Factors influencing the stability of H-bonds between guanidinium ions and counter ions or neutral ligands

Abstract

Guanidinium ions can form H-bonds either with counter ions with proton acceptor sites or with neutral ligands. In solvents as nitrobenzene their formation constants Kh depend a priori on the pKa of the guanidinium ion in water. Nevertheless the sequence of these constants is perturbed by two other factors: the number of equivalent N−H sites and the steric hindrance of the substituents of the guanidinium ion. In this work the stability constants Kh of guanidinium and methyl or phenyl substituted guanidinium ions with counter ions and neutral ligands were determined from the conductimetric data in nitrobenzene at 25°C. The constants are first divided by the number of available proton donor sites yielding Kh/*. Considering only the cases where steric hindrance is not important, correlations are established between log Kh/* and the pKa of the guanidinium ion in water, also corrected for the number of available sites. The effect of steric hindrance in the other systems can be estimated from the comparison of the actual value of Kh/* with that of Kh/** predicted by the correlations. To a rough approximation, the effect of the steric hindrance can be described by factors αi characterizing the guanidinium ion and αj characterizing the partner as expressed by the relation log10Kh**/Kh* = αiαj.