Infrared intensities of liquids VIII. Accurate baseline correction of transmission spectra of liquids for computation of absolute intensities, and the 1036 cm−1 band of benzene as a potential intensity standard

Abstract

FT-IR transmission spectra of liquids in well-made and firmly held cells with KBr or NaCl windows are usually very reproducible except that their baselines often show unexpected variations. To obtain absolute absorption intensities from these spectra such baseline differences must be corrected. The problem is illustrated with absorbance spectra of the 1036 cm−1 band of benzene and the absorption index spectra calculated from them via the National Research Council of Canada program 46. Distinction is made between the experimental absorbance spectrum, the ideal experimental absorbance spectrum, and the absorbance spectrum, and a soundly-based method to correct the baselines is presented. We describe a modification of the NRC program 46 that effects the correction and calculates, on a laboratory computer, the complex refractive indices from a transmission spectrum of a liquid.The method is applied to 15 transmission spectra of the 1036 cm−1 band of benzene. It improves the agreement between the 15 peak absorption index values obtained from these spectra from 4% to 2.5%, and improves the agreement between the baseline absorption index values from 5% to 0.01%, all percentages being of the peak value. A table of the average real and imaginary refractive indices and molar absorption coefficients is given. The average peak absorption index value and the area under the band agree closely with those obtained in 1980, and with earlier values from transmission or ATR measurements. These areas are all distinctly lower than those calculated from early measurements of dispersion. These results form the basis for a possible recommendation to the International Union of Pure and Applied Chemistry for a secondary intensity standard. We encourage others to measure this band to help ascertain that systematic errors do not significantly influence our results.To summarise our numerical results at 25 ± 1 °C and 1.0 cm−1 nominal resolution: The average peak absorption index is 0.0710 with a 95% confidence limit of 0.0004, which is within 0.0001 of the less precise value measured in 1980 with a dispersive instrument that was calibrated against a primary intensity standard. The area under the absorption index band between 1095.8 and 912.6 cm−1 is 1.358 cm−1 with a 95% confidence limit of 0.009 cm−1, and that under the molar conductivity band is 3.69 ± 0.03 km/mol. These areas convert to 15.64 ± 0.1 km/mol and 6.79 ± 0.05 km/mol for the more commonly used measures of integrated absorption, the areas under the Naperian and decadic molar absorption coefficients, respectively.Key words: infrared, absorption intensity, optical constants, FT-IR spectroscopy, benzene (liquid), intensity standard.