Blue-shifting C-H…Y intramolecular hydrogen bonds - DFT and AIM analyses

B3LYP/6-311++G(d,p) calculations have been performed on intramolecular C-H…Y (Y = O, N, S) hydrogen bonded model systems, all species were fully optimized. The results show that for these systems weak H-bonds are formed which are not resonance assisted H-bonds in spite of the existence of conjugated bonds. IR and geometry analyses show that the H-bond formation for the systems studied leads to the shortening of the proton donating bonds and hence they may be classified as blue-shifting hydrogen bonds.