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Two-Dimensional Correlation Analysis, Fourier Self-Deconvolution, and Second-Derivative Studies on Assignments of the CH2 Stretching Bands for Cast Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane
Volume 54, Number 5 (May 2000) Page 692-698
Wang, Hai-Shui; Ozaki, Yukihiro
Infrared spectra have been measured for cast films of three kinds of 2-alkyl-7,7,8,8-tetracyanoquinodimethanes (CnTCNQ) prepared on KBr plates. Each infrared spectrum shows two components for the CH2 antisymmetric stretching band (2926 and 2918 cm-1), and the relative intensity of the two bands changes with the length of the alkyl chain. In contrast to the characteristic doublet band in the antisymmetric stretching band region, only one band seems to appear near 2850 cm-1 in the corresponding CH2 symmetric stretching band region. In order to reveal the number of the bands appearing in the CH2 antisymmetric and symmetric stretching regions and to explore the origins of these bands, Fourier self-deconvolution (FSD) and second-derivative methods have been applied to the infrared spectra of the cast films. The FSD analysis of the spectra for the three kinds of CnTCNQ derivatives measured at a 4 cm-1 resolution demonstrates that the CH2 symmetric stretching band also consists of two components (near 2853 and 2848 cm-1). The same conclusion has been obtained by the second derivative of the spectra measured at 1 cm-1 resolution. An asynchronous two-dimensional (2D) correlation spectrum created from time-dependent infrared spectra for a one-layer Langmuir-Blodgett (LB) film develops cross peaks at (2927 and 2916 cm-1) and (2854 and 2846 cm-1), suggesting that the bands at 2927 and 2916 cm-1 come from different species and those at 2854 and 2846 cm-1 also have different origins. The bands at 2918 and 2848 cm-1 may be due to the interdigitated part of the alkyl chain in the cast film, which has ordered (trans-zigzag) form, while the bands near 2926 and 2853 cm-1 are probably ascribed to the noninterdigitated part of the alkyl chain, which has disordered form with several gauche conformations.