Paradigms and paradoxes: why is the electron affinity of the azide radical, N3, so large?

Abstract

The consensus value for the electron affinity of azide radical is 261 kJ mol -1 , anomalously higher than many species that are also made of highly electronegative elements. N 3 -has two equivalent resonance structures analogous to NO 2 -that has a lower electron affinity. Electronegativity trends rationalize why the electron affinity of N 3 is higher than that of P 3 ; however, those of N and N 2 are lower than those of P and P 2 . We suggest the reason for the observed high electron affinity of azide radical is Coulombic stabilization in the ionic triplet resonance structure, -N=N ? =N -.The chemistry of azides has been explored for more than a century since the discovery of organic azides by P. Griess and hydrazoic acid by T. Curtis [1]. The area of azides can formally be divided into organic and inorganic azides [2][3][4][5]. Most organic azides of the type R-N 3 as well as many non-metal inorganic azides such as HN 3 and XN 3 (X = F, Cl, Br, I) are of predominantly covalent nature and contain an asymmetric and slightly bent azide group. Decomposition of these covalent azides proceeds as a result of rupture of the longest N-N bond which is the N a -N b bond (connectivity: R-N a -N b -Nc) (Eq. 1). Many of the covalently bonded organic and non-metal azides are explosive. In contrast, purely ionic metal azides of the type M ? N 3 -(e.g., NaN 3 , KN 3 ) contain a linear and symmetrical azide anion and are usually not explosive. Covalent binding of the type which produces an asymmetric azide group occurs when the ionization potential of the metal is greater than ca. 9.0 eV [6]. Such metal azides with a significant covalent character are often thermally less stable and are usually more sensitive than the ionic azide salts. Sodium and potassium azide, for example, are comparatively stable, whereas the azides of copper and lead are quite sensitive as shown by their explosive character. It has been shown that the ionization potential of the metal forming the compound is increased in the series KN 3 , TlN 3 , AgN 3 , CuN 3 so the valence electron becomes more and more under the influence of the metal. This is also shown by the fact that the M-N crystallographic distance diminishes in this order. Moreover, the increasing ionization potential of the metal atom-forming compounds in the series KN 3 , TlN 3 , AgN 3 , CuN 3 results in a decrease in the activation energy for the formation of a neutral azide group, hence, in contrast to the above discussed covalent organic and non-metal azides these explosive metal azides form the metal M and an azide This paper is dedicated to H. Donald B. Jenkins on the occasion of his ''official'' retirement.

Group Transfer to an Aliphatic Bond: A Biomimetic Study Inspired by Nonheme Iron Halogenases

“…Indeed, the electron affinity of N 3 was shown to be considerably larger than that of NO 2 . 38 Although there is no experimental data on the ground state [Fe IV (O)(TPA)(X)] + (X = NO 2 − /N 3 − ) species, Mossbauer spectroscopy and EXAFS (extended X-ray absorption finestructure) methods strongly indicate a ground-state triplet when the chloride anion is present. 39 In all cases, we find that the singlet and septet spin states are higher in energy, and therefore, those results are relegated to the Supporting Information.…”

“…As such, the N 3 – ligand is a weaker electron donor and lowers the σ* x 2– y 2 orbital over that found for X = NO 2 – . Indeed, the electron affinity of N 3 was shown to be considerably larger than that of NO 2 …”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 21, 2010) and the discipline

“…These authors discuss metal azides. We note a recent paper that discusses the anomalously high electron affinity of azido radical [175] and thus the stability of species with the azide ion such as these metal azides. In the theoretical study by Wu et al [177], DFT was used for the purpose of studying the structures and electronic properties of endohedral metallofullerenes TM@C 24 and TM@C 24 H 12 (TM = Cr, Mo, W).…”

Interplay of thermochemistry and Structural Chemistry, the Journal (volume 22, 2011, issues 1–3) and the discipline

“…Klapötke et al [58] studied high electron affinity of the azide radical 261 kJ mol -1 , which is unusually high and being comparable to that of atomic or diatomic halogens. Coulombic stabilization in the ionic triplet resonance structure, of -N=N ?…”