Parametrized valence bond studies of the origin of the N-F bond lengthenings of FNO2 and FNO

The origins of the lengthening of the N-F bonds of FNO2 and FNO relative to that of NH2F are examined via STO-6G valence bond calculations. The calcul ations were parametrized to reproduce approximately the UHF/cc-pVQZ molecul ar orbital estimates for the nitrogen and oxygen spin densities of NO2 and NO This procedure was used in a recent valence-bond study of asym N2O3. The results of the calculations show that the N-F lengthening for FNO2 arises from the delocalization of oxygen lone-pair electrons into the AOs of the N -F sigma bond, whereas this effect and the presence of a "bent" N-F sigma b ond are primarily responsible for the N-F lengthening in FNO. For compariso n with the experimental geometries of FNO2, FNO, and NH2F, B3LYP/6-31+G(d) molecular orbital estimates of their geometries and those for related syste ms are reported. Consideration is also given to a spin-coupled valence-bond representation of the electronic structure of FNO2: and a valence-bond rep resentation for the reaction FNO2 + O-3 --> FNO + 2O(2) is provided via the use of increased-valence structures for the reactants.