Determination of the rotational barrier about the nitrogen-carbon bond in two chalcogen replaced N,N-dimethylamides

Abstract

Hindered rotation (HR) about the N-C amido bond in samples of 3-dimethylaminoacrylonitrile and (dimethylaminoethylene)malonitrile has been examined by proton magnetic resonance. Substitution of the =CH2 moeity in place of chalcogens in the amide structure leads to a large decrease in the barrier to rotation about the amido N-C bond, but substitution of electronegative groups for hydrogen in =CH2 then causes an increase in the barrier. The effects of CN substitution are manifest by stabilization of the carbanion resonance structure. In turn, the free energy of activation for HR is found to increase approximately with the square of the total mesomeric interacting potential of the substituents. Eight-site formulation of the chemical exchange for 3-dimethylaminoacrylonitrile requires consideration of parity in the signs of long-range couplings. Thus it is suggested that both of the 4J(CH3-H) and ®J(CH3~H) couplings have the same sign. This is substantiated by line shape analysis and the magnitudes of the averaged couplings in the fast exchange limit. The magnitudes of all couplings are reported and they are considered to be dependent upon the extent of tt conjugation and/or configuration interaction.

Solvent Effects on the Barrier to C−N Bond Rotation in N,N-Dimethylaminoacrylonitrile

“…Although applicable to somewhat different rate constant regimes, and therefore somewhat different temperatures, the two methods yielded results that were in excellent agreement with each other. The free energy barriers determined in chloroform (12.63 kcal/mol) and in dichloromethane (12.71 kcal/mol) also agree quite closely with results reported previously in carbon tetrachloride (12.9 kcal/mol) 3 Experimentally Determined Barriers to Rotation about the Conjugated C−N Bond in N , N -Dimethylaminoacrylonitrile (kcal/mol) solventε a S b Δ G ⧧ (273) c Δ H ⧧ Δ S ⧧ gas phase 1.0 −0.556 9.3 ± 0.5 d methylcyclohexane- d 14 2.0 −0.324 11.0 ± 0.2 toluene- d 8 2.4 −0.237 12.05 12.9 +3.0 dibutyl ether- d 18 3.1 −0.286 11.54 12.9 +5.0 chloroform- d 4.8 −0.200 12.63 12.8 +0.7 dichloromethane- d 2 8.9 −0.189 12.71 12.6 −0.5 acetone- d 6 20.6 −0.175 12.80 13.4 +2.3 methanol- d 4 32.7 +0.050 12.96 12.5 −1.7 acetonitrile- d 3 35.9 −0.104 13.34 13.8 +1.8 nitromethane- d 3 35.9 −0.134 13.13 12.7 −1.7 water- d 2 78.0 ≤13.0 a Dielectric constant.…”

“…However, as long as exchange is fairly rapid compared to longitudinal relaxation, whether this assumption is correct makes almost no difference for the computed rate constant. Perrin has used a much more sophisticated equation, having seven adjustable parameters and making fewer assumptions, to model his SIR data . However, we have chosen the simple equations above in the hopes that, with only two adjustable parameters, any serious problems with the data would reveal themselves as a nonlinear plot.…”

The vibrational and NMR spectra, conformations and ab initio calculations of aminomethylene, propanedinitrile and itsN-methyl derivatives

“…By NMR spectroscopy the barrier to hindered rotation around the C--N amino bond in DMAM was examined [13,14].…”