chemia - Studia

TO WHAT EXTENT THE NMR “MOBILE PROTONS” ARE RELEVANT FOR RESTRICTED ROTATIONAL …
we entitled “slow free rotation” (see the later discussion) while the SER N-
ligand, more solvated at room temperature, displayed, at 80 o C, a typical
slow exchange status between unequally populated sites [1a].
Furthermore, since computational data (Table 4) predicted the same
bond order concerning C(s-triazine)-N(exocyclic) connections, it followed
that the above delay in the dynamic behaviour of the two N-ligands was
dictated mainly by solvation and not by electronic effects.
If so, we calculated the so-called “temperature gradients”
[Δδ(NH)/ΔT]×10 3 of protons NH for the major rotational diastereomer in
series I-2 and II-3 (Table 5) [1b].
Table 5. Temperature gradients of protons NH of compounds I-2a-c and II-3a-c
Compd. δ H (ppm) (T, K) [Δδ(NH)/ΔT]×10 3
(as major rotamer)
(ppb/K) a
T (K) D-NH SER-NH D-NH SER-NH
I-2a (a-a) 298 7.62 6.86
353 7.01 6.41 b -11.1 -8.2
I-2b (a-a) 303 7.62 6.77 -12.0 -6.8
353 7.02 6.43
I-2c (a-a) 303 - 6.57 - -6.6
353
6.24
II-3a (s-s) 298 8.03 6.92 -10.0 -6.2
353 7.48 6.58
II-3b (s-s) 298 8.05 6.83 -10.0 -6.5
353 7.50 6.47
II-3c (s-s) 303 8.01 6.64 -8.2 -4.8
353 7.60 6.40
a Calculated as [δ(NH) 353 K - δ(NH) r.t. ] / (353 – T r.t. );
b At 353 K, in all cases, as signal displayed by the major SER-NH rotational site.
Thus, as recently Simanek observed in the case of elaborated aminos-triazines
[8], although temperature gradient is usually applied to peptides
and proteins [1b], it is generally accepted and indicative that if this coefficient
is more negative than -4 ppb/K in aqueous solution, the NH group was,
initially, exposed to solvent and not involved in intramolecular hydrogen bonds.
Conversely, a temperature gradient less negative than -4 ppb/K indicates the
NH protons being, primarily, involved in intramolecular hydrogen bonding.
Our temperature gradients (Table 5) were consistent with the below
assignments:
i) Although, at room temperature, D-NH protons were by far more
chelated by the solvent, upon heating, they faster “escaped” from the solvent
cage, in agreement with the slow free rotating status reached by these
“closed-chain” N-ligands. The D-NH-solvation in our chlorodiamino-s-triazines
was not dependent on the type of D-anchorage, axial or equatorial.
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