### Strength of Intramolecular Hydrogen Bonds

JIANG Xiaoyu1,WU Wei2,MO Yirong3,*()

1. 1 College of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350108, P. R. China
2 The State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China
3 Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, USA
• 收稿日期:2017-07-17 发布日期:2017-12-18
• 通讯作者: MO Yirong E-mail:yirong.mo@wmich.edu

### Strength of Intramolecular Hydrogen Bonds

Xiaoyu JIANG1,Wei WU2,Yirong MO3,*()

1. 1 College of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350108, P. R. China
2 The State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China
3 Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, USA
• Received:2017-07-17 Published:2017-12-18
• Contact: Yirong MO E-mail:yirong.mo@wmich.edu

The concept of resonance-assisted hydrogen bonds (RAHBs) highlights the synergistic interplay between the π-resonance and hydrogen bonding interactions. This concept has been well-accepted in academia and is widely used in practice. However, it has been argued that the seemingly enhanced intramolecular hydrogen bonding (IMHB) in unsaturated compounds may simply be a result of the constraints imposed by the σ-skeleton framework. Thus, it is crucial to estimate the strength of IMHBs. In this work, we used two approaches to probe the resonance effect and estimate the strength of the IMHBs in the two exemplary cases of the enol forms of acetylacetone and o-hydroxyacetophenone. One approach is the block-localized wavefunction (BLW) method, which is a variant of the ab initio valence bond (VB) theory. Using this approach, it is possible to derive the geometries and energetics with resonance shut down. The other approach is Edmiston’s truncated localized molecular orbital (TLMO) technique, which monitors the energy changes by removing the delocalization tails from localized molecular orbitals. The integrated BLW and TLMO studies confirmed that the hydrogen bonding in these two molecules is indeed enhanced by π-resonance, and that this enhancement is not a result of σ constraints.

Abstract:

The concept of resonance-assisted hydrogen bonds (RAHBs) highlights the synergistic interplay between the π-resonance and hydrogen bonding interactions. This concept has been well-accepted in academia and is widely used in practice. However, it has been argued that the seemingly enhanced intramolecular hydrogen bonding (IMHB) in unsaturated compounds may simply be a result of the constraints imposed by the σ-skeleton framework. Thus, it is crucial to estimate the strength of IMHBs. In this work, we used two approaches to probe the resonance effect and estimate the strength of the IMHBs in the two exemplary cases of the enol forms of acetylacetone and o-hydroxyacetophenone. One approach is the block-localized wavefunction (BLW) method, which is a variant of the ab initio valence bond (VB) theory. Using this approach, it is possible to derive the geometries and energetics with resonance shut down. The other approach is Edmiston's truncated localized molecular orbital (TLMO) technique, which monitors the energy changes by removing the delocalization tails from localized molecular orbitals. The integrated BLW and TLMO studies confirmed that the hydrogen bonding in these two molecules is indeed enhanced by π-resonance, and that this enhancement is not a result of σ constraints.