What is the mechanism for the auto-dissociation of nitrogen tetraoxide
Dinitrogen tetroxide $\ce{N2O4}$ is a colourless liquid composed of planar dimers of nitrogen dioxide $\ce{NO2}$.$^{[1]\ \text{[a]}\ [2]}$



As Martin noted and is to be expected from the structure of $\ce{N2O4}$, the process

$$\ce{N2O4 &lt;=&gt; NO+ + NO3-}$$

is not very likely to happen. Autoionisation of $\ce{NO2}$ is more common$^{[1]}$.

$$\ce{NO2 -&gt; NO2+ +  e^-\\ NO2 +  e^- -&gt; NO2-}$$

Ionisation energy of $\ce{NO2}$ is $9.8\ \mathrm{eV}$ while its electron affinity equals $1.62\ \mathrm{eV}$.$^{[1]}$ Technically you could look at this as heterolytic bond dissociation $\ce{N2O4}$ but that is probably not what is happening. This autoionisation is, in a way, realised for aqueous solutions$^{[2]}$:

$$\ce{\underset{electron transfer}{NO2 + NO2} + H2O -&gt; \underset{really just HNO3}{NO2OH} +  HNO2},$$
$$\ce{\underset{electron transfer}{NO2 + NO2} + 2KOH -&gt; KNO3 + KNO2 +  H2O}.$$

Table 1. Melting and poiling points of $\ce{NO2}$ and $\ce{N2O4}$. Note the "equality" of melting points.$^{[2]\ [3]}$
$$\begin{array}{|c|c|c|}
\hline
\mathbf{Molecule} &amp; \mathbf{mp\ (\deg  \mathrm{C})} &amp;  \mathbf{bp\ (\deg  \mathrm{C})}\\
\hline
\ce{NO2} &amp; -11.2 &amp; 21 \\
\ce{N2O4} &amp; -11.2 &amp; 22 \\ 
\hline
\end{array}$$

Table 2. Percentage of  $\ce{NO2}$ and $\ce{N2O4}$ at various temperatures (pressure $1 \ \mathrm{atm}$).$^{[2]}$
$$\begin{array}{|c|c|c|c|}
\hline
\mathbf{Molecule} &amp; \text{below}\  \mathrm{mp}\ (\mathrm{\%}) &amp;  \mathbf{40\ \deg  \mathrm{C}\ (\mathrm{\%})} &amp; \text{over}\mathbf{\  150\deg  \mathrm{C}\ (\mathrm{\%})}\\
\hline
\ce{NO2} &amp; 0 &amp; 31 &amp; 100 \\
\ce{N2O4} &amp; 100 &amp; 69 &amp; 0 \\ 
\hline
\end{array}$$



$\text{[a]}$ To the best of my knowledge, the enthalpy $-55\ \mathrm{kJ}$ applies at $20 \deg \mathrm{C}$ and $1\  \mathrm{atm}$ for both components in the gas phase. NIST gives $-57.12\ \mathrm{kJ}$ at $25 \deg \mathrm{C}$ and $1\  \mathrm{bar}$. See $\ce{N2O4}$ and $\ce{NO2}$.



$[1]$ N. N. Ahmetov. Anorgaaniline keemia. (1974) (pages 375, 376)

$[2]$ H. Karik, Kalle Truus. Elementide keemia. (2003) (pages 424, 426)

$[3]$ Wikipedia:


Nitrogen dioxide
Dinitrogen tetroxide