W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting

Abstract

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm(-1), ET = 0.093 cm(-1), and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = - 0.306 cm(-1), EQ = 0.0137 cm(-1), and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = - 0.203 cm(-1), ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

“… The contribution of the spin‐orbit (SO) coupling to parameter D is one order of magnitude greater than the contribution of the spin–spin (SS) interaction. The main SO contribution is the SOMO→SOMO spin‐flip interaction between excited singlet and ground triplet states (Table S3). The PBE/Ahlrichs‐DZ level of theory is universal and describes the experimental results for all known triplet phosphinidenes (Table ), as well as for high‐spin nitrenes The calculated components ( A x and A y ) of hyperfine structure tensor A are similar to those obtained from the experimental spectrum, see Table . …”

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

“… The contribution of the spin‐orbit (SO) coupling to parameter D is one order of magnitude greater than the contribution of the spin–spin (SS) interaction. The main SO contribution is the SOMO→SOMO spin‐flip interaction between excited singlet and ground triplet states (Table S3). The PBE/Ahlrichs‐DZ level of theory is universal and describes the experimental results for all known triplet phosphinidenes (Table ), as well as for high‐spin nitrenes The calculated components ( A x and A y ) of hyperfine structure tensor A are similar to those obtained from the experimental spectrum, see Table . …”

“…Our interest in the EPR spectra of organic phosphinidenes was motivated by the study of the heavy‐atom effect on magnetic anisotropy as a continuation of our previous studies . Our initial EPR measurements in photolyzed frozen solutions with precursors 1 a and 1 b showed that the spectrum of photolytic product 2 (Scheme ) is significantly different from that reported in Ref.…”

“…The PBE/Ahlrichs‐DZ level of theory is universal and describes the experimental results for all known triplet phosphinidenes (Table ), as well as for high‐spin nitrenes …”

EPR spectroscopy of multicomponent, multispin molecular system obtained by the photolysis of 2,4,6‐triazido‐3‐cyano‐5‐fluoropyridine in solid argon

“…However, the best conditions for stabilization and further electron paramagnetic resonance (EPR) studies of all types of septet trinitrenes are achieved on works in solid inert gases at 4–16 K . Using this matrix‐isolation technique, it has been shown that the photolysis of 1,3,5‐triazidobenzenes results in the formation of septet trinitrenes in more than 95% yield . On contrast, the photolysis of 2,4,6‐triazido‐1,3,5‐triazine and 2,4,6‐triazidopyrimidines gives the corresponding septet trinitrenes only as transient species that undergo further photodecomposition to form triplet nitrenes NCN and CNN as well as triplet carbenes NCCCN, HCCN, and HCCCCN.…”