Ab initio and AIM studies on typical π-type and pseudo-π-type halogen bonds: Comparison with hydrogen bonds.

Series of typical π-type and pseudo-π-type halogen-bonded complexes B ···ClY and B ···BrY and hydrogen-bonded complex B ···HY (B = C2H4, C2H2, and C3H6; Y = F, Cl, and Br) have been investigated using the MP2/aug-cc-pVDZ method. A striking parallelism was found in the geometries, vibrational frequencies, binding energies, and topological properties between B ···XY and B ···HY (X = Cl and Br). It has been found that the lengths of the weak bond d(X ···π)/ d(H ···π), the frequencies of the weak bond ν(X ···π)/ν(H ···π), the frequency shifts Δν(XY)/Δν(HY), the electron densities at the bond critical point of the weak bonds ρc(X ···π)/ρc(H ···π), and the electron density changes Δρc(XY)/Δρc(HY) could be used as measures of the strengths of typical π-type and pseudo-π-type halogen/hydrogen bonds. The typical π-type and pseudo-π-type halogen bond and hydrogen bond are noncovalent interactions. For the same Y, the halogen bond strengths are in the order B ···ClY < B ···BrY. For the same X, the halogen bond strength decreases according to the sequence F > Cl > Br that is in agreement with the hydrogen bond strengths B ···HF > B ···HCl > B ···HBr. All of these typical π-type and pseudo-π-type hydrogen-bonded and halogen-bonded complexes have the 'conflict-type' structure. Contour maps of the Laplacian of π electron density indicate that the formation of B ···XY halogen-bonded complex and B ···HY hydrogen-bonded complex is very similar. Charge transfer is observed from B to XY/HY and both the dipolar polarization and the volume of the halogen atom or hydrogen atom decrease on B ···XY/B ···HY complex formation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]