A valence bond study of the low-lying states of the NF molecule

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.