Attractive strain: the disadvantages of rigid multiple H-bond donors and acceptors. A theoretical analysis of the hydrogen-bonding interactions in complexes of tetraazaanthracenedione with pyridylureas.
We report density functional theory calculations at the B3LYP/D95(d,p) level on the hydrogen-bonding complexes of tetraazaanthracenedione, 1, with N-(pyridin-2-yl)urea, 2H, or N-(6-aminopyridin-2-yl)urea, 2N. The interaction energy of the 1-2H complex exceeds that of 1-2N, despite the fact that 1-2N contains a strong N-H...O interaction in place of a weak C-H...O interaction in 1-2H. We show that the 1-2N interaction is weaker than the sum of the four normal individual H-bonding interactions because the steric constraints of the complex prevent the H-bonding donors and acceptors from optimally approaching each other to form the two central H-bonds. This steric phenomenon, which we call attractive strain, is likely present to some extent in most H-bonding systems that contain more than two H-bonds between rigid monomers. Attractive strain is unusually important in 1-2N. Attractive strain can be conceived of as an enthalpic cost for the entropic benefits of freezing the dihedral angles of the multiple H-bond donors and acceptors by designing rigid systems.
Oliva
Antonio
A
Departament de Química, Unitat de Química Física, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
Bertran
Juan
J
Dannenberg
J J
JJ
D000880:Anthraquinones / Q000737:chemistry*; D001372:Aza Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D013816:Thermodynamics; D014508:Urea / Q000031:analogs & derivatives* / Q000737:chemistry*
D000880:Anthraquinones; D001372:Aza Compounds; C527919:tetraazaanthracenedione; D014508:Urea