Nuclear magnetic resonance quantitative interpretation

Spectroscopic techniques, such as ultra-violet (9), Infrared (25), Nuclear Magnetic Resonance (24), and Fluorescence spectroscopies (5-8), constitute direct probes of specific events occurring at the molecular scale. When a quantitative interpretation is possible, spectroscopy provides very detailed microscopic information. Unfortunately however, the interpretation of spectra in terms of molecular events is often complex. Yet another approach that probes events at the molecular scale involves the use of tracers, such as chromophores (1-225). Again, the complexity of the tracer imposes limitations on the extent to which the data can be interpreted quantitatively. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

A wide variety of chemical and spectroscopic techniques has been used to determine functionality in humic substances. Although nuclear magnetic resonance (NMR) spectroscopy has been used for a much shorter period of time than most other techniques for determining functional group concentrations, this technique has provided far more definitive information than all other methods combined. However, substantially more work must be done to obtain the quantitative data that are necessary for both structural elucidation and geochemical studies. In order to increase the accuracy of functional group concentration measurements, the effect of variations in nuclear Overhauser enhancement (NOE) and relaxation times must be evaluated. Preliminary results suggest that spectra of fractions isolated from humic substances should be better resolved and more readily interpreted than spectra of unfractionated samples. 

The nuclear magnetic resonance (NMR) spectrometer has become in recent years one of the most popular and useful tools available to the chemist for structural studies. In the usual high-resolution NMR spectrum of a liquid the scalar parameter called the chemical shift is readily obtained and may be interpreted in an approximate and qualitative fashion to establish features of the overall structure. The task of placing the interpretation of the chemical shift on a more quantitative and rigorous basis is not an easy one but considerable progress has been made in this direction. The potential rewards of improved understanding in this area seem most attractive. 

This section provides correlation charts and operational information for the design and interpretation of ultraviolet-visible spectrophotometric (UV-Vis) measurements. While UV-Vis is perhaps not as information-rich as infrared or nuclear magnetic resonance, it nonetheless has value in structure determination and sample identification. Moreover, it is extremely valuable in quantitative work. Typical UV-Vis instruments cover not only the UV and visible spectrum, but the near-infrared as well. Although there is overlap among the ranges, the approximate breakdown is ... 

Chapter 9 looks at mercury intrusion porosimetry, which before the advent of nuclear magnetic resonance was the best available technique for studying the pore structure of cementitious materials. Although the technique has been much maligned, we believe it can give good quantitative information about the pore structure if specimens are prepared correctly (not oven dried) and the interpretation of the results takes into account how the technique works. 

Abstract This tribute to the work by Carl Johan Ballhausen focuses on the emergence of quantitative means for the study of the electronic properties of complexes and molecules. Development, refinement, and application of the orbital picture elucidated electric and magnetic features of ranges of molecules when used for the interpretation of electronic transitions, electron spin resonance parameters, rotatory dispersion, nuclear quadrupole couplings as well as geometric bonding patterns. Ballhausen s profound impact on the field cannot be overestimated. 

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