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.