Theoretical determination of the 1H NMR spectrum of ethanol

GIAO calculations of the ¹H NMR chemical shifts for ethanol at the SCF and DFT levels of theory are presented. The importance of molecular geometry and basis set is discussed. Vibrational correction to the hydroxyl proton chemical shift is also considered in calculations for the monomer of ethanol. The final theoretical results for the monomer obtained at the optimized DFT/B3LYP/6-311G(d,p) geometry with the 6-311G++ (d,p) basis set for NMR are in very good agreement with gas phase experimental data. For the liquid phase ethanol the hydrogen bonding effects are taken into account by performing calculations on various clusters of ethanol. It is shown that inaccuracy due to molecular geometry and basis set in the monomer of ethanol is magnified significantly in calculations for its clusters. In this context the structure of liquid ethanol as predicted recently by quantum cluster equilibrium (QCE) theory is discussed.     

23Na NMR study of the local order in the Na1/2Bi1/2TiO3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the ²³Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na_1/2Bi_1/2TiO₃ structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] _p direction and a small displacement statistically or dynamically disordered over six equally probable [100] _p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] _p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.     

Structure of molecules in crystalline lattice obtained by a modified method of molecular mechanics: Calculations of<Superscript>13</Superscript>C chemical shifts

A program for the calculation of the geometric structure of molecular crystals on the basis of the methods of molecular mechanics (MM) has been developed. A standard MM method has been modified by including force fields taking into account the specific (H-bond and van der Waals) interactions and the periodicity of the crystal lattice of an arbitrary form and symmetry. The geometric parameters of the molecule in a crystal calculated by this method are in agreement with the experimental X-ray data within reasonable accuracy. The nuclear magnetic resonance¹³C chemical shifts have been calculated for the molecular geometry obtained by the quantum chemical UB3LYP/6-31G(d, p) method. The results of the calculations have been used to explain some unusual NMR spectral data.     

The 2‐norbornyl cation: a retrospective

Decades of research on the structure of the 2‐norbornyl cation afford a new understanding of bonding in carbon compounds and of the electron‐donor capacity of carbon–carbon σ bonds. The bridged or “nonclassical” structure of the 2‐norbornyl cation, which features 3‐center, 2‐electron bonding, is supported by (inter alia) X‐ray crystallography, NMR spectroscopy, calculations, thermochemical studies, and copious mechanistic evidence. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental and four‐component relativistic DFT studies of tungsten carbonyl complexes

We present a theoretical and experimental study of the structure and nuclear magnetic resonance (NMR) parameters of the pentacarbonyltungsten complexes of η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine, η²‐norbornene, and imidazolidine‐2‐thione. The three complexes have a pseudo‐octahedral molecular structure with the six ligands bonded to the tungsten atom. The η¹‐2‐(trimethylstannyl)‐4,5‐dimethylphosphinine‐pentacarbonyl tungsten complex was synthesized for the first time. For all compounds, we present four‐component relativistic calculations of the NMR parameters at the Dirac–Kohn–Sham density functional level of theory using hybrid functionals. These large‐scale relativistic calculations of NMR chemical shifts and spin–spin coupling constants were compared with available experimental data, either taken from the literature or measured in this work. The inclusion of solvent effects modeled using a conductor‐like screening model was found to improve agreement between the calculated and experimental NMR parameters, and our best estimates for the NMR parameters are generally in good agreement with available experimental results. The present work demonstrates that four‐component relativistic theory has reached a level of maturity that makes it a convenient and accurate tool for modeling and understanding chemical shifts and indirect spin–spin coupling constants of organometallic compounds containing heavy elements, for which conventional non‐relativistic theory breaks down. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessment of the aromaticity of borepin rings by spectroscopic,crystallographic and computational methods: a historical overview

This review presents a chronological discussion of the evolution of our conceptual and experimental understanding of aromaticity as pertaining to the borepin ring structure. Borepin is the boron‐containing charge‐neutral analogue of the carbocyclic tropylium ion, and many molecular variations involving the borepin motif have been synthesized over the past half century. The aromaticity of the borepin system has been probed with ultraviolet–visible (UV–vis), photoluminescence and infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X‐ray crystallography and computational analysis. Recently, the focus of borepin‐containing compounds has shifted to π‐electron materials, building on the foundation of a firm understanding of the physical organic properties of the borepin motif that will allow for electronic fine‐tuning toward desired applications. Copyright © 2015 John Wiley & Sons, Ltd.     

氨基酸原子电距矢量表达与核磁共振碳谱模拟

提出以原子电性距离矢量(VAED)描述20种天然氨基酸中不同等价碳原子的化学环境,结合γ效应校正与碳原子类型,建立核磁共振碳谱(¹³CNMR)化学位移(CS)的五参数线性模型.用于氨基酸分子中4类不同的等价碳原子化学位移的估计,复相关系数分别为R=0.9864,0.9513,0.9463,0.9567;均方根误差分别为RMS=0.5544,2.5232,11.3096,4.9098ppm经交互校验,模型稳定性较好.并综合几种处理方法,找到一种较好的建模方法,将它用于4个外部样本化学位移的定量预测,结果良好.     

High-resolution solid-state NMR spectroscopy as a tool for investigation of enantioselective inclusion complexation

In this paper, we showed the application of solid state-NMR (SS NMR) spectroscopy in structural studies of chiral compounds employing sample of (E)-1-diphenylphosphinoylpent-3-en-2-ol 1 as a model compound. Racemate of 1 was fully characterized by NMR techniques (both in liquid and solid phase) and X-ray crystallography. Theoretical calculations employing the GIAO approach were used to explain the influence of hydrogen bonding on 31P NMR shielding parameter in racemate. Enantioselective inclusion complexation (EIC) method with TADDOL as host molecule was applied to separate of enantiomers. The formation of host-guest complex and decomplexation procedure was monitored by means of the SS NMR. The liquid-state NMR, due to similarity of 13C and 31P spectral parameters was not able to distinguish racemate from enantiomer. In the solid phase, owing to distinction of hydrogen bonding and molecular packing in the crystal lattice, racemate and enantiomers were easy recognized by NMR spectroscopy.     

双分子缬氨酸氢膦烷构型的确定

以缬氨酸为原料,和三氯化磷反应合成了双分子缬氨酸氢膦烷．采用¹H NMR、¹³C NMR、¹H-¹H COSY、DEPT和HSQC等技术确证了目标化合物的结构;对其¹H NMR和¹³C NMR信号进行了归属,同时利用IR和ESI-MS验证了其结构．³¹P NMR显示所得产物为缬氨酸氢膦烷的一对非对映异构体,利用NOESY技术对分离得到的其中一种异构体绝对构型进行了确定,该结果得到了X-射线单晶衍射的验证．     

Synthesis,spectroscopic characterization,and computed optical analysis of green fluorescent cyclohexenone derivatives

Cyclohexenone containing chalcones core is one important class of materials, which exhibit high nonlinear optical (NLO) responses and good crystallizability. The present study reports the successful development of six new fluorescent cyclohexenone derivatives (CDs) via conventional Robinson annulation method. The molecular structures of these newly synthesized CDs were confirmed by using various analytical techniques such as ¹H NMR, ¹³C NMR, FTIR, EIMS, UV–Vis spectroscopy and single crystal X‐ray diffraction. The crystallographic data revealed that the spatial structure of the representative CD (4BE) belongs to monoclinic, P2₁/c space group. The results from luminescence studies show that the CDs molecules apparently emit intense green light at room temperature in aqueous media. The relative polarity and molecular chemical stability of the CDs molecules were predicted by measuring the molecular electrostatic potential and frontier molecular orbital energy. In addition, the UV–Vis spectra, transition character and electronic structures of these CDs were computed by using quantum chemical methodology. It was interesting to note that the values of computed and experimental electronic transitions (λ_max) were in good agreement and these CDs display high hyperpolarizability (β) values. The present work will be helpful for systematical understanding of the structures and the optical properties of CDs for studying the structure–activity relationship that will suggest their potential application in photonic devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Conformational flexibility of xanthene‐based covalently linked dimers

The conformational flexibility of three covalently linked dimers consisting of two xanthene‐based moieties connected by a diphenyl ether linker was studied using NMR spectroscopy, X‐ray crystallography, and density functional theory (DFT) calculations. The three dimers interconvert as a function of pH: the doubly cationic dimer (Xan⁺)₂ exists in acidic solutions (pH < 0.5), the mono‐alcohol monocation Xan⁺–Xan‐OH at intermediate pH values (pH = 1–3), and the neutral diol at the highest pH‐values (pH > 3). Each dimer exhibits conformational degrees of freedom associated with rotations of either the xanthene moiety or of the diphenyl ether (DPE) linker. The barriers for rotation of the xanthylium moiety were evaluated using DFT calculations, yielding values of 23 kcal/mol for (Xan⁺)₂ and 11 kcal/mol for (Xan‐OH)₂, respectively. The rotational barrier for the diphenyl ether linker in Xan⁺–Xan‐OH (15 kcal/mol) was experimentally determined using variable temperature NMR measurements. The relative orientation of the two –OH groups in (Xan‐OH)₂ diol was investigated in solution and the solid state using NMR spectroscopy and X‐ray crystallography. The conformer observed in the solid state was found to be the In–Out conformer, while free rotation of the xanthenol units is thought to occur on the NMR timescale at room temperature. These studies are relevant for the design of linkers for efficient water oxidation catalysts. Copyright © 2016 John Wiley & Sons, Ltd.     

A study of a moleculartweezer host-guest system by a combination of quantum-chemical calculations and solid-state NMR experiments

A study of a host-guest system consisting of a naphthalene-spaced tweezer with a 1,4 dicyanobenzene guest molecule is presented. The complex is investigated using a combination of quantum-chemical calculations and solid-state NMR experiments. The advantages of such an approach are illustrated. The focus is on the calculation of (1) 1H NMR and (2) 13C NMR chemical shifts for model fragments of the solid-state structure, (3) the analysis of host-guest interactions important for molecular recognition, and (4) the investigation of the process of a guest molecule rotation. For modeling the solid-state structure, up to three host-guest units are considered and the convergence with respect to the size of the solid-state fragment is investigated.     

Molecular basis of structural make-up of feeds in relation to nutrient absorption in ruminants,revealed with advanced molecular spectroscopy: A review on techniques and models

Progress in ruminant feed research is no more feasible only based on wet chemical analysis, which is merely able to provide information on chemical composition of feeds regardless of their digestive features and nutritive value in ruminants. Studying internal structural make-up of functional groups/feed nutrients is often vital for understanding the digestive behaviors and nutritive values of feeds in ruminant because the intrinsic structure of feed nutrients is more related to its overall absorption. In this article, the detail information on the recent developments in molecular spectroscopic techniques to reveal microstructural information of feed nutrients and the use of nutrition models in regards to ruminant feed research was reviewed. The emphasis of this review was on (1) the technological progress in the use of molecular spectroscopic techniques in ruminant feed research; (2) revealing spectral analysis of functional groups of biomolecules/feed nutrients; (3) the use of advanced nutrition models for better prediction of nutrient availability in ruminant systems; and (4) the application of these molecular techniques and combination of nutrient models in cereals, co-products and pulse crop research. The information described in this article will promote better insight in the progress of research on molecular structural make-up of feed nutrients in ruminants.     

Electronic structure of binary zirconium compounds: Cluster calculations using different molecular orbital methods

Cluster calculations of the electronic structure and charge distribution in the refractory compounds ZrX(X = C, N, O) and ZrO₂ have been performed using different molecular orbital methods; a semiempirical LCAO model based on the Mulliken-Wolfsberg-Helmholz approximation, and the Hartree-Fock-Slater model in both the Discrete Variational X_α and Multiple Scattering X_α versions. The main features of chemical bonding are discussed and illustrated by contour level diagrams of some bonding molecular orbitais of the valence band. Finally, the results are compared with X-ray emission and X-ray photoelectron spectra, and also with band structure calculations.     

Towards a high-accuracy kinetic database informed by theoretical and experimental data: CH3 + HO2 as a case study

The reliability of predictive simulations for advanced combustion engines depends on the availability of accurate data and models for thermochemistry, chemical kinetics, and transport. In that regard, accurate data are critically important for both their direct use in predictive simulations and for benchmarking improved theoretical methodologies that can similarly produce accurate data for predictive simulations. The use of informatics-based strategies for the determination of accurate thermochemical data with well-defined uncertainties, e.g. the Active Thermochemical Tables (ATcT), has revolutionized the field of thermochemistry–providing thermochemical data of unprecedented accuracy for predictive combustion simulations and has served as a key enabler of ab initio electronic structure methodologies of equally impressive accuracy. Clearly, pursuit of informatics-based analogs in chemical kinetics would be similarly worthwhile. Here, we present results and analyses for the kinetics of CH₃ + HO₂ reactions that demonstrate some key elements of any approach to developing analogs for kinetics, including: the importance of raw data for quantifying the information content of experimental data, the utility of theoretical kinetics calculations for constraining experimental interpretations and providing an independent data source, and the subtleties of target data selection for avoiding unphysical parameter adjustments to match data affected by structural uncertainties. Notably, we find the optimization performed here using the MultiScale Informatics (MSI) approach applied to carefully selected (mostly raw) experimental data yields an opposite temperature dependence for the channel-specific CH₃ + HO₂ rate constants as compared to a previous rate-parameter optimization. While both optimization studies use the same theoretical calculations to constrain model parameters, only the present optimization, which incorporates theory directly into the model structure, yields results that are consistent with theoretical calculations.     

On the crystallographic structure of small particles of platinum supported on graphite

The shape and crystallography of small metallic particles of platinum on a graphite substrate are studied by high resolution weak beam dark field and microdiffraction electron microscopy techniques. It is found that they have the fcc bulk crystal structure and their shape is a cubooctahedron bounded by {111} and {100} faces. The implications for the catalytic reaction of neopentane on Pt/C catalyst are discussed.     

Analysis of hydrogen bonding and weak interactions in the crystal structure of (E)-N-(4-ethylphenyl)-2-(4-hydroxybenzylidene)thiosemicarbazone: experimental and theoretical studies

Hydrogen-bonding interactions play an important role in the rational design of crystal systems with desirable architectures. The novel thiosemicarbazone derivative described herein, namely (E)-N-(4-ethylphenyl)-2-(4-hydroxybenzylidene)thiosemicarbazone, C₁₆H₁₇N₃OS, (I), was prepared and characterised by ¹H NMR, IR and single-crystal X-ray crystallography techniques. The compound is arranged in the lattice by O–H···S and N–H···S bonded polymeric ribbons that extend along the crystal b-axis, and the intermolecular N–H···S hydrogen bonds formed R2 2(8) ring motifs. More importantly, C–H···π interaction stabilises the supramolecular structure of (I). Hirshfeld surface and their associated two-dimensional fingerprint plot analyses are presented to illustrate the supramolecular connectivity in the solid state. The result shows that the short H···H contacts is dominated in the total Hirshfeld surface. As well as we report on n→π* interactions in thiosemicarbazone derivatives by using the reduced density gradient function and natural bond orbital analyses. Besides, molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) analysis of the title compound are also investigated by theoretical calculations.     

Solid-state NMR studies of supercapacitors

Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.