The Experimental Determination of the Intensities of Infra-Red Absorption Bands. III. Carbon Dioxide, Methane, and Ethane

Abstract

The absolute intensities of the chief infra‐red absorption bands of carbon dioxide, methane, and ethane have been determined experimentally to be (in cycles per centimeter at N.T.P.): $$\begin{array}{ccc}\mathrm{Carbon\; dioxide}& \mathrm{Methane}& \mathrm{Ethane}\\ \text{670\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{band}{\text{:560×10}}^{10}& \text{1306\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{band}{\text{:450×10}}^{10}& \text{\hspace{0.17em}820\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{band}{\text{:\hspace{0.17em}\hspace{0.17em}73×10}}^{10}\\ \text{2350\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{band}{\text{:8600×10}}^{10}& \text{3020\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{band}{\text{:900×10}}^{10}& \text{1450\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{bands}{\text{:\hspace{0.17em} 230×10}}^{10}\\ & & \text{2950\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\hspace{0.17em}\mathrm{bands}{\text{:2200×10}}^{10}\end{array}$$ These results are in satisfactory agreement with available data on infra‐red dispersion and atomic polarization. They may be interpreted in terms of dipole moments, μ′, of bonds between vibrating atoms, and their rates of change with internuclear distance, ∂μ′/∂r. When this is done for the CH bond, however, unexpected variations in these quantities are found for the different vibrations and molecules studied. The average values are about μ′=0.4×10⁻¹⁸, ∂μ′/∂r=±0.6×10⁻¹⁰. For the CO bond the values are considerably larger.