On the geometry of 3-amino-sydnones

The X-ray structure of molsidomine (3-morpholine-sydnoneimine) reveals a widening of the CNC angle of morpholine substituted at the N-3 atom of the sydnone moiety. This might be due to lone-pair delocalization, since sydnone is a mesoionic aromatic heterocycle, and its N-3 nitrogen atom is expected to undergo an important a-electron loss. With a view to clarifying this problem, the results of an extensive analysis of the literature and theoretical computations carried out with standard empirical and ab initio methods have been discussed with the help of the IML-EH method, a version of the EHT method which includes hybridization, or localization and mutual charge adjustment. Although the computational results can be interpreted as favouring the prediction that N-2 of sydnones is a-electron deficient, no significant delocalization of the lone pair of the amine substituent is predicted. The CNC angle widening appears to be due to hybridization-polarization effects. In the course of the paper, numerical information relevant to further research on the sydnones and on the N-N bond is presented and discussed. (C) 1998 Elsevier Science B.V. All rights reserved.