15N14N

15N14N is the diatomic molecule formed by one nitrogen-15 nucleus and one nitrogen-14 nucleus. It is an isotopologue of molecular nitrogen (N2), sharing the same electronic structure and chemical behavior, including a triple bond and a ground electronic state of X1Σg+. The substitution of one 14N with 15N affects mainly the molecular mass, producing only small changes in the rovibrational spectra through a different reduced mass.

Spectroscopic properties of 15N14N differ from those of 14N14N primarily in terms of reduced mass, which shifts vibrational and rotational constants slightly. The bond length is essentially the same, so the electronic structure remains that of N2. Because both nuclei carry the same electric charge, 15N14N has, to first approximation, no permanent electric dipole moment. Consequently, pure rotational transitions and infrared-active vibrational transitions are extremely weak or forbidden; rovibrational spectra arise mainly from higher-order processes such as quadrupole transitions or Raman scattering. Nevertheless, spectroscopic data for all N2 isotopologues allow measurement of isotopic effects and, in some cases, extraction of isotopic ratios.

Natural abundance and occurrence: 15N has about 0.366% natural abundance, while 14N is about 99.634%. Therefore, in a sample of molecular nitrogen, approximately 0.7% of N2 molecules are the mixed 15N14N isotopologue, about 0.001% are 15N15N, and the remainder are 14N14N. This distribution makes 15N14N relevant for studies of nitrogen fractionation and isotopic composition in chemistry, geology, and astrophysics.