Magnetic Inequivalence Research Articles

How Solid Surfaces Control Stability and Interactions of Supported Cationic CuI(dppf) Complexes─A Solid-State NMR Study

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex-solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(Lx)+ (dppf = 1,1'-bis(diphenylphosphino)ferrocene, Lx = mono- and bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by 31P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al2O3 resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of 31P nuclei verified by 31P-31P RFDR and 1H-31P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad 31P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(Lx)+ complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

31P nuclear spin singlet lifetimes in a system with switchable magnetic inequivalence: experiment and simulation

31P NMR nuclear spin singlet order is reported for the first time. Experiments and molecular dynamics simulations indicate that the singlet order relaxes primarily via chemical shift anisotropy at high magnetic field strengths.

Long live the singlet state!

The field of long-lived states in NMR is reviewed. The relationship of long-lived-state phenomena to those associated with spin isomerism is discussed. A brief overview is given of key developments in the field of long-lived states, including chemical symmetry-switching, the role of magnetic equivalence and magnetic inequivalence, long-lived coherences, hyperpolarized NMR involving long-lived states, quantum-rotor-induced polarization, and parahydrogen-induced hyperpolarization. Current application areas of long-lived states are reviewed, and a peer into the crystal ball reveals future developments in the field.

Crystallographic elucidations of indium(III) porphyrin conformations, morphology and aggregation behaviour: Comparative optical study of free base porphyrins and their indium(III) derivatives at varying pH

The supramolecular structural architectures of 5, 10, 15, 20-Tetrakis-(4-methoxyphenylporphyrinato) indium (III) chloride (I) and 5, 10, 15, 20-Tetrakis-(4-methylphenylporphyrinato) indium (III) chloride (II) have been determined from their three dimensional X-ray diffraction data. Complex I crystallizes in the orthorhombic space group, Pbca with a = 23.0083 (4) Å, b = 15.2447 (3) Å, c = 23.3737 (4) Å, α = β = γ = 90°, V = 8198.5 (3) Å3 and Z = 8, whereas complex II crystallizes in the triclinic space group, P-1 with a = 12.0737 (5) Å, b = 12.1000 (5) Å, c = 14.1682 (6) Å, α = 88.560 (3)°, β = 78.730 (3)°, γ = 78.280 (3)° V = 1987.40 (14) Å3 and Z = 2. The geometries around In+3 ions in both the complexes are square pyramidal giving dome shaped architectures to the metalloporphyrins. In their respective crystal lattices, the monomeric units of complex I and II are packed in J-type (head-to-tail) supramolecular aggregation. These monomers are linked into their three dimensional supramolecular networks by various non covalent forces. Further the packing of monomers of each complex in their respective unit cells leaves significant void volumes (porosities). The optical absorption spectra of complex I and II and their free base analogues i.e. 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP and 5, 10, 15, 20-Tetrakis-(4-methylphenyl) porphyrin, H2TTP have also been investigated in CH2Cl2 at different ionic strengths of the media. The investigations revealed that both the free bases exist as simple monomers, j-aggregates and dianionic monomers at neutral (pH = 7), acidic (pH = 4 and lower) and basic (pH = 10) media conditions. Also at high ionic strength of the media, the free bases undergo charge transfer electronic transitions. The two In3+ porphyrin exists as simple monomers at pH = 7. However at pH = 5 and lower, the In3+ ions in complex II is remarkably easily replaced by the protons of the acid to generate the corresponding protonated free base monomers which simultaneously aggregate in J-type stacking mode. In contrast to complex II that exhibits low stability in acidic media, the complex I remain intact in such media reflecting extremely high stability of the complex. The insertion of In3+ ions into the porphyrinic cores of free bases causes bathochromic shift followed by increase in the intensities of absorption bands. The appearance of two set of doublets for ortho phenyl protons in the 1H NMR spectra of each complex is attributed to the magnetic inequivalence of their ortho phenyl protons caused by the pulling of In+3 ions above the mean porphyrin planes.

Weak coupling between magnetically inequivalent spins: The deceptively simple, complicated spectrum of a 13C-labeled trimethylated amine

Magnetic inequivalence of nuclear spins is well known to cause additional splittings that complicate spectral analysis. Here, we present an extraordinary case of magnetic inequivalence, manifested in the 13-spin system of a 13C,15N-labeled trimethylated amine. All methyl group protons are chemically equivalent due to the molecular symmetry, but not all are magnetically equivalent as they have different 1JCH and 3JCH couplings. In general, spectra of such a large spin system can be expected to be extremely complicated by the presence of hundreds if not thousands of extra lines, caused by the strong coupling between inequivalent nuclei. Surprisingly, the 1H spectrum presented consists of very few lines, in a pattern of the utmost simplicity. Using sub-spectral analysis we show that this is due to weak coupling between the magnetically inequivalent nuclei, as a consequence of the particular combination of coupling constants. We find that the 4JHH geminal methyl coupling constant is 0.43Hz and 2JCC is ∼0Hz. In addition, we demonstrate that homo-decoupling can be used to transform the spin system to a set of fully equivalent spins, resulting in disappearance of 4JHH-splittings. We believe this curious case is a highly instructive example of magnetic inequivalence. The spectra may be considered deceptively simple, as fewer lines are observed than one would anticipate. At the same time, the spectra are deceptively complicated, as they can very well be approximated by intuitive reasoning.

The exceptional magnetic inequivalence in helical form I of poly-1-pentene

A sample of isotactic poly-1-pentene has been synthesized by using a metallocene catalyst, and two specimens have been prepared displaying the most stable Form I of this polymer, reported to adopt a 31 helical conformation. The solid-state 13CNMR study of this helical form shows an exceptional splitting of the resonances, indicative of an important magnetic inequivalence in the unit cell. The maximum separation (amounting to 4.7 ppm) is obtained for carbon 2B: the methylene next to the terminal methyl in the lateral branch. This exceptional splitting is interpreted as originating from either intermolecular (packing) or intramolecular (conformational) differences (or both), leading to well non-equivalent sites for all the carbons. The comparative analysis of the relaxation times in the rotating frame between isotactic poly-1-pentene and isotactic polypropylene indicates that the two polymers display a rather similar relaxation behavior for both the crystalline and amorphous components.

Accessing long lived 1H states via 2H couplings

In this paper we demonstrate long-lived states involving a pair of chemically equivalent protons, with lifetimes ∼30 times T1 up to a total lifetime of ∼117s at high field (8.45T). This is demonstrated on trans-ethylene-d2 in solution, where magnetic inequivalence gives access to the long-lived states. It is shown that the remaining J-coupling between the two quadrupolar deuterium spins, JQQ, splits the conditions for optimally generating proton singlet states. Detailed simulations of the spin evolution are performed, shedding light on the coherent evolution during singlet–triplet conversion as well as on the incoherent evolution that causes relaxation. Subsequently, the simulations are compared with experimental results validating the theoretical insights. Possible applications include storage of hyperpolarization in the proton long-lived state. Of particular interest may be utilization of parahydrogen induced polarization to directly induce the examined long-lived states.

Insights into the electronic structure of Cu(II) bound to an imidazole analogue of westiellamide.

Three synthetic analogues of westiallamide, H3L(wa), have previously been synthesized (H3L(1-3)) that have a common backbone (derived from l-valine) with H3L(wa) but differ in their heterocyclic rings (imidazole, oxazole, thiazole, and oxazoline). Herein we explore in detail through high-resolution pulsed electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy in conjunction with density functional theory (DFT) the geometric and electronic structures of the mono- and dinuclear Cu(II) complexes of these cyclic pseudo hexapeptides. Orientation-selective hyperfine sublevel correlation, electron nuclear double resonance, and three-pulse electron spin echo envelope modulation spectroscopy of [Cu(II)(H2L(1))(MeOH)2](+) reveal delocalization of the unpaired electron spin onto the ligating and distal nitrogens of the coordinated heterocyclic rings and that they are magnetically inequivalent. DFT calculations confirm this and show similar spin densities on the distal heteroatoms in the heterocyclic rings coordinated to the Cu(II) ion in the other cyclic pseudo hexapeptide [Cu(II)(H2L(2,3,wa))(MeOH)2](+) complexes. The magnetic inequivalencies in [Cu(II)(H2L(1))(MeOH)2](+) arise from different orientations of the heterocyclic rings coordinated to the Cu(II) ion, and the delocalization of the unpaired electron onto the distal heteroatoms within these N-methylimidazole rings depends upon their location with respect to the Cu(II) d(x(2)-y(2)) orbital. A systematic study of DFT functionals and basis sets was undertaken to examine the ability to reproduce the experimentally determined spin Hamiltonian parameters. Inclusion of spin-orbit coupling (SOC) using MAG-ReSpect or ORCA with a BHLYP/IGLO-II Wachters setup with SOC corrections and ∼38% Hartree-Fock exchange gave the best predictions of the g and A((63)Cu) matrices. DFT calculations of the (14)N hyperfine and quadrupole parameters for the distal nitrogens of the coordinated heterocyclic rings in [Cu(II)(H2L(1))(MeOH)2](+) with the B1LYP functional and the SVP basis set were in excellent agreement with the experimental data, though other choices of functional and basis set also provided reasonable values. MCD, EPR, mass spectrometry, and DFT showed that preparation of the dinuclear Cu(II) complex in a 1:1 MeOH/glycerol mixture (necessary for MCD) resulted in the exchange of the bridging methoxide ligand for glycerol with a corresponding decrease in the magnitude of the exchange coupling.

Magnetic percolation and inequivalence of Fe sites in YFe x Co2−x (x = 0.03 and 1) Laves phase compounds

YCo2 compound is an exchange-enhanced Pauli paramagnet on the verge of being magnetic. Topological or chemical disorder can induce in it ferromagnetic long-range ordering. The influence of Fe substitution and quenched-in topological disorder on the magnetic properties of YFe0.03Co1.97 and YFeCo is studied. Structural and magnetic properties of these compounds are analyzed by means of X-ray diffraction, vibrating sample magnetometry, Mossbauer spectroscopy and AC magnetic susceptibility measurements. All samples crystallize in cubic MgCu2-type phase with lattice constant changing from 7.223 A for YCo2 through 7.221 A for YFe0.03Co1.97 to 7.313 A for YFeCo. Fe atoms are responsible for stabilization of magnetic moments on Co, and mictomagnetism is observed in YFe0.03Co1.97 sample. Upon further substitution, as in YFeCo, long-range ordering appears. The Mossbauer spectra permitted distinction between two magnetically inequivalent Fe sites, as reported earlier for YFe2.

An Al/P-based frustrated Lewis pair as an efficient ambiphilic ligand: coordination of boron trihalides, rearrangement, and formation of HBX₂ complexes (X = Br, I).

The Al/P-based frustrated Lewis pair (FLP) Mes2P-C(═CH-Ph)-Al(CMe3)2 (1) reacted with boron halides BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF3 adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes2P-(μ-C═CH-Ph)(μ-HBX2)-AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2P-C(═CH-Ph)-AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.

Theoretical study of homonuclear J coupling between quadrupolar spins: Single-crystal, DOR, and J-resolved NMR

The theory describing homonuclear indirect nuclear spin–spin coupling (J) interactions between pairs of quadrupolar nuclei is outlined and supported by numerical calculations. The expected first-order multiplets for pairs of magnetically equivalent (A2), chemically equivalent (AA′), and non-equivalent (AX) quadrupolar nuclei are given. The various spectral changeovers from one first-order multiplet to another are investigated with numerical simulations using the SIMPSON program and the various thresholds defining each situation are given. The effects of chemical equivalence, as well as quadrupolar coupling, chemical shift differences, and dipolar coupling on double-rotation (DOR) and J-resolved NMR experiments for measuring homonuclear J coupling constants are investigated. The simulated J coupling multiplets under DOR conditions largely resemble the ideal multiplets predicted for single crystals, and a characteristic multiplet is expected for each of the A2, AA′, and AX cases. The simulations demonstrate that it should be straightforward to distinguish between magnetic inequivalence and equivalence using J-resolved NMR, as was speculated previously. Additionally, it is shown that the second-order quadrupolar-dipolar cross-term does not affect the splittings in J-resolved experiments. Overall, the homonuclear J-resolved experiment for half-integer quadrupolar nuclei is demonstrated to be robust with respect to the effects of first- and second-order quadrupolar coupling, dipolar coupling, and chemical shift differences.

Iridium(III) hydrido N-heterocyclic carbene-phosphine complexes as catalysts in magnetization transfer reactions.

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

Fluorine Tagging of Polyoxometalates: The Cyclic [{MoV2O4(H2O)}4{O3PC(CF3)(O)PO3}4]12–

AbstractWe have discovered a simple and efficient strategy to incorporate fluorine into polyoxometalates by reaction of thefluorinated diphosphonate H2O3PC(CF3)(OH)PO3H2 with [MoV2O4(H2O)6]2+ in aqueous medium (pH = 6.4) resulting in [{MoV2O4(H2O)}4{O3PC(CF3)(O)PO3}4]12– (1), which was structurally characterized in solution by multinuclear NMR spectroscopy and in the solid state by single‐crystal X‐ray diffraction, infrared, thermogravimetric and elemental analyses. Polyanion 1 crystallizes as the mixed sodium/guanidinium salt Na6.5[(H2N)3C]5.5[{MoV2O4(H2O)}4{O3PC(CF3)(O)PO3}4]·8H2O (1a) in the monoclinic system, space groupC2/c with the unit‐cell parameters a = 34.0685(95) Å, b = 8.0874(19) Å, c = 34.4101(10) Å, β = 100.20(2)°, V = 9331.02(4) Å3 and Z = 2. The novel polyanion 1 has a cyclic, saddle‐like structure with four {Mo2O4(H2O)} units linked by four fully deprotonated diphosphonate moieties. In addition, the (CF3)C–OH moiety is deprotonated, due to the electronic effects of the neighboring CF3 group, leading to the formation of an Mo–O(C) bond. Solution 31P and 19F NMR studies of 1 indicate magnetic inequivalence of the two phosphorus atoms in each diphosphonate moiety. The solid‐state packing of the title polyanions in 1a by loosely bound Na+ counterions leads to an AAA‐type arrangement.

Para-Hydrogen Induced Polarization without Incorporation ofPara-Hydrogen into the Analyte

The cationic iridium complexes [Ir(COD)(PR3)2]BF4 (1a-c) (a, R = Ph; b, R = p-tolyl; c, R = p-C6H4-OMe) react with parahydrogen in the presence of pyridine to give trans, cis, cis-[Ir(PR3)2(py)2(H)2]+ (2a-c) and small amounts of fac, cis-[Ir(PR3)(py)3(H)2]+ (3a-c), each of which exhibit polarized hydride resonances due to the magnetic inequivalence associated with the resultant AA"XX" spin system when 15N-labeled pyridine is employed. The pyridine ligands in 2 are labile, exchanging slowly into free pyridine with a rate constant of 0.4 s(-1) for 2a at 335 K in a dissociative process where DeltaH(double dagger) = 134 +/- 1 kJ mol(-1) and DeltaS(double dagger) = 151 +/- 5 J mol(-1) K(-1). Pyridine ligand exchange in 2 proves to be slower than that determined for 3. Parahydrogen induced polarization (PHIP) based on the hydride ligands of 2 and 3 is transferred efficiently to the 15N nuclei of the bound pyridine ligand by suitable insensitive-nuclei-enhanced-by-polarization-transfer (INEPT) based procedures. Related methods are then used to facilitate the sensitization of the free pyridine 15N signal by a factor of 120-fold through ligand exchange even though this substrate does not contain parahydrogen. This therefore corresponds to the successful polarization of an analyte by parahydrogen induced polarization methods without the need for the actual chemical incorporation of any parahydrogen derived nuclei into it.

Accurate Calculation of the Phenyl Radical's Magnetic Inequivalency, Relative Orientations of Its Spin Hamiltonian Tensors, and Its Electronic Spectrum

The phenyl radical's electronic structure, magnetic inequivalency, spin Hamiltonian tensor components, and the relative orientation of their principal axes are computed by Neese's coupled-perturbed Kohn-Sham hybrid density functional (CPKS-HDF) technique in a moderate amount of time without resorting to expensive post-Hartree-Fock techniques. The g tensor component values are in excellent agreement with those determined experimentally and differ by less than 370 ppm. The computed hydrogen nuclear hyperfine tensors, A(1H), are also found to be in very good agreement with their experimental counterparts. The correlation of the radical's electronic structure with its g and A numerical values corroborates that it has a 2A(1) ground state. In accordance with our previous studies on the equivalency of planar radicals that possess C(2v) symmetry, the in-plane g and A(1H) principal axes should not be parallel to one another. Consequently, the spatially equivalent ortho (1H(2), 1H(6)) and meta (1H(3), 1H(5)) proton pairs should be magnetically inequivalent. This was confirmed in both the present computations and the simulation of the EPR solid-state spectrum. To the best of our knowledge, this is the first aromatic in-plane sigma-type radical whose magnetic inequivalency is studied both computationally and experimentally. To properly interpret the radical's electronic excitation spectra, the spectroscopy-oriented dedicated difference configuration interaction (SORCI) procedure was employed. Aside from a slight overestimation, the method seems to be capable of reproducing the C(6)H(5)* electronic vertical excitation energies in the range of 0-50,000 cm(-1). These vertical excitations, in conjunction with the corresponding orbit and spin orbit matrix elements, were also used to compute the g tensor components, employing the sum-over-states technique. Due to the limited number of computed roots and excited states, the results were marginally inferior to those obtained using the CPKS-HDF method.

Transition metals in ZnGeP 2 and other II–IV–V 2 compounds

Semiconductors that exhibit room-temperature ferromagnetism are central to the development of semiconductor spintronics. Transition metal (TM)-doped A IIB IVC V 2 are a promising class of such system. These ternary compound semiconductors have two metal sites A and B that can be substituted by the TMs. A site preference for TM incorporation is crucial for a possible explanation of ferromagnetism since dependent on the TM valent state holes or electrons can be released. For low Fe- and Cr-doped ZnGeP 2, our EPR investigations revealed in addition to the well-known native defects the presence of substitutional Fe 2+ (3d 6, S = 2 ) on Zn site, Fe 3+ (3d 5, S = 5 2 ) and Cr 4+ (3d 2, S = 1 ) on Ge site. A photo-induced recharging of Fe 2+ to Fe + is observed. The Cr 4+ center exhibits a well-resolved phosphorus ligand hyperfine splitting. For Fe 2+ and Fe 3+, the magnetic site inequivalence of each of the both Zn and Ge sites, respectively, has been detected. Moreover, anti-ferromagnetically coupled Mn 2+–Mn Zn 2+ pairs have been observed in Mn-doped ZnGeP 2.

The hyperfine structure in the electronic A 2E″←X 2E′ system of the pseudorotating lithium trimer

Sub-Doppler laser spectroscopy of the A 2E″←X 2E′ band of Li3 shows well-resolved and characteristic hyperfine structures of rovibronic transitions which can be explained in all detail by the interplay between the underlying Fermi-contact interaction, the symmetry species of the rovibronic states and the Coriolis coupling of rotational and pseudorotational motions. Ab initio calculated electron spin densities reproduce the observed line splittings to within 3%. In this Jahn–Teller distorted molecule magnetic equivalence of identical nuclei is strictly valid only for rovibronic states of A symmetry. However, it also holds for rovibronic states of E symmetry if they are of pure A vibronic origin, and it is enforced for many E states of pure E vibronic origin by Coriolis coupling. In contrast, those transitions which involve states with dominant Kc=0 rotation functions and/or with strong vibronic E/A mixing show significant changes in the hyperfine patterns due to magnetic inequivalence, sometimes called “hyperfine doubling.”

Solid-State 19F MAS, 19F CRAMPS, and 19F → 13C CP/MAS NMR Study of an Amorphous Perfluoropolymer

Solid-state 19F high-speed MAS, 19F CRAMPS, and 19F → 13C CP/MAS NMR spectra have been investigated for an amorphous fluoropolymer (CYTOP) having a five-membered heteroring in the main chain. In the 19F MAS spectra, spinning at 35 kHz is necessary to ensure no spectral overlap of spinning sidebands (SSB) with the isotropic signals. The integrated areas obtained are consistent with the molecular structure, and the signal assignments were confirmed using shielding calculations by the GIAO method at a density functional level of theory, B3LYP/6-31G(d). A 19F CRAMPS spectrum measured at a spinning speed of 6 kHz gives slightly narrower half-height widths despite SSB overlap. All fluorine signals show monoexponential decays in the 19F spin-lock experiment with the same , indicating the structure is monophasic. Two fluorines connected to the same carbon give separate peaks at ambient temperature, which reflects the magnetic inequivalence at the axial and equatorial positions of the heteroring. However, they sta...

NMR of 57Fe in RFe1−xMnxO3 orthoferrites

The spin echo method is used to investigate the NMR spectrum of the 57Fe nuclei in manganese-substituted Tm and Er orthoferrites. It is shown that the Jahn–Teller effect for the Mn3+ impurity ion is manifested in a proportional enhancement of the E-type deformation of the Fe3+–6O2− octahedron. In the region of the spin-reorientation transition a magnetic inequivalence of the sublattices is observed both for the Fe3+ ions and for the Mn3+ impurity ions.

Magnetic Inequivalency, Electron Paramagnetic Resonance, Electronic Structure, Optimal Geometry, and Electronic Spectra of the 4,5-Bis(trifluoromethyl)-1,3,2-dithiazol-2-yl Radical†

The geometry of the 4,5-bis(trifluoromethyl)-1,3,2-dithiazolyl radical, predicted by full geometry optimization, is found to be in very good agreement with that determined experimentally by electron diffraction. Post-SCF configuration-interaction (CI) computations must be performed to generate the weak transitions in the spectrum which are responsible for its characteristic blue color. The energy barrier due to the rotation of the two CF3 groups along their C−C axes is estimated to be approximately 60 kcal/mol. Thus, they are not expected to rotate freely at low temperatures and the six 19F atoms should be magnetically inequivalent. This is verified by electron paramagnetic resonance (EPR) spectroscopy of the isolated radical in an Ar matrix at 12 K and is the first experimental evidence of magnetic inequivalency in a main group inorganic cyclic complex. The inhomogeneous broadening due to the magnetic inequivalency of the 19F is large enough to mask their hyperfine splittings. The specific expressions that give rise to these magnetic inequivalencies in conjunction with the g and hyperfine tensors, as a function of the molecular orbital (MO) coefficients, are derived and are required to fully comprehend and accurately simulate the EPR spectra. The magnitudes and signs of the MO coefficients are independently estimated by computing its electronic structure using the B1LYP hybrid density functional method. The simulation of the experimental EPR spectra followed by the comparison of the experimental and computed spin Hamiltonian tensor components reveal that the complex has a 2B ground state. Its spin Hamiltonian parameters are found to be gxx = 2.0020, gyy = 2.0004, gzz = 2.0124, Axx(14N) = 29.097 G, Ayy(14N) = 2.717 G, and Azz(14N) = 2.246 G. The high intensity at the low magnetic field end of the EPR spectrum is due to an extra “off-principal axes” resonance occurring in the xz molecular plane.