Absolute Infrared Intensities of Vibration Bands in Ammonia and Phosphine

Abstract

The absolute absorption intensities of the fundamental vibration bands in ammonia and phosphine have been measured. Normal coordinates are calculated for each of the vibrations and the intensities interpreted in terms of bond moments μ and their derivatives ∂μ/∂r. Values of ∂μ/∂r for the NH and PH bonds in the A₁ mode are 0.6 and 1.2 D/A, respectively; in the E mode the corresponding values are 0.2 and 0.8 D/A. Values of the vibrational bond moments found are 1.0 D (A₁) and 0.5 D (E) for the NH bond; 0.7 or 0.5 D (A₁) and 1.2 or 0.6 D (E) for the PH bond. The relation of these vibrational bond moments to the molecular dipole moment and the moment (μ_u.p.) of the unshared pair of electrons is discussed in detail. After corrections have been applied for changes in magnitude and direction of μ_u.p. during the vibrations, the results for NH₃ are compatible with the following static moments: $${\mu }_{\mathrm{NH}}\text{∼0.7D(}{\mathrm{H}}^{+}{\mathrm{),\mu }}_{\mathrm{u.p.}}\text{∼0.7D.}$$ The data for PH₃ are less internally consistent. ``Likely'' moments, taken from the A₁ bending vibration, are $${\mu }_{\mathrm{PH}}\text{∼0.2(}{\mathrm{H}}^{+}{\mathrm{),\hspace{0.17em}\hspace{0.17em}\mu }}_{\mathrm{u.p.}}\text{∼0.2D.}$$