Experimental and ab initio MO studies on the IR spectra and structure of 4-hydroxyacetanilide (paracetamol), its oxyanion and dianion

The spectral and structural changes taking place during the course of the conversion of 4-hydroxyacetanilide (paracetamol), HO-C6H4-NH-COCH3, into the corresponding oxyanion, O--C6H4-NH-COCH3, and dianion, O--C6H4-(N) over bar-COCH3, have been followed by both quantitative infrared spectra and ab initio HF/6-31G force-field calculations. The changes accompanying the first deprotonation concern mainly the oxyphenylene fragment; those resulting from the second one are spread over the whole dianion. Analysis of the atomic charge changes shows that over 90% of the first (oxyanionic) charge remains localized within the oxyphenylene fragment. The second (nitranionic) charge delocalizes over the acetyl (0.51e(-)) and phenylene (0.26e(-)) groups, nitranionic (0.14e(-)) and oxyanionic (0.09e(-)) centres. The trans conformers (with respect to phenylene and methyl groups) have been calculated to be more stable than the cis ones in all cases studied. (C) 1998 Elsevier Science B.V. All rights reserved.