Strong para-substituent effects on rotational barriers in N-alkoxy-4-R-2,6-dinitrobenzenamines. Dynamic H-1 NMR study

Dynamic H-1 NMR measurements reveal strong para-substituent effects on the rotational barriers about N-arom-O bond in N-alkoxy-4-R-2,6-dinitrobenzenam ines (alkoxy = OCH3, OC2H5, R = NO2, COOH, CF3, CH3). An exceptionally high value (Delta G* > 93 kJ/mol) for R = NO2, intermediate (Delta G* =46.0 kJ/ mol) for R = COOH and too low to be determined by H-1-NMR for R = CF3 and C H3 have been observed. These effects are discussed in terms of Hammett nu(p )-substituent constants and pK(a)-values of para-substituted anilines.