Infrared Intensities of Liquids XI: Infrared Refractive Indices from 8000 to 2 cm −1,Absolute Integrated Intensities,and Dipole Moment Derivatives of Methanol at 25°C

This paper reports infrared absorption intensities of liquid methanol at 25°C between 8000 and 2 cm⁻¹. Measurements were made by attenuated total reflection spectroscopy by four different workers between 1984 and 1991, with the use of CIRCLE cells of two different lengths and with several different alignments of the cell in the instrument. Steps were taken to ensure that as few parameters as possible remained unchanged throughout the series of measurements, to try to reveal systematic errors. The reproducibility was better than ±2.5% in regions of significant absorption. In order to allow comparison between different methods, results of all methods were converted to real and imaginary refractive index spectra. Measurements were also made by transmission spectroscopy in regions of weak absorption, with results that agreed excellently with those from ATR. The ATR and transmission results were combined to give a spectrum between 7500 and 350 cm⁻¹. This spectrum agreed excellently with literature results from 350 to 2 cm⁻¹, and the two sets of measurements were combined to yield a spectrum from 7500 to 2 cm⁻¹. The imaginary refractive index was arbitrarily set to zero between 7500 and 8000 cm⁻¹, where it is always less than 2 × 10⁻⁶, in order that the real refractive index can be calculated below 8000 cm⁻¹ by Kramers-Kronig transform. The results are reported as graphs and as tables of the real and imaginary refractive indices between 8000 and 2 cm⁻¹, from which all other infrared properties of liquid methanol can be calculated. The accuracy is estimated to be ±3% below 5000 cm⁻¹ and ±10% above 5000 cm⁻¹ for the imaginary refractive index and better than ±0.5% for the real refractive index. To obtain molecular information from the measurements, one calculates the imaginary molar polarizability spectrum, vs. , under the Lorentz local field assumption, and the area under bands is separated into contributions from different vibrations under several approximations. Much accuracy is lost in this process. The changes of the dipole moment during normal vibrations, and during OH, CH, and CO bond stretching and COH torsional motion, are presented.