1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
Obviously, only two tautomers are present since two sets of signals are detected (Figure 19).
That appears feasible since structures T3 – T6 displayed in Scheme 33 involve directly
neighbored hydrogen atoms being sterically demanding and therefore energetically
unfavorable. Thus, those structures will not be observable and the following discussion will
focus on T1 and T2.
Because of the superimposition of the corresponding two spectra in the downfield region,
the determination of the tautomeric ratio and of interchange rates has to be deduced from
the NH resonances. For that purpose, line-shape analysis can be applied taking two
processes into account which are illustrated in Scheme 34. Firstly, both considered
tautomers can interconvert (T1 ⇄ T2 and T1 * ⇄ T2 * ), whereby both protons move to
different pyrrolic moieties. Secondly, the protons are able to change their positions within
the same tautomer (T1 ⇄ T1 * and T2 ⇄ T2 * ). 103
52
O
O
N
NH
N 1 H2 N
N
T1
T2
O
N
NH H N 2 H1 N
N
H
O
N
N N
N
N
1
H2 H1 H2 Scheme 34. Potentially observable intramolecular proton exchange processes within 53.
Assuming that tautomerism (i.e. T1 ⇄ T2 and T1 * ⇄ T2 * ) is significantly faster than inherent
proton interchange within the tautomers, the observed line-shapes give rise to an
approximate 9 : 1 ratio in favor of the thermodynamically more stable tautomer. 103
Unfortunately, we have not been able to identify which tautomer that is. Following
CROSSLEY 101 , the electron withdrawing ketone moiety as 2-substituent should be in favor of
N
T1 *
T2 *