Preparation and Characterization of Metastable trans-Dinitrogen Tetroxide

Under atmospheric conditions, NO₂ is in equilibrium with its dimers, N₂O₄, which can exist in the form of constitutional isomers and stereoisomers whose relative stabilities and reactivities are still being debated. Experimental limitations facing the spectroscopic characterization of the isomers of N₂O₄ prevent us from determining their relative contributions to reaction mechanisms possibly causing discrepancies in the reported reaction orders and rates. Using reflection-absorption infrared spectroscopy, molecular beam deposition, and matrix isolation techniques, it is shown that the relative abundances of NO₂ and its dimers can be controlled by heating or cooling the deposited gas. The comparison of spectra acquired from samples prepared using molecular beam deposition with those obtained using tube dosing deposition demonstrates how the N₂O₄ isomer distributions are sensitive to details of the experimental conditions and sample preparation protocols. These observations not only provide a better understanding of a possible source for the disagreements found in the literature, but also a methodology to control and quantify the chemical speciation in NO₂ vapors in terms of the relative abundances of NO₂ and of the various isomers of N₂O₄.