IR and NMR spectroscopy. All these hydrogen bonds show large proton polarizability. In the case of the heteroconjugated hydrogen bonds the proton transfer equilibrium shifts from compounds 1 to 3 to the left hand side since the interaction of the hydrogen bond with the solvent environment decreases in this series of compounds. With compound 1 the hydrogen bonds are slightly weaker and longer, hence the wavenumber dependence of the intensity of the continuum caused by these hydrogen bonds is slightly changed with compound 1 compared with compound 2. In the case of compound 3 the intensity of the continuum decreases because of increasing screening of the hydrogen bonds. In the series of homoconjugated hydrogen bonds, from compound 4 to 6 the intense continuum vanishes, and only the band of the 0–1 proton transition at 950 cm⁻¹ remains. The vanishing of the continuum is caused by increasing screening of the hydrogen bonds against their solvent environments by bulky groups, and thus, this change demonstrates again that the interaction of the hydrogen bond with large proton polarizabilities is a necessary prerequisite for IR continua to appear.