Chemistry Reference
In-Depth Information
Figure 9.7 If adopting an extended conformation, (a) strands 10 and 11 should form
H bonded polymer chains, (b) strands 10
0
and 11 should form an 8-H bonded duplex, and
(c) strand 12 should form H bonded polymer chains.
the formation of supramolecular AB copolymers (Fig. 9.7a). In contrast, if strand 10
0
adopted an extended conformation, it would be completely complementary to an
extended strand 11, leading to the formation of an 8-H bonded, extended duplex
(Fig. 9.7b). Similar to the design of 10 and 11, linking two self-complementary
ADAD units in a head-to-head fashion leads to oligoamide strand 12 (Fig. 9.6).
Based on similar reasoning made with 10 and 11, if strand 12 adopted an extended
conformation, it would also self-associate into H bonded homopolymeric aggregates
(Fig. 9.7c).
The signal of the aniline proton of 11 appeared at 9.66 ppm in the
1
H spectrum
of 11 dissolved in CDCl
3
(1 mM, 23 8C). The signal of the same proton moved to
10.03 ppm in the spectrum of the 1:1 mixture of 1 and 2 in CDCl
3
(1 mM, 23 8C).
Thus, strands 10 and 11 did associate via intermolecular H bonding. However, the
1
H-NMR spectrum of the mixture of 1 and 2 contained very well defined, sharp
signals, which was inconsistent with the presence of polymeric aggregates.
Similarly, the
1
H-NMR spectrum of strand 12, which presumably would form
H bonded polymers, also showed a set of sharp, well-resolved peaks, suggesting
the presence of a single, discrete species in solution. Furthermore, comparing the
1
H-NMR spectrum of 12 with that of 12
0
revealed a surprising result. Oligomer 12
0
,
with its self-complementary ADAD array, was known to dimerize into a self-comp-
lementary, 4-H bonded duplex (Gong et al. 1999). However, the signal of the (internal)
aniline proton of 12 appeared at 10.59 ppm whereas the corresponding proton of 12
0
