Improper, blue-shifting hydrogen bond

The spectral manifestation and the nature of improper, blue-shifting hydrogen-bonded complexes are entirely different from those of standard hydrogen-bonded complexes. While the latter class of complexes is characterized by an elongation of the X–H bond and a concomitant red shift of the respective stretch frequency, a contraction of the X–H bond and a blue shift of the X–H stretch frequency are typical for the former class of complexes. Both classes of complexes exhibit electron-density transfer from the proton acceptor to the proton donor. In the case of hydrogen-bonded complexes, the electron-density transfer is mostly larger then in the improper complexes. The role of the charge transfer and electrostatic interaction is discussed. The importance of the charge transfer is documented by natural bond orbital analysis, orbital interaction diagrams, and by the insufficient description of the interaction by the electrostatic model only.