1 $\begin{array}{*{20}{c}}
{Ph - C - O{C_2}{H_5}}\\
{||\,\,\,\,\,\,\,\,\,\,\,}\\
{{O_{18}}\,\,\,\,\,}
\end{array}$
2 $\begin{array}{*{20}{c}}
{{C_2}{H_5} - C - \mathop O\limits^{18} - Ph}\\
{||\,\,\,\,\,\,}\\
{O\,\,\,\,\,\,}
\end{array}$
3 $\begin{array}{*{20}{c}}
{{C_2}{H_5} - C - OPh}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{O^{18}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}$
4 $\begin{array}{*{20}{c}}
{Ph - C - \mathop O\limits^{18} - {C_2}{H_5}}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}$