Modern Polymer Spect..

5.3 Amidc Modes 255
Figure 5-1. Optimized confonxatioiis
of Ala dipeptide
(p, ti/ in parentheses). Peptide
groups-1: C6N7, 7: C12N17.
Hydrogen bonds are indicated by
broken lines. (From [33])
alter this: for C5, even though the CO sl force constant (1 1.040) is larger than the
CO s2 force constant (10.922), the predominantly CO sl mode is at a lower frequency
(1 668 cm-') than CO s2 (1 698 cm-I). This is also counterintuitive in terms
of the (admittedly weak) hydrogen bond to C02, whereas the relative frequencies
are in the expected order for C7E and C7A. In distinction to the nonhydrogenbonded
structures, NH ib is now a significant component of the amide I modes of
the hydrogen-bonded structures (C7E and C7A). (To a lesser extent, this is also true
for the Ala dipeptide in a crystal conformation similar to that of C5 but in which
both peptide groups are fully hydrogen bonded [74].)
The amide I1 mode frequencies generally reflect the influence of hydrogen bonding,
which manifests itself primarily through the magnitudes of such force constants
as NH ib and CN s. (It should be noted that explicit hydrogen bond coordinates are
not included in the coordinate basis [33, 341, so the force constants implicitly reflect
the influence of this interaction.) As expected, the higher frequency modes are
associated with the peptide group having a hydrogen-bonded NH. In addition, the
modes in C5 involve coupled NH ib displacements [34], which makes it difficult in
general to infer the explicit dependence of amide I1 on conformation.
The amide I11 modes demonstrate an increasing complexity compared with the