text: In chemistry, one usually ignores distinctions between isotopes of the same element. However, in some situations (for instance in Raman, NMR, or microwave spectroscopy) one may treat different isotopes of the same element as different elements. In the second case, two molecules with the same number of atoms of each isotope but distinct bonding schemes are said to be structural isotopomers.
  Thus, for example, ethene would have no structural isomers under the first interpretation; but replacing two of the hydrogen atoms (H) by deuterium atoms (H) may yield any of two structural isotopomers (1,1-dideuteroethene and 1,2-dideuteroethene), if both carbon atoms are the same isotope.  If, in addition, the two carbons are different isotopes (say, C and C), there would be three distinct structural isotopomers, since 1-C-1,1-dideuteroethene would be different from 1-C-2,2-dideuteroethene.)  And, in both cases, the 1,2-dideutero structural isotopomer would occur as two stereo isotopomers, cis and trans.