ENERGIES AND GEOMETRIES OF ISOGRAPHIC HYDROGEN-BONDED NETWORKS .1. THE R(2)(2)(8) GRAPH SET

The bidentate hydrogen-bonding patterns frequently formed by the side chains of the amino acids arginine, asparagine, and glutamine can be d escribed, using graph-set notation, as R(2)(2)(8) (ring pattern of eig ht atoms with two donor and two acceptor hydrogen bonds). It is shown that this hydrogen-bonding pattern is present in approximately 34% of the high-resolution crystal structures in the Cambridge Structural Dat abase (CSD) that have the potential to form this particular pattern. M ost of these crystal structures involve an interaction between two ami de groups, but a significant number involve an interaction between an amidinium cation and a carboxylate anion. The mean H...O bond distance s and the N-H...O bond angles found for these two types of interaction are nearly the same. Ab initio molecular orbital calculations are use d to investigate the structure, charge distribution, and decomposition enthalpies of several gas phase hydrogen-bonded complexes which have these types of R(2)(2)(8) interactions. Calculated H...O bond distance s and N-H...O bond angles for the amide-amide type of interaction lie between the upper and lower bounds for these geometrical parameters fo und in the CSD. On the other hand, calculated H...O bond distances for several amidinium ion-carboxylate ion structures are found to be sign ificantly less than the lower bound of this distance found in the CSD study, apparently due to a strong electrostatic interaction between th e ion fragments in the gas phase.