Infrared intensities of liquids. Part XXIII. Infrared optical constants and integrated intensities of liquid benzene-d1 at 25°C

Abstract

This paper presents the first absolute infrared absorption intensities of liquid benzene-d1, C6H5D, at 25°C. It also presents what, surprisingly, seems to be the first complete assignment of an infrared spectrum of liquid benzene-d1 recorded with post-1950 instrumentation. The spectra of the real and imaginary refractive indices are given as graphs and tables between 6200 and 500cm−1, and a table is given of the peak wavenumbers, absolute integrated intensities and vibrational assignments between 4800 and 500cm−1. Errors in the imaginary refractive index, k, values are estimated to be 5–7% for peaks and 5–20% for the baseline between 6200 and 4700cm−1, 0.3–2.0% for both peaks and baseline between 4700 and 825cm−1, 3–4% for both peaks and baseline between 825 and 620cm−1, and 40–50% between 620 and 500cm−1 where the very strong peak near 600cm−1 was too intense for us to measure accurately. Errors in the real refractive indices, n, are estimated to be 0.25% at 8000cm−1 increasing to 0.5% near 800cm−1 and, due to the uncertain intensity of the peak near 600cm−1, to range up to 10% between 710 and 500cm−1. The refractive index spectra were converted to spectra of the real and imaginary dielectric constants, ϵ′ and ϵ″, the molar absorption coefficient, Em, and the real and imaginary molar polarizabilities under the Lorentz local field, α′m and α″m. The peak heights and wavenumbers in the spectra of the different absorption quantities are compared for the most intense bands. Integrated intensities were determined as Cj, the area under bands in the ν̃α″m spectrum, for all bands between 4800 and 500cm−1. The contributions from the different bands were separated by fitting the spectrum with classical damped harmonic oscillator bands. The estimated errors in the integrated intensities range from 2 to 10% for most bands, although they may reach 100% for very weak bands and shoulders. The integrated intensities of the fundamentals and the corresponding transition dipole moments are summarized and compared with literature values for the gas. Crawford's F-sum rule shows that the measured integrated intensities of C6H5D are nicely consistent with those reported recently for C6H6 and C6D6. The total integrated intensity of the first overtone of the CH stretches is ∼20 times smaller than that of the fundamentals.