My PhD dissertation - Institut Fresnel

4.4.1 The Use of Variable-Temperature NMR to Study Intramolecular
Mobility
54
Intramolecular movements in rotational isomers can in principle be studied by
various spectroscopic methods e.g. Raman, infrared, microwave or NMR
spectroscopy
enriched materials
[ , ] 203 204 or by kinetic measurements of the racemization process for enantio-
[ ] 205 . However, each one of these techniques remains limited, to a certain
extent, either by the rate of the rotational process considered, or by the structural complexity
of the substrate investigated or even by the ease to access optically active substrates.
Vibration spectroscopy techniques can be used for the determination of rotamer populations
in case of very fast exchange processes with activation energy lower than 5 kcal·mol -1 [204] .
Furthermore, these techniques are solely applicable to relatively simple structures since
absorption bands for both rotamers should be clearly distinct in the spectrum to afford
accurate data. NMR spectroscopy has an advantage over vibration spectroscopies in the study
of rotational isomers owing to the fact that the intensities of signals are directly proportional
to the number of nuclei at the magnetic site. In addition, NMR technique, due to its "time
scale", afford the study of slower rotational process with activation energies of 5 to
-1 [ a, ]
25 kcal·mol 206 207 , which constitute the usual range of value for sterically little
encumbered biphenyls.
In the rotation about a bond in a molecule, there are certain energetically favoured
conformations. For instance, in biphenyl derivatives where free rotation about the pivot bond
is hindered by ortho- substituents, a coplanar arrangement of the aromatic rings corresponds
to an energy maximum. As a consequence, such kind of substrate are twisted in the ground
state to minimize their energy level
C
D
[ ] 208 .
A
B
k r
C
B
Ia Ib
A
D