Why Do Nitroso Compounds Dimerize While their Oxime Tautomers Do Not? A Structural Study of the Trans-Dimer of 2-Chloro-2-methyl-3-nitrosobutane and Higher Level ab Initio Study of Thermodynamic Stabilities and Electronic Structures of Isomers of Diazene Dioxides


NOCl addition to 2-methyl-2-butene produces a white crystalline solid which was shown by low-temperature X-ray diffraction to contain trans-dimers 2 of 2-chloro-2-methyl-3-nitrosobutane (1n):  C10H20N2O2Cl2, Mr = 271.18, monoclinic, I2/a, a = 11.982(4), b = 7.1297(7), c = 15.735(5) Å, β = 90.20(2)°, V = 1344.2(6) Å3, Z = 4, Dx = 1.340(2) g cm-3, (Mo Kα) = 0.70930 Å, μ = 4.7 cm-1, F(000) = 1152, T = 173 K, R = 0.030, Rw = 0.032 for 791 observed reflections of 898 unique data. Diazene dioxide 2 also is the dominant species in solution although some spectroscopic and chemical observations indicate the presence of monomeric nitroso compound 1n and of the tautomeric oxime 1o at least in small quantities. In conjunction with the experimental study, conformational and configurational equilibrium structure preferences and the bonding in diazene dioxides R(O)NN(O)R' (3:  R = R' = H, 4:  R = H, R' = Me, 5:  R = R' = Me) were studied at the MP2(full)/6-31G* level. The 1A' and 3A'' states of HNO and MeNO and the oxime tautomer of MeNO were considered. Isomer energies, rotational barriers, excitation energies, tautomerization energies, and dimer formation energies were determined at levels up to QCISD(T)/6-311G**//MP2(full)/6-31G* + ΔVZPE(MP2(full)/6-31G*). The trans-isomer is favored in all cases, and the trans preference energy increases from 4.0 kcal/mol in (HNO)2 to 13.2 kcal/mol in (MeNO)2. The thermodynamic stabilities of the trans-configured diazene dioxides vary by less than 1 kcal/mol as the result of successive H/Me replacement; the calculated dimerization enthalpies for the trans-isomers are -9.0 (3), -9.9 (4), and -9.4 kcal/mol (5). Natural bond orbital analysis and dipole and quadrupole moments show dimer formation to increase the polarity of the NO bond, and this increase is only slightly larger for the Me-substituted NO bond. The driving force for dimer formation is the electron density transfer to the most electronegative element and this density transfer is made possible by N-rehybridization. The analogous dimerization of oximes via Nσ lone pair [2 + 2] addition would result in an unfavorable polarization reversal for the C-atom.



International Standard Serial Number (ISSN)

0022-3263; 1520-6904

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Article - Journal

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