Ab initio calculations of protonated ethylenediamine-(water)(3) complex: Roles of intramolecular hydrogen bonding and hydrogen bond cooperativity

Ab initio density functional calculations on the structural isomers, the hy dration energies, and the hydrogen bond many-body interactions for gauche-, trans-protonated ethylenediamine-(water)(3) complexes (g-enH(+)(H2O)(3), t -enH(+)(H2O)(3)) have been performed. The structures and relative stabiliti es of three representative isomers (cyclic, tripod, open) between g-enH(+)( H2O)(3) and t-enH(+)(H2O)(3) are predicted to be quite different due to the strong interference between intramolecular hydrogen bonding and water hydr ogen bond networks in genH(+)(H2O)(3), Many-body analyses revealed that the combined repulsive relaxation energy and repulsive nonadditive interaction s for the mono-cyclic tripod isomer, not the hydrogen bond cooperativity, a re mainly responsible for the greater stability of the bi-cyclic isomer.