Modern Polymer Spect..

184 3 T/ihrutionul Spectra as u Probe of Strzicturul Order
overlapped by the stronger CH3 U mode near 1375 cm-' . The expected progression
of CH2 twisting (t) must be very weak in IR while CH? rocking (PI must be strong/
medium and clearly noticeable from 720 to near - 1100 cni-'. C-C stretchings are
weak in IR. For the band progression of the P modes we expect to observe nine
components of B2 species corresponding to quasi-phonons with odd j values.
J = I, 3, and 5 turn out to be almost overlapping and the others must he observed in
the frequency range -750-1 100 cm-' . The observed band progressions are clearly
displayed in Figure 3-46.
The infrared bands at 1375 and 890 an-' are unquestionably assigned to the
umbrella (U) and rocking ([I) modes of the methyl groups respectively (Figure 3-46).
The Raman spectrum provides further information. It is generally known that in
the Raman spectra only the limiting k i 0 modes for the infinite chain appear as
strong or easily observable transitions. This is also the case for C19 where only the
k 2 0 modes are observed.
The most important information conies from the observation of the band progression
of the LAM modes (Section 3.16) with k = 1,3, and 5 components
observed at 124.5, 342, and 490 cni-'. Using equations 3.53 and 3.54, the frequency
of LAM1 at 124.5 c1n-l proves that the chain comprises exactly 19 C atoms.
The conclusions are as follows:
Conclusion no. 1
The infrared and Raman spectra provide unquestionable evidence that C 19 below
22.8"C consists of trnns-planar molecule with 17 CH2 units capped by two CH3
groups. Further observation of the infrared spectrum for the orthorhombic phase
shows the splitting (factor group splitting) of the 6 modes and of many components
of P modes into doublets with a measurable intensity ratio (Figure 3-48). Analogously,
the Raman line associated to k - 0 of the 6 mode near 1460 cni-' is split
into a doublet (Figure 3-47). The number of components of the splitting is related to
the number of molecules (2) in the unit cell. If the orthorhombic lattice is accepted,
the intensity ratio of the components of the splitting in the infrared provides the
value of the setting angle of the hydrocarbon chains in the unit cell (8 e 42"). The
shape of the Ranian scattering in the 3000 and 1450 cni-' range can be interpreted
in terms of Fermi resonances (Section 3.17) uniquely between trans-planar chains.
Conclusion no. 2
IR and Raman spectra provide evidence that two trms-planar molecules are
organized in the orthorliombic lattice, with setting angle 8 = 42".
3.19.3 Temperature-Dependent Spectra
Next, we study the temperature-dependent infrared and Raman spectra of C19
through phase transition (Ta) (T, = 22.8 "C) and melting (T,) (TI,, = 32 "C). We
first focus on the spectroscopy of C19. Each experimental observation is labeled and
will be included in the overall analysis. In the IR spectrum: