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1780-1550 cm-1 region constructed from the polarization angle-dependent (between -90° and 90°) polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under a dc electric field of +40 V. (Reproduced with permission from Ref. No. 29. Copyright
(2000) American Chemical Society.)
groups on the behavior of the two aromatic rings. The 2D correlation analysis should provide a clear answer for this.
Figure 10.27(A) and (B) shows the synchronous and asynchronous 2D correlation spectrum in the region 1780-1550 cm-1 generated from the polarization angle-dependent IR spectra of FLC-1.29 An autopeak is developed at 1606 cm-1 in the synchronous map, showing that the intensity of the band due to the
2D IR Study of Ferroelectric Liquid Crystals
ring stretching modes changes significantly with the polarization angle. Two negative cross peaks at (1736, 1606) and (1721, 1606 cm-1) indicate that the two C=O stretching bands and the ring stretching band show the polarization angle-dependent intensity changes in the reverse directions.
The corresponding asynchronous map (Figure 10.27(B)) shows three cross peaks at (1608, 1736), (1608, 1728), and (1608, 1715 cm-1). This result is clear evidence for the existence of three C=O stretching bands. The splitting of the band near 1721 cm-1 due to the chiral carbonyl group into two bands at 1728 and 1715 cm-1 may be ascribed to the rotational isomerism around the O-C bonds; there may be two conformers around the O-C bonds. The signs of the cross peaks indicate that the phases of the intensity changes in the two C=O stretching bands at 1728 and 1715 cm-1 are delayed compared with that of the C=O stretching band at 1736 cm-1. Moreover, the signs of cross peaks between the C=O stretching bands and the ring stretching band suggest that the phases of the band intensity changes are delayed in the order of bands at 1736, 1606, 1728, and 1715 cm-1. This conclusion is consistent with the observation in Figure 10.26. Another notable observation in the asynchronous spectrum is the appearance of cross peaks near 1600 cm-1. It seems that the ring stretching bands of the benzene and naphthalene rings are observed separately in the cross peaks. The existence of the cross peaks in the asynchronous spectrum suggests that the directions of the transition moments of the ring stretching modes of two aromatic rings are different, and the rotations of two rings are hindered.
Figure 10.28(A) and (B) shows synchronous and asynchronous maps between the regions 1780-1550 cm-1 and 3000-2800 cm-1 calculated from the
1780 1720 1660 1600
1780 1720 1660 1600
Figure 10.28 (A) Synchronous and (B) asynchronous 2D IR correlation spectra between
the 1780-1550 cm-1 and 3000-2800 cm-1 regions generated from the polarization angle-dependent (between -90° and 90°) polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under a dc electric field of +40 V. (Reproduced with permission from Ref. No. 29. Copyright (2000) American Chemical Society.)
Generalized 2D Correlation Studies of Polymers and Liquid Crystals
polarization angle-dependent spectral variations (between -90° and +90°) of FLC-1 in the Sm-C* monodomain at 60 °C under a dc electric field of +40 V.29 Of note in the asynchronous spectrum is that the band at 2945 cm-1 has a positive cross peak with the band at 1606 cm-1, while the band at 2965 cm-1 shares a negative cross peak with the same band. The original spectra show only one band due to the asymmetric CH3 stretching mode at 2960 cm-1. It seems that both bands at 2965 and 2945 cm-1 are due to the asymmetric CH3 stretching mode of different CH3 groups of FLC-1 whose intensities show different polarization angle dependence. The two CH3 bands at 2965 and 2945 cm-1 show different signs in the cross peaks at (2965, 1606) and (2945, 1606 cm-1), suggesting that the two bands have completely different polarization angle dependences. Nagasaki et at.29 assigned the band at 2945 cm-1 to the chiral CH3 groups, because it is likely that only this band, whose transition moment is in the direction of the short molecular axis, appears separately. This study has demonstrated the usefulness of 2D correlation spectroscopy in the detection of slight differences in polarization angle-dependent intensity variations.
1. I. Noda, in Handbook of Vibrational Spectroscopy, Vol. 3 (Eds J. M. Chalmers and P. R. Griffiths), John Wiley & Sons, Ltd., Chichester, 2002, pp. 2135-2172.
2. Y. Ozaki, in Handbook of Vibrational Spectroscopy, Vol. 3 (Eds J. M. Chalmers and P. R. Griffiths), John Wiley & Sons, Ltd., Chichester, 2002, pp. 2135-2172.
3. I. Noda, in Modern Polymer Spectroscopy (Ed. G. Zerbi) Wiley-VCH, Weinheim, 1999, pp. 1-32.
4. Y. Ozaki and I. Noda, Two-Dimensional Correlation Spectroscopy, American Institute of Physics, New York, 2000.
5. I. Noda, A. E. Dowrey, C. Marcott, Y. Ozaki, and G. M. Story, Appl. Spectrosc., 54, 236A (2000).