A pulsed EPR study of the catalytic oxidation of CO over Cu/SnO2

The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO₂ with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO₂ gel and (iii) impregnation onto calcined SnO₂. Two types of Cu(II) species were identified in these calcined Cu/SnO₂ materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the¹³C hyperfine coupling of enriched¹³CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of^{117, 119}Sn and¹³C and at twice these Larmor frequencies. Although these signals indicate that^{117, 119}Sn and¹³C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for^{117, 119}Sn an isotropic hyperfine constant,a _iso, of ±4.0 MHz and an anisotropic component,T _?, of ±2.0 MHz. Pulsed ENDOR spectra also showed^{117, 119}Sn signals which agree with the above values. The¹³C cross peaks yielda _iso=±1.0 MHz andT _?=±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO₂ after the adsorption of CO₂. Based on the same hyperfine couplings in the samples exposed to¹³CO and¹³CO₂ the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed.     

Pulsed ESR investigations of anisotropic interactions in M@C82 (M=Sc,Y,La)

82 , Y@C₈₂, and La@C₈₂ in frozen solutions. We were able to determine the g tensors of these molecules by analysing magnetic field spectra at X-band (9.5 GHz) and W-band (94 GHz) frequencies. Moreover, in Y@C₈₂ we have investigated the hyperfine interaction of the ⁸⁹Y nuclear spin (I=1/2) with the electron spin on the C₈₂ cage. The principal values of the hyperfine tensor A and the relative orientation of g and A tensors were determined by applying three- and four-pulse electron spin echo envelope modulation techniques (ESEEM). Received: 3 September 1997/Accepted: 10 November 1997     

Complete Determination of Nitrogen Quadrupole and Hyperfine Tensors in an Oxovanadium Complex by Simultaneous Fitting of Multifrequency ESEEM Powder Spectra

One-dimensional C- and X-band as well as two-dimensional X-band ESEEM experiments were performed on the complex oxobis(2-methylquinolin-8-olato) vanadium(IV) in frozen solution. A¹⁴N ESEEM simulation strategy based on initial first- and second-order perturbation analysis of peak positions in orientationally selected ESEEM spectra is presented. The constraint parameters extracted enable one to reduce the number of free fitting parameters for each nitrogen from 10 to 4. These are the α, β resp. the φ, θ Euler angles of the NQI and the HFI tensor defined in the coordinate system of the axialgtensor. The local symmetry of the complex allows one to reduce the number of free parameters to two angles only. Subsequently, a grid search in the remaining Euler space produced the starting parameters for the final fit of the¹⁴N hyperfine and quadrupole tensors. The anisotropic nitrogen hyperfine interaction tensor was found to be strongly nonaxial (0.06, 0.51, −0.57) MHz with the components significantly smaller than the isotropic hyperfine constant −6.18 MHz. In contrast, the quadrupole tensor withK= 0.58 MHz is close to axial (η = 0.13). These tensors share the principal axis normal to the ligand plane (as imposed by the local symmetry). The axes in the ligand plane are, however, rotated 50° with respect to each other. The orientation of the quadrupole tensor axes correlate within 10° with the orientation of the ligand plane following from the X-ray structure.     

Nonperturbative simulation of two-pulse electron spin-echo-envelope modulation spectra. Comparison with experimental single-crystal spectra

The direct method of calculating magnetic observables provided by the Liouville formalism is utilized for a nonperturbative numerical treatment of electron spin-echo-envelope modulation (ESEEM). Calculated time-domain signals are compared with experimental ones. The particular system studied is the C0₂ anion trapped in NaHC₂O₄·H₂O single crystals. The experimental ESEEM spectra show modulations due to both proton and sodium nuclei. The hyperfine and quadrupole tensors, determined independently by ENDOR spectroscopy, are used as input for calculating the ESEEM spectra.     

Structures and electronic states of photoexcited states in a system of two nitroxide radicals linked to fullerene studied by two-dimensional pulsed nutation and time-resolved electron paramagnetic resonance spectroscopy

Magnetic properties of fulleropyrrolidine adducts with two stable nitroxide radicals (2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO) were studied in toluene solution by continuous-wave time-resolved (TR) and pulsed electron paramagnetic resonance (EPR) spectroscopy in the ground and photoexcited states. Four isomers of the bisadduct,trans-1,trans-2,trans-3, and equatorial forms, having the second pyrrolidine ring at different [6-6] bonds were synthesized. In the ground states, the exchange interaction between two TEMPOs is so small that the spin state of the bisadduct is a doublet in nature. By means of spectral simulations of the EPR spectra in frozen solution at 70 K, the upper limit of the exchange interaction was estimated to be 5 MHz for thetrans-1 andtrans-2 and 10 MHz for thetrans-3 and equatorial isomers. The simulation was also made to determine relative positions of the two TEMPO groups with respect to the pyrrolidine ring. Photoexcited states of the bisadducts with excitation of the 532 nm laser pulse were studied in frozen toluene solution at 5–100 K by using two-dimensional (2-D) pulsed nutation EPR and TREPR. The spin multiplicity of the excited state was determined by the nutation frequency. All of the four bisadducts showed strong exchange couplings between two TEMPOs and fullerene triplet³C ₆₀ ^* , resulting in the generation of the excited quintet and triplet states. The excited triplet states have been observed and assigned for the first time in strongly coupled triplet-radical systems. The zero-field splittings of the quintet state determined from the 2-D nutation EPR spectra were analyzed as the sum of the spinspin interactions among the three paramagnetic centers, two TEMPOs and³C ₆₀ ^* . On the basis of these analyses, the spin distribution on the³C ₆₀ ^* part and the geometry of two TEMPOs are discussed.     

Knight shift tensors and π-spin densities in the organic metals αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2

¹³C-MASS spectra of pure BEDT-TTF and of the organic metals α_t-(BEDT-TTF)₂I₃ and (BEDT-TTF)₂Cu(NCS)₂ were recorded atν _L = 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts arechemical shifts while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned ingroups to theinner, middle andouter carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the π-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the π-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecule, i.e., on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.     

Theoretical spin density in nitroxides

The evolution of the hyperfine tensors in the nitroxide series with increasing alkyl substitution on the NO group has been studied theoretically for radicals from H₂NO to C₅H₁₀NO. A projection technique has been applied to the UHF wave-functions in order to correct spin densities for quartet contamination. The magnitude of the isotropic and anisotropic couplings reflects the substitution effect already observed on the spin distribution maps, that is a spin transfer from oxygen to nitrogen when alkyl groups are substituted to hydrogens in H₂NO. The alternation of the signs of the couplings along the chain as well as the cos² γ law for the coupling constants of atoms (C or H) in β position are verified in the series. The orientation of the anisotropic tensors with respect to the chemical bonds depends on the position of the atoms in the molecule (radical site, α, β positions); it is not affected by further substitution, except for strongly asymmetric configurations.     

Magnetization and Mössbauer studies of 57Fe doped SrCoO3

⁵⁷Fe (1%) doped SrCoO₃ obtained by high-pressure method, has been investigated by magnetization and Mössbauer spectroscopy studies (MS) in the temperature range 4.2 K to 300 K. The ferromagnetic ordering temperature T _C obtained is 272(2) K. Isothermal magnetization curves have been measured at various temperatures, from which the saturation moments (M _sat) have been deduced. The ⁵⁷Fe MS spectra display standard six-line patterns with an isomer shift typical of Fe^3?+? and a very small quadrupole splitting (QS = 0.14(1) mm/s above T _C). The magnetic hyperfine field at 4.2 K is 276(1) kOe. The temperature dependencies of the iron hyperfine field and M _sat (1.83 µ _B at 5 K) are almost identical. This shows that the Fe^3?+? is replacing Co^4?+?, both of the same electronic configuration. They also interact similarly, namely the Fe–Co exchange is almost identical to the Co–Co exchange.     

ELDOR-detected NMR at Q-Band

In recent years, electron–electron double resonance detected nuclear magnetic resonance (EDNMR) has gained considerable attention as a pulsed electron paramagnetic resonance technique to probe hyperfine interactions. Most experiments published so far were performed at W-band frequencies or higher, as at lower frequencies detection of weakly coupled low-γ nuclei is hampered by the presence of a central blind spot, which occurs at zero frequency. In this article we show that EDNMR measurements and a meaningful data analysis is indeed possible at intermediate microwave frequencies (Q-band, 34 GHz), once experimental parameters have been optimized. With highly selective detection pulses and Gaussian shaped electron–electron double resonance (ELDOR) pulses it is possible to detect low-γ nuclei coupled to paramagnetic Mn²⁺. Weakly coupled ¹⁴N resonances, which are separated from the zero frequency by only 2.8 MHz, were readily detected. In systems where different spin active nuclei are coupled to the electron spin, particular care has to be taken when using higher powered ELDOR pulses, as combination frequencies from the two nuclei (?m _S = ±1, ?m _I,1 = ±1, ?m _I,2 = ±1) can lead to severe line broadening and complicated EDNMR spectra. We also compare the EDNMR spectra of ¹³C-labeled Mn–DOTA to ¹³C-Mims electron–nuclear double resonance to get a better insight into the similarities and differences in the results of the two techniques for ¹³C hyperfine coupling.     

Solid state NMR studies and chemical shift calculations of a gold(I) complex with a diphosphacyclobutadiene cobaltate sandwich anion

The ³¹P and ¹³C solid state nuclear magnetic resonance spectra of a neutral gold(I) complex with bis(diphosphacyclobutadiene) cobaltate anions, [Au{Co(P₂C₂tPent₂)₂}₂(PMe₃)₂], are reported. Complete ³¹P resonance assignments have been derived from saturation transfer, radio-frequency driven recoupling (RFDR) and RTOBSY experiments and confirmed further by ab-initio calculations of magnetic shielding tensors by density functional theory, with consideration of relativistic effects. Coordination of the diphosphacyclobutadiene ring with gold(I) results in a high-frequency shift of the ³¹P signal of the directly coordinated P atom, whereas a low-frequency shift is observed for the P atom at the opposite end of that ring. Based on these results, a previous assignment made for the complex salts [Au(PMe₃)₄][Au{Co(P₂C₂Ad₂)₂}₂] and [K(18-crown-6)(thf)₂][Au{Co(P₂C₂Ad₂)₂}₂] (Ad=adamantyl) must be corrected.     

Automatic computer simulations of ESR spectra

A versatility of automatic ESR simulation procedures has been developed to obtain high quality of fitting and produce additional information. The following examples are treated: derivation of long-range proton hyperfine coupling constants from unresolved lines; determination of¹³C hf couplings from the naturally abundant isotope satellites; analyzing chemical exchange phenomena with two-sites model; decomposition of superimposed spectra consisting of poorly resolved components. In order to achieve best agreement between calculated and experimental spectra within a reasonable number of iteration cycles, we combined various approaches, like consecutive independent parameter optimization, least-square approach, optimization on “serpentines” and optimization of compound parameters. The spectra can be computed both for liquid and solid state samples, for non-oriented, partially oriented and single crystal samples. Second order perturbation formulas are used for the spin Hamiltonian includingg- and hyperfine tensors that have isotropic, axial and rhombic symmetry. For chemical exchange the modified Bloch equation for two-site model is used. Various lineshapes, including Lorentzian, Gaussian, mixed, modulation and dispersion distorted forms are applied. Third order parabolic interpolations are used for building up spectra from individual lines. In order to correct sweep non-linearity a third order interpolation converts the spectrum to become equidistant. This procedure can occasionally improve square deviation by an order of magnitudes for well resolved spectra. In the analysis of strongly overlapping superimposed spectra, simultaneous adjustment of two independent experimental spectra can give an exact decomposition. The inclusion of long-range couplings offers highly perfect fitting and allows one to resolve contributions of naturally abundant¹³C isotopes.     

Refocused primary echo: A zero dead time detection of the electron spin echo envelope modulation

We report on the two-dimensional (2D) implementation of the refocused primary electron spin echo envelope modulation (ESEEM) technique, its theory and experimental application to a model system and a system of biological interest. This technique is virtually free of dead time along one time coordinate. The ESEEM obtained by integration of the 2D time-domain data of the refocused primary ESEEM over one of the time coordinates shows the intensity of the sum combination harmonics proportional to k² for k 1 and proportional to k for k 1 (k is a usual notation for the modulation amplitude factor). This feature, in combination with the adjustment of k by means of variation of the operational frequency of the spectrometer, was found to be very useful for detection of protons with distributed hyperfine interaction parameters situated close to the electron spin.     

Pulsed EPR studies of polycrystalline cesium hexamethyl hexacyclen sodide

The electron spin echo envelope modulation (ESEEM) technique of pulsed EPR spectroscopy has been used to measure weak¹³³Cs hyperfine couplings to trapped electrons in polycrystalline cesium hexamethyl hexacyclen sodide. Two magnetically distinct groups of weakly coupled¹³³Cs nuclei were found — one with an isotropic electron-nuclear hyperfine (Fermi contact) coupling of 0.34 MHz and the other with a contact coupling less than 0.1 MHz. Analysis of these data, by computer simulation, shows that the number of Cs nuclei that give rise to these interactions and their distance from the paramagnetic center is consistent with the hypothesis that the trapped electrons occupy vacant anion sites. The results indicate that the spread of unpaired electron spin density, along the axis formed by the “contact ion pair,” may be greater than that in the plane perpendicular to this axis.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Optical and ESR spectra of the C4H radical in rare gas matrices at 4 K

C₄H (butadinyl) was trapped in Ar and Ne matrices at 4 K by the uv photolysis of a mixture of C₄H₂ and the rare gas. CC stretching and CH bending modes have been identified in the ir at 2060 and 735 cm^?1. A band system appearing in the 2500–3000 Å region with (0, 0) at 33 800 cm^?1 has been analyzed and tentatively assigned to a ²Π_r ← X²Σ transition. C₄D: Ar spectra confirm the optical assignments. The ESR spectrum exhibits a proton hyperfine splitting b = A_⊥(H) = 16.5 MHz, at g_⊥ = 2.0004(5). The spin-doubling constant, γ, is estimated to be +0.0006(2) cm^?1.     

The influence of the metalloid elements on the magnetic properties of amorphous Fe40Ni40PyB20−y

The Curie temperatures (T_C) and the hyperfine interactions of amorphous Fe₄₀Ni₄₀P_yB_20?y(0≤y>≤20) have been determined by ⁵⁷Fe Mössbauer spectroscopy. In this series of amorphous solid with a fixed transition-metal content, the metalloid atoms are shown to have a noticeable effect on T_C, isomer shift and hyperfine field distribution.     

Nuclear Quadrupole Coupling Constants of Chlorine and Microwave Spectrum, Structure, and ab Initio Calculation of 1-Chloro-1,1,2-trifluoroethane

The microwave spectrum of the ³⁵Cl and ³⁷Cl isotopic species of 1-chloro-1,1,2-trifluoroethane (HCFC-133b) has been investigated in the frequency region 10 to 50 GHz using a Stark modulation microwave spectrometer. A pulsed jet Fourier transform microwave spectrometer was also used for the measurement of hyperfine splittings. A least-squares analysis of the observed b-type Q- and R-branch transition frequencies gave rotational and centrifugal distortion constants and components of the chlorine nuclear quadrupole coupling constant tensors in the principal axes system as follows: A=4625.161 (3) MHz, B=2004.127 (2) MHz, C=1875.813 (2) MHz, Δ_J=0.144 (9) kHz, Δ_JK=1.0748 (8) kHz, Δ_K=1.57 (1) kHz, δ_J=0.01376 (4) kHz, δ_K=−0.146 (4) kHz, χ_aa=−57.958 (10) MHz, χ_bb=21.231 (11) MHz, and χ_cc=36.727 (11) MHz for ³⁵ClCF₂CH₂F species, and A=4607.684 (6) MHz, B=1960.565 (2) MHz, C=1834.823 (2) MHz, Δ_J=0.106 (7) kHz, Δ_JK=1.022 (3) kHz, Δ_K=1.48 (1) kHz, δ_J=0.0142 (2) kHz, δ_K=−0.18 (2) kHz, χ_aa=−46.268 (11) MHz, χ_bb=17.319 (13) MHz, and χ_cc=28.950 (13) MHz for ³⁷ClCF₂CH₂F species. The structural parameters are calculated from the observed six rotational constants by assuming the partial structure of ab initio calculation. The electronic properties of the C-Cl bond are evaluated from the observed nuclear quadrupole constants of chlorine. These molecular properties are compared with those of other related molecules.     

Observation of potassium hyperfine interactions in x-irradiated KH2AsO4 through the method of electron spin echo envelope modulation

Hyperfine interaction frequencies of ¹H and ³⁹K nuclei near the AsO^4-₄ radical in X-ray irradiated KH₂AsO₄ (KDA) have been observed through the method of electron spin echo envelope modulation (ESEEM). This method enabled us to record nuclear hyperfine interaction (ENDOR-like) spectra around the ferroelectric phase transition of KDA for the first time. The ESEEM spectrum of ³⁹K exhibits a clear change when passing the ferroelectric phase transition temperature, but that of close protons does not. The result for close protons is in agreement with the symmetry breaking of the AsO^4-₄ site as observed via the EPR spectrum [5]. Finally, at 4.2 K the hyperfine interaction parameters of a ³⁹K nucleus near the AsO^4-₄ unit could be determined through the ESEEM method.     

Transient and pulsed EPR study of17O-substituted methyl-naphthoquinone as radical anion in the A1 binding site of photosystem I and in frozen solution

Quinones have been studied in considerable detail as functional cofactors in membrane-bound protein-cofactor systems, in particular in reaction centers (RCs) of photosynthesis. For both types of RCs, they act primarily as one-electron gates during light-induced charge separation but at very different redox potential. Hydrogen bonding between the RC protein and the two, 1,4-quinone carbonyl groups constitutes a major protein-cofactor interaction in control of function. In contrast to symmetric H-bonding for quinones in isotropic solution, asymmetric H-bonding is a characteristic feature of the quinone binding sites in RC proteins. A simple valence bond model correlates the asymmetry of respective H-bond strength with the asymmetric spin density distribution derived from observable hyperfine couplings of the quinone anion state. Among all quinone-protein systems studied so far, the A₁ acceptor site in photosystem (PS) I exhibits the highest asymmetry. Since the carbonyl groups carry most of the total unpaired electron spin density, isotopic labelling of the carbon (¹³C) and oxygen (¹⁷O) appears to be the proper way to characterize the H-bond asymmetry by hyperfine couplings. Indeed, recent¹³C hyperfine studies, together with data for protons in specific ring substituents, confirm the high asymmetry correlated with only one dominant H-bond in the A₁ site of PS I, which is consistent with the structure model derived from X-ray structure (1JB0) for the ground state of the PS I protein complex.¹⁷O hyperfine tensors measured for the A₁ site of PS I yield high hyperfine coupling constants but very small asymmetry for the two carbonyl groups. The asymmetry is even three times smaller than the already small one observed for the Q_A site of purple bacterial RCs. A small asymmetry is however consistent with previous studies on model systems which showed an insensitivity of the¹⁷O hyperfine coupling to H-bond-induced changes of the unpaired electron spin density. The large¹⁷O hyperfine coupling itself appears to depend on the electrostatics seen by the radical anion. It is slightly larger when A ₁ ^? is part of the functional transient radical ion pair state as compared with the photoaccumulated stable radical anion. Possible explanations and consequences of these results are discussed.     

Microwave spectra of the Cl and Cl isotopomers of dichloromethylene: Nuclear quadrupole-, spin-rotation-, and nuclear shielding constants from the hyperfine structures of rotational lines

The rotational spectra of the isotopomers C³⁵Cl³⁷Cl and C³⁷Cl₂ of dichloromethylene in the ground vibronic state were recorded in the range 10-33 GHz using a molecular beam Fourier transform microwave spectrometer. CCl₂ was generated by flash pyrolysis using different precursors. The observed spectra were analyzed to yield rotational and centrifugal distortion constants, as well as the complete Cl nuclear quadrupole coupling tensors and the spin-rotation interaction constants from the hyperfine structure of the rotational lines. With inclusion of data from previous work on the most abundant species C³⁵Cl₂ [N. Hansen, H. Mäder, F. Temps, Phys. Chem. Chem. Phys. (3) (2001) 50-55.] a refined r₀ structure was determined. The spin-rotation interaction constants of all three isotopomers were used to derive ³⁵Cl and ³⁷Cl principal inertial axis nuclear magnetic shielding components which have not yet been determined by NMR spectroscopy.