Absolute Infrared Intensities of the Fundamental Vibrations of NF3

The absolute infrared intensities of the fundamental vibrations of NF₃ have been measured. The results have been interpreted in terms of the N–F bond moment (μ), its derivative $\mathrm{(\partial \mu /\partial r)}$ , and the unshared pair moment on the N atom (μ_u.p.). The bond moment obtained from the A₁ bending mode is found to be in the vicinity of 1 D. μ_u.p. is in the vicinity of either 1.2 D or 1.7 D depending upon the sign assumed for the molecular dipole moment. The bond moment calculated from the E mode is quite sensitive to the potential function. Since this function is not known with high accuracy, one is not able to determine whether the bond moments from the two different symmetry species are in agreement. There is a striking difference in the values of $\mathrm{\partial \mu /\partial r}$ from the two symmetry species with $\mathrm{\partial \mu /\partial r}$ (A₁) in the range of 1.5 to 2 D/A and $\mathrm{\partial \mu /\partial r}$ (E) in the vicinity of 4.5 D/A. A qualitative explanation for this difference is offered.