1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
3.2.3.3.2 VT-NMR at Lowered Temperatures
At low temperatures, dynamic processes are slowed down significantly making them
accessible even for relatively slow methods like NMR spectroscopy. Therewith a better
resolution for the signals in the aromatic region should be obtainable. Corresponding
experiments were conducted at 400 MHz measuring frequency in a temperature range from
-10 °C down to -90 °C in dichlorodideuteromethane, CD2Cl2, as appropriate solvent. The
obtained spectra are assorted in Figure 19.
50
*
applied θ
↓
+20 °C
-10 °C
-20 °C
-30 °C
-40 °C
-50 °C
-70°C
-90 °C
9 8.5 8 7.5 7 ← δ [ppm] -1→ -1.5 -2
Figure 19. 1 H VT-NMR spectra of
53 at 400 MHz at the given
temperatures as solution in
CD2Cl2. The reference spectrum
(+20 °C) was taken under the
same conditions but in CDCl3 (*).
The above shown spectra deliver the expected better resolution in the aromatic area at
temperatures down to -20 °C. The broad signals for the phenyl rings positioned on carbon
atoms 10 and 15 then clearly show up as distinct doublets. But further lowing of the
temperature seemingly leads to a brake-down of couplings which is also observed for the β-
pyrrolic resonances. Additionally, the signal for the inner ring protons splits upon cooling.
More precisely, at +20 °C, the inner ring protons only show one sharp resonance signal (s,
-1.62 ppm). Cooling then provides a significant line broadening and, from -20 °C on, the NH-
signal begins to split into two separated singlets being clearly resolved at -50 °C (-1.83 and
-1.97 ppm). At that temperature furthermore, a second set of two singlets appears, which