Theoretical Study of Heteroatom Resonance-Assisted Hydrogen Bond: Effect of Substituent on π-delocalization

The concept of Resonance Assisted Hydrogen Bond (RAHB), which usually occurs in b diketons, has a remarkable role in chemistry. These molecules, which contain heteroatom particularly O and N, are species with biological interest in protein folding and DNA pairing. Therefore, the amplification of hydrogen bonds strength by substituents may be important in life sciences. In the current research, we have shown that the nature of hydrogen bond in the enol form of heteroatom RAHB systems has partial covalent and electrostatic character. Nonetheless, the strength of hydrogen bonds increases by means of three different groups of substituents, which cannot be attributed to the contributions of resonance structures. Parameters such as bond ellipticity, p delocalization indices and bond equalizations cannot help to prove the effect of conjugation on the strength of hydrogen bond in RAHB systems. It is shown by NBO that the primary and secondary hyperconjugative charge transfer caused by substituents help to amplify this type of hydrogen bond. We showed that the existence of p-conjugation is necessary only for transformation of electron from substituents to the hydrogen bond as an acceptor of electrons.The strength of RAHB has a rather good correlation with the distance between of two non-connected heteroatoms O and N according to reference [62].