A Valence Bond Study of the Low‐Lying States of the NF Molecule |
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| Authors: | Peifeng Su Wei Wu Prof Sason Shaik Prof Philippe C Hiberty Prof |
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| Institution: | 1. 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), Fax: (+86)?592‐218‐6207;2. Department of Organic Chemistry and The Lise Mertner–Minerva Center for Computational Quantum Chemistry, The Hebrew University, Jerusalem 91904 (Israel);3. Laboratoire de Chimie Physique, Bat 490, Université de Paris‐Sud, CNRS UMR 8000, 91405 Orsay (France), Fax: (+33)?1?69?15?61?88 |
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| Abstract: | 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 O2 molecule, the ground state 3Σ? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet 1Δ and 1Σ+ excited states the π‐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 π bonding contributes at least 24, 35 and 42 kcal mol?1 to the total bonding energies of the 3Σ?, 1Δ and 1Σ+ 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. |
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| Keywords: | ab initio calculations dipole moment dissociation energy electronic structure valence bond theory |
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