Modern Polymer Spect..

Frequency (cm-'1
100%
b
700 500 300 100 (ern-')
Figure 5-9. Infrared spectra of a-poly(L-alanine). (a) Mid-IR region. (From [85]).(b) Far-IR
region. (From [I 371). (-) Electric vector perpendicular to stretching direction. (- - -) Electric vector
parallel to stretchin2 direction.
5.4.2.2 a-Helix
The a1-PLA structure is well determined from X-ray diffraction [ 1041, and therefore
permits a secure refinement of an empirical force field. The helical symmetry results
in three optically active modes, defined by the phase difference between similar
vibrations in adjacent uiiits of the helix: A(0") - 28 modes, Raman, IR( 11); El (99.6')
- 29 doubly degenerate modes, Raman, IR (1); E2 (199.1")-30 doubly degenerate
modes, Raman. Extensive IR and Raman studies on a-PLA and a-PLA-ND [5]
have provided good experimental data. Polarized IR spectra [85, 1371 are shown in
Figure 5-9 and Raman spectra [138] in Figure 5-10. The force field for the normalinode
calculation [lo61 was refined using that of /?-PLA [22] as a starting point.
The frequencies and PEDs [lo61 are given in Table 5-9; the iins frequency error is
1.6 cm-'.