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Li AY, Yan XH: Electronic properties of multifurcated bent hydrogen bonds CH3..Y and CH2...Y.. Phys Chem Chem Phys. 2007 Dec 21;9(47):6263-71. Epub 2007 Oct 17.
H-bonding angle angleYHX has an important effect on the electronic properties of the H-bond Y...HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl (-) and Br (-) at the four high levels of theory: MP2/6-311++G (d,p), MP2/6-311++G (2df,2p), MP2/6-311++G (3df,3pd) and QCISD/6-311++G (d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7-12 kcal mol (-1), and large blue shifts, delta r (HC) = -0.0025 --0.006 A and delta v (HC) = 30-90 cm (-1). The natural bond orbital analysis shows that the blue shifts of these H-bonds Y...HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n (Y) --> sigma*(CZ), in that the electron density from n (Y) of the proton acceptor is transferred not to sigma*(CH), but to sigma*(CZ) of the donor; intramolecular hyperconjugation n (Z) --> sigma*(CH), in that the electron density in sigma*(CH) comes back to n (Z) of the donor such that the occupancy in sigma*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y...H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y...H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon. |
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