Red- versus Blue-Shifting Hydrogen Bonds:  Are There Fundamental Distinctions?

The OH covalent bond of conventional hydrogen bonds of the OH···O type undergoes a lengthening as a result of the complex formation, and the OH stretching frequency shifts to the red; the same is true of many CH···O H-bonds. However, a subset of CH···O H-bonds behave in an opposite fashion, with the CH bond contracting, and its stretching frequency shifting to the blue. Ab initio calculations are performed to determine whether the two contrasting behaviors can be traced to different patterns of electron density shifts within the complex or they are associated with very different geometrical perturbations within the proton donor molecule. The calculations reveal that the density shifts of red- and blue-shifting H-bonds are very much alike, as are the covalent bond length changes that occur within the donor molecule. It is concluded that there are no fundamental distinctions between the two sorts of H-bonds.