An unusual substituent effect in a cycloaddition of an azolium ylide 1,3-dipole: An inverted V-shaped hammett plot

An unusual inverted V-shaped Hammett plot has been observed for substituent effects at the N-terminus of 1,2,3-triazolium-1-imide 1,3-dipoles in cycloaddition reactions. Theoretical studies (3-21G and 6-31G) suggest strong resonance interactions separately stabilizing both ends of the dipole.