Modern Polymer Spect..

3.13.1 Mass Defects
Let us introduce a CD2 group as a defect in n-alkanes (CH3-(CHl),,CH3) or n-alkyl
chain (CH3(CH?),X or in any polyniethylene chain. This is a typical case of ‘mass
defect’ which leaves the force constant matrix (Eq. 3-31) unchanged because of
Born-Oppenheimer approximation. The site where the CD2 group is located can
be changed along the chain. Chemically, this can be easily achieved and selectively
deutei-ated n-alkane or alkyl derivatives are either coininercially available or can be
(and have been) easily synthesized.
Calculations have shown that the frequency of the rocking motion of the unit
CD? embedded in a CH2 host lattice changes by changing the conformation of
its surroundings [102, 1031. The calculated and observed frequencies for the CD?
rocking motion when the adjacent torsional angles are changed are reported in
Table 3-1. These motions are classical well-localized gap-inodes as they occur in the
frequency gap between the CH2 rocking and the CCC bending dispersion branches
of the host lattice.
Table 3-1. Calculated and experimental CD2 ’gap frequencies’ for selectively deuterated nonadecane
as function of the torsional angle about C-C bonds.
Calculated
Observed
4 3 2 1
-CH2-CH>-CH>-CD>-CH3
T T
G T
T G
G G
615 622
615 632
658 658
654
13 12 11 10 9 8 7
CH2-CH2-CH2-CD2 -CH: -CH:-CH?
T (T T) T
G (T T) T
G (T T) G
G’ (T T) G
T (T G) T
G (T G) T
G (T G) G
G‘ (T G) T
T (G G) T
T (G G) G - 671
G [G G) G
-615 622
- 650 650