Modern Polymer Spect..

144 3 Vibrutionnl Spcctrtr as N Prohc OJ Structurnl Order
Table 3-2. Characteristic calculated and expeiimental ‘in band’ frequencies fot the conformers
-CH~’CHL’~’CH~-CHI and -CHP(~)CH~(~’CH~-CH~ at the end of the n-nonadecdnr molecule
v (cm-’)
Calculated
Observed
TG 1348 1342 (IR)
879 866 (R), 866 (IR)
769 766 (IR)
GT 1367
851 844 (R), 845 (IR)
3.14 Case Studies
3.14.1 Conformational Mapping of Fatty Acids Through Mass
Defects
It becomes apparent that the use of selective deuteration becomes a powerful tool
for the mapping, site by site, of the coiifoiniation of polymethylene chains. This has
been applied for the conformational mapping of n-alkane and of fatty acids. The
idea was first proposed by Snyder and Poor for n-alkanes [lo21 and was later
applied to the case of fatty acids [lo71 and has settled the issue on the conformation
in the solid state which has long been matter of great debate [108].
Fatty acids are typical of a class of compounds for which the existence of premelting
phenomena has been claimed with the formation of ‘liquid-like’ structures
for the molecules while still in the solid. The structure of these systems has been
studied by many workers. The molecules in the solid are dimeric since pairs of carboxyl
groups face each other and are held together by hydrogen bonds. Moreover,
complex polymorphic phenomena have been revealed depending on the way crystallization
is carried out (from the melt, from solution, froin the kind of solvent,
etc.) [107, 1091. More recent works based on the study of LAM inodes in the
Raman suggested that the molecules of fatty acids, independent of the crystalline
form, are never all-trans planar and contain somewhere along the chain a gauche
distortion [108]. These information were at odds with our studies on LAM modes of
the same materials which were shown to be fully trans-planar [110, 11 I].
The use of defect gap modes using selectively deuterated materials have definitely
solved the problem. We have recorded the infrared spectra of solid perhydrogenated
stearic and palmitic acids and of a few selectively deuterated derivatives.
For palmitic acid, single CD2 group were placed in position 2, 3, or 4 (carbon atom
1 is that of the carboxyl group); for stearic acid, the single CD? group was placed in
position 2, 3, 4, 5, 6, 7, or 13. As an example of the way we proceeded, Figure 3-25
illustrates the infrared spectra of: (i) normal undeuterated palmitic acid (Figure