The structural features of a recently introduced class of 1,3-dipolar reagents have been computed by density functional theory and ab initio methods. the reagents are formally derived from Munchnones by replacement of the C = O group with a PR3 unit. The parent species shows a long P...O interaction (2.55 at the B3LYP/6-31+G(d)level) together with a nonplanar ring, and is best described as a weakly chelated acylamino-phosphonium ylide. the corresponding acyclic form, in which the P center dot center dot center dot O interaction is absent is predicted to be 2-3 kcal mol(-1) higher in enthalpy. Variation of the phosphorous substitutes exerts marked effect on the P...O distance, with electron-withdrawing groups favoring a covalent interaction [P center dot center dot center dot O 197 angstrom for PR3 = Pph(catechyl)] and electron-donating groups favoring a weak interaction [P center dot center dot center dot O 3.92 angstrom for PR3 = PPh3]. However, this variation has little effect on the relative energies of the cyclic and acyclic forms. The barriers for concerted cycloadditions with ethylene are 22.8 kcal mol(-1) (PH3), 31.7 kcal mol(-1) (PPh3), and 16.2 kcal mol(-1) [PPh(catechyl) with axial O], which correspond with experimental observations and follow the same trend as the energies required to distort the dipole to the TS geometry.