Infrared Intensities of Liquids XII: Accurate Optical Constants and Molar Absorption Coefficients between 6225 and 500 cm−1 of Benzene at 25°C, from Spectra Recorded in Several Laboratories

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Abstract
This paper presents the results of a study to obtain accurate infrared absorption intensities of liquid benzene at 25°C. To achieve this we have determined the agreement between the intensities measured by different spectroscopists using the same instrument in the same laboratory and also by different spectroscopists in different laboratories using instruments made by different manufacturers. The agreement between integrated intensities over specific wavenumber ranges has been found to average about 2%. The spectra from the different laboratories have been averaged, unweighted, to give intensity spectra of benzene that are presented as the best available. The use of data from different instruments in different laboratories has reduced the influence of systematic instrumental errors, so that the agreement presented should be a better approximation to the accuracy of the intensities than would be the case from an extensive study by one person on one instrument. The intensity data presented agree with the only measurements that have been made against a primary standard, the evaluated uncertainty of which is about 6%. The results are presented in both graphic and tabular forms as spectra of the molar absorption coefficient, Em(ν˜), (also called the molar absorptivity, ε, formerly the extinction coefficient) and the real and imaginary refractive indices, n(ν˜) and k(ν˜), between 6225 and 500 cm−1. The peak heights and the areas under the bands in the imaginary refractive index spectrum are reported, together with the peak heights and the areas under the bands in the molar absorption coefficient spectrum. Imaginary refractive index, k(ν˜), and molar absorption coefficient, Em(ν˜), values are believed accurate to an average ±2% at the peaks of stronger bands, ±3.5% at the peaks of the weaker bands below 4100 cm−1, and ±2.5% above 4100 cm−1. The baseline k(ν˜) values are accurate to ∼10% above 1200 cm−1 and 1% to 5% below 1200 cm−1. The areas under bands or band groups in k(ν˜) and Em(ν˜) spectra are accurate to 2% on average, or 1.5% when measured above a baseline for calibration purposes. The real refractive index, n(ν˜), values are believed accurate to 0.2%.