Song Jong-Won, Lee Ho-Jin, Choi Young-Sang, Yoon Chang-Ju
Department of Chemistry, Korea University, 1 Anam-dong, Seoul 136-701, Korea.
J Phys Chem A. 2006 Feb 9;110(5):2065-71. doi: 10.1021/jp055755c.
Hydrazine passes through two transition states, TS1 (phi = 0 degrees ) and TS2 (phi = 180 degrees ), in the course of internal rotation around its N-N bond. The origin of the corresponding rotational barriers in hydrazine has been extensively studied by experimental and theoretical methods. Here, we used natural bond orbital (NBO) analysis and energy decomposition of rotational barrier energy (DeltaE(barrier)) to understand the origin of the torsional potential energy profile of this molecule. DeltaE(barrier) was dissected into structural (DeltaE(struc)), steric exchange (DeltaE(steric)), and hyperconjugative (DeltaE(deloc)) energy contributions. In both transition states, the major barrier-forming contribution is DeltaE(deloc). The TS2 barrier is lowered by pyramidalization of nitrogen atoms through lowering DeltaE(struc), not by N-N bond lengthening through lowering DeltaE(steric). Higher pyramidality of nitrogen atoms of TS2 than that of TS1 explains well why the N-N bond of TS2 is longer than that of TS1. Finally, the steric repulsion between nitrogen lone pairs does not determine the rotational barrier; nuclear-nuclear Coulombic repulsion between outer H/H atoms in TS1 plays an important role in increasing DeltaE(struc). Taken together, we explain the reason for the different TS1 and TS2 barriers. We show that NBO analysis is a useful tool for understanding structures and potential energy surfaces of compounds containing the N-N bond.
联氨在围绕其N-N键进行内旋转的过程中会经过两个过渡态，即TS1（φ = 0°）和TS2（φ = 180°）。通过实验和理论方法对联氨中相应旋转势垒的起源进行了广泛研究。在此，我们使用自然键轨道（NBO）分析和旋转势垒能量（ΔE(势垒)）的能量分解来理解该分子扭转势能面的起源。ΔE(势垒)被分解为结构（ΔE(结构)）、空间交换（ΔE(空间)）和超共轭（ΔE(离域)）能量贡献。在两个过渡态中，形成势垒的主要贡献都是ΔE(离域)。TS2的势垒通过降低ΔE(结构)使氮原子呈棱锥形而降低，而非通过降低ΔE(空间)使N-N键伸长。TS2中氮原子的棱锥形比TS1更高，这很好地解释了为什么TS2的N-N键比TS1的更长。最后，氮孤对之间的空间排斥并不决定旋转势垒；TS1中外层H/H原子之间的核-核库仑排斥在增加ΔE(结构)方面起着重要作用。综上所述，我们解释了TS1和TS2势垒不同的原因。我们表明NBO分析是理解含N-N键化合物的结构和势能面的有用工具。
