Intramolecular resonance-assisted hydrogen bonds: A theoretical description by means of atomic charges and charge fluxes

The characterization of intramolecular H-bonds in terms of atomic charges and charge fluxes (at the B3LYP/cc-pVTZ level of theory) has been extended to the case of the so called resonance-assisted (RA) H-bonds. A quadratic correlation between the charge fluxes phi(H) and the molecular IR absorption coefficients E that includes the entire family of the studied systems (31 of them) containing both intra- and intermolecular hydrogen bonds (O-H center dot center dot center dot O/N) confirmed the critical importance of the charge fluxes on the IR intensity enhancements. Since they reflect changing of the atomic charge distribution during the normal modes of vibrations, the dynamic nature of hydrogen bonding properties has been re-emphasized. The changes of the charge flux of the hydroxyl hydrogen in an RA intramolecular H-bond are between those for free OH bonds and the values calculated for intermolecular H-bonds. The transition free -> intramolecular -> intermolecular is gradual and therefore the hydrogen charge flux can be considered as practically sufficient to give quantitative measure to the intuitively obvious statement that intramolecular H-bonding is somehow in between no H-bonding situation and intermolecular H-bonding and thus provide a quantitative and yet simple parameterization of H-bond strength. In strictly planar molecules, the difference of the sums of charges of atoms participating in the 6-membered H-bond ring Delta Sigma can serve as a measure of the charge delocalization after the H-bond is formed. The electronic charge is withdrawn from the group of six atoms when the H-bond is formed in nitrophenol (Delta Sigma = -0.07), while the opposite is true (Delta Sigma = +0.03) for 2-hydroxy benzylidene amine. The corresponding values of the geometrical resonance parameter Delta are 0.39 and 0.37, respectively, similar to those found for 2-hydroxy acetophenone and 2-hydroxy benzaldehyde. The extent of the pi-electron delocalization as measured by the resonance parameter Delta does not follow the strength of H-bond as measured by the charge flux phi(H). (C) 2013 Elsevier B.V. All rights reserved.