text: Enumerating or counting structural isomers in general is a difficult problem, since one must take into account several bond types (including delocalized ones), cyclic structures, and structures that cannot possibly be realized  due to valence or geometric constraints, and non-separable tautomers.
  For example, there are nine structural isomers with molecular formula CHO having different bond connectivities. Seven of them are air-stable at room temperature, and these are given in the table below.
  Two structural isomers are the enol tautomers of the carbonyl isomers (propionaldehyde and acetone), but these are not stable.