Gas-phase proton affinities of guanidines with heteroalkyl side chains

The gas-phase proton affinities (PAs) of four novel guanidine derivatives, with three of them incorporating heteroalkyl groups capable of forming intramolecular hydrogen bonds, are determined by the extended kinetic method. In addition, the PAs of two other guanidines are evaluated using the simple variant of the kinetic method. The proton affinities of the investigated bases fall in the range of 251–264 kcal mol−1 and are thus 16–29 kcal mol−1 larger than the proton affinity of the parent compound guanidine. It is shown that the formation of intramolecular hydrogen bonds, where possible, significantly contributes to the basicity of the guanidine bases under study.