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1.
Inorg Chem ; 58(24): 16434-16444, 2019 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-31749359

RESUMO

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac)2(bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (gx = 2.388, gy = 2.417, gz = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm-1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

2.
J Phys Chem A ; 122(45): 8931-8937, 2018 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-30359039

RESUMO

Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a ( D = 0.9930 cm-1, E = 0.0261 cm-1), 1c ( D = 1.244 cm-1, E = 0.030 cm-1), and 1d ( D = 1.369 cm-1, E = 0.093 cm-1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin-spin interactions ( DSS ∼ 1 cm-1), but drastically differ in the spin-orbit coupling parameter (from DSOC = 0.087 cm-1 for 1a to DSOC = 0.765 cm-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of DSOC of nitrenes 1a-f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a-1f, the smaller the N--Br distance and the larger DSOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.

3.
Dalton Trans ; 46(23): 7540-7548, 2017 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-28573307

RESUMO

We report a combined experimental characterization and theoretical modeling of the hexa-coordinated high-spin Co(ii) complex cis-[Co(hfac)2(H2O)2] (I). The magnetic static field (DC) data and EPR spectra (measurements were carried out on the powder samples of diluted samples cis-[Co0.02Zn0.98 (hfac)2(H2O)2]) were analyzed with the aid of the parametric Griffith Hamiltonian for the high-spin Co(ii) supported by the ab initio calculations of the crystal field (CF) parameters, g-factors and superexchange parameters between H-bonded Co(ii) ions in the neighboring molecules in a 1D network. This analysis suggests the presence of the easy axis of magnetic anisotropy and also shows the existence of a significant rhombic component. The detected frequency dependent (AC) susceptibility signal shows that complex I exhibits slow paramagnetic relaxation in the applied DC field belonging thus to the class of non-uniaxial field induced single ion magnets with a negative axial component of anisotropy. It is demonstrated that the main contributions to the relaxation come from the direct one-phonon process dominating at low temperatures, while the contribution of the two-phonon Raman process becomes important with increasing temperature.

4.
Inorg Chem ; 55(19): 9696-9706, 2016 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-27632142

RESUMO

In this article we report the synthesis and structure of the new Co(II) complex Et4N[CoII(hfac)3] (I) (hfac = hexafluoroacetylacetonate) exhibiting single-ion magnet (SIM) behavior. The performed analysis of the magnetic characteristics based on the complementary experimental techniques such as static and dynamic magnetic measurements, electron paramagnetic resonance spectroscopy in conjunction with the theoretical modeling (parametric Hamiltonian and ab initio calculations) demonstrates that the SIM properties of I arise from the nonuniaxial magnetic anisotropy with strong positive axial and significant rhombic contributions.

5.
J Phys Chem A ; 119(11): 2413-9, 2015 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-25350487

RESUMO

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.

6.
J Phys Chem A ; 117(33): 8065-72, 2013 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-23895219

RESUMO

The ESR spectrum of compact nitroxide (NO)-substituted nitronyl nitroxide (NN) triplet diradical N-tert-butyl-N-oxidanyl-2-amino-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) was recorded in solid argon matrix at 15 K. The zero-field splitting (ZFS) parameters of 1 were derived from the recorded ESR spectrum: |D| = 0.0248 cm(-1) and E = 0.0025 cm(-1). Quantum chemical calculations have been performed using DFT and multiconfigurational ab initio (CAS) methods in order to establish equilibrium geometries of the conformational isomers resulting from twisted conformations of NO and NN moieties. The ZFS parameters of 1 were calculated at these levels of theory to test validity of the calculated structures. The calculation results were analyzed using the measured ZFS parameters and magnetic and structural data from the previous studies (Suzuki, S.; et al. J. Am. Chem. Soc. 2010, 132, 15908; Tretyakov, E. V.; et al. Russ. Chem. Bull. 2011, 60, 2608). It was found that the ab initio method is most successful for accurate predictions of molecular and magnetic parameters. Diradical 1 has only one stable enantiomeric pair in pseudoeclipsed conformations. The two chiral isomers exist in racemic crystals 1 and in solid matrices with molecular parameters close to those attributed to a free molecule. The analysis of the spin density distribution suggests that one unpaired electron occupies NO group at the equilibrium geometry, whereas the torsion of NO group governs the spin density distribution of the second unpaired electron on a conjugated fragment in NN group. The increase in planarity by torsion of NO group enhances the trimethylenemethane-type properties and, therefore, gives rise to larger ferromagnetic exchange interaction. More planar equilibrium geometry and greater (three times) exchange interaction constant J were predicted for hypothetical diradical 1a, where bulky tert-butyl group is replaced by a methyl group in the nitroxide fragment.

7.
J Chem Phys ; 138(20): 204317, 2013 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-23742485

RESUMO

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.

8.
J Chem Phys ; 137(6): 064308, 2012 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-22897274

RESUMO

High-spin organic molecules with dominant spin-orbit contribution to magnetic anisotropy are reported. Quintet 4-azido-3,5-dibromopyridyl-2,6-dinitrene (Q-1), quintet 2-azido-3,5-dibromopyridyl-4,6-dinitrene (Q-2), and septet 3,5-dibromopyridyl-2,4,6-trinitrene (S-1) were generated in solid argon matrices by ultraviolet irradiation of 2,4,6-triazido-3,5-dibromopyridine. The zero-field splitting (ZFS) parameters, derived from electron spin resonance spectra, show unprecedentedly large magnitudes of the parameters D: ∣D(Q1)∣ = 0.289, ∣D(Q2)∣ = 0.373, and ∣D(S1)∣ = 0.297 cm(-1). The experimental ZFS parameters were successfully reproduced by density functional theory calculations, confirming that magnetic anisotropy of high-spin organic molecules can considerably be enhanced by the "heavy atom effect." In bromine-containing high-spin nitrenes, the spin-orbit term is dominant and governs both the magnitude and the sign of magnetic anisotropy. The largest negative value of D among septet trinitrenes is predicted for 1,3,5-trinitrenobenzene bearing three heavy atoms (Br) in positions 2, 4, and 6 of the benzene ring.

9.
Phys Chem Chem Phys ; 14(6): 2032-9, 2012 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-22234556

RESUMO

The ESR spectrum of 5-methylhexa-1,2,4-triene-1,3-diyl (1) was recorded in an argon matrix at 15 K. The derived zero-field splitting (ZFS) parameters (D = 0.5054 ± 0.0006 cm(-1) and E = 0.0045 ± 0.0002 cm(-1)) fall between those determined previously for propargylene (2) and vinylcarbene (3). DFT and ab initio (CAS and MRCI) quantum-chemical calculations of the ZFS parameters of 1, 2, and 3 were performed. These calculations indicate that multireference methods are needed to successfully predict ZFS parameters of delocalized carbenes/biradicals such as 1-3. The calculated singly occupied MOs and spin density distributions show that the spin is more delocalized in 1 than in 2 and 3, indicating that 1 is a "hybrid" of the constituent ethynyl- and vinylcarbenes, 2 and 3, respectively. The dominant contribution to the D-value in 1 and 2 is found to result from spin-spin interactions on the C atoms of the propylidene moiety, which is strongly affected by spin polarization. Accurate values for the D-parameter are also predicted for other types of delocalized triplet carbenes such as HC(5)H and HCCN.

10.
J Chem Phys ; 133(6): 064101, 2010 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-20707555

RESUMO

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.

11.
Phys Chem Chem Phys ; 12(31): 8863-9, 2010 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-20535405

RESUMO

The EPR spectrum of the Y@C(82) molecules isolated in solid argon matrix was recorded for the first time at a temperature of 5 K. The isotropic hyperfine coupling constant (hfcc) A(iso) = 0.12 +/- 0.02 mT on the nucleus (89)Y as derived from the EPR spectrum is found in more than two times greater than that obtained in previous EPR measurements in liquid solutions. Comparison of the measured hfcc on a metal atom with that predicted by density-functional theory calculations (PBE/L22) indicate that relativistic method provides good agreement between experiment in solid argon and theory. Analysis of the DFT calculated dipole-dipole hf-interaction tensor and electron spin distribution in the endometallofullerenes with encaged group 3 metal atoms Sc, Y and La has been performed. It shows that spin density on the scandium atom represents the Sc d(yz) orbital lying in the symmetry plane of the C(2v) fullerene isomer and interacting with two carbon atoms located in the para-position on the fullerene hexagon. In contrast, the configuration of electron spin density on the heavier atoms, Y and La, is associated with the hybridized orbital formed by interaction of the metal d(yz) and p(y) electronic orbitals.

12.
J Org Chem ; 74(19): 7238-44, 2009 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-19725551

RESUMO

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.


Assuntos
Magnetismo , Nitrocompostos/química , Tolueno/análogos & derivados , Espectroscopia de Ressonância de Spin Eletrônica , Estrutura Molecular , Nitrocompostos/síntese química , Fotólise , Tolueno/síntese química , Tolueno/química
13.
Inorg Chem ; 48(18): 8723-8, 2009 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-19694454

RESUMO

Xenon trifluoride radicals were generated by the solid-state chemical reaction of mobile fluorine atoms with XeF(2) molecules isolated in a solid argon matrix. On the basis of spectroscopic and kinetic FTIR measurements and performed quantum chemical calculations, two infrared absorption bands at 568 (strong) and 523 (very weak) cm(-1) have been assigned to asymmetric and symmetric Xe-F stretching vibrational modes of radical (*)XeF(3), respectively. Chemical reaction of fluorine atom with XeF(2) in a solid argon cage obeys specific kinetic behavior indicating the formation of a long-lived intermediate complex under the condition that the diffusing fluorine atom is attached to isolated XeF(2) at temperatures 20 K < T < 27 K. Subsequent thermally activated conversion in the complex is the main source of novel xenon-containing radical species (*)XeF(3). The rate constant and energy barrier are estimated for the reaction in an argon cage, [XeF(2)-F] --> (K(r)) [XeF(3)], as K(r) approximately 7 x 10(-5) c(-1) at 27 K and E approximately 1.2 kcal/mol, respectively. Quantum chemistry calculations reveal that radical (*)XeF(3) has a planar C(2v) structure. DFT calculations show that formation of the third Xe-F bond in the (*)XeF(3) radical is exothermic, and the binding energy of the third Xe-F bond is 8-20 kcal/mol.

14.
J Chem Phys ; 122(3): 34503, 2005 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-15740205

RESUMO

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

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