Ab Initio/IGLO/NMR Investigation of nido-C(4)B(7)H(11) Configurations: Structural Assignment of the Three Known Isomers and Reconfirmation of Empirical Carbon Placement Patterns

The ab initio/IGLO/NMR method has been successfully applied to establish the structures of the three known isomers of nido-C(4)B(7)H(11). The method confirms the previously proposed structure, nido-7,8,9,10-C(4)B(7)H(11), 1a, as one of the three known isomers. Of four candidates considered for the second isomer, one of the previously proposed structures, nido-1,7,8,10-C(4)B(7)H(11), 2b, is selected. Of four candidates considered for the third isomer, structure nido-2,7,9,10-C(4)B(7)H(11), 3b, which had not been previously proposed, is established. The relative order of stability is 1a > 2b > 3b. A comparison of the relative energies of the nine cage structures considered in this study shows that, in complete agreement with previous empirically determined patterns, the most stable structures are those in which the carbons occupy low coordinate sites. This preference is more important than avoiding carbon-carbon connections.     

Synthesis of a new zeolite structure ITQ-24, with intersecting 10- and 12-membered ring pores

A new 10- and 12-membered ring zeolite, named ITQ-24, has been synthesized, and its structure has been solved. It has been found that this zeolite structure is topologically identical to that proposed for the hypothetical polymorph C of the SSZ-33/SSZ-26/CIT-1 family. This new zeolite has been achieved by using a rational approach of introducing Ge in the framework that has a directing effect toward zeolite structures with double-four-membered rings as secondary building units. Notoriously, active catalytic centers, such as Ti and Al, have been incorporated into this new zeolite, demonstrating that it is catalytically active for alkylation of aromatics.     

3-苯基-1-丁炔-3-醇质谱碎裂中[C8H7]^+的结构及分子内质子迁移反应

报道了3-苯基-1-丁炔-3-醇的常规电子轰击质谱(EIMS)。利用碰撞诱导解离(CID)技术研究了质谱碎裂过程中产生的[C8H7]^+的气相离子结构。同时, 氘代同位素交换、亚稳(MI)和CID实验进一步证实了m/z 103离子的形成并不是分子离子的质谱碎裂中顺次失去甲基自由基和中性CO分子的直接氢迁移的协同反应, 而是在失去CO分子前后发生了二次质子迁移反应的逐步过程。在此基础上提出了一种独特的双分子质子键合复合物中间体的碎裂机理。     

Chalcogenide based all-solid-state thin electroplated ion-selective membrane for Hg(II) flow-injection determinations

The response to Hg(II) of a thin all-solid-state Te-doped silver chalcogenide membrane, described by the general formula Ag₂ + δSe_{1 − x}Te_x, which has been electrochemically prepared following a previously proposed approach, has been investigated. The kinetics of formation of the membrane's secondary dynamic response to Hg(II) has been successfully combined with the precise timing and transient signal, typical for flow-injection (FI) measurements, in developing a sensitive and reliable mercury FI detector. Under optimized stream conditions it exhibits a linear Nernstian response, with a double slope of the calibration graph of 59 mV dec⁻¹, over the mercury(II) concentration range 10⁻⁶ − 10⁻³ M, the typical sample throughput amounting to about 70 samples per hour. The observed chemical amplification of the signal is due to the specificity of the processes dominating the initial step in formation of the steady-state signal of the membrane to mercury. The analytical performance of the Hg(II) FI detector, as regards sensitivity, reproducibility, selectivity and long-term stability has been thoroughly investigated. The exact procedure for membrane electrodeposition is given and the potential of the proposed approach as a cost-effective way for preparing chalcogenides of unique structure and properties has been outlined in the above context.     

Predicting physical-chemical properties of compounds from molecular structures by recursive neural networks

In this paper, we report on the potential of a recently developed neural network for structures applied to the prediction of physical chemical properties of compounds. The proposed recursive neural network (RecNN) model is able to directly take as input a structured representation of the molecule and to model a direct and adaptive relationship between the molecular structure and target property. Therefore, it combines in a learning system the flexibility and general advantages of a neural network model with the representational power of a structured domain. As a result, a completely new approach to quantitative structure-activity relationship/quantitative structure-property relationship (QSPR/QSAR) analysis is obtained. An original representation of the molecular structures has been developed accounting for both the occurrence of specific atoms/groups and the topological relationships among them. Gibbs free energy of solvation in water, Delta(solv)G degrees , has been chosen as a benchmark for the model. The different approaches proposed in the literature for the prediction of this property have been reconsidered from a general perspective. The advantages of RecNN as a suitable tool for the automatization of fundamental parts of the QSPR/QSAR analysis have been highlighted. The RecNN model has been applied to the analysis of the Delta(solv)G degrees in water of 138 monofunctional acyclic organic compounds and tested on an external data set of 33 compounds. As a result of the statistical analysis, we obtained, for the predictive accuracy estimated on the test set, correlation coefficient R = 0.9985, standard deviation S = 0.68 kJ mol(-1), and mean absolute error MAE = 0.46 kJ mol(-1). The inherent ability of RecNN to abstract chemical knowledge through the adaptive learning process has been investigated by principal components analysis of the internal representations computed by the network. It has been found that the model recognizes the chemical compounds on the basis of a nontrivial combination of their chemical structure and target property.     

Structural studies of Vgamma2Vdelta2 T cell phosphoantigens

Human gammadelta T cells containing the Vgamma2Vdelta2 (Vgamma9Vdelta2) T cell receptor are stimulated by a broad variety of small, phosphorus-containing antigenic molecules called phosphoantigens. The structures of several species present in both Mycobacteria (TUBags1-4) and in Escherichia coli have been reported to contain a formyl-alkyl diphosphate core. Here we report the synthesis of the lead member of the series, 3-formyl-1-butyl diphosphate. This compound has low activity for gammadelta T cell stimulation, unlike its highly active isomer (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate, necessitating a revision of the structure of TUBag1. Likewise, the structure of the species identified as the pentyl analog (TUBag 2) is revised to 6-phosphogluconate. These results indicate that neither TUBag1 nor the m/e 275 species proposed for TUBag2 are 3-formyl-1-alkyl diphosphates, leading to the conclusion that none of the natural phosphoantigens (TUBags1-4) possess the structures reported previously.     

Regioselective Oxazolination of C(70)(2-) and Formation of cis-1 C(70) Adduct with Respect to the Apical Pentagon

Oxazolination of C(70) has been achieved via the aerobic oxidation of C(70)(2-) in the presence of PhCN. Only one C(70) oxazoline regioisomer (1) is obtained, indicating that the oxazolination of C(70)(2-) occurs with an unusual regioselectivity. Further benzylation of 1(2-) with benzyl bromide leads to the formation of the first cis-1 C(70) derivative with respect to the apical pentagon (2), as shown by the X-ray single-crystal structure and various spectral characterizations. The structure of the obtained C(70) oxazoline (1) is resolved with H/D labeling benzylation and HMBC (heteronuclear multiple bond coherence) NMR on the basis of the structure of 2. The result shows that for compound 1, the O atom is selectively bonded to the C1, while the N atom is bonded to the C2 of C(70). The exhibited regioselectivity for the orientation of oxazolino group on C(70) is further rationalized with computational calculations, and a reaction mechanism for the oxazolination of C(70)(2-) is proposed.     

The structure of arachno-[B6H11]-, at -25 degrees C in (CD3)2O,is resolved via the ab initio/IGLO-GIAO/NMR procedure

The arachno-[B6H11]- solution structure at -25 degrees C was clarified as fluxional compound 2 by applying the ab initio/IGLO/NMR method. The anion 2 can be derived from arachno-B6H12, 1, by the removal of the B2/B3 bridging hydrogen (2). No minimum on the potential energy surface could be found for an asymmetric complex, a, between [B5H8]- and BH3, which had been proposed originally. A Cs-symmetric [mu-(BH3)B5H8]- complex, A, only 3.2 kcal mol-1 higher in energy than 2, is the intermediate in the fluxional rearrangement observed on the NMR time scale. The transition structure [D] connecting 2 (Erel = 0.0) and A (Erel = 3.2) has a relative energy of 9.7 kcal mol-1. The elimination of both a and A as "most stable structure" candidates of arachno-[B6H11]- reinforces the early geometrical bonding systematics for boranes and carboranes.     

Fluorescence Dynamics of Monocyclodextrin– and Bis(thiol‐cyclodextrin)–Coumarin C153 Complexes

The solvation and confinement of coumarin C153 within supramolecular host/guest complexes based on β‐cyclodextrin (β‐CD) and 6‐deoxy‐6‐thio‐β‐cyclodextrin (β‐CD‐SH) in water are studied by fluorescence spectroscopy. For β‐CD/C153, the 1:1 complex is proposed, and for β‐CD‐SH/C153 both the 1:1 and 2:1 complexes are believed to be formed. The 2:1 β‐CD‐SH/C153 complex has an association constant of 4.2×10⁵ M ^?1 and a C153 population of 82 %, which are interestingly high values, indicating that the proposed β‐CD‐SH dimers structure are connected by covalent disulfide bonds; this is supported by mass spectrometry. Solvation related to fast hydrogen‐bond rearrangement as a part of fluorescence relaxation is determined by the ultrafast components of time‐resolved spectroscopy to be 3 and 7 ps for the 1:1 β‐CD/C153 and 2:1 β‐CD‐SH/C153 complexes, respectively.     

Stereoselective total syntheses and reassignment of stereochemistry of the freshwater cyanobacterial hepatotoxins cylindrospermopsin and 7-epicylindrospermopsin

A stereoselective total synthesis of the structure 1 proposed for the freshwater cyanobacterial heptatotoxin cylindrospermopsin has been accomplished in approximately 30 operations starting from commercially available 4-methoxypyridine. Utilizing methodology developed by Comins, the tetrasubstituted piperidine A-ring unit of the hepatotoxin was efficiently constructed. The two remaining stereocenters in the natural product were then set by a stereospecific intramolecular N-sulfinylurea Diels-Alder cyclization/Grignard ring opening/allylic sulfoxide [2,3]-sigmatropic rearrangement sequence previously developed in these laboratories, leading to key intermediate 29. The stereochemical assignment of alcohol 29, which contains all six of the stereogenic centers of the natural product, was confirmed by an X-ray crystal structure determination of a derivative. Installation of the D-ring uracil moiety was effected by using our new methodology developed for this purpose, and construction of the C-ring guanidine completed the total synthesis of racemic structure 1. However, the (1)H NMR data for this compound do not match that of cylindrospermopsin, but instead agree with the data reported for 7-epicylindrospermopsin, a minor toxic metabolite that co-occurs with cylindrospermopsin. Therefore, we propose a revision of the stereochemical assignments of these natural products such that cylindrospermopsin is now represented as structure 2 and 7-epicylindrospermopsin is 1. This reassignment was further confirmed by Mitsunobu inversion of the C-7 alcohol 51 to epimer 52, and conversion of this compound to tetracyclic diol 57, which has previously been transformed to cylindrospermopsin (2).     

甲基丙烯酸丁酯在Nd-Al-α,α'-联吡啶催化剂体系中的聚合反应

 研究了Nd（naph）3－Al（i－Bu）3－α,α’－联吡啶（naph＝环烷酸）催化体系中甲基丙烯酸丁酯的聚合反应．用核磁共振和红外光谱表征了聚合物的结构．用凝胶色谱测定了聚合物的分子量和分子量的分布．结果表明,聚合物的结构以全同和无规为主,反应机理为配位聚合,聚合反应对单体浓度呈一级关系,表观活化能为30．2kJ／mol．     

Total Synthesis and Structure Revision of Didemnaketal B

Didemnaketal B, a structurally complex spiroacetal that exhibits potent HIV‐1 protease inhibitory activity, was originally discovered by Faulkner and his colleagues from the ascidian Didemnum sp. collected at Palau. Its absolute configuration was proposed on the basis of degradation/derivatization experiments of the authentic sample. However, our total synthesis of the proposed structure of didemnaketal B questioned the stereochemical assignment made by Faulkner et al. Here we describe in detail our first total synthesis of the proposed structure 2 of didemnaketal B, which features 1) a convergent synthesis of the C7–C21 spiroacetal domain by means of a strategy exploiting Suzuki–Miyaura coupling, 2) an Evans syn‐aldol reaction and a vinylogous Mukaiyama aldol reaction for the assembly of the C1–C7 acyclic domain, and 3) a Nozaki–Hiyama–Kishi reaction for the construction of the C21–C28 side chain domain. The NMR spectroscopic discrepancies observed between synthetic 2 and the authentic sample as well as careful inspection of the Faulkner’s stereochemical assignment led us to postulate that the absolute configuration of the C10–C20 domain of 2 has been erroneously assigned. Accordingly, the total synthesis of the revised structure 65 was achieved to show that the NMR spectroscopic properties of synthetic 65 were in good agreement with those of the authentic sample. Furthermore, application of the phenylglycine methyl ester (PGME) method to the C7–C21 spiroacetal domain enabled us to establish the absolute configuration of didemnaketal B.     

N2O多层膜局域结构的多重散射簇理论研究

利用多重散射簇(multiple scattering cluster, MSC)方法计算了N2O多层膜中氮原子的1s芯态近边X射线吸收精细结构(near edge X-ray absorption fine structure, NEXAFS)谱,首次给出N2O多层膜局域结构的模型. MSC研究显示多层膜中N2O分子以短程有序的分层错位链结构排列,并求得链中相邻分子间距为0.233 nm和相邻分子层之间距离为0.240～0.245 nm.用自洽场离散变分(discrete variation, DV)Xα方法计算的N2O多层膜电子结构支持了MSC的计算结果;阐明了NEXAFS谱中弱结构的物理起源.对N2O多层膜中分子之间相互作用的分析显示N2O多层膜的结构具有分子自组装的特性.     

用分子对接方法预测HIV-1整合酶与金精三羧酸抑制剂的相互作用

用分子对接方法(Docking)研究了HIV-1整合酶与其抑制剂金精三羧酸的结合过程.为弄清金属离子在结合中所起的作用,选择含有一个Mg+2或不含Mg+2的两种不同的整合酶受体分别与金精三羧酸对接.结果表明, Mg+2对稳定配体与受体的结合起了重要作用. 金精三羧酸配体与含有一个金属Mg+2的整合酶受体对接,最优结合自由能为-45.19 kJ/mol. 当Mg+2失去后,整合酶的活性中心构象将发生变化,使金精三羧酸抑制剂与整合酶的结合自由能(-24.35 kJ/mol)明显增加. 预测了未知的HIV-1整合酶与其抑制剂金精三羧酸的复合物结构, 并可对基于结构的抗HIV-1整合酶的药物设计提供重要信息.     

Communication: spectroscopic measurements for HfF+ of relevance to the investigation of fundamental constants

The properties of the HfF(+) cation are thought to be well-suited for investigations of the electron electric dipole moment (eEDM) and temporal variations of the fine structure constant. Precision spectroscopic measurements involving the X(1)Σ(+) and low-lying (3)Δ(1) states have been proposed to measure both. Due to the lack of data for HfF(+), the design of these experiments has relied entirely on the predictions of electronic structure calculations. Spectroscopic characterizations of the X(1)Σ(+), (3)Δ(1), (3)Δ(2) and (3)Δ(3) states are reported. The results further support the contention that HfF(+) is a viable candidate for eEDM measurements. The spacings between adjacent X(1)Σ(+) and (3)Δ(1) levels are found to be less favorable for the proposed studies of the fine structure constant.     

Two Calcium Coordination Polymers with Multi-Stimuli-Responsive Properties

Two skeleton isomers, [CaL(H₂O)(DMF)₂]•DMF ( 1 ) (H₂L=5-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)isophthalic acid) with a 1 dimensional chain structure and [CaL(DMF)_1.72]• (DMF)_0.28 ( 2 ) with a 2 dimensional layer structure were synthesized. Mechanochromic luminescence studies reveal that 1 and 2 exhibit luminescent red-shift and blue-shift respectively under grinding stimuli. The different influence of grinding stimuli on the π⋅⋅⋅π interaction in the two structures are proposed to be the main reason for such different responses. In addition, 1 has a rare broad sensing ability for halogenated hydrocarbons and can distinguish seven halogenated hydrocarbons out of 17 solvents. 1 can also be used for rapid detection of trace water in DMF with a calculated detection limit of 0.0078 v/v %. The relevant recognition mechanism is the decomposition of the structure during the recognition process due to the water instability of the ionic bond. The above results reveal that both compounds have the potential to be used as multi-stimulus-responsive materials.     

磷酸钒钠Na_3V_2(PO_4)_3电化学储能研究进展

锂离子电池在全球范围内的广泛应用加剧了对锂资源的消耗,其成本和原料将限制其未来发展。钠与锂具有相似物理化学性质,并且储量丰富。根据锂离子"摇椅式"电池原理,富钠离子化合物可类似富锂离子正极材料,提供可脱嵌的钠离子及结构。钠离子较锂离子大,其可逆脱嵌反应要求材料结构具有较大的容钠位与离子迁移通道。聚阴离子体磷酸钒钠Na_3V_2(PO_4)_3属于钠离子超导体(NASICON)材料,其NASICON结构骨架形成了稳定的容钠位,并且开放的三维离子迁移通道利于提高钠离子的扩散。Na_3V_2(PO_4)_3作为电池正极材料,具有理想的比容量、电压平台与循环稳定性,从而受到了广泛关注。本文首先介绍了Na_3V_2(PO_4)_3结构特点,其次结合团队已有的工作基础对Na_3V_2(PO_4)_3在钠离子电池、混合离子电池、水系电池,混合超级电容器等体系中的应用与反应机理进行了阐述;总结了基于Na_3V_2(PO_4)_3设计的复合材料与结构并探讨了Na_3V_2(PO_4)_3可能存在的问题与未来发展趋势。     

Crystal structure of an epoxycembradienol, 3,15-epoxy-4-hydroxycembra-7(Z), 11(Z)-diene

The crystal structure of the title compound, C₂₀H₃₄O₂, has been determined by single crystal X-ray diffraction methods at 295 K from diffractometer data using direct methods and refined by least squares to a residual of 0.06 for 2152 “observed” reflections. Crystals are orthorhombic, P2₁2₁2₁, a = 32.770(8), b = 10.960(3), c = 10.850(3) Å, Z=8. The asymmetric unit comprises two independent molecules, both of which confirm the structure proposed in the preceding paper.     

Synthesis of Two Metabolites of the Antiarrythmicum Amiodarone

The metabolism of the potent antiarrythmic drug amiodarone (AMI; 1 ) has yet not been fully investigated. Recently, in vitro experiments revealed that in rabbit‐liver microsomes, AMI ( 1 ) and its main metabolite MDEA ( 2 ) were biotransformed to the hydroxylated derivatives 3′‐OH‐AMI ( 3 ) and 3′‐OH‐MDEA ( 4 ), respectively. To establish the chemical structure of 3 and 4 , we developed a total synthesis of these two metabolites of AMI ( 1 ). ¹H‐ and ¹³C‐NMR Signal assignment from HSQC and HMBC 2D NMR data of synthesized 4 showed that the proposed structure of metabolite 4 is correct. Even the structure of 3 was found to be correct by comparing its HPLC/MS‐MS/MS with the data described earlier.     

Discussion of the catalytic pathway of cysteine proteases based on AM1 calculations

The deacylation reaction of the cysteine protease papain was examined by AM1 reaction-coordinate calculations. The transition-state (TS) structure was extracted from the reaction pathway as corresponding to the maximum point along this minimum-energy pathway. Consistent with experimental kinetic data revealing that deacylation is about 60 times faster for thioester (---C(O)---S---) than dithioester (---C(S)---S---) intermediates, calculated E_a values are about 10 kcal mol⁻¹ lower for the former than the latter. The calculated partial atomic charges indicate that the C=O carbon in the thioester is a good site for nucleophilic attack whereas the corresponding C=S carbon in the dithioester is a poor site. The present calculations reveal that the enzyme's oxyanion hole contributes about 9 kcal mol⁻¹ toward reducing E_a for the anionic tetrahedral intermediate and TS structure. On the other hand, the presence of Asn in the putative Asn-His-Cys catalytic triad contributes only about l kcal mol⁻¹ toward reducing their E_a value. The presence of this Asn, however, did appear to stabilize His in its protonated form (ImH⁺) over its unprotonated form (Im). Two novel mechanisms are introduced to explain the unusual effect of a remote X substituent on the deacylation kinetics of the substrate family under consideration. The first mechanism invokes a “field effect” while the second mechanism embodies the concepts of induction and homoconjugation. The unique feature of these two mechanisms is that, unlike other proposed models, they circumvent the requirement for a close N…S interaction which has stimulated controversy.