Distribution of Europium Ions in Ion-Exchanged NaX and NaY Zeolites

The chemical environments of europium-exchanged NaX (Si/Al =1.16) and NaY (Si/Al = 2.29) zeolites have been investigated by means of ¹²⁹Xe NMR and isotherm measurements of adsorbed xenon. EuNaX and EuNaY samples with varied concentrations of Eu cations were subjected to diverse chemical and thermal treatments, namely dehydration, reduction, oxidation, and re-reduction. Thermal analyses of hydrated EuNaX and EuNaY samples indicate that both the structural stability and the saturation concentration of water increase with increasing Eu content. For dehydrated EuNaY zeolites, the Eu³⁺ cations tend to replace Na⁺ ions at S2 sites and tend to be located in framework supercages; similar behavior is found for Eu²⁺ ions after reduction. After subsequent oxidation, Eu³⁺ ions migrate from supercages into small sodalite and/or D6R cages; similar results were deduced for samples after re-reduction. In contrast to the behavior observed in EuNaY, Eu³⁺ ions tend to exchange for Na⁺ ions in the sodalite and/or D6R cages in dehydrated EuNaX zeolites, regardless of the thermal treatment; this behavior is ascribed to the existence of unlocalized S3 Na⁺ in EuNaX samples.     

Coking and Deactivation of H-ZSM-5 Zeolites during Ethylbenzene Disproportionation: I. Formation and Location of Coke

The mode of deactivation and coke formation in a H-ZSM-5 zeolite during ethylbenzene disproportionate has been investigated by TGA, adsorption, IR, and ¹²⁹Xe NMR techniques. The initial rate of deactivation and the total coke content are found to increase with increasing conversion level. Within the range of conversion (54-63%) covered in the present study, the location and extent of coke deposition can be divided into three distinct regions. At low coke content, coke is found to deposit on the Brønsted acid sites and resulted in a notable decrease in catalytic activity. As the amount of coke increases (≤ 7 wt%), while the catalytic activity decreases slightly, most of coke still tends to deposit within the zeolite channels. The slight decrease in catalytic activity is ascribed to steric hindrance by coke residues. When the coke content exceeds 7 wt%, while the activity remains almost constant, the deposition of coke on the external surface of the zeolite crystallites is evident. It is found that neither the pore opening nor the acid site were completely blocked off by coke residues even at the highest coke loading.     

不同镧含量LaNaY沸石的^129Xe NMR研究

用＾１２９ＸｅＮＭＲ技术研究了ＬａＮａＹ沸石在不同镧含量时的离子定位情况，发现经３５０℃抽真空预处理后，即使在低镧含量时也有部分镧离子留在超笼内，其数目取决于沸石中总的镧含量，当平均每单胞内镧离子数达到８个后，在３５０℃下方钠石笼中出现镧离子对，不可逆地形成桥式镧氧（氢氧）化物；但低于此含量时，镧离子处于不同方钠石笼中，面成对发生反应，通常认为Ｎａ＾＋离子与Ｘｅ的化学位移无关，但＾１２９ＸｅＮＭＲ     

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized ¹²⁹Xe gas spins to ¹H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized ¹²⁹Xe into solution, we can enhance ¹H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study ¹²⁹Xe interactions in solutions.     

Cucurbit[6]uril Hyperpolarized Chemical Exchange Saturation Transfer Pulse Sequence Parameter Optimization and Detectability Limit Assessment at 3.0T

Molecular imaging is the future of personalized medicine; however, it requires effective contrast agents. Hyperpolarized chemical exchange saturation transfer (HyperCEST) can boost the signal of Hyperpolarized ¹²⁹Xe MRI and render it a molecular imaging modality of high efficiency. Cucurbit[6]uril (CB6) has been successfully employed in vivo as a contrast agent for HyperCEST MRI, however its performance in a clinical MRI scanner has yet to be optimized. In this study, MRI pulse sequence parameter optimization was first performed in CB6 solutions in phosphate-buffered saline (PBS), and subsequently in whole sterile citrated bovine blood. The performance of four different depolarization pulse shapes (sinusoidal, 3-lobe sinc (3LS), rectangular (block), and hyperbolic secant (hypsec) was optimized. The detectability limits of CB6 in a clinical 3.0T MRI scanner was assessed using the optimized pulse sequences. The 3LS depolarization pulses performed best, and demonstrated 24 % depletion in a 25 μM solution of CB6 in PBS. It performed similarly in blood. The CB6 detectability limit was found to be 100 μM in citrated bovine blood with a correspondent HyperCEST depletion of 30 % ±9 %. For the first time, the HP ¹²⁹Xe HyperCEST effect was observed in red blood cells (RBC) and had a similar strength as HyperCEST in plasma.     

Spin‐orbit ZORA and four‐component Dirac–Coulomb estimation of relativistic corrections to isotropic nuclear shieldings and chemical shifts of noble gas dimers

Hartree–Fock and density functional theory with the hybrid B3LYP and general gradient KT2 exchange‐correlation functionals were used for nonrelativistic and relativistic nuclear magnetic shielding calculations of helium, neon, argon, krypton, and xenon dimers and free atoms. Relativistic corrections were calculated with the scalar and spin‐orbit zeroth‐order regular approximation Hamiltonian in combination with the large Slater‐type basis set QZ4P as well as with the four‐component Dirac–Coulomb Hamiltonian using Dyall's acv4z basis sets. The relativistic corrections to the nuclear magnetic shieldings and chemical shifts are combined with nonrelativistic coupled cluster singles and doubles with noniterative triple excitations [CCSD(T)] calculations using the very large polarization‐consistent basis sets aug‐pcSseg‐4 for He, Ne and Ar, aug‐pcSseg‐3 for Kr, and the AQZP basis set for Xe. For the dimers also, zero‐point vibrational (ZPV) corrections are obtained at the CCSD(T) level with the same basis sets were added. Best estimates of the dimer chemical shifts are generated from these nuclear magnetic shieldings and the relative importance of electron correlation, ZPV, and relativistic corrections for the shieldings and chemical shifts is analyzed. © 2015 Wiley Periodicals, Inc.     

用于氙灯阵列的渐开线翼面反射器

 用光线追踪法分析反射翼几何形状对氙灯反射器光传输效率的影响。提出一种新的反射器几何构型——仿渐开线型,其外形与标准渐开线相似,但翼展宽度短于标准渐开线反射器。用蒙特卡罗法模拟氙灯光能量在激光泵浦腔内的反射和吸收过程,计算结果显示：在合理选择反射翼的几何修正系数前提下,“仿渐开线型”反射器的光传输效率优于“短翼渐开线型”的光传输效率。在腔长、灯内径等设计约束条件受限的条件下,采用“仿渐开线”设计方法,能够最大限度地增大氙灯阵列输出的总泵浦能量。     

NMR Study of Laser polarized ~(129) Xe in Low Pressure Natural Xenon Gas

Ｉｎｔｒｏｄｕｃｔｉｏｎ　　Ｂｙｔｈｅｍｅｔｈｏｄｏｆｌａｓｅｒｏｐｔｉｃａｌｐｕｍｐｉｎｇｓｐｉｎ ｅｘｃｈａｎｇｅ ,ｔｈｅＮＭＲｓｉｇｎａｌｆｒｏｍ 1 2 9Ｘｅｇａｓｃａｎｂｅｇｒｅａｔｌｙｅｎｈａｎｃｅｄ[1 ] .Ｔｈｅｏｂｓｅｒｖｅｄｒａｔｉｏｏｆｓｉｇｎａｌｔｏｎｏｉｓｅｉｓｂｅｔｔｅｒｔｈａｎ 10 0 .Ｔｈｅａｍｐｌｉｆｉｃａｔｉｏｎｆａｃｔｏｒｏｆｔｈｅｎｕｃｌｅａｒｓｐｉｎｐｏｌａｒｉｚａｔｉｏｎｏｆｌａｓｅｒ ｐｏｌａｒｉｚｅｄ 1 2 9Ｘｅｉｓ 10 4ｂｙｃｏｍｐａｒｉｓｏｎｗｉｔｈｔｈｅＢ…     

Prediction of the thermal properties of CO2, CO, and Xe laser media

Approximation functions describing the experimental data of thermal conductivity and viscosity of chosen gases (CO₂, N₂, He, Xe, CO, O₂, Ar) are given in the paper. Introduced formulas allow to predict thermal conductivity and temperature distribution of typical high-power laser gas mixture. Examples of temperature distribution in RF excited CO₂, CO, and Xe laser media are shown. Knowledge of the temperature distribution in the laser cavity can be useful for predicting the general properties of laser.     

高灵敏度的脉冲氦离子化检测器分析混合气体中的Kr、Xe

使用高灵敏度的脉冲氦离子化检测器（PDHID）气相色谱法对Kr、Xe浓度进行定量分析,并通过采用反应床除去杂质气体或与Kr、Xe难以分离的干扰组分N2、CH4和CO等,可使最低检测限达到Kr为7．931×10^-8、Xe为7．186×10^-8．对Kr的测量相对偏差在2％以内,对Xe的测量相对偏差在6％以内．