Sergei V. Chapyshev

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

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.

Matrix Isolation and EPR Spectroscopy of Septet 3,5-Difluoropyridyl-2,4,6-trinitrene

Septet 3,5-difluoropyridyl-2,4,6-trinitrene along with quintet 2-azido-3,5-difluoropyridyl-4,6-dinitrene, quintet 4-azido-3,5-difluoropyridyl-2,6-dinitrene, triplet 2,6-diazido-3,5-difluoropyridyl-4-nitrene, and triplet 2,4-diazido-3,5-difluoropyridyl-6-nitrene have been obtained by photolysis of 2,4,6-triazido-3,5-difluoropyridine in solid argon at 4 K. The electronic and magnetic properties of the matrix-isolated nitrenes were studied using electron paramagnetic resonance (EPR) spectroscopy in combination with density functional theory (DFT) calculations. The fine-structure parameters of the nitrenes were determined with high accuracy from computer spectral simulations. All signals in the EPR spectra of the nitrenes randomly oriented in the solid phase were unambiguously assigned on the basis of eigenfield calculations of the Zeeman energy levels and angular dependencies of resonance fields from the direction of the applied magnetic field.

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).

Molecular Structure and Magnetic Parameters of Septet 2,4,6-Trinitrenotoluene

Septet 2,4,6-trinitrenotoluene is the major paramagnetic product formed during the photolysis of 2,4,6-triazidotoluene in cryogenic matrices. This trinitrene displays different electron paramagnetic resonance (EPR) spectra in solid argon and in 2-methyltetrahydrofuran (2MTHF) glass, corresponding to septet spin states with the zero-field splitting (ZFS) parameters D(S) = -0.0938 cm(-1), E(S) = -0.0040 cm(-1) and D(S) = -0.0934 cm(-1), E(S) = -0.0015 cm(-1), respectively. Analysis of these parameters shows that the molecular and electronic structure of the septet trinitrene derived from the EPR spectrum in argon is in good agreement with the expectations from DFT calculations. The very small parameter E(S) in 2MTHF glass is explained by significant changes of the spin densities on the three nitrene units due to interactions of the nitrogen atom with surrounding 2MTHF molecules.

High resolution electron paramagnetic resonance spectroscopy of quintet pyridyl-2,6-dinitrene in solid argon: Magnetic properties and molecular structure

The high resolution X-band electron para magnetic resonance (EPR) spectrum of quintet pyridyl-2,6-dinitrene was recorded after the photolysis of 4-amino-2,6-diazido-3,5-dichloropyridine in solid argon matrix at 15 K. This spectrum represents a new type of powder EPR spectra that are characteristic for quintet spin states with zero-field splitting parameters |E(q)/D(q)| approximately 1/4. All EPR lines of the quintet dinitrene were unambiguously assigned based on the eigenfield calculations of the Zeeman energy levels and angular dependencies of resonance magnetic fields. Owing to the high resolution of the experimental EPR spectrum, zero-field splitting parameters of the quintet dinitrene were determined with a high accuracy: D(q)=0.2100+/-0.0005 cm(-1) and E(q)=-0.0560+/-0.0002 cm(-1). These parameters provide correct information regarding the molecular angle Theta and distance r between two triplet sites in the molecule of quintet dinitrene. The measured molecular angle Theta=114.2 degrees+/-0.2 degrees is in excellent agreement with results of the density functional theory calculations. The analysis of the magnetic parameters shows that the spin population on the nitrene units in the quintet dinitrene is greater than that on the nitrene unit in the triplet nitrene.

Matrix Isolation, Zero-Field Splitting Parameters, and Photoreactions of Septet 2,4,6-Trinitrenopyrimidines

The key intermediates of decomposition of high-energy 2,4,6-triazidopyrimidine and its 5-chloro-substituted derivative, the detonation of which is used for preparation of carbon nitrides, were investigated using electron paramagnetic resonance (EPR) spectroscopy in combination with quantum chemical calculations. The decomposition of the triazides was carried out photochemically, using the matrix isolation technique. The photodecomposition of both triazides with 254 nm light in argon matrices at 5 K occurred selectively to subsequently give the corresponding triplet 4,6-diazido-2-nitrenopyrimidines, quintet 4-azido-2,6-dinitrenopyrimidines, and septet 2,4,6-trinitrenopyrimidines. The latter were photochemically unstable and decomposed to form triplet nitrenes NCN and NNC as well as triplet carbenes NCCCN, HCCN, and HCCCCN. The results obtained provide important information about exchange interactions in high-spin nitrenes with the pyrimidine ring and the mechanism of the formation of carbon nitrides during thermolysis of 2,4,6-triazidopyrimidine.

Heavy atom effect on magnetic anisotropy of matrix-isolated monobromine substituted septet trinitrene.

The heavy atom effect on the magnetic anisotropy of septet trinitrenes is reported. Septet 1-bromo-3,5-dichloro-2,4,6-trinitrenobenzene (S-1) was generated in a solid argon matrix by ultraviolet irradiation of 1,3,5-triazido-2-bromo-4,6-dichlorobenzene. This trinitrene displays an electron spin resonance (ESR) spectrum that drastically differs from ESR spectra of all previously studied septet trinitrenes. The zero-field splitting (ZFS) parameters, derived from the experimental spectrum, show the parameter |D| = 0.1237 cm(-1) and the unprecedentedly large ratio of E/D = 0.262 that is close to the rhombic limit E/D = 1/3 for high-spin molecules. The CASCI (based on state-averaged CASSCF) and DFT methods were applied to calculate the ZFS tensor focusing on the heavy (bromine) atom effects on the spin-orbit term. These calculations show that the multiconfigurational ab initio formalism and the CASCI method are the most successful for accurate predictions of the spin-orbit term in the ZFS tensor of high-spin nitrenes containing heavy bromine atoms. Due to the presence of the bromine atom in S-1, the contribution of the spin-orbit term to the total parameter D is dominant and responsible for the unusual orientation of the easy Z-axis lying in the molecular plane perpendicular to the C-Br bond. As a result, the principal values D(XX), D(YY), and D(ZZ) of the total tensor D̂(Tot) have such magnitudes and signs for which the ratio E/D is close to the rhombic limit, and the total parameter D is large in magnitude and positive in sign.

Matrix isolation ESR spectroscopy and magnetic anisotropy of D3h symmetric septet trinitrenes

The fine-structure (FS) parameters D of a series of D3h symmetric septet trinitrenes were analyzed theoretically using density functional theory (DFT) calculations and compared with the experimental D values derived from ESR spectra. ESR studies show that D3h symmetric septet 1,3,5-trichloro-2,4,6-trinitrenobenzene with D = -0.0957 cm(-1) and E = 0 cm(-1) is the major paramagnetic product of the photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in solid argon matrices at 15 K. Trinitrenes of this type display in the powder X-band ESR spectra intense Z1-transition at very low magnetic fields, the position of which allows one to precisely calculate the parameter D of such molecules. Thus, our revision of the FS parameters of well-known 1,3,5-tricyano-2,4,6-trinitrenobenzene [E. Wasserman, K. Schueller, and W. A. Yager, Chem. Phys. Lett. 2, 259 (1968)] shows that this trinitrene has [line]D[line] = 0.092 cm(-1) and E = 0 cm(-1). DFT calculations reveal that, unlike C2v symmetric septet trinitrenes, D3h symmetric trinitrenes have the same orientations of the spin-spin coupling tensor D[^]SS and the spin-orbit coupling tensor D[^]SOC and, as a result, have negative signs for both the DSS and DSOC values. The negative magnetic anisotropy of septet 2,4,6-trinitrenobenzenes is considerably strengthened on introduction of heavy atoms in the molecules, owing to an increase in contributions of various excitation states to the DSOC term.

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

Summary In contrast to theoretical expectations, the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine in argon at 5 K gives rise to EPR peaks of just two triplet mononitrenes, two quintet dinitrenes, and a septet trinitrene. EPR spectral simulations in combination with DFT calculations show that observable nitrenes can be assigned to triplet 2,4-diazido-3-chloro-5-fluoropyridyl-6-nitrene (D T = 1.026 cm−1, E T = 0), triplet 2,6-diazido-3-chloro-5-fluoropyridyl-4-nitrene (D T = 1.122 cm−1, E T = 0.0018 cm−1), quintet 4-azido-3-chloro-5-fluoropyridyl-2,6-dinitrene (D Q = 0.215 cm−1, E Q = 0.0545 cm−1), quintet 2-azido-3-chloro-5-fluoropyridyl-4,6-dinitrene (D Q = 0.209 cm−1, E Q = 0.039 cm−1) and septet 3-chloro-5-fluoropyridyl-2,4,6-trinitrene (D S = −0.1021 cm−1, E S = −0.0034 cm−1). Preferential photodissociation of the azido groups located in ortho-positions to the fluorine atom of pyridines is associated with strong π-conjugation of these groups with the pyridine ring. On photoexcitation, such azido groups are more efficiently involved in reorganization of the molecular electronic system and more easily adopt geometries of the locally excited predissociation states.

Synthesis, thermolysis, and mass spectrometry of perfluorinated di- and triazidopyridines

Abstract2,4-Diazido-3,5,6-trifluoropyridine and 2,4,6-triazido-3,5-difluoropyridine were obtained by the reaction of pentafluoropyridine with sodium azide in aqueous acetone. Under the action of electron impact the 2,4-diazidopyridine undergoes sequential fission of the azide groups in positions 2 and 4 of the pyridine ring and ring contraction with the formation of a characteristic [M-2N2-F]+ ion. On the other hand thermolysis of the same diazide is accompanied by the selective decomposition of its γ-azide group with the formation of 4-amino-2-azido-3,5,6-trifluoropyridine. The effect of selective decomposition of the azide groups in 2,4-diazidopyridines on thermolysis and under electron impact is caused by the different distribution of bonding orbital density at the α- and γ-azide groups respectively in the initial diazide and its radical cation. One of the routes of the triazide decomposition under electron impact is the form! ation of the [M-N22]+ ion due to decomposition of the α-azide group.