ESCA: Electron Spectroscopy for Chemical Analysis

A substance on which X-rays fall emits photoelectrons and Auger electrons. The energy spectra of the electrons emitted provide information about the electronic structure in the specimen, ranging from the innermost atomic levels and their dependence on the chemical environment to the molecular orbitals of the valence electrons and the band structure in solids. Electron spectra of this nature can now be recorded with high-resolution instruments; their analysis offers new aspects for investigation of chemical composition. The method of electron spectroscopy developed for this purpose, which has now been developed to a high degree of perfection, will be referred to in the following discussion as ESCA (Electron Spectroscopy for Chemical Analysis).     

Comparative analysis of the applicability of AM1 and CNDO/2 methods for quantitative prediction of the35Cl NQR frequencies of RC-Cl compounds

The³⁵Cl NQR frequencies of a series of RC-Cl compounds were calculated by the AM1 and CNDO/2 methods in the Townes-Dailey approximation. It was shown that neither of these methods can be directly used for the quantitative prediction of NQR frequencies, and their use in the correlation approach is only possible in narrow series. The AM1 method gives better results for saturated compounds.Institute of Organic and Physical Chemistry, Kazan' Scientific Center, Russian Academy of Sciences, 420083 Kazan'. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1078–1082, May, 1992.     

Correlating light and thermal stability of silver carboxylate complexes by infrared and 13C NMR spectroscopy

Bond nature of carboxylic groups can play an important role in Ag(I)-carboxylate compounds because light and thermal stability are important requirements for future applications such as antibacterial additives. A linear correlation between bond character and light stability of silver carboxylate complexes has been predicted by a direct relationship of infrared and ¹³C NMR spectroscopic data. This correlation is in agreement with the thermogravimetric analysis and provides a new approach to explore the interaction and the physical properties of metal-carboxylate bonds.     

Two-Dimensional Solid-State NMR Spectroscopy: New Possibilities for the Investigation of the Structure and Dynamics of Solid Polymers [New Analytical Methods (38)]

NMR spectroscopy is an effective method not only for examining liquid samples but also for characterizing molecular sturcture, order and dynamics in amorphous and ordered solids. Recent developments in the area of solid-state NMR spectroscopy span from model-dependent studies of conventional one-dimensional spectra to the more definitive two-dimensional (2D) spectra which provide more specific information. For example, with 2D-NMR spectroscopy it is possible to determine the orientational distribution functions of molecular segments in drawn polymers and to distinguish different mechanisms of complex molecular motions. Following an introduction to basic NMR spectroscopy, an overview of the current state-of-the-art of 2D methods in solid-state NMR spectroscopy is presented and demonstrated with selected examples.     

Two-Dimensional NMR Spectroscopy: Background and Overview of the Experiments [New Analytical Methods (36)]

An exact knowledge of the structure, dynamics, and reactions of molecules provides the key to understand their functions and properties. NMR spectroscopy has developed, through the introduction of two-dimensional methods, into the most important method for the investigation of these questions in solution. A great variety of different techniques is available. However, for their successful application not only the appropriate equipment is required, but also the right choice of experiments and optimum measurement parameters, as well as a careful evaluation of the spectra. This contribution describes the necessary background for modern NMR spectroscopy. With the aid of the so-called product operator formalism it is possible to understand pulsed Fourier transform NMR spectroscopy both qualitatively and quantitatively. Very few, readily understandable assumptions are sufficient for confident application of these methods. This article attempts to introduce in a simple manner this formalism as well as phase cycles necessary for the understanding of pulse sequences, and to train the reader through the discussion of several 2D NMR techniques. An overview of the most important techniques is given in the second part of this article.     

Extending the scope of NMR spectroscopy with microcoil probes

Capillary NMR (CapNMR) spectroscopy has emerged as a major breakthrough for increasing the mass-sensitivity of NMR spectroscopic analysis and enabling the combination of NMR spectroscopy with other analytical techniques. Not only is the acquisition of high-sensitivity spectra getting easier but the quality of CapNMR spectra obtained in many small-molecule applications exceeds what can be accomplished with conventional designs. This Minireview discusses current CapNMR technology and its applications for the characterization of mass-limited, small-molecule and protein samples, the rapid screening of small-molecule or protein libraries, as well as hyphenated techniques that combine CapNMR with other analytical methods.     

Determination of rice type by 1H NMR spectroscopy in combination with different chemometric tools

A 400‐MHz ¹H nuclear magnetic resonance (NMR) spectroscopy and multivariate data analysis were used in the context of food surveillance to discriminate 46 authentic rice samples according to type. It was found that the optimal sample preparation consists of preparing aqueous rice extracts at pH 1.9. For the first time, the chemometric method independent component analysis (ICA) was applied to differentiate clusters of rice from the same type (Basmati, non‐Basmati long‐grain rice, and round‐grain rice) and, to a certain extent, their geographical origin. ICA was found to be superior to classical principal component analysis (PCA) regarding the verification of rice authenticity. The chemical shifts of the principal saccharides and acetic acid were found to be mostly responsible for the observed clustering. Among classification methods (linear discriminant analysis, factorial discriminant analysis, partial least squares discriminant analysis (PLS‐DA), soft independent modeling of class analogy, and ICA), PLS‐DA and ICA gave the best values of specificity (0.96 for both methods) and sensitivity (0.94 for PLS‐DA and 1.0 for ICA). Hence, NMR spectroscopy combined with chemometrics could be used as a screening method in the official control of rice samples. Copyright © 2013 John Wiley & Sons, Ltd.     

Microstructure elucidation of poly(epichlorohydrin) modified with thiol compounds using one- and two-dimensional NMR spectroscopy

Poly(epichlorohydrin) has been modified chemically using aromatic and aliphatic thiol compounds. The NMR results show that using both aromatic and aliphatic thiols, one achieves degrees of modification of up to 90% without any elimination side reaction. As a consequence no degradative chain-scission takes place. A microstructural analysis of the modified polymers has been carried out by ¹³C NMR, ¹H NMR and ¹³C DEPT spectroscopy. Additionally, 2D heteronuclear correlated spectroscopy (HMQC and HMBC) were used in order to determine the chemical shifts of quaternary carbons.     

Dynamic NMR spectroscopy in studies of the kinetics of photoinduced chemical exchange in solutions

Modern NMR-based methods of studying the kinetics and mechanisms of reversible photochemical reactions in solutions are surveyed. Detailed consideration of peculiar features of the experimental techniques based on NMR lineshape analysis and double resonance NMR and used for the determination of the effective rate constants for and quantum yields of photoinduced chemical exchange processes is presented. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1631–1641, October, 2006.     

123Sb NQR and 19F NMR study of potassium,rubidium, and cesium heptafluorodiantimonates(iii)

The dynamics of crystal lattices of potassium, rubidium, and cesium heptafluorodiantimonates(iii) and specific features of internal rotations of the Sb₂F₇ fluoride groups in these compounds were studied using ¹²³Sb NQR in the temperature interval from 77 to 325 K and ¹⁹F NMR in the temperature interval from 240 to 470 K in combination with X-ray diffraction and thermogravimetric analyses. The distinctions in the dynamic behavior of the fluoride ions with changing the size (polarizability) of outer-sphere cations are discussed. The structural phase transition in CsSb₂F₇ was revealed at 425—430 K accompanied by the appearance of a high ion conductivity ( 1.3·10^–3 S cm^–1 at 450 K). A second type phase change can exist at 220—270 K.     

Study of the EFG tensor at 75As nuclei in Ge‐As‐Se chalcogenide glasses

Asymmetry parameters of the electric field gradient tensor at ⁷⁵As nuclei were determined for chalcogenide glassy semiconductors (CGS) of the Ge‐As‐Se system by comparing the experimental and simulated ⁷⁵As nuclear quadrupole resonance nutation interferograms. The electric field gradient asymmetry in CGS was analyzed, and it is believed that a structural change in these glassy semiconductors takes place at = 2.425. Electron paramagnetic resonance spectra of the Ge‐As‐Se system were obtained for the first time. A comparison was made between the results of analysis of the Ge‐As‐Se system by nuclear quadrupole resonance and electron paramagnetic resonance methods, and this allowed us to make the supposition that a structural phase transition occurs at = 2.4 from two‐dimensional to three‐dimensional CGS structure. Copyright © 2013 John Wiley & Sons, Ltd.     

ChemInform Abstract: Characterization of the Oxo-Anions of Bromine BrO- x (x = 1-4) by IR,NMR, and Bromine K-Edge Extended X-Ray Absorption Fine Structure Techniques.

This detailed spectroscopic and structural study of the complete series BrO- x (x = 1-4) for the first time yields a value for the bond length in the hypobromite ion.     

Structure of pravastatin and its complex with sodium dodecyl sulfate micelles studied by NMR spectroscopy

The aim of this work was to study the mechanisms of interaction between pravastatin and cell membranes using model membranes (sodium dodecyl sulfate micelles) by nuclear magnetic resonance spectroscopy methods. On the basis of the nuclear magnetic resonance experiments, it was established that pravastatin can form intermolecular complexes with sodium dodecyl sulfate micelles by the interaction of its hydrophilic groups with the polar surface of the micelle. Conformational features of pravastatin molecule were also studied. Copyright © 2014 John Wiley & Sons, Ltd.     

Structure analysis of polyether-based thermoplastic polyurethane elastomers by FTIR, 1H NMR and 13C NMR

One kind of unknown structure sequence and composition ratio of thermoplastic polyurethane elastomers were characterized by nuclear magnetic resonance spectroscopy, Fourier transformed infrared spectroscopy, and gel permeation chromatography (GPC). The results showed that the polyurethane (PU) was obtained from poly(tetramethylene glycol) (PTMG) as soft segment, 1,4-butanediol (BDO) as chain extender, and 4,4′-methylenediphenyl diisocyanate (MDI) as hard segment. Furthermore, the composition ratio of MDI:PTMG:BDO was 2.07:1.22:1.00. At last, the molecular weight of PU was determined by GPC, and the number average molecular weight (M_n) and weight average molecular weight (M_w) are 63,300 and 133,800?g?mol^?1, respectively.     

Unexpected conformational properties of 1-trifluoromethyl-1-silacyclohexane, C5H10SiHCF3: gas electron diffraction, low-temperature NMR spectropic studies, and quantum chemical calculations

The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol%/equatorial=42(12) mol%) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length Si--C 1.934(10) A. A low-temperature 19F NMR experiment results in an axial/equatorial ratio of 17(2) mol%:83(2) mol% at 113 K and a DeltaG (not equal) of 5.5(2) kcal mol-1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100-300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.