\centerline{ \structure{\nopositioncheck\atom{C}\esingle\atom{C}\nnwsingle \atom{C}\sswsingle\atom{C}} \hskip 1truecm \structure{\nopositioncheck\atom{C}\nsingle\atom{C}\sesingle \atom{C}\swsingle\atom{C}} \hskip 1truecm \structure{\nopositioncheck\atom{C}\nsingle\atom{C}\esingle \atom{C}\ssingle\atom{C}\wsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nwsingle\atom{C}\nesingle\atom{C}\sesingle \atom{C}\swsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nnwsingle\atom{C}\nesingle\atom{C}\sesingle \atom{C}\sswsingle\atom{C}\wsingle\atom{C}} } \medskip \centerline{ \structure{\atom{C}\nwsingle\atom{C}\nsingle\atom{C}\nesingle \atom{C}\sesingle\atom{C}\ssingle\atom{C}\swsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nnwsingle\atom{C}\nnesingle\atom{C}\esingle \atom{C}\ssesingle\atom{C}\sswsingle\atom{C}\wsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nnwsingle\atom{C}\nnesingle\atom{C}\esingle \atom{C}\sesingle\atom{C}\ssingle\atom{C}\swsingle\atom{C} \wsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nwsingle\atom{C}\nnwsingle\atom{C}\nesingle \atom{C}\esingle\atom{C}\sesingle\atom{C}\sswsingle\atom{C} \swsingle\atom{C}} } \medskip \centerline{ \structure{\atom{C}\nwsingle\atom{C}\nsingle\atom{C}\nesingle \atom{C}\esingle\atom{C}\sesingle\atom{C}\ssingle\atom{C} \swsingle\atom{C}\wsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nwsingle\atom{C}\nnwsingle\atom{C}\nsingle \atom{C}\nesingle\atom{C}\esingle\atom{C}\sesingle\atom{C} \ssingle\atom{C}\sswsingle\atom{C}\swsingle\atom{C}} \hskip 1truecm \structure{\atom{C}\nwsingle\atom{C}\nnwsingle\atom{C}\nnesingle \atom{C}\nesingle\atom{C}\esingle\atom{C}\sesingle\atom{C} \ssesingle\atom{C}\sswsingle\atom{C}\swsingle\atom{C}\wsingle \atom{C}} } \medskip\centerline{\bf Figure~2. \rm Three- to ten-membered carbon rings.}