$(I)$ $ CH_3 - NO_2$
$(II)$ $NO_2 - CH_2 - NO_2$
$(III)$ $ CH_3 - CH_2 - NO_2 $
$(IV)$ $\begin{array}{*{20}{c}}
{N{O_2} - CH - N{O_2}} \\
{|\,\,\,\,\,\,} \\
{\,N{O_2}}
\end{array}$
$(1) H_3C - C = C⊝\,(2) H - C = C⊝ \,(3) $ ${H_3}C\,\, - \,\,\mathop C\limits^\Theta {H_2}$
