A THEORETICAL-STUDY OF THE RESONANCE EFFECT BY MEANS OF THE NH2 GROUP

In this work, we carried out a theoretical study at the INDO level of relative basicity processes involving N-protonation of X-substituted nitrogen compounds (X = NO2, NH2). We determined the protonation energy changes (delta(X)DELTA-E(p) for all the processes concerned and developed a general model for evaluating the contribution of resonance to the delta(X)DELTA-E(p) value of an NH2 substituent (R(NH2)). We found the evaluated resonance effect (R(NH2)) to depend on two parameters, namely the change in the pi-charge of the NH2 substituent during the N-protonation process (delta-q(NH2)) and that between the protonated and neutral parent compounds (delta-q(c)) at the carbon atom bonded to the NH2 group. Good linear correlation was found in both cases. The reliability of INDO predictions is assessed by 3-21G ab initio calculations. We also studied changes in the R(NH2) value arising from bond structure modifications in some aromatic nitrogen compounds and found the calculated R(NH2), value to be larger if formation of conjugated double bond between positions 2 and 3 of the pyridine ring undergoing the N-protonation is hindered.