Combined gas-phase electron diffraction and coupled cluster determination of the molecular structure of 3,4-dinitrofurazan - A propellant ingredient

Abstract

The equilibrium molecular structure of the first pernitroheterocycle, 3,4-dinitrofurazan, in terms of the dynamic model with relaxation of all structural parameters, has been calculated for the first time using gas-phase electron diffraction (GED) with coupled cluster calculations up to CCSD(T)-AE/aug-cc-pwCVQZ level of theory. A model with C2 symmetry was found to be the best match for the observed scattering intensities. The structural parameters of 3,4-dinitrofurazan were compared to those of the parent furazan molecule calculated at the CCSD(T)-AE/aug-cc-pwCVQZ level of theory. The mean amplitudes and vibrational corrections necessary for the GED analysis were computed at the B3LYP/SNST//MP2/aug-cc-PVTZ level of theory using quadratic and cubic force fields. With the help of several topological, magnetic, and orbital descriptors, peculiar characteristics of the electronic and geometric structure of 3,4-dinitrofurazan were studied. Comparison of the structures obtained at the CCSD(T)-AE/aug-cc-pwCVQZ and CCSD(T)-AE/cc-pwCVQZ levels shows that, when going from furazan to 3,4-dinitrofurazan, the presence of the electron-withdrawing NO2 group decreases the bond lengths and increases the C–C=N bond angles of the furazan ring. According to ICSSZZ aromaticity descriptors, the aromaticity of furoxan and 3,4-dinitrofurazan is lower than that of furazan. This is in agreement with the calculated ring current densities induced by an external magnetic field.