INTERPRETATION OF INFRARED SPECTRA OF THE MOLECULE OCTACHLORODIBENZO-P-DIOXINE

Klimenko V. G. Central New Mexico Community College
525 Buena Vista Dr SE, Albuquerque vklimenko@cnm.edu

INTERPRETATION OF INFRARED SPECTRA OF THE MOLECULE OCTACHLORODIBENZO-P-DIOXINE

The infrared spectra of the octachlorodibenzo-p-dioxine molecule are measured and all normal vibrational modes of the molecule are calculated. Each vibrational mode was assigned to the vibrations of certain functional groups of atoms in the molecule, taking into account the local symmetry characteristics of the vibration mode. A correlation of vibrational modes by their shape was established in a series of molecules: dibenzo-p-dioxine, 2,3,7,8-tetrachlorodibenzo-p-dioxine, and octachlorodibenzo-p-dioxine.
The vibrational structure of the low-temperature electronic spectra provides important information on mechanisms of the intramolecular interaction governing processes of quenching of excited electronic states of molecules [1]. To obtain such data, detailed information is needed on normal vibrational modes, first of all, on their symmetry and shape. Molecules studied in this paper belong to known ecotoxicants, namely, the polychloro-derivatives of dibenzo-p-dioxines (dioxines). The study of optical spectra of these compounds is of interest because they can be used for analytical purposes. Vibrational modes of dioxines were studied in paper [2]. In [3], vibrational frequencies observed in IR spectra of fifteen different dioxines in the spectral region from 1650 to 700 cm–1 were interpreted. Although the use of molecular spectra for analytical purposes does not require their detailed interpretation, the correlation between vibrational frequencies in a series of related chemical compounds is often used. In this case, as a rule, it is assumed that spectral bands with close frequencies belong to the same vibrational mode. Such a correlation of the IR spectral bands for a number of dioxines was given in [3]. However, such an approach may lead to erroneous results. Thus, a comprehensive analysis [4] of vibrational modes of dibenzo-p-dioxine (DD) and in the region from ~50 to 1650 cm–1 2,3,7,8- tetrachlorodibenzo-p-dioxine (1) molecules based on calculations and experimental IR and phosphorescence spectra showed that the correlation [3] of vibrational frequencies observed in IR spectra should be substantially reexamined. This shows the importance of detailed vibrational analysis of dioxines. In this paper, we studied experimentally and theoretically the intramolecular vibrational modes of the octachlorodibenzo-p-dioxine (2) molecule and examined the effect of the number of Cl atoms on frequencies and shape of vibrational modes of molecules DD, 1, and 2.



Fig.1. Molecules 2,3,7,8- tetrachlorodibenzo-p-dioxine (1) and octachlorodibenzo-p-dioxine (2)
The Raman spectra (Fig. 2) were recorded with an U-1000 Jobin Ivon spectrometer and IR spectra (Fig. 3) were recorded with a Perkin-Elmer, model 821 spectrometer.  The spectral bands corresponding to fundamental vibrational modes are denoted by letters, as explained below.



The assignment of experimental frequencies to the vibrational modes of a certain type and symmetry was made by comparison with calculations taking into account the selection rules for the D2h symmetry. The vibrational modes of the planar (2) molecule were calculated in the harmonic approximation using the known computer programs. As in the vibrational analysis of the DD and (1)  molecules, the potential field of a xanthone molecule was used in the calculations [5].



For calculation of in-plane vibrations, this field was modified due to the replacement of hydrogen atoms by chlorine. The corresponding force constants were taken from [6]. The calculation of (2) performed with the modified field provided satisfactory results: The mean deviation of calculated frequencies from the experimental ones did not exceed 20 cm-1. The out-of-plane vibrations of the (2) molecule were calculated using the field of the xanthone molecule without any modifications. Unfortunately, because of the lack of experimental data for out-of-plane vibrations of the (2) molecule, it is impossible to judge whether or not they are adequately described in this approximation. On the other hand, similar calculations of out-of-plane vibrations of the DD and (1) molecules provided satisfactory results [1,6,7].
Upon vibrational correlation in a series of molecules, normal vibration is assigned to vibrations of some functional molecular groups, although most of the normal vibrational are modes delocalized. Our calculations of vibrational spectra of many molecules containing benzene fragments (BF) showed that the traditional use of the calculated amplitudes of natural vibrational coordinates as an only criterion for such an assignment is often inefficient. For this reason, we interpreted each normal vibration taking into account the approximately high symmetry (') to which a set of amplitudes of the natural vibrational coordinates of the BF molecules can be assigned [7].
Thus, IR absorption spectra of (2) molecule were obtained, the normal vibrational modes of this molecule were calculated, and an assignment of each vibrational mode to vibrations of certain functional group of atoms was proposed. A correlation is established between the modes by their shape in a row of molecules: dibenzo-p-dioxine, 2,3,7,8-tetrachlorodibenzo-p-dioxine, and (2). The obtained data are necessary for detailed interpretation of electronicvibrational spectrum of (2). This interpretation will permit investigation of the mechanism of radiational deactivation of electronically excited states of the (2) molecule [8].
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