Intramolecular multi-bond strain: the unrecognized side of the dichotomy of conjugated systems

Yirong Mo; Huaiyu Zhang; Peifeng Su; Peter D Jarowski; Wei Wu

doi:10.1039/c6sc00454g

Intramolecular multi-bond strain: the unrecognized side of the dichotomy of conjugated systems

Chem Sci. 2016 Sep 1;7(9):5872-5878. doi: 10.1039/c6sc00454g. Epub 2016 May 20.

Authors

Yirong Mo^{1

2}, Huaiyu Zhang^{1

2}, Peifeng Su¹, Peter D Jarowski³, Wei Wu^{1

4}

Affiliations

¹ The State Key Laboratory of Physical Chemistry of Solid Surfaces , Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China.
² Department of Chemistry , Western Michigan University , Kalamazoo , Michigan 49008 , USA . Email: ymo@wmich.edu.
³ University of Surrey , Advanced Technology Institute , Guildford , GU2 7XH , UK . Email: peterjarowski@gmail.com.
⁴ iChEM , Xiamen University , Xiamen , Fujian 361005 , China . Email: weiwu@xmu.edu.cn.

Abstract

Electron conjugation stabilizes unsaturated systems and diminishes the differences among bond distances. Experimentally, Kistiakowsky and coworkers first measured and noticed the difference between the hydrogenation heats of carbon-carbon double bonds in conjugated systems. For instance, the hydrogenation heat of butadiene is 57.1 kcal mol^-1, which is less than two times that of the hydrogenation heat of 1-butene (30.3 kcal mol^-1), and the difference (3.5 kcal mol^-1) is the extra stabilization due to the resonance between two double bonds in the former, and is referred to as the experimental resonance energy. Following Kistiakowsky's definition, Rogers et al. studied the stepwise hydrogenation of 1,3-butadiyne and concluded that there is no conjugation stabilization in this molecule. This claim received objections instantly, but Rogers and coworkers further showed the destabilizing conjugation in 2,3-butanedione and cyanogen. Within resonance theory, the conjugation energy is derived "by subtracting the actual energy of the molecule in question from that of the most stable contributing structure." The notable difference between the experimental and theoretical resonance energies lies in that the former needs other real reference molecules while the latter does not. Here we propose and validate a new concept, intramolecular multi-bond strain, which refers to the repulsion among π bonds. The π-π repulsion, which is contributed to by both Pauli exchange and electrostatic interaction, is quantified with the B₄H₂ model system (16.9 kcal mol^-1), and is compared with the σ-σ repulsion in B₂H₄ (7.7 kcal mol^-1). The significance of the π-π repulsion can be demonstrated by the much longer carbon-nitrogen bond in nitrobenzene (1.486 Å) than in aniline (1.407 Å), the very long and weak nitrogen-nitrogen bond (1.756 Å) in dinitrogen tetroxide, and the instability of long polyynes. This new concept successfully reconciles the discrepancy between experimental and theoretical conjugation energies. However, we maintain that by definition, electron conjugation must be stabilizing.