On long and short hydrogen-bonds

Abstract

Abstract Experimental results of OH-stretching frequencies, O-O- and O-H-distances are compared with each other and are discussed in connection with results of the same characteristic parameters as well as their complexation energies as determined on the basis of the theoretical ab initio calculations of Almlof for [Cl ⋯H⋯ Cl]− complexes. The comparison of the various results shows that the deviation of the X-H-distance ΔrOH and ΔrClH, respectively, from the central position seems to be a helpful tool in order to distinguish between asymmetric H-bond potentials and potentials in which the lowest energy level is above the potential barrier (more or less symmetric H-bonds). The theoretical results support the idea that the vOH−ROO correlation is not continuous at ROO ⋍ 2.44 A - the O-O-distance where the strongest H-bonds are to be expected. The theoretical results indicate, that the correlation of the H-bond energy ΔE(v=0) with the XH-frequency (or frequency shift) is not linear and is different for asymmetric and more or less symmetric H-bonds. However, for relative frequency shifts of more than approximately 30% the change in H-bond energy is expected to be of the order of kT. This indicates, that the influence of the surrouding of a H-bond will become very important for comparatively strong H-bonds (approx. −22kcal/mol H-bond for ClH-complexes).