High resolution electron paramagnetic resonance spectroscopy of septet pyridyl-2,4,6-trinitrene in solid argon: Fine-structure parameters of six electron-spin cluster.

The high resolution 9 GHz electron paramagnetic resonance (EPR) spectrum of septet pyridyl-2,4,6-trinitrene was recorded after the photolysis of 2,4,6-triazido-3,5-dichloropyridine in solid argon matrix at 15 K. Owing to the high resolution of the experimental EPR spectrum, the zero-field splitting parameters of the septet trinitrene were determined with a high accuracy: D(s)=-0.1019+/-0.0004 cm(-1) and E(s)=0.003 25+/-0.000 15 cm(-1). All EPR transitions of the septet trinitrene were, for the first, unambiguously assigned based on the eigenfield calculations of the Zeeman energy levels. The spectrum of the septet trinitrene represents a new type of EPR spectra of septet spin states with nonzero zero-field splitting parameter E(s). The nonvanishing parameter E(s) of the septet trinitrene arises due to magnetic nonequivalence of three triplet centers in the molecule and is manifested in the appearance in the spectrum of separate x and y transitions. The septet spin states of this type display at very low magnetic fields two intense z transitions since the mid R:3D(s)mid R: energy gap between zero-field energy levels W(+/-1) and W(+/-2) fits the quantum of microwave irradiation of a 9 GHz EPR spectrometer. Analysis of the magnetic parameters shows that semiempirical description of the fine-structure tensor for six electron-spin cluster in the septet trinitrene is appropriate for precise estimations of the parameter D(s) but it is too crude to estimate small value of the parameter E(s).