Three-pulse Electron Spin-echo Modulation Research Articles

ESR, ESEM을 이용한 이온 교환된 MoH-SAPO-34에 대한 Mo의 화학종, 위치 및 흡착상호작용에 관한 연구

<TEX>$MoO_3$</TEX>와 H-SAPO-34의 고체상 반응은 상자기성의 Mo(V) 화학종을 띤다. 탈수하면 Mo(V) 화학 종이 약하게 나타나지만 계속적으로 활성화 시키면 ESR로 규명할 수 있는 <TEX>$Mo(V)_{5c}$</TEX>와 <TEX>$Mo(V)_{6c}$</TEX>와 같은 Mo(V) 화학종이 생성된다. ESR과 ESEM 자료들은 <TEX>$(MoO_2)^+$</TEX>나 <TEX>$(MoO)^{3+}$</TEX> 같은 옥소-몰리브덴 화학 종을 보여준다. <TEX>$(MoO_2)^+$</TEX> 화학 종이 다음과 같이 더 합리적인 것 같이 보여진다. H-SAPO-34는 낮은 골격전하를 갖기 때문에 높은 양전하를 갖는 <TEX>$(MoO)^{3+}$</TEX>는 쉽게 안정화 되지 못한다. 소성된 H-SAPO-34와 도데카몰리브덴 규산 용액 사이의 용액 상태 반응은 단지 <TEX>$MoO^{2+}$</TEX> 화학 종만을 발생한다. 마름모형 ESR 신호는 <TEX>$D_2O$</TEX>, <TEX>$CD_3OH$</TEX>, <TEX>$CH_3CH_2OD$</TEX>와 <TEX>$ND_3$</TEX>를 흡착할 때 관측되었다. Mo(V) 화학 종의 배위구조와 위치는 트리 펄스 전자 스핀반향 자료로 측정하였다. MoH-SAPO-34에 메탄올, 에틸렌 암모니아와 물이 흡착될 때 3분자, 1분자, 1분자와 1분자가 <TEX>$(MoO_2)^+$</TEX>에 각각 직접 배위하였다. A solid-state reaction of <TEX>$MoO_3$</TEX> with as-synthesized H-SAPO-34 generated paramagnetic Mo(V) species. The dehydration resulted in weak Mo(V) species, and subsequent activation resulted in the formation of Mo(V) species such as <TEX>$Mo(V)_{5c}$</TEX> and <TEX>$Mo(V)_{6c}$</TEX> that are characterized by ESR. The data of ESR and ESEM show the oxomolybdenum species, to be <TEX>$(MoO_2)^+$</TEX> or <TEX>$(MoO)^{3+}$</TEX>. The <TEX>$(MoO_2)^+$</TEX> species seems to be more probable. Since H-SAPO-34 has a low framework negative charge, <TEX>$(MoO)^{3+}$</TEX> with a high positive charge can not be easily stabilized. A solution reaction between the solution of silico-molybdic acid and calcined H-SAPO-34 resulted in only <TEX>$(MoO_2)^+$</TEX> species. A rhombic ESR signal is observed on adsorption of <TEX>$D_2O$</TEX>, <TEX>$CD_3OH$</TEX>, <TEX>$CH_3Ch_2OD$</TEX> and <TEX>$ND_3$</TEX>. The Location and coordination structure of Mo(V) species has been determined by three-pulse electron spin-echo modulation data and their simulations. After the adsorption of methanol, ethylene, ammonia, and water for MoH-SAPO-34, three molecules, one molecule, one and one molecule, respectively, are directly coordinated to <TEX>$(MoO_2)^+)$</TEX>.

Electron Spin Resonance and Electron Spin Echo Modulation Studies of Cu(II)-Exchanged Silicoaluminophosphate Type 18 Molecular Sieve

The silicoaluminophosphate type 18 (SAPO-18) is a medium-pore molecular sieve and is of interest as heterogeneous catalysts. A SAPO-18 material was synthesized with an Al:P:Si ratio of 10:9:1 with the silica substituting selectively for the phosphorus sites, thus giving a negatively charged framework. Divalent copper ion exchanged into SAPO-18 from an aqueous solution was used as a spin probe to examine its location in SAPO-18 and its interaction with small molecular adsorbates. To monior the Cu(II) interactions with the local environment, electron spin resonance (ESR) and three-pulse electron spin echo modulation (ESEM) were used. ESEM has been used to detect 31P modulation to determine the distance and number of 31P from the Cu(II) ion at various stages of hydration of the material. It was found that in freshly ion-exchanged Cu−SAPO-11 at 400 °C the Cu(II) is directly coordinated to a hexagonal window inside of the pear-shaped large cavity in SAPO-18. Using deuterium-labeled adsorbates including D2O, ND...

Synthesis and characterization of a (Zn, Ti)-substituted layered silicate

The incorporation of TiIV into the framework of zinc-substituted fluorohectorite is reported. Various spectroscopic techniques point to the substitution of TiIV into predominantly the octahedral rather than tetrahedral sublattice of the smectite framework. For clay particles prepared from a reactant composition with Ti/Ti + Si = 0.063 (ZTFH), energydispersive X-ray analysis of the particles observed by transmission electron microscopy indicates that TiIV accounts for ca. 6% of the Ti + Si. X-Ray absorption near-edge spectra indicate that the TiIV in these clay particles is predominantly in an octahedral coordination. This is supported by electron paramagnetic resonance (EPR) spectra of an oriented film sample in which TiIV is reduced to TiIII by heating the sample to 450 °C in 30 Torr of CO. The observed single-ion TiIII EPR spectrum shows dramatic dependence of the orientation of the clay film in the magnetic field. This orientation dependence is consistent with the principal g-tensor axis of the TiIII site being perpendicular to the clay layers. The EPR parameters of this TiIII species are consistent with TiIII occupying an octahedral site. Three-pulse electron spin-echo modulation data taken at the TiIII EPR line position shows modulation at the 7Li nuclear frequency and simulation indicates that TiIII ions are at a distance of 0.30–0.35 nm from LiI ions, a distance that is consistent with these two ions occupying adjacent sites in the octahedral sheet. The MAS NMR spectrum of the zinc/titanium-substituted fluorhectorite consists of an intense Q3 resonance at –96.4 ppm and a weak shoulder at ca.–92.5 ppm. These two resonances are interpreted as due to variations in the octahedral sheet composition. Therefore TiIV appears to have a strong preference for occupation of the octahedral sites in the zinc-substituted fluorohectorite framework.

Solvate structures in water‐methanol solutions of MRI contrast agents: Electron spin echo envelope modulation in gadolinium chelates

Electron Spin Echo Envelope Modulation (ESEEM) data are reported for three gadolinium (III) chelates, Gd-EDTA, Gd-DTPA, and Gd-TTHA, in frozen D2O/CH3OD glasses. This series includes Gd-DTPA, an MRI contrast agent that is in clinical use. Three-pulse electron spin echo modulation patterns can be modeled to yield geometrical information; e.g., distances and numbers of Gd+3-OD interactions. Interpreted by a spherical shell model, the data for Gd-TTHA are accounted for by Gd+3 surrounded by seven deuterons at an effective distance of 3.9 A. We attribute these deuterons to second-coordination sphere solvent molecules, since it is likely that Gd+3 is fully coordinated by a TTHA ligand. For analysis of ESEEM from Gd-EDTA and Gd-DTPA, the second-sphere structures of Gd-EDTA, Gd-DTPA, and Gd-TTHA are assumed equivalent. 2H-ESEEM from inner-sphere solvent molecules gives the effective distance between deuterons and Gd+3 to be approximately 2.7 A for the inner-sphere water in both EDTA and DTPA complexes. The results of simulations also give the isotropic hyperfine and quadrupole coupling constants for solvent deuterium in these systems. Possible sources of error in the determination of r and q by this method are discussed.

Effects of quadrupole interaction on electron spin echo modulation patterns in disordered systems

Effects of quadrupole interactions on two- and three-pulse electron spin echo modulation patterns are studied by calculating the modulation patterns for model systems. It is found that a small quadrupole coupling gives a characteristic pattern for two-pulse modulation patterns but has a considerably smaller effect on three-pulse modulation patterns.

Analysis of three-pulse electron spin-echo modulation: Phase reversal

Abstract Three-pulse electron spin-echo modulation patterns are analyzed with particular attention paid to the phase reversal of the modulation. For zero isotropic hyperfine coupling a simple expression is derived which relates the time of phase reversal to the dipolar distance to the interacting magnetic nuclei. However, even a small isotrophc hyperfine coupling dramatically affects the time of phase reversal. The absence of phase reversal in the three-pulse echo modulation from trapped electrons in methyltetrahydrofuran glass is shown to be consistent with the results obtained from two-pulse echo modulation studies.