Theoretical study of linear and bifurcated H-bonds in the systems Y•••H2CZn (n=1, 2; Z = O, S, Se, F, Cl, Br; Y=Cl-, Br-)

The linear and bifurcated H-bonds in the systems Y center dot center dot center dot H(2)CZ(n) (n = 1, 2; Z = O, S, Se, F, Cl, Br; Y = Cl-, Br-) are studied at the MP2 level of theory with the basis sets 6-311++G(d,p) and 6-311++G(2df,2p). The linear H-bonds are red-shifted but the bifurcated H-bonds are blue-shifted. The red shifts of the linear H-bonds are caused by direct intermolecular hyperconjugation; the blue shifts of the bifurcated H-bonds are caused by rehybridization, indirect intermolecular hyperconjugation and decrease of intramolecular hyperconjugation. As concerns the topological properties of electron density, both the linear and bifurcated H-bonds satisfy the definitions of H-bonds proposed by both Bader and Popelier. In the bifurcated complexes, there are three intermolecular critical points: one bond critical point between the acceptor atom Y and each hydrogen atom, and a ring critical point inside the tetragon YHCH. (C) 2008 Elsevier B.V. All rights reserved.