The isomerization by concerted ring expanding of singlet cyclopropyl n
itrene has been studied using the self-consistent field (SCF) method w
ith the 3-21G basis set. The transition state has been obtained. The d
isrotation-conrotation mechanism has been proposed and discussed with
the Woodward-Hoffmann approach. The barrier height for the ring expans
ion is 38.89 kJ/mol and the released energy is 232.21 kJ/mol. The rela
tive structure data are given.