On the structure and the antisymmetric ν3 IR mode of covalent azide group

► The IR stretching ν3 mode frequency and the bond length asymmetry are correlated in organic and inorganic azides. ► An analytical expression as ν3 = 1997 + 1.17 × 103 Δr with R = 0.972 is gained on the database of more than fifty azides. ► The blue shift Δν3 from the free azide value varies in direct proportion with the N3 group asymmetry.