NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts

(1)H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host-guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.     

Characterization of active phosphorus surface sites at synthetic carbonate-free fluorapatite using single-pulse 1H, 31P, and 31P CP MAS NMR

The chemically active phosphorus surface sites defined as PO(x), PO(x)H, and PO(x)H2, where x = 1, 2, or 3, and the bulk phosphorus groups of PO4(3-) at synthetic carbonate-free fluorapatite (Ca5(PO4)3F) have been studied by means of single-pulse 1H,31P, and 31P CP MAS NMR. The changes in composition and relative amounts of each surface species are evaluated as a function of pH. By combining spectra from single-pulse 1H and 31P MAS NMR and data from 31P CP MAS NMR experiments at varying contact times in the range 0.2-3.0 ms, it has been possible to distinguish between resonance lines in the NMR spectra originating from active surface sites and bulk phosphorus groups and also to assign the peaks in the NMR spectra to the specific phosphorus species. In the 31P CP MAS NMR experiments, the spinning frequency was set to 4.2 kHz; in the single-pulse 1H MAS NMR experiments, the spinning frequency was 10 kHz. The 31P CP MAS NMR spectrum of fluorapatite at pH 5.9 showed one dominating resonance line at 2.9 ppm assigned to originate from PO4(3-) groups and two weaker shoulder peaks at 5.4 and 0.8 ppm which were assigned to the unprotonated PO(x) (PO, PO2-, and PO3(2-)) and protonated PO(x)H (PO2H and PO3H-) surface sites. At pH 12.7, the intensity of the peak representing unprotonated PO(x) surface sites has increased 1.7% relative to the bulk peak, while the intensity of the peaks of the protonated species PO(x)H have decreased 1.4% relative to the bulk peak. At pH 3.5, a resonance peak at -4.5 ppm has appeared in the 31P CP MAS NMR spectrum assigned to the surface species PO(x)H2 (PO3H2). The results from the 1H MAS and 31P CP MAS NMR measurements indicated that H+, OH-, and physisorbed H2O at the surface were released during the drying process at 200 degrees C.     

On the reactions of molybdenum and tungsten carbonyls with trimethyl- and triethyl-aluminium

Reactions of hexacarbonylmolybdenum, hexacarbonyltungsten and arene complexes of tricarbonylmolybdenum and tricarbonyltungsten with trimethyl- and triethyl-aluminium have been studied. It has been found, based on IR and NMR spectra, that trialkylaluminium does not form complexes with hexacarbonyls of molybdenum and tungsten. Arene (mesitylene, toluene and benzene) complexes of tricarbonylmolybdenum form 1 : 1 complexes with triethylaluminium, and arene complexes of tricarbonyltungsten form complexes with trimethyl- and triethyl-aluminium. Regardless of the molar ratios of reactants (arene)M(CO)₃/AlEt₃, only one of the three CO groups bonded to molybdenum or tungsten forms a complex with AlEt₃. Fast exchange between free and complexed trialkylaluminium and an exchange of trialkylaluminium between all three carbonyl groups have been observed in benzene, toluene and decalin solutions. In the ¹H NMR spectra of the products of the reactions of (mesitylene)Mo(CO)₃ with AIEt₃ and AIMe₃, signals at –9 to –14ppm (characteristic for molybdenum hydrides) were present. It confirmed an alkylation of molybdenum followed by β- or α-hydrogen elimination with the formation of the corresponding molybdenum hydrides, the actual catalyst of aromatic hydrocarbon hydrogenation.     

Investigating sorption on iron-oxyhydroxide soil minerals by solid-state NMR spectroscopy: a 6Li MAS NMR study of adsorption and absorption on goethite

High-resolution 2H MAS NMR spectra can be obtained for nanocrystalline particles of goethite (alpha-FeOOH, particle size approximately 4-10 nm) at room temperature, facilitating NMR studies of sorption under environmentally relevant conditions. Li sorption was investigated as a function of pH, the system representing an ideal model system for NMR studies. 6Li resonances with large hyperfine shifts (approximately 145 ppm) were observed above the goethite point of zero charge, providing clear evidence for the presence of Li-O-Fe connectivities, and thus the formation of an inner sphere Li+ complex on the goethite surface. Even larger Li hyperfine shifts (289 ppm) were observed for Li+-exchanged goethite, which contains lithium ions in the tunnels of the goethite structure, confirming the Li assignment of the 145 ppm Li resonance to the surface sites.     

NMR studies of the thermal degradation of a perfluoropolyether on the surfaces of gamma-alumina and kaolinite

Solid-state nuclear magnetic resonance (NMR) methods are used to follow the thermal degradation of Krytox 1506, a common perfluoropolyether, following adsorption onto the surfaces of gamma-Al2O3 and a model clay (kaolinite). The alumina studies are complemented with thermogravimetric analysis (TGA) to follow the degradation process macroscopically. Molecular-level details are revealed through 19F magic-angle spinning (MAS), 27Al MAS, and 19F --> 27Al cross-polarization MAS (CPMAS) NMR. The CPMAS results show the time-dependent formation of probable VIAl(O6 - nFn) (n = 1, 2, 3) species in which the fluorine atoms are selectively associated with octahedrally coordinated aluminum atoms. For the alumina system, the changes in peak shapes of the CP spectra over time suggest the early formation of catalytically active degradation products, which in turn lead to the formation of additional perfluoropolyether degradation products. Similar to the alumina system, the kaolinite system also displays new resonances in both the 27Al MAS and 19F --> 27Al CPMAS spectra after thermal treatment at 300 degrees C for up to 20 h but reveals a more distinct species at -15.5 ppm that forms at the expense of an initial species (3 ppm), which is in greater abundance at shorter heating times.     

Incorporation of aluminum in the calcium silicate hydrate (C-S-H) of hydrated portland cements: a high-field 27Al and 29Si MAS NMR investigation

The calcium silicate hydrate (C-S-H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO(2) solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by (27)Al and (29)Si MAS NMR spectroscopy. (27)Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the (27)Al isotropic chemical shift (delta(iso)) and quadrupolar product parameter (P(Q) = C(Q)) for tetrahedrally coordinated Al incorporated in the C-S-H phase and for a pentacoordinated Al site. The latter site may originate from Al(3+) substituting for Ca(2+) ions situated in the interlayers of the C-S-H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (delta(iso) = 5.0 ppm and P(Q) = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C-S-H phase has been quantitatively determined from (27)Al MAS spectra at 14.09 T and indirectly observed quantitatively in (29)Si MAS NMR spectra by the Q(2)(1Al) resonance at -81.0 ppm. A linear correlation is observed between the (29)Si MAS NMR intensity for the Q(2)(1Al) resonance and the quantity of Al incorporated in the C-S-H phase from (27)Al MAS NMR for the different samples of hydrated wPc. This correlation supports the assignment of the resonance at delta(iso)((29)Si) = -81.0 ppm to a Q(2)(1Al) site in the C-S-H phase and the assignment of the (27)Al resonance at delta(iso)((27)Al) = 74.6 ppm, characterized by P(Q)((27)Al) = 4.5 MHz, to tetrahedrally coordinated Al in the C-S-H. Finally, it is shown that hydration of wPc in a NaAlO(2) solution results in a C-S-H phase with a longer mean chain length of SiO(4) tetrahedra and an increased quantity of Al incorporated in the chain structure as compared to the C-S-H phase resulting from hydration of wPc in water.     

不同模板剂含量MCM-49分子筛的CTAB修饰及在苯甲醚酰基化反应中的应用

芳香族化合物的Friedel-Crafts酰基化反应是制备芳香酮的一类重要反应,传统的Lewis酸(如AlCl3)和质子酸(如H2SO4)催化剂易制备,价格便宜,但存在对环境污染严重、与产物难分离等问题.因此,近年来研究者一直致力于环境友好催化剂的研发,其中分子筛因选择性好、与产物易分离、可再生和无污染等优点而日益受到人们的重视,尤其是MWW分子筛,由于具有较多的外表面酸性位,而在酰基化反应中表现出良好的抗积碳性能.本课题组曾讨论了对一系列不同模板剂(六亚甲基亚胺, HMI)含量的MCM-49分子筛进行NaOH与十六烷基三甲基溴化铵(CTAB)共处理的情况,而本文重点考察的是只用CTAB处理不同HMI含量的MCM-49分子筛时,其结构与酸性会如何变化,改性前后样品的结构与酸性借助XRD, N2吸脱附等温线测试,29Si与27Al MAS NMR, NH3-TPD, Py-IR与漫反射傅里叶变换红外光谱(DRIFTS)等技术进行表征.另外,将一系列样品用于催化苯甲醚(AN)与乙酸酐(AA)的酰基化反应,考察CTAB处理对MCM-49分子筛催化性能的影响.将含模板剂HMI的MCM-49分子筛原粉在不同温度(250,350,450或550°C)下焙烧,得到一系列不同孔道系统内保留HMI的样品,随后采用CTAB溶液(0.27 mol/L)对其在70°C下进行后处理1 h.结构表征的结果表明,在脱除模板剂HMI及CTAB处理的过程中,分子筛骨架结构基本未被破坏,同时, CTAB处理不能向分子筛中引入任何形式的介孔,无论是晶内介孔还是晶间介孔.在CTAB处理时,可以清除分子筛中部分无定形物种,尤其是无定形Al物种,同时也可能发生部分非骨架Al原子重新进入分子筛骨架的情况,造成骨架铝的比例相对提高.根据29Si与27Al MAS NMR结果可以推断,进入分子筛骨架的Al原子可能会取代T3位置上的Si原子,亦或直接进入T2或T3位置缺陷位.酸性表征的结果表明, CTAB修饰后样品Br?nsted酸量比HMCM-49明显提高,并随HMI含量的逐渐降低呈现先增加后降低的趋势,而Lewis酸量则相对于HMCM-49有所降低.将一系列CTAB修饰前后的MCM-49样品用于催化AN与AA的酰基化反应,反应条件为：压力1.0 MPa、温度110°C、总质量空速WHSV(AN+AA)为10.2 h?1及原料中AN与AA的摩尔比为5：1.反应数据表明,相对于HMCM-49样品, CTAB修饰后样品对应的AA转化率显著提高(初始转化率由51.4%最高增加至85.0%),并随CTAB处理过程中HMI含量的逐渐降低呈现先增加后降低的趋势,而产物选择性则基本没有变化.各样品的AA初始转化率与其Br?nsted酸量基本呈现正向关联.通过间二甲苯歧化反应预积碳堵塞超笼及碱性探针分子2,4-二甲基喹啉吸附覆盖表面半超笼的方法,研究CTAB修饰前后各样品的不同孔道系统对酰基化反应的催化贡献.结果表明,酰基化反应主要发生在MCM-49分子筛的表面半超笼,其次为超笼,正弦孔道的贡献很小.另外, CTAB修饰后样品催化活性的提高主要来自于不含HMI的孔道系统的贡献,进一步验证在CTAB处理过程中,改性作用主要发生在MCM-49分子筛中不含HMI的区域.结合表征和反应评价结果,提高酰基化反应活性需尽可能提高催化剂的Br?nsted酸量,这是分子筛催化剂今后改进的一个主要方向.     

Intramolecular CO ⋯ H interaction in Arene(tricarbonyl)-chromium complexes

Unlike benzene(tricarbonyl)chromium which displays two carbonyl stretching vibrations bands in the IR spectrum, analogous tricarbonylchromium complexes of the general formula (C₆H₅ZMe)Cr(CO)₃ [Z = O, CH(OH), N(Pr), CH=CH] are characterized by three carbonyl bands, one of which is displaced to the low-frequency region. The appearance of that band was rationalized in terms of intramolecular interaction between hydrogen atoms in the substituent on the benzene ring and carbonyl groups.     

Tricarbonylchromium complexes of [5]- and [6]metacyclophane: an experimental and theoretical study

Tricarbonylchromium complexes of [5]- and [6]metacyclophane were prepared and the interaction between the Cr(CO)₃ tripod and the cyclophane fragment was evaluated by both an experimental and a theoretical study. The tricarbonylchromium complex of [5]metacyclophane could only be obtained in solution and was characterized by its ¹H NMR spectrum. The tricarbonylchromium complex of [6]metacyclophane was isolated and an X-ray crystal structure was obtained, which reveals that no significant geometric changes occur upon coordination of the severely distorted aromatic ring. Computations on the tricarbonylchromium complexes of m-xylene, [5]- and [6]metacyclophane furthermore demonstrate that the corresponding complexation energy is remarkably unaffected by the degree of distortion of the aromatic ring. Theoretical analyses of the above model systems as well as complexes of planar and artificially deformed benzene with Cr(CO)₃ show that this is primarily the result of two counteracting effects: (i) a stabilization due to an increased back-donation from the metal center to the benzene and (ii) a destabilization due to the increasing strain in the aromatic ring.     

Isotropic chemical shifts in magic-angle spinning NMR spectra of proteins

Here we examine the effect of magic-angle spinning (MAS) rate upon lineshape and observed peak position for backbone carbonyl (C') peaks in NMR spectra of uniformly-(13)C,15N-labeled (U-(13)C,15N) solid proteins. 2D N-C' spectra of U-(13)C,15N microcrystalline protein GB1 were acquired at six MAS rates, and the site-resolved C' lineshapes were analyzed by numerical simulations and comparison to spectra from a sparsely labeled sample (derived from 1,3-(13)C-glycerol). Spectra of the U-(13)C,15N sample demonstrate large variations in the signal-to-noise ratio and peak positions, which are absent in spectra of the sparsely labeled sample, in which most 13C' sites do not possess a directly bonded 13CA. These effects therefore are a consequence of rotational resonance, which is a well-known phenomenon. Yet the magnitude of this effect pertaining to chemical shift assignment has not previously been examined. To quantify these effects in high-resolution protein spectra, we performed exact numerical two- and four-spin simulations of the C' lineshapes, which reproduced the experimentally observed features. Observed peak positions differ from the isotropic shift by up to 1.0 ppm, even for MAS rates relatively far (a few ppm) from rotational resonance. Although under these circumstances the correct isotropic chemical shift values may be determined through simulation, systematic errors are minimized when the MAS rate is equivalent to approximately 85 ppm for 13C. This moderate MAS condition simplifies spectral assignment and enables data sets from different labeling patterns and spinning rates to be used most efficiently for structure determination.     

Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additions of anions, cations, and radicals.

A computational and experimental study of additions of electrophiles, nucleophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomplexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylchromium)-2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-phenylethane arising from addition to the complexed arene. Radical conditions (SmI(2)) afford two isomeric products, reflecting a 2:1 preference for radical addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzene and (benzene)tricarbonylchromium show that addition to the complexed aromatic ring is faster than attack on the noncomplexed species by a factor of at least 100,000. Density functional theory calculations using the B3LYP method, employing a LANL2DZ basis set for geometry optimizations and a DZVP2+ basis set for energy calculations, for all three reactive intermediates showed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radical addition leads to exclusive reaction on the complexed arene ring as predicted by the computations. The intramolecular radical reaction involves initial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homolytic cleavage of an exo bond in the intermediate cyclohexadienyl radical complex. Mechanisms are explored for electrophilic addition to complexed arenes. The calculations strongly favor a pathway in which the cation initially adds to the metal center rather than to the arene ring.     

95Mo magic angle spinning NMR at high field: improved measurements and structural analysis of the quadrupole interaction in monomolybdates and isopolymolybdates

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis.     

Determining secondary structure in spider dragline silk by carbon-carbon correlation solid-state NMR spectroscopy

Two-dimensional (2D) (13)C-(13)C NMR correlation spectra were collected on (13)C-enriched dragline silk fibers produced from Nephila clavipes spiders. The 2D NMR spectra were acquired under fast magic-angle spinning (MAS) and dipolar-assisted rotational resonance (DARR) recoupling to enhance magnetization transfer between (13)C spins. Spectra obtained with short (150 ms) recoupling periods were utilized to extract distinct chemical shifts for all carbon resonances of each labeled amino acid in the silk spectra, resulting in a complete resonance assignment. The NMR results presented here permit extraction of the precise chemical shift of the carbonyl environment for each (13)C-labeled amino acid in spider silk for the first time. Spectra collected with longer recoupling periods (1 s) were implemented to detect intermolecular magnetization exchange between neighboring amino acids. This information is used to ascribe NMR resonances to the specific repetitive amino acid motifs prevalent in spider silk proteins. These results indicate that glycine and alanine are both present in two distinct structural environments: a disordered 3(1)-helical conformation and an ordered beta-sheet structure. The former can be ascribed to the Gly-Gly-Ala motif while the latter is assigned to the poly(Ala) and poly(Gly-Ala) domains.     

Direct solid-state NMR spectroscopic evidence for the NH4AlF4 crystalline phase derived from zeolite HY dealuminated with ammonium hexafluorosilicate

Multinuclear (1)H, (19)F, and (27)Al MAS (magic angle spinning) and corresponding 2D HETCOR (heteronuclear correlation) NMR spectroscopy, in combination with powder XRD measurements, provide the direct evidence for the NH(4)AlF(4) crystalline phase, which was formed from zeolite HY dealuminated with an aqueous (NH(4))(2)SiF(6) solution at 80 degrees C. The NH(4)AlF(4) crystalline phase exhibits a characteristic second-order quadrupolar-induced (27)Al NMR line shape spreading from 0 to -90 ppm (in a magnetic field of 11.7 T) and two (19)F resonances at -151 and -166 ppm in the (19)F NMR spectrum. An( 27)Al quadrupolar coupling constant (C(Q)) of 9.5 MHz and an asymmetry parameter (eta) of 0.1 were identified, for the first time, for the NH(4)AlF(4) crystalline phase observed. On the basis of the (19)F{(27)Al} TRAPDOR (transfer population in double resonance) NMR results, the (19)F resonances at -151 and -166 ppm are therefore assigned to (19)F spins associated with the fluorines in the terminal Al-F and the bridging Al-F-Al groups, respectively.     

Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by 31P and 7Li solid state NMR spectroscopy

Here we report the observation of electron delocalization in nano-dimension xLiFePO(4):(1 - x)FePO(4) (x = 0.5) using high temperature, static, (31)P solid state NMR. The (31)P paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct (31)P resonances arising from FePO(4) and LiFePO(4) are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 °C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. (7)Li MAS NMR spectra of nanometre sized xLiFePO(4):(1 - x)FePO(4) (x = 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO(4) and FePO(4) domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the (7)Li MAS spectrum of LiMnPO(4), which despite being iso-structural with LiFePO(4) yields a remarkably different (7)Li MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO(4) the higher (7)Li MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM ≈ 500 Hz) is attributed to an additional unpaired electron in the t(2g) orbital as compared to LiFePO(4) which has δ(iso) = -11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO(4) is iso-electronic and iso-structural with LiMnPO(4). This similarity is readily observed in the (7)Li MAS spectrum of xLiFePO(4):(1 - x)FePO(4) (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO(4). Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO(4) and the Li-environments generated upon cycling.     

Spectroscopic characterization of natural calcite minerals

The FT-IR, FT-Raman, NMR spectral data of ten different limestone samples have been compared. FT-IR and FT-Raman spectral data show that calcium carbonate in limestone, principally in the form of calcite, as identified by its main absorption bands at 1426, 1092, 876 and 712 cm(-1). The sharp diffractions at the d-spacings, 3.0348, 1.9166 and 1.8796 confirm the presence of calcite structure and the calculated lattice parameters are: a=4.9781 A, c=17.1188 A. The range of 13C chemical shifts for different limestone samples is very small, varying from 198.38 to 198.42 ppm. The observed chemical shifts are consistent with the identical C-O bonding in different limestone samples. 27Al MAS NMR spectra of the samples exhibit a central line at 1 ppm and another line at 60 ppm corresponding to octahedral and tetrahedral Al ions, respectively. The five component resonances were observed in 29Si MAS NMR spectrum of limestone and these resonances were assigned to Si (4 Al), Si (3 Al), Si (2 Al), Si (1 Al) and Si (0 Al) from low field to high field.     

Synthesis,structure, and NMR spectra of a new ferrocenylbenzene tricarbonylchromium complex

Complex 1 (1,2,4-triferrocenylbenzene tricarbonylchromium), possessing four heterometals, has been synthesized efficiently by a new method in one pot. Through detailed study on the NMR chemical shifts of 1 and the ligand 2, the NMR chemical shifts of 1 shift upfield from those of 2. The NMR spectra of 1 have reduced magnetic anisotropy of benzene after Cr(CO)₃ coordination with 2. The ferrocenyl groups have electron donating effect on the central benzene ring of 1.     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

An experimental study of various arenetricarbonylchromium complexes

For a series of arenetricarbonylchromium complexes ZCr(CO)₃, analysis of the electric dipole moments in benzene and carbonyl stretching frequencies in cyclohexane suggest the following: for unsubstituted ZCr(CO)₃, with Z = diphenyl, fluorene and cis-stilbene, the π-basicity to be considered is that of the ring directly united to the tricarbonylchromium group, and not that of the arene as a whole. In general, the substituent effect in p-substituted diphenyltricarbonylchromium complexes (Z = 4-methyl-, 4-amino- and 4,4′-dimethyldiphenyl), in which the tricarbonylchromium group is bonded to the substituted phenyl ring, is markedly weaker than that in the corresponding benzenetricarbonylchromium complexes. In 4-fluorodiphenyltricarbonylchromium, the tricarbonylchromium group is attached to the unsubstituted phenyl ring. 4-Aminodiphenyltricarbonylchromium exists in two distinct forms: one with the nitrogen lone pair away from, and the other close to, the metal atom. Diphenylmethanetricarbonylchromium (and its 4,4′-diamino derivative), indanetricarbonyl- and 4,4′-difluorodiphenyltricarbonylchromium have also been examined.