THE WEAKLY-BOUND DINITROGEN TETROXIDE MOLECULE - HIGH-LEVEL SINGLE REFERENCE WAVE-FUNCTIONS ARE GOOD ENOUGH

Ab initio studies of dinitrogen tetroxide (N2O4) have been performed t o predict the equilibrium geometry, harmonic vibrational frequencies, and fragmentation energy (N2O4-->2NO(2)). The structure was optimized at the self-consistent field, configuration interaction, and coupled-c luster levels of theory with large basis sets. At the highest level of theory, the N-N bond distance was 1.752 Angstrom, in excellent agreem ent with the experimental value of 1.756 +/- 0.01 Angstrom. In additio n, the harmonic vibrational frequencies were predicted with an average absolute error of 51 cm(-1) relative to experimental fundamental valu es with differences largely attributed to anharmonic effects. The frag mentation 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 mul tireference wavefunction may be necessary to accurately describe the b iradical nature of N2O4, Single reference treatments with large basis sets and high levels of electron correlation yield molecular parameter s remarkably close to experimental values. (C) 1997 American Institute of Physics.