The electric dipole polarity of the ground and low-lying metastable excited states of NF

NF (nitrogen monofluoride, fluoroimidogen) is isoelectronic with O₂, and, like O₂, it has a triplet configuration in the ground state, with two low-lying metastable singlet excited states. The dipole moment of the a ¹Δ excited state was measured in 1973 to be 0.37 ± 0.06 D; at the time its polarity was assumed to be normal (i.e., with the negative charge on the fluorine). However, high-level electronic structure calculations, which reproduce with high accuracy the known spectroscopic constants of the ground and excited states of NF, predict a dipole moment of -0.388 D for a ¹Δ NF, indicating that, despite the electronegativities, this molecule carries a positive charge on fluorine. The other singlet state is predicted to have an even larger negative dipole moment; the ground-state triplet should have a very small positive moment. Singlet NF resembles in this respect CO and BF, from the N₂ isoelectronic series, both of which also have negative dipole moments.