On a hard/soft hydrogen bond interaction

Abstract

Various physical properties of AH⋯B hydrogen bonded complexes are analyzed from the point of view of possible proton transfer processes. As a general parameter of the proton-donor–acceptor ability of interacting components the normalized Δp K N=Δp K a−Δp K a(crit) parameter was assumed, where Δp K a=p K a(B +H)−p K a(AH). Δp K a(crit) refers to the range where the proton transfer degree reaches 50%. Two kinds of correlations between the given physical property and Δp K N can be distinguished: of sigma and delta type. To the first one belong dipole moments, 15N NMR chemical shifts, NQR frequencies and the bond lengths in the solid state, while to the second one infra-red protonic band positions, their intensities and 1H NMR chemical shift. A general equation expressing the relationship between any physical property and Δp K N was derived in which the central factor exp(2.303 ξΔp K N) describes the behavior in the critical region. It has been shown that the ξ parameter can be used for the expression of the hardness. The value of this parameter is the higher the harder is interaction its maximum being equal 1. This measure very well correlates with the polarizability in transition state of hydrogen bonds. The detailed analysis of data in literature collected so far shows a good agreement of estimated ξ values by using different physical properties. The influence of various factors on the ξ value is discussed. Most important are chemical properties of interacting components but it was clearly shown the importance of the environment. With increasing permittivity a marked augmentation of the ξ parameter is generally observed.