The weakly bound dinitrogen tetroxide molecule: High level single reference wavefunctions are good enough

Ab initio studies of dinitrogen tetroxide (N2O4) have been performed to predict the equilibrium geometry, harmonic vibrational frequencies, and fragmentation energy (N2O4→2 NO2). The structure was optimized at the self-consistent field, configuration interaction, and coupled-cluster levels of theory with large basis sets. At the highest level of theory, the N–N bond distance was 1.752 Å, in excellent agreement with the experimental value of 1.756±0.01 Å. In addition, the harmonic vibrational frequencies were predicted with an average absolute error of 51 cm−1 relative to experimental fundamental values with differences largely attributed to anharmonic effects. The fragmentation energy corrected for zero point vibrational energy and basis set superposition error was 7.2 kcal/mol, in fair agreement with the experimental value of 12.7 kcal/mol. Despite the suggestion that a multireference wavefunction may be necessary to accurately describe the biradical nature of N2O4, single reference treatments with large basis sets and high levels of electron correlation yield molecular parameters remarkably close to experimental values.