\LtetrahedralS{0==N;2==H$\bf_3$C;3==H$\bf_3$C;
1s==\put(25,25){\sixheteroh[bdf]{}{1==(yl);1==;
4==\tetrahedral{0==N;2==(yl);
4D==\tetrahedral{0==N;2==(yl);
4==\sixheteroh[bdf]{}{1==(yl);4==SO$\bf_3^-$}[]
}}
}[]}
}
\put(550,-50){\sffamily\bfseries a}
\hspace{7cm}
\LtetrahedralS{0==N$^+$;2==H$\bf_3$C;3==H$\bf_3$C;
1s==\put(90,25){\sixheteroh[be]{}{1==(yl);1D==;
4D==\tetrahedral{0==N;2==(yl);
4==\tetrahedral{0==NH;2==(yl);
4==\sixheteroh[bdf]{}{1==(yl);4==SO$\bf_3^-$}[]
}}
}[]}
}
\put(600,-50){\sffamily\bfseries b}