The equilibrium molecular structure of 3-methyl-4-nitro- and 4-methyl-3-nitrofuroxans by gas-phase electron diffraction and coupled cluster calculations

The equilibrium molecular structures of the isomeric 3-methyl-4-nitro- and 4-methyl-3-nitrofuroxans have been determined for the first time by gas-phase electron diffraction (GED) supported by coupled cluster calculations up to CCSD(T)/cc-pVTZ level of theory, in frame of dynamic model with relaxation of all structural parameters. The best fit of the experimental scattering intensities was obtained for a model of Cs symmetry. The small differences between similar geometric parameters were constrained at the theoretical values. To compare structural parameters of the titled compounds, the geometries of parent furazan and furoxan molecules were calculated at the CCSD(T)-AE/aug-cc-pVQZ and CCSD(T)-AE/cc-pVQZ levels of theory, respectively. Mean amplitudes and vibrational corrections necessary for GED analysis were computed at the B3LYP/SNDS levels of theory using quadratic and cubic force fields. Peculiar features of electronic and geometric structures of the studied isomeric nitrofuroxans were analyzed with the use of different topological, magnetic and orbital descriptors and the reasons of the significant distinctions between 4- and 3-nitroisomers structures were revealed. Due to hyperconjugation a lone electron pair of exocyclic oxygen atom in both isomers was found to delocalize into antibonding orbital of adjacent ON endocyclic bond, nπ(O)→σ∗(ON). The peculiarity of the 3-nitroisomer structure was provided with the additional interaction of exocyclic and nitro group oxygen atoms. [Display omitted] •Isomeric 3-/4-nitrofuroxans were characterized by gas-phase electron diffraction.•Structural parameters were refined by high-level quantum-chemical calculations.•The use of orbital descriptor revealed distinctions between 4- and 3-nitroisomers.