N‐arylamides and N‐arylcarbamates NCO internal rotation barrier study by molecular modeling

Abstract

Amides and carbamates present an energetic barrier associated to NAC(O) bond rotation, which determines two different equilibrium geometries. In this work, the conformational equilibrium of formanilide, acetanilide, methyl and t-butyl phenylcarbamates, and their N-methylderivatives was studied by AM1 and B3LYP/6-31G(d,p) calculations. The effect of aryl p-substituents (MeO, Me, Cl, Br, CN, and NO 2 ) was also studied. Amide barriers were found by DFT calculation between 12 and 21 kcal/ mol. Carbamates, on the other hand, showed barriers between 11 and 15 kcal/mol. AM1 underestimates the energetic barriers and provides values around half those obtained by B3LYP/6-31G(d,p) calculations. Electron withdrawing substituents on aryl group decrease the barrier.

Accurate Oxidation Potentials of 40 Benzene and Biphenyl Derivatives with Heteroatom Substituents

“…Density functional theory calculations showed that the neutral N -methylacetanilides 24a – c all prefer a cis conformation with a nearly planar amide geometry, as measured by the Winkler–Dunitz torsion angle τ (0.3°, 1.7°, and 3.5°, respectively), and large N–Ar twist angles (87.1°, 81.0°, and 74.2°, respectively). These results are consistent with both experiments and other MO calculations on N -methylacetanilide conformations . As shown in Table , the computed energy differences between the cis and trans conformers in vacuo for 24b and 24c are comparable (−2.6 and–3.0 kcal/mol, respectively) and considerably smaller for 24a (−0.83 kcal/mol).…”

“…We begin by discussing our computational results for acetanilides 25a – c . Density functional theory calculations (B3LYP/6-31+G**) using the SM8 solvation model to simulate acetonitrile showed that 25a – c all prefer a trans conformation (see Table and below), consistent with both experimental evidence and other MO calculations . As also shown in Table , our calculations revealed a strong energetic preference for the trans conformer for the acetanilide cation radicals 25a – c +• .…”

“…For example, N -methylacetanilides 24a – c all have oxidation potentials that are greater than those of the corresponding acetanilides 25a – c . We hypothesized that the differences in the oxidation potentials of the acetanilides and the N -methylacetanilides might be related to conformational factors, as it is well-known that acetanilides lacking ortho substituents are generally planar whereas N -methylacetanilides have large twist angles about the N–Ar bond . This hypothesis was tested experimentally by measuring the oxidation potentials of 2-oxindole ( 26 ) and N -methyl-2-oxindole ( 27 ), which are constrained to have planar structures.…”

Schwingungsspektroskopische Untersuchung der Aggregation und Isomerisierung von Modellpeptiden in der Jet-Expansion am Beispiel kleiner sekundärer Amide

“…Über die Höhe der Isomerisierungsbarriere ∆E A finden sich kaum Angaben in der Literatur. Die einzig verfügbaren Daten liefern B3LYP-Rechnungen, die je nach gefundenem Übergangszustand Barrierenhöhen von 68.2-88.2 kJ•mol −1 angeben[276,287] und eine NMR-Analyse bei 349 K, die eine freie Aktivierungsenthalpie von 74.1(8) kJ•mol −1 angibt[273]. Diese Barrierenhöhen lassen die Annahme zu, dass während der Expansion keine Relaxation stattfindet, die Besetzungsverhältnisse im thermodynamischen Gleichgewicht vor der Expansion also "eingefroren" werden[116].…”