A density functional theory study of the zero-field splitting in high-spin nitrenes

Abstract

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn-Sham and the Pederson-Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%-12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of D(SS) into n-center (n=1-4) interactions shows that the major contribution to D(SS) results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S=2 and 3, if their molecules are constructed from the first-row atoms.

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

“…The most realistic theoretical estimation of E Q in dinitrene 16 is obtained from PBE/DZ calculations of the spin–spin interaction parameter E Q SS (Table 3). This fact indicates that the contribution of the spin–orbit interactions to the total parameter E Q of dinitrene 16 is quite small, as is typical for most of quintet dinitrenes [32]. …”

“…Previous studies of nitrenes 6a – c have shown that the most accurate theoretical evaluations of D S and E S by the DFT approach were obtained by using the PBE and BLYP functionals in combination with the DZ or EPRII basis sets [32]. Extensive DFT calculations performed in the present work show that the best agreement between the experiment and theory is observed at the BLYP/EPRII level of theory, overestimating the experimental D S and E S values of trinitrene 18 by only 6% (Table 2).…”

“…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]. Such calculations are also the most accurate for estimations of D Q in dinitrene 15 (Table 2).…”

“…3). Unfortunately, as in the case of many other triplet mononitrenes [32], such calculations are too crude for estimations of the parameters E T , predicting E T = 0.006 ± 0.001 cm −1 for all isomers of 12 – 14 .…”

“…Nowadays, the EPR spectral identification of high-spin nitrenes is based on comparison of their zero-field splitting (ZFS) parameters derived from experimental EPR spectra and calculated by quantum chemistry methods. The most accurate theoretical estimations are obtained at the PBE/DZ level of theory, which overestimates the experimental ZFS values of nitrenes by only about 10% [32]. This accuracy of calculations is sufficient for reliable identification of isomeric dinitrenes 4a – c and 5a – c formed during the photolysis of triazides 1a – c (Scheme 1).…”