Chemistry Reference
In-Depth Information
appeared at 9.89 ppm. These results suggest that themolecules of 12 associated through
much stronger H bonding interactions than those of 12
0
. This result was unexpected
because if the molecules of 3 adopted an extended conformation and associated in a
staggered fashion as required by its ADAD units, the strength of its intermolecular
H bonds would be similar to those of the 4-H bonded self-dimer 12
0
†12
0
.
Further
1
H-NMR experiments in CDCl
3
containing 10-20% DMSO-d
6
confirmed
that the intermolecular H bonding interactions between 10 and 11 and among the
molecules of 12 were very strong. For example, at 1 mM, the aniline NH signals of
the 1:1 mixture of 10 and 11 and those of 12 showed insignificant shifts with increas-
ing percentage (up to 20%) of DMSO-d
6
in CDCl
3
. Diluting a sample of 10 and 11
(1:1) or that of 12 in a mixed solvent containing 10% DMSO-d
6
(down to 10 mM)
in CDCl
3
did not lead to any apparent change in the chemical shifts of the aniline
NH signals involved in intermolecular H bonding. The stabilities of the H
bonds of 10 and 11 and those of 12 are in sharp contrast to those of the corresponding
4-H bonded (i.e., DDAD/AADA and DADA/ADAD) duplexes (Gong et al.
1999; Zeng et al. 2002), whose association constants were previously shown to
be in the 10
4
M
21
range in chloroform and could be easily determined by detecting
the
concentration-dependent
shifts of
amide protons using NMR dilution
experiments.
These observations suggested that strands 10 and 11 and strand 12 did not assem-
ble into H bonded polymers as expected. Instead, they seemed to have formed highly
stable, well-defined discrete species. Such a conclusion was confirmed by VPO and
mass spectrometry measurements. VPO experiments showed that, in solution, 10 and
Figure 9.8 Hydrogen bonded duplex consisting of folded strands: (a) heteroduplex 10†11
and (b) homoduplex 12†12.
