Reactions of photogenerated fluorine atoms with molecules trapped in solidargon 3. Formation of stabilized (NH2)-N-center dot radicals and (NH2)-N-center dot-HF radical-molecule complexes in the reactions of fluorine atoms with NH3 molecules

Isolated radicals (NH2)-N-. and radical-molecule complexes (NH2)-N-.-HF, wh ich are products of the reactions of mobile fluorine atoms with NH3 molecul es in solid argon, were identified by EPR spectroscopy. The isotropic HFC c onstants of the complex (a(N) = 1.20, a(H) = 2.40, and a(F) = 0.70 mT) were determined experimentally. The constant of isotropic HFC with the nucleus of hydrogen atom of the HF molecule is less than 0.1 mT. This assignment wa s confirmed in the experiments on isotope substitution of atoms (H --> D, N -14 --> N-15) in the NH3 molecule. According to quantum-chemical calculatio ns, the free complex (NH2)-N-.-HF has a planar structure with C-2 nu symmet ry and a binding energy of 12 kcal mol(-1). Optimization of the arrangement of the complex in the crystal showed that its structure is only slightly d istorted in the Ar lattice so that the equilibrium configuration is close t o that obtained from gas-phase calculations. Different ratios of relative i ntensities of the proton triplet lines in the EPR spectra of isolated (NH2) -N-. radicals and (NH2)-N-.-HF complexes were qualitatively explained by di fferent heights of the barriers to rotation of the NH2 fragment in the Ar l attice.