This is the last of four papers that present the detailed measurements and results that led to the acceptance by the International Union of Pure and Applied Chemistry of Secondary Infrared Absorption Intensity Standards for liquids. In this paper accurate infrared absorption intensities of liquid dichloromethane at 25°C are presented. The accuracy was estimated from the ±1.5% average agreement of integrated intensities over specific wavenumber ranges between spectra measured by five spectroscopists in four laboratories. The use of data from different instruments in different laboratories has significantly included the effect of systematic instrumental errors. The spectra from the different spectroscopists have been averaged, unweighted, to give intensity spectra of dichloromethane that are presented as the best available. The results obtained agree with the only measurements that have been made against a primary standard, the estimated accuracy of which is about 5.5%. The spectra of the molar absorption coefficient and or the real and imaginary refractive indices are reported as tables and graphs between 6500 and 800 cm<sup>-1</sup>. Also reported are the peak heights and the areas under band groups in the molar absorption coefficient and imaginary refractive index spectra. The imaginary refractive index, <i>k</i>(<i>v</i>), and molar absorption coefficient, <i>E</i><sub>m</sub>(<i>v</i>), values are believed to be accurate to an average ±2.3% over the 36 measured bands. The baseline <i>k</i>(<i>v</i>) values are believed to be accurate to ~8% below 6000 cm<sup>-1</sup>, ~1% below 4500 cm<sup>-1</sup>, and ~25% above 6000 cm<sup>-1</sup>, where the absorption is extremely weak. The areas under band groups in the <i>k</i>(<i>v</i>) and <i>E</i><sub>m</sub>(<i>v</i>) spectra are believed to be accurate to 1.5% averaged over the 15 measured band groups and to 1.0% over the 10 band groups below 3600 cm<sup>-1</sup>. The real refractive index, <i>n</i>(<i>v</i>), values are believed to be accurate to 0.2%.
John E. Bertie, Zhida Lan, R. Norman Jones, and Yoram Apelblat, "Infrared Intensities of Liquids XVIII: Accurate Optical Constants and Molar Absorption Coefficients between 6500 and 800 cm-1 of Dichloromethane at 25°C, from Spectra Recorded in Several Laboratories," Appl. Spectrosc. 49, 840-851 (1995)