Effect of Intramolecular Hydrogen Bonds on the Gas-Phase Basicity of Guanidines

Three series of novel trisubstituted guanidines containing at least one hydrogen bond accepting (HBA) group were modelled using B3LYP/6-311+G(2df, p)//B3LYP/6-31G(d) calculations. Their structure was modified by incorporating a variety of different HBA groups covering a wide range of hydrogen bond strengths. Calculated gas-phase basicities (GBs) ranged from 1035 to 1181 kJ mol(-1) depending on the nature of the substituent. To rationalise changes in the GB, a correlation of GB against two independent variables (pK(HB) and sigma(4B)) was conducted where pK(HB) served as the descriptor of the hydrogen bond strength and sigma(4B) was introduced to describe changes in the GBs in the open-chain model systems, i.e. in the absence of intramolecular hydrogen bond (IMHB), caused by the electronic effect of the propyl-HBA substituent. A very good correlation of the calculated gas-phase basicities against these two independent variables was established for all three sets of the bases.