Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Temperature dependence of the optical properties of liquid benzene in the infrared between 25 and 50 °C
Introduction
An area of study over the past number of years in this laboratory has been an investigation of how the intermolecular interactions in a liquid affect vibrational intensities. As part of this work, the optical constants and polarizability spectra of several aromatic and aliphatic hydrocarbons have been determined in the liquid phase [1], [2], [3], [4], [5], [6], [7], [8]. Sometime ago, we published [9] a comparison of the experimental liquid intensities with the gas intensities based on the integrated intensities of benzene and benzene-d6. This initial study found that the experimental integrated intensities of the so-called butterfly motion, ν4 (A2u) are the same within experimental error in both phases. However, it was found that the integrated intensities of all the E1u fundamentals (ν12, ν13, ν14) varied significantly in the two phases. This left the question unresolved as to whether or not this is due to changes in the intensities of each of the internal coordinate components (CH stretch, CC stretch, HCC bend), or is the change in the intensity of one internal coordinate motion affecting all three E1u fundamentals. Recently [10], using the experimental intensities of benzene, benzene-d6 and benzene-d1, we determined that the variation in the intensities of the E1u fundamentals in the liquid and gas phases are essentially due to a difference in the dipole moment derivative with respect to the CH stretch motion in the two phases. This now raises a new question: will the dipole moment derivative with respect to the CH stretch vary uniformly as the temperature of the liquid is raised to the boiling point and thus approach the dipole moment derivative in the gas phase? Which means, is it dependent on the distance between the molecules?
To gain insight into the effect of temperature on these interactions, the optical constants of liquid benzene were determined through transmission measurements between 7400 and 800 cm−1. The measurements were taken over a range of 30–50 °C in 5 °C increments. The optical properties were used to calculate the dielectric constants, molar absorption coefficients, and the imaginary molar polarizability spectra under the Lorentz local field model. A similar study has been done in this laboratory on toluene [11].
Section snippets
Experimental
The benzene was purchased from BDH chemicals. It was further purified through fractional freezing and kept over molecular sieve to ensure the samples did not contain any water. The experimental absorption spectra were collected on a Nicolet Impact 410 FTIR and a Thermo-Nicolet 6700 FTIR spectrometer with Ever-Glo mid-IR sources, Ge-on-KBr beam splitters, and DTGS detectors. A Spectra-Tech HT-32 heated transmission cell along with an Omega CN8501 temperature controller allowed the temperature of
Results and discussion
The imaginary molar polarizability spectrum of liquid benzene at 30 °C is shown in Fig. 4. The peaks labeled A–M and the regions labeled α–φ are discussed in more detailed below.
The imaginary molar polarizability spectra at 30, 40, and 50 °C for peaks A, B, C, D, G, and H are shown in Fig. 5 and those for peaks J, L, M, and N are shown in Fig. 6. In all cases, the peak maxima in the 30 °C spectrum are the highest and in the 50 °C spectrum are the lowest. In the baseline, the situation is reversed.
Summary
This paper presents a study of the temperature dependence of the optical properties of liquid benzene between 25 and 50 °C. It was found that:
- (i)
there is very little temperature dependence on the peak frequencies,
- (ii)
the peak heights decrease linearly with temperature,
- (iii)
the peak widths increase linearly with temperature,
- (iv)
the integrated intensities do not show a temperature dependence, and
- (v)
isoentosic points were detected in the spectra.
It was concluded that the variations in the liquid phase spectra with
Acknowledgements
CDK thanks the Office of Research and Academic Institutes at Cape Breton University, the Natural Sciences and Engineering Research Council of Canada (NSERC), Enterprise Cape Breton Corporation, the Atlantic Coastal Opportunities Agency, the Canada Research Chairs program, the Canadian Foundation for Innovation and the Nova Scotia Research and Innovation Trust for their support of this work. EALG thanks NSERC for an undergraduate student research award.
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