Rotations and inversions in organic molecules are readily recognizable from the temperature dependence of the NMR spectra. The study of the effects of substituents on the activation barriers of such processes permits the elucidation of reaction mechanisms, and the stability limits of isomers can also be determined. The knowledge of the stability limits is necessary for the specific synthesis of stable rotamers and invertomers.

Detection of Hindered Rotation and Inversion by NMR Spectroscopy

Rotations- und Inversionsprozesse in organischen Molekulen geben sich durch die Temperaturabhangigkeit der NMR-Spektren zu erkennen. Das Studium der Substituenteneffekte auf die Aktivierungsbarrieren solcher Prozesse gestattet die Aufklarung von Reaktionsmechanismen, ferner lassen sich die Stabilitatsgrenzen von Isomeren ermitteln. Die Kenntnis der Stabilitatsgrenzen ist notwendig, um gezielt stabile Rotamere und Invertomere synthetisieren zu konnen.

Nachweis gehinderter Rotationen und Inversionen durch NMR‐Spektroskopie

Enamines: recent advances in synthetic, spectroscopic, mechanistic, and stereochemical aspects—I

Glyoxal derivatives—I

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H^+) by the addition of trifluoroacetic acid The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C═N double bond, leading to isomerization Treating Z-H^+ with base (K_(2)CO_3) yields a mixture of E and “metastable” Z isomers The latter thermally equilibrates to reinstate the initial isomer ratio The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state) However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E_T) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism) These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways The highly negative entropy values obtained for both the forward (E → Z-H^+) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone–azo tautomerization followed by rotation around a C–N single bond, as opposed to the more common rotation mechanism around the C═N double bond

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E. H. Wagener

Papers

Restricted rotation about the N-N single bond Linear correlation of rate with substituent

Structural organic chemistry by nmr. III. Isomerization of compounds containing the carbon-nitrogen double bond