Isotopids

An isotopid is a concept within theoretical chemistry that describes a molecule or ion that possesses the same number of electrons as a given reference molecule or ion, but with a different nuclear composition. Essentially, isotopids are isoelectronic species that differ in their isotopic makeup. For instance, if we consider the molecule N2, which has 14 electrons, an isotopid would be another species with 14 electrons where at least one atom has a different isotopic abundance than that found in typical N2. A simple example would be N2 where one or both nitrogen atoms are isotopically enriched, such as 15N2. This molecule still has 14 electrons (7 from each nitrogen atom), but its atomic nuclei are heavier than those in the standard 14N2.

The term "isotopid" is not as commonly used as related terms like "isotope" or "isoelectronic." It is primarily employed when discussing the subtle effects of isotopic substitution on molecular properties that are not directly related to electronic structure. These properties can include vibrational frequencies, rotational constants, and reaction rates. For example, the vibrational frequency of a bond can be slightly altered by replacing an atom with one of its heavier isotopes, leading to a new isotopid. While the electronic configuration remains the same, the mass difference influences the molecule's overall energy levels and dynamics.