General Chemistry and the NO2 /N2 O4 System

The NO2/N2O4 system is prevalent in general chemistry texts as a model for basic chemical concepts, including formal charges and exceptions to the octet rule. Nitrogen dioxide (NO2) is commonly used to illustrate deviations from the octet rule for chemical bonding in which a correct ground state structure must be chosen based on rules given in general chemistry texts. These rules are not absolute however, and unexpected subtleties must be taken into account in the presence of relevant data. Here we present this juxtaposition of the simple rules of chemistry and the alterations that experimental data elicit. Upon dimerization, what seems to be the correct structure of NO2 based on rules prescribed in general chemistry texts does not result in the known structure of the dimer. Several scenarios can explain this behavior and recent theoretical data from ELF/AIM studies of the NO2/N2O4 system provide an explanation. This topic proves to be an excellent enrichment teaching tool and an opportunity to expose students to the fact that chemistry concepts that have been taught for years are still pursued by researchers to gain a deeper understanding of the science behind them.