Ab initio studies of the structure, vibrational frequencies, and intensities have been carried out on the open-chain species HNnH, HNnF, and FNnF (n=3,4). Particular attention has been focused on the species HN3H, which is isoelectronic with NO2 and exhibits many of the same features in its electronic structure. HN3H is shown to have a planar trans structure with R(N-N)=1.252 Angstroms, indicating considerable double-bond character. HN3H also exhibits a low-lying doublet A2 state separated from the ground state by about 36 kcal/mol. Replacement of one or more of the hydrogens in HN3H by fluorines alters the electron distribution, spin density, and geometry, most importantly the NNN angle. The biradical species N4 in its trans planar structure is shown to be of significantly higher energy than two N2 molecules.