VIBRATIONAL ASSIGNMENTS FOR THE LOW FREQUENCY RAMAN ACTIVE PHONONS IN TCNQ SINGLE CRYSTALS
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Date
1986
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Ohio State University
Abstract
A satisfactory assignment for the low frequency, Raman active phonons of 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) in the solid state has eluded spectroscopists ever since the first polarized Raman spectra of the single crystal were $reported.^{1}$ Several attempts have been made to assign the observed transitions as either intramolecular vibrations or lattice phonons, but it has been thought that significant coupling prevents a well defined demarcation in frequency between the internal and external modes in the TCNQ crystal. In order to resolve this problem, we have measured the single crystal Raman spectra of TCNQ in the range $5-190 cm-^{1}$ for six different polarization configurations. The data exhibit some significant differences from results that have already been reported, particularly for the bands observed at 40, 63, and $75 cm-^{1}$. A lattice dynamical calculation using a minimized potential for the TCNQ lattice has also been performed. A comparison of the theoretical and experimental results shows excellent agreement and offers a complete determination of the lattice phonon frequencies, symmetries, and eigenvectors of TCNQ.
Description
1A, Girlando and C. Pecile, Spectrochim. Acta, 29A, 1859 (1973). Address of White, Brose, and Eckhardt: Department of Chemistry, University of Nebraska, Lincoln, Nebraska, 68588-0304.
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