Intramolecular resonance-assisted hydrogen bonds: Insights from symmetry adapted perturbation theory

Abstract

The accurate description of hydrogen bonds (H-bonds) is one of the primary challenges when describing intermolecular forces. Intramolecular hydrogen bonds (IMHB) are even more challenging because of their ill-defined energy and their interplay with π-systems. In this contribution, we investigate the role of π-resonance in the enhancement of IMHBs. This is accomplished by comparing a number of π-conjugated systems to their saturated planar counterparts using intramolecular symmetry adapted perturbation theory (I-SAPT). The change in the composition of the IMHB energies (electrostatics, exchange, induction, and dispersion) due to π-conjugation is examined in detail. The biggest changes are observed in the induction and electrostatics contributions. However, electrostatics is nearly canceled by its exchange counterpart. The importance of induction term is also observed when plotting rotational potential curves for the different energy components. Once the distance between the H-bond donor and acceptor is frozen, the stabilization of IMHBs mostly occurs through the decrease in the exchange term leading to less repulsion. In any case, all results are consistent with expected trends in the resonance-assisted hydrogen bonds (RAHBs), and corroborate the fragmentation procedures used in this study. The closeness of induction contribution to the whole interaction energy and the existence of a linear correlation between the IMHB energy and the induction component show that the induction term is a good predictor of the intramolecular RAHBs stability trends.