Comparative quantum chemical investigation of structures and properties of diazocyclopropane and other diazoalkanes

The electronic structures and dissociation energies of diazocyclopropane (1), diazomethane (2), 2-diazopropane (3), and diazocyclobutane (4) were calculated at the density functional B3LYP and the ab initio MP2 levels using the 6-31G(d) basis set and at the G2(MP2,SVP)//B3LYP/6-31G(d) level. Distinctive features of diazocyclopropane 1 are the low energy of dissociation with loss of the nitrogen molecule; ΔE = 18.7 kcal mol⁻¹, B3LYP; 9.2 kcal mol⁻¹, G2 at 0 K) and a nonplanar structure, in which the C=N bond forms an angle of 115.7° with the plane of the cyclopropane ring. The behavior of molecules 1 and 2 in the 1,3-dipolar cycloaddition to ethylene (5), acrylonitrile (6), and methyl acrylate (7) was studied. The reactions of 1 with 6 and 7 have very low activation barriers (ΔE _a = 4.7 and 4.4 kcal mol⁻¹, respectively; at the B3LYP level). For these reactions, the G2 method gives even smaller activation parameters (1.8 and 0.3 kcal mol⁻¹, respectively). The results of our calculations provide a good explanation for high reactivity of diazocyclopropane 1. Dedicated to Academician N. K. Kochetkov on the occasion of his 90th birthday. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1072–1076, May, 2005.