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

Abstract

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).

High-spin intermediates of the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine

“…Due to different substituents in positions 3 and 5 of the pyridine ring, trinitrene 18 has different angles Θ 1 , Θ 2 , Θ 3 between the nitrene C–N bonds and different Mulliken spin populations on the nitrene units (Table 1). Theoretically, such asymmetric trinitrenes should show rather large E S values [30]. Nevertheless, the E S value of trinitrene 18 is even smaller than that of C 2 v symmetric trinitrene 6c [31].…”

“…DFT calculations show that trinitrene 18 , on comparison with C 2 v symmetric trinitrenes 6a-c , has slightly different orientations of the principal magnetic axes due to the presence of only one heavy chlorine atom. Thus, the principal axes D XX SS and D XX Tot of C 2 v symmetric trinitrene 6a were directed along the line connecting the nitrene units in positions 2 and 6 of the pyridine ring and were parallel to the D ZZ SO axis of the spin–orbit tensor D SO connecting two heavy chlorine atoms in positions 3 and 5 of the pyridine ring [30]. In the case of trinitrene 18 , the D ZZ SO axis of the spin-orbit tensor D SO almost coincides with the C(3)–Cl bond (Fig.…”

“…Thus, for instance, quintet dinitrene 5a showed D Q = 0.202 cm −1 and E Q = 0.0554 cm −1 [30], and its difluoro-derivative 5b displayed D Q = 0.213 cm −1 and E Q = 0.0556 cm −1 [29]. The most accurate theoretical estimations of the magnetic parameters in dinitrene 5a – c were obtained by using the PBE/DZ calculations [32].…”

“…As a rule, triplet pyridyl-2-nitrenes show D T = 1.03 ± 0.02 cm −1 and E T ≈ 0 cm −1 , while triplet pyridyl-4-nitrenes show D T = 1.13 ± 0.02 cm −1 and E T ≈ 0 cm −1 [29–31]. When chlorine atoms are set in ortho -positions to the nitrene center, such triplet pyridylnitrenes show E T > 0.003 cm −1 [30]. Since one of triplet mononitrenes obtained in the present work shows D T = 1.026 cm −1 and E T = 0 cm −1 , this nitrene can safely be assigned to 12 (Scheme 2).…”

EPR Studies on Branched High‐Spin Arylnitrenes

“…Assuming that similar overestimations are also valid for D SS of the quintet dinitrenes 6 and 7 as well as of septet trinitrene 8, these nitrenes are predicted to show D values of~0.202, 0.157, and~0.089 cm À1 , respectively. [4][5][6][7][8]12] According to Equation (4), a decrease from V = 1208 by 18 results in an increase of j E S j by approximately 0.0011 cm À1 . Consequently, it seems that the photolysis of triazide 3 occurs selectively, generating subsequently triplet mononitrene 4, quintet dinitrene 6, and septet trinitrene 8.…”