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

A new correlation ν3 = 1997 + 1.17 × 103 Δr between the stretching mode ν3 of the IR band frequency and the bond length asymmetry expressed as Δr ≡ r(Nα − Nβ) − r(Nβ − Nγ) is established in molecular azides RNαNβNγ with the correlation coefficient R = 0.972. The database represents recent spectral and structural information for more than fifty organic molecules and inorganic metal complexes. Because the absolute term practically coincides with the ν3 frequency value for the free azide group, this correlation shows that the blue shift Δν3 (in cm−1) of the ν3 from the free azide value varies in direct proportion with the N3 group asymmetry Δr (in Å) (Δν3 = 1.17 × 103 Δr) at formation of different azide structures.