ESR of N2− in uv-Irradiated Single Crystals of Anhydrous Barium Azide

Abstract

Single crystals of anhydrous barium azide have been irradiated at room temperature with 2537 Å ultraviolet light. Upon cooling to 77°K, an ESR spectrum is observed, the angular variation of which is expressed by the spin-Hamiltonian H = βH·g·S + I·A·S with gx≈gy = 1.997 ± 0.001, gz = 1.979 ± 0.0001, Ax = 3.6 ± 0.2 G, Ay = 20.0 ± 0.2 G (see text), Az = 4.1 ± 0.2 G. The spectrum is interpreted in terms of the theoretical predictions of Gelerinter and Silsbee for an N2− molecular ion in a field of rhombic symmetry. The ESR data are used to speculate on the orientation of the azide ions in the Ba(N3)2 lattice, the structure of which is as yet unknown. The optical absorption spectrum of thin single crystals in the range 200-300 mμ has been measured at room temperature and is correlated with the wavelength threshold for production of the paramagnetic center. It is postulated that the initial step in production of the center is the creation of an excited azide ion by the ultraviolet light.

Reductive Activation of the Nitrogen Molecule at the Surface of “Electron-Rich” MgO and CaO. The N2- Surface Adsorbed Radical Ion

“…However, on the basis of theoretical calculations (vide infra), the only reasonable alternative is A yy > 0, while A xx and A zz < 0, since all other combinations lead to unacceptable results. The choice of signs implies that a iso > 0, and it is also consistent with that adopted for the hyperfine analysis of N 2 - trapped in various single crystals, − for which a iso is expected to be positive . The largest value of the hyperfine tensor corresponds to the y direction determined by π y *, i.e., the orbital constituting the SOMO.…”

“…The adsorbed species identified on MgO and CaO can be classified in terms of a true N 2 - radical anion, and in the limits of a “chemical” approach, this represents the best description for the observed species. This anion has been previously observed only in the bulk of solid (matrixes including single crystals of various azides (K, Na, Ba) − or alkali-halides) and formed by the photodecomposition of N 3 - . The reported g and A tensors were remarkably similar to those found in this work (Table ).…”

Nitrogen Fixation via Splitting into Nitrido Complexes

“…Discrete redox steps are formalized by the N 2 redox series (Table ) that ranges from neutral N 2 , via diazenide N 2 2– , which is isoelectronic with O 2 and likewise adopts a triplet (S = 1) ground state, all the way through to the highly reduced hydrazide N 2 4– ion (S = 0). A limited number of compounds with trianionic radical ligands (N 2 3– , S = 1 / 2 ) have been reported, and monoanionc N 2 – (S = 1 / 2 ) has only been confirmed in solid matrices at low temperatures, as might be expected from the low electron affinity of N 2 . − The free radical HN 2 is unknown, and computational analysis suggests exothermic decay into N 2 and H atoms (Δ E = 9 kcal·mol –1 ) . This observation emphasizes the unfavorable nature of one-electron N 2 -reduction as a general bottleneck for N 2 -functionalization chemistry.…”

N2− Radical Anions Trapped in Bulk Polycrystalline MgO

“…The N 2 anion has been previously observed in the bulk of solid matrices including single crystals of various azides (K, Na, Ba) [4][5][6] or alkali halides. 7 In general, the reported g and A tensors for these systems are remarkably similar to those found in this work (Table 1).…”

“…As an example, N 2 impurities have been reported in the case of irradiated azides and trapped at negative anion vacancies in different alkali halides. [4][5][6][7] In particular, the N 2 species in KCl has been fully characterized by electron paramagnetic resonance (EPR) spectroscopy a long time ago, becoming soon a classical example of a molecular radical ion trapped in a solid. 7 The formation of N 2 radical anions has also been reported to occur at the surface of "electronrich" alkaline earth oxides 8 (MgO and CaO) via a direct surfaceto-adsorbate electron transfer; [9][10][11][12] however no reports are available on the identification of this radical anion in the bulk of alkaline earth oxides.…”