The effect of formation of second hydrogen bond in adjacent two-ring resonance-assisted hydrogen bonds - Ab initio and QTAIM studies

Two-ring intramolecular resonance-assisted hydrogen bonds (RAHBs) existing in 1,5-dihydroxy-1,4-diene-3-pentanone and some its symmetrically substituted derivatives have been studied at the MP2/6-311++G∗∗ level of theory. The influence of the coexistence of two intramolecular hydrogen bonded rings in these molecular systems on the structure, intramolecular hydrogen bonding and π-electron delocalization was investigated. The results of calculations show that such coexistence increases the π-electron delocalization but it leads also to the weakening of hydrogen bonding. The Quantum Theory of ''Atoms in Molecules'' (QTAIM) of Bader was also applied here to get more details about the nature of hydrogen bonds. Correlations between the hydrogen bond strength and different parameters were also analyzed. It was found that topological characteristics of critical points (bond critical points and ring critical points) are also useful to estimate the strength of intramolecular hydrogen bonds in two-ring RAHB systems.