The structure of dinitrogen tetroxide N2O4: Neutron diffraction study at 100, 60, and 20 K and ab initio theoretical calculations

Single crystal neutron diffraction studies are reported for cubic dinitrogen tetroxide (space group Im3; Z = 6). The crystals were grown from ${\mathrm{NO}}_2$ vapor in situ on the diffractometer by precise cryostatic control. The lattice parameter, measured at seven temperatures, increases from 7.6937(6) Å at 20 K to 7.7925(6) Å at 140 K. The nuclear positional and thermal parameters were refined using diffraction data $(\sin {\text{\hspace{0.17em}ϑ/λ⩽0.79\hspace{0.17em}Å}}^{\text{−1}})$ measured at 20, 60, and 100 K. Final fit indices ${\mathrm{R(F}}^2)$ are 0.028, 0.034, 0.037, respectively. The observed N–N bond length and O–N–O angle are invariant between 20 and 100 K at values 1.7562(±4) Å and 134.46(±6)° with individual e.s.d.’s of 0.001 Å and 0.1°. The observed N–O bond length increases linearly from 1.1855(9) Å at 100 K to 1.1893(5) Å at 20 K; the extrapolated zero‐point value is 1.191 Å. Ab initio self‐consistent field calculations using a two‐configuration wave function which allows partial occupation of the σ* MO (antibonding with respect to the two nitrogens) yields a reasonable value for the N–N bond length: 1.80 Å is obtained when the 4‐31G basis is employed, and an estimate for the effect of polarization functions leads to a corrected value of 1.74 Å.