Conformational dynamics of substituted N-acetyl-N-phenylbenzylamines. H-1-DNMR and AM1-MO study
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Activation parameters for the rotation around the methylene-nitrogen single bond found in a series of four 2,2'-disubstituted N-acetyl-N-phenylbenzylamines, dissolved in DMSO-d(6), were measured employing the temperature dependence of the H-1-NMR spectrum lineshape between 340 and 400 K. The results are (I) 2 - [N-acetyl -N- (2-acetamidophenyl) aminomethyl] phenyl acetate DeltaG(not equal) =79.9 +/-2.0 kJ/mol (370K), DeltaH(not equal) =96.2 +/-6.0 kJ/mol and DeltaS(not equal) =+45 +/- 20 J/Kmol; (II) 2 [N - acetyl - N - (2-acetyioxybenzyl)amino]benzylacetate DeltaG(not equal) =82.0 +/-2.0 kJ/mol (370K), DeltaH(not equal) =79.1 +/-6.0 kcal/mol and DeltaS(not equal) =8 +/- 20 J/Kmol; (III) 2 - [N - Acetyl - N - (2-nitrobenzyl)amino]benzylacetate DeltaG(not equal) =80.8 +/-2.0 kJ/mol (380K), DeltaH(not equal) =60.7 +/-6.0 kcal/mol and DeltaS(not equal) =53 +/- 20 J/Kmol; (IV) 2-[N-Acetyl-N-(2-acetyloxybenzyl)amino]phenylacetate DeltaG(not equal) =77.0 +/-2.0 kJ/mol (370K), DeltaH(not equal) =65.3 +/-6.0 kJ/mol and DeltaS(not equal) =-32 +/- 20 J/Kmol. Substitution at positions 2 and 2' with bulky groups appears to be essential to freeze the gauche structure at room temperature and consequently, the main contribution to the barrier arises from the steric hindrance between these two groups. Structure I shows an anomalous behavior, possibly due to the formation of a hydrogen bond between the NH at position 2' and the solvent. This interaction increases DeltaH pi and freezes the rotation around the aryl-nitrogen bond, increasing DeltaS(not equal).