The H-index unambiguously discriminates between hydrogen bonding and improper blue-shifting hydrogen bonding

Abstract

X–H···Y contacts are characterised by a red shift of the X–H stretch frequency. We have shown recently (P. Hobza and Z. Havlas, Chem. Re., 2000, 100, 4253) that these contacts can also be characterised by a blue shift of the X–H stretch frequency. An H-index, defined as the ratio of electron density transferred from the proton donor (Y) to the σ* antibonding orbital of the X–H bond and the total electron density transferred between proton donor and proton acceptor unambiguously discriminates between both types of H-bonding. Its value for proper H-bonding lies between 1.0 and 0.7 while values between 0.3 and 0 are typical for improper (blue-shifting) H-bonding.