Nuclear magnetic resonance spectroscopy aromatic compounds

The tautomerism of 2- and 3-aminothiophenes was mentioned by Hartough in his review of thiophenes/ but the first definite evidence became available in 1961 when Hoffman and Gronowitz showed conclusively by nuclear magnetic resonance spectroscopy that these compounds both exist in the amino form. In agreement with this finding, 3-aminothiophene generally behaves as an aromatic amine. ... 

Figure 15.16. H Relaxation of 1-naphthol protons with increasing humic acid concentration at pH 7. All protons are observed to relax at a similar rate, suggesting a nonselective interaction between the protons of 1-naphthol and humic acid. Reprinted from Simpson, M. I, Simpson, A. J., and Hatcher, R G. (2004). Noncovalent interactions between aromatic compounds and dissolved humic acid examined by nuclear magnetic resonance spectroscopy. Environ. Toxi. Chem. 23, 355-362, with permission from the Society of Environmental Toxicology and Chemistry.

Simpson, M. J., Simpson, A. J., and Hatcher, E G. (2004c). Noncovalent interactions between aromatic compounds and dissolved humic acid examined by nuclear magnetic resonance spectroscopy. Environ. Toxicol. Chem. 23, 355-362. 

Nuclear magnetic resonance spectroscopy ( C CP/MAS Solid State NMR) and Fourier transform infrared spectroscopy (FT-IR) were also performed for the freeze dried NOM sample. The results were both very noisy and paramagnetic compounds such as iron and manganese interfered with the - C NMR analysis. After 20 h of run time the sample showed mostly alkyl and alkyl-oxygen carbon, thus very little aromatic compounds. 

ASTM Test Method D5292, Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy, permits determination of aromatic hydrogen and aromatic carbon content of gas oils, lubricating oils, and other hydrocarbon fractions that are completely soluble in chloroform and carbon tetrachloride at ambient temperatures. Concentrations as low as 0.1 mol-% hydrogen and 0.5 mol-% carbon can be determined. Olefins and phenolic compounds above 1 mass-% interfere. 

This includes benzene with six t-electrons but also systems with 2,10,14, etc., Jt-electrons. If the ring has 4 t-electrons, it is described as antiaromatic—including cyclobutadiene with four electrons but also systems with 8, 12, 16, etc., t-electrons. What properties are specific to aromatic compounds We have already seen that the additional stability is reflected in heats of hydrogenation, and this is also true for other thermodynamic parameters such as heat of combustion or heat of formation. Aromatic molecules are planar, or nearly so—this is essential for proper interaction of the p orbitals. Substitution, rather than addition, is the characteristic reaction. The easiest practical measurement of whether a compound shows aromatic character or not comes from nuclear magnetic resonance spectroscopy, which we will explore further in Chapter 6. 

The structures of vanicosides A (1) and B (2) and hydropiperoside (3) were established primarily by one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy techniques and fast atom bombardment (FAB) mass spectrometry (MS).22 The presence of two different types of phenylpropanoid esters in 1 and 2 was established first through the proton (4H) NMR spectra which showed resonances for two different aromatic substitution patterns in the spectrum of each compound. Integration of the aromatic region defined these as three symmetrically substituted phenyl rings, due to three p-coumaryl moieties, and one 1,3,4-trisubstituted phenyl ring, due to a feruloyl ester. The presence of a sucrose backbone was established by two series of coupled protons between 3.2 and 5.7 ppm in the HNMR spectra, particularly the characteristic C-l (anomeric) and C-3 proton doublets... 

To return to the problem of the general invisibility or atoms, how does the chemist follow the course of a reaction ir the molecules cannot be imaged One way is to use spectroscopy. Thus the conversion or methanol, first to dimethyl ether, then to the higher aliphatic and aromatic compounds Round in gasoline, can be followed by nuclear magnetic resonance (NMR) spectroscopy (Fig. 1.6). As the reaction proceeds, the concentration or the methanol (as measured by the intensity or the NMR peak at <550 ppm) steadily decreases. The first product, dimethyl ether ( 60 ppm), increases at first and then decreases as the aliphatic and aromatic products eventually predominate. 

Obtain infrared and nuclear magnetic resonance spectra following the procedures of Chapters 19 and 20. If these spectra indicate the presence of conjugated double bonds, aromatic rings, or conjugated carbonyl compounds obtain the ultraviolet spectrum following the procedures of Chapter 21. Interpret the spectra as fully as possible by reference to the sources cited at the end of the various spectroscopy chapters. 

Fraenkel G, Dayagi S, Kobayashi S (1968) Nuclear Magnetic Resonance and Ultraviolet Spectroscopy of Substituted Aromatic Organometallic Compounds of Lithium, Magnesium, and Calcium. J Phys Chem 72 953-961... 

The organic cations generated and accumulated by the cation pool method can be observed by nuclear magnetic resonance (NMR) spectroscopy or infrared spectroscopy (IR). A solution can contain organic cations at concentrations of 0.1-0.05 M. An iV-acyliminium ion generated by the cation pool method can be used for the Friedel-Crafts alkylation reaction of aromatic compounds. 

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