ESR Spectrum and Structure of HNO2− in KCl

Abstract

If a KCl melt containing traces of OH− and CN− is subjected to controlled oxygenation, then ultraviolet photolysis of crystals grown from this melt produces the HNO2− radical anion as a substitutional impurity. The radical is also produced by photolysis of KCl:OH−:NO2− crystals. Formation of this radical is attributed to reaction of H atoms, produced by photolytic decomposition of OH−, with NO2− ions, which in the former case are produced by oxidation of CN−. At 77°K the electron spin resonance (ESR) spectrum of this radical is an isotropic nitrogen hyperfine structure (hfs) triplet. At 4°K, however, the spectrum is anisotropic and contains both nitrogen and hydrogen hfs. The magnetic constants of HNO2− are: g1 = 2.0020, g2 = g3 = 2.0074; A1(N) = 39.2, A2(N) = A3(N) = 12.0 Oe; and A1(H) = − 5.0, A2(H) = − 12.5, A3(H) = 13.0 Oe. Axis 2 is perpendicular to the only symmetry plane of the slightly nonplanar molecule. Axes 1 and 3 are, respectively, approximately perpendicular and parallel to the NH bond. Axes 1 and 2 lie in the same {110} crystal plane, and axis 1 makes a 26.6° angle with the (100) crystal axis contained in this plane. At roughly 25°K a spectrum of (111) symmetry is observed which is the result of motional averaging about that (111) crystal axis which makes the smallest angle with molecular axis 2. It can be shown from the nitrogen and proton hfs constants that the deviation of the molecule from planarity is roughly 10°, and that the unpaired electron density on the nitrogen atom is 0.57.