Infrared Intensities of Liquids XV: Infrared Refractive Indices from 8000 to 350 cm−1, Absolute Integrated Intensities, Transition Moments, and Dipole Moment Derivatives of Methanol-d, at 25°C

Abstract

This paper reports infrared absorption intensities of liquid methanol- d, CH3OD, at 25°C, between 8000 and 350 cm−1 Measurements were made by multiple attenuated total reflection spectroscopy with the use of the CIRCLE cell, and by transmission spectroscopy with a variable-path-length cell with CaF2 windows. The results of these two methods agree excellently and were combined to yield an imaginary refractive index spectrum, k(ν˜) vs. ν˜, between 6187 and 350 cm−1. The imaginary refractive index spectrum was arbitrarily set to zero between 6187 and 8000 cm−1 where k is always less than 2 × 10−6, in order that the real refractive index can be calculated below 8000 cm−1 by Kramers-Krönig transformation. The results are reported as graphs and as tables of the real and imaginary refractive indices between 8000 and 350 cm−1, from which all other infrared properties of liquid methanol- d can be calculated. The accuracy is estimated to be ± 3% below 5900 cm−1 and ± 10% above 5900 cm−1 for the imaginary refractive index and better than ± 0.5% for the real refractive index. In order to obtain molecular information from the refractive indices, the spectrum of the imaginary polarizability multiplied by wavenumber, ν˜ vs. ν˜, was calculated under the assumption of the Lorentz local field. The area under this ν˜ spectrum was separated into the integrated intensities of different vibrations. Molecular properties were calculated from these integrated intensities—specifically, the transition moments and dipole moment derivatives of the molecules in the liquid, the latter under the harmonic approximation. The availability of the spectra of both CH3OH and CH3OD enables the integrated intensities and the molecular properties of the C-H, O-H, O-D, and C-O stretching and CH3 deformation vibrations to be determined with confidence to a few percent. Further work with isotopic molecules is needed to improve the reliability of the integrated intensities of the C-O-H(D) in-plane bending, H-C-O-H(D) torsion, and CH3 rocking vibrations.