Tuning the strength of the resonance-assisted hydrogen bond in acenes and phenacenes with two o-hydroxyaldehyde groups: the importance of topology

Abstract

The fact that intramolecular resonance-assisted hydrogen bonds (RAHBs) are stronger than conventional ones is attributed to the partial delocalization of the pi-electrons within the hydrogen bond (HB) motif, the so-called quasi-ring. If an aromatic ring is involved in the formation of the RAHB, previous studies have shown that there is an interplay between aromaticity and HB strength. Moreover, in 1,3-dihydroxyaryl-2-aldehydes, some of us found that the position of the quasi-ring formed by the substituents interacting through RAHB influences the strength of the H-bonding, the HBs being stronger when a kinked-like structure is generated by formation of the quasi-ring. In this work, we explore this concept further by considering a set of acenes and phenacenes of different sizes with two o-hydroxyaldehyde substituents. Calculations with the CAM-B3LYP/6-311+G(d,p)+GD3B method show that for long acenes or phenacenes, once the substituent effect loses importance because quasi-rings are pull apart far from each other, the different topologies rule the HB distances. This fact can be explained in most cases using an extended Clar's aromatic pi-sextet model. In some kinked systems, however, the justification from the Clar model has to be complemented by taking into account the repulsion between hydrogen atoms. Triphenylene-like compounds with different number of benzene rings have been studied finding out a very good relationship between aromaticity of the ipso- and quasi-rings with the RAHB distances. This result confirms the importance of the communication of the pi-systems of the ipso- and quasi-rings