ELECTROSTATIC PROPERTIES OF SYDNONE DERIVATIVES

The bonding type and characteristic chemical reactions of the sydnone ring have been investigated by determination of the electron density d istribution and electrostatic potential by both experimental and theor etical methods. Thirteen sydnone derivatives with known crystal struct ures were studied by theoretical AM1, MNDO, and ab initio calculations . Most of these compounds are 3-phenyl- or 3-pyridylsydnone derivative s. The net atomic charges, bond orders, and deformation density distri bution maps are presented in comparison with the experimental results from X-ray diffraction. The net atomic charges, bond lengths, and bond orders of these compounds confirm the semiaromatic bonding type for t he sydnone ring which was previously assigned by consideration of the experimental bond distances. The large range of twist angles between t he sydnone and phenyl (pyridyl) rings experimentally found in these co mpounds shows that little resonance interaction exists between the 3-p henyl (pyridyl) group and sydnone ring. The highly negative net atomic charge of C4 suggests its susceptibility to electrophilic substitutio n at this position, in agreement with the known chemical reactions of sydnone derivatives. The well-known 1,3-dipolar cycloaddition of the s ydnone can be understood by frontier orbital analysis. The electrostat ic potential map calculated by ab initio methods reveals clearly the p ossible protonation sites of the sydnone ring at O1, N2, and O6 with t he deepest potential energy of -84.6 kcal/mol near the exocyclic O6.