Preparation and Characterization of Metastable trans -Dinitrogen Tetroxide.

Under atmospheric conditions, NO 2 is in equilibrium with its dimers, N 2 O 4 , 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 N 2 O 4 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 NO 2 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 N 2 O 4 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 NO 2 vapors in terms of the relative abundances of NO 2 and of the various isomers of N 2 O 4 .