Su Peifeng, Wu Wei, Shaik Sason, Hiberty Philippe C
Department of Chemistry, College of Chemistry and Chemical Engineering and State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, Fujian 361005, China.
Chemphyschem. 2008 Jul 14;9(10):1442-52. doi: 10.1002/cphc.200800143.
The electronic structures of the three lowest-lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self-consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9-12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O(2) molecule, the ground state (3)Sigma(-) possesses both a sigma bond and 3-electron pi bonds. However, here the polar sigma bond contributes the most to the overall bonding. It is augmented by a fractional (19%) contribution of three-electron pi bonding that arises from pi charge transfer from fluorine to nitrogen. In the singlet (1)Delta and (1)Sigma(+) excited states the pi-bonding component is classically covalent, and it contributes 28% and 37% to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that pi bonding contributes at least 24, 35 and 42 kcal mol(-1) to the total bonding energies of the (3)Sigma(-), (1)Delta and (1)Sigma(+) states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.
采用现代价键（VB）方法，如VB自洽场（VBSCF）、呼吸轨道VB（BOVB）和VB组态相互作用（VBCI）方法，研究了NF三个最低能态的电子结构。这三个态的波函数由9 - 12个VB结构表示，这些结构可进一步简化为三或四个经典的路易斯结构，并对其权重进行了定量估计。尽管波函数简洁，但BOVB和VBCI方法能很好地再现这三个态的光谱性质和偶极矩，与先前的计算研究和实验值吻合良好。与等电子体O₂分子类似，基态(³Σ⁻)既有一个σ键又有3电子π键。然而，这里极性σ键对整体键合的贡献最大。它通过氟向氮的π电荷转移产生的三电子π键合的部分贡献（19%）得到增强。在单重态(¹Δ)和(¹Σ⁺)激发态中，π键合成分是典型的共价键，分别对这两个态的整体键合图像贡献28%和37%。计算了共振能，结果表明π键合分别对(³Σ⁻)、(¹Δ)和(¹Σ⁺)态的总键能贡献至少24、35和42 kcal mol⁻¹。根据简单的价键图像解释了NF分子的一些不寻常性质，如激发时平衡距离缩短和键能增加、偶极矩的反直觉值以及键拉伸时偶极矩的反转。分子在基态时的整体极性非常小，而在单重态激发态中与N和F的相对电负性相反。三个态的偶极矩值由计算得到的VB结构权重定量解释。
