Pyridines nuclear magnetic resonance

The 9ai7-quinolizine structure (82) for the labile adduct from 3,5-dimethylpyridine was clearly established by the nuclear magnetic resonance studies of Richards and Higham, and subsequent work showed the labile adduct from 3-methylpyridine was analogous. As the labile adducts from all the pyridines and benzopyridines so far examined have very similar infrared absorption spectra in the 5-7 yn. (carbonyl and aromatic) region and within quite close limits very similar ultraviolet absorption spectra, it can be concluded that all are derivatives of 9aH-quinolizine,... 

C Nuclear Magnetic Resonance Spectra. The sohd-state CNMR spectra of 6,7-dimethyl-2,3-di(pyridin-2-yl)quinoxahne (130) and its salts have been used to complement X-ray information (see above) on fine structure. A study of the spectra of mono- to tetramethylquinoxalines has made possible the analysis of mixtures of such methylated quinoxalines obtained from ambiguous primary syntheses. 

Changes In nuclear magnetic resonance measurements of an extensive suite of Australian coals on heating and exposure to pyridine are used to elucidate the molecular conformation of coal macerals Two types of fusible material are Identified In these coals One Is associated with llptlnltes of all ranks and Is typified by fusion commencing at temperatures below 475 K. The other Is associated with vltrlnltes and some Inertlnltes of bituminous coals only and Is characterized by a sharp onset of fusion at temperatures above 625 K. The temperature of onset of fusion Increases with rank for both types The effect of pyridine on the molecular stability of bituminous coals at ambient conditions Is strongly dependent on maceral composition at 86% C and on rank at higher carbon contents ... 

Solvent destabilization of the molecular structure of organic materials can be quantified by simple proton nuclear magnetic resonance ( H NMR) measurements at ambient temperatures. Such measurements have shown that up to 60% of a coal s molecular structure can be destabilized by pyridine and, by the same token, that at least 40% Is Impervious to this solvent (15-18). 

Extensive nuclear magnetic resonance and ultraviolet spectroscopy methods were reviewed in <1996CHEC-II(7)363>, as well as mass spectral fragmentation patterns of [l,2,3]triazolo[4,5-/ ]pyridines (Section 7.10.8.1). More recently, furoxan rearrangement of some pyridofuroxan derivatives has been studied by H, and... 

Equivalent Weight. Three reliable analytical methods are available to determine the equivalent weight of CTPB prepolymer (1) titration by 0.1 N sodium methylate in pyridine solution to the thymol blue end point, (2) infrared spectroscopy, and (3) nuclear magnetic resonance. Satisfactory agreement has been obtained between these instrumental analyses and the acid content as determined by titration (Table XVI). 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Dega-Szafran, Z., Szafran, M., Stefaniak, L., Brevard, C., and Bourdonneau, M., Nitrogen-15 nuclear magnetic resonance studies of hydrogen bonding and proton transfer in some pyridine trifluoroacetates in dichloromethane, Magn. Reson. Chem., 24, 424, 1986. 

The water slurry of SP-300 was acidified to pH 2 with 10% HCL to obtain two forms of precipitates one was lumpy and the other was powder-like. After separation from the solution, each form of precipitate was dried. The former was designated as fraction I and the latter, fraction J. The solid product from SP-300 was acidified, separated from the solution and dried. Then the product was extracted with pyridine at room temperature with a solvent/sample ratio of 10. The soluble portion was called fraction K. Thus SP-300 was divided into four fractions fractions I, J, K, and Pyridine-Insoluble. Likewise, SP-320 was divided into four fractions fractions I Jf, K, and Pyridine-Insoluble. Figure 1 summarizes the procedures used in the preparation of all CDL products. Elemental compositions, molecular weight, nuclear magnetic resonance and infrared spectra were obtained for each major fraction. Details of the preparation of specific fractions of HVL-P and solubilization products and analyses can be found elsewhere (13,14). 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

Although the determination of HA or HB selectivity is relatively straightforward the techniques for isolation of pyridine nucleotides from the reaction mixtures are tedious and time consuming. Two more recent techniques use either proton magnetic resonance or electron impact and field desorption mass spectrometry. The technique of Kaplan and colleagues requires a 220 MHz nuclear magnetic resonance spectrometer interfaced with a Fourier transform system [104], It allows the elimination of extensive purification of the pyridine nucleotide, is able to monitor the precise oxidoreduction site at position 4, can be used with crude extracts, and can be scaled down to /nmole quantities of coenzyme. The method can distinguish between [4-2H]NAD+ (no resonance at 8.95 8) and NAD+ (resonance at 8.95—which is preferred) or between [4A-2H]NADH (resonance at 2.67 8, 75 4B = 3.8 Hz) and [4B-2H]NADH (resonance at 2.77 8, J5 4A = 3.1 Hz). 

Effect of nonaqueous solvents on acid-base properties of NGu) (Acidic in dimethyl-formamide, pyridine and acet basic in HAc and formic acid) 26) E. Ripper, Explosivst 17 (7), 145-51 (1969) Bt CA 72, 48454(1970) (a-and /3-Nitroguanidine) (Reinvestigation of both forms by IR UV, Nuclear Magnetic Resonance (NMR), Differential Thermal Analysis (DTA),... 

Golubev, N. S., Smirnov, S. N., Gindin, V. A., Denisov, G. S., Benedict, H., and Limbach, H.-H., Formation of charge relay chains between acetic acid and pyridine observed by low-temperature nuclear magnetic resonance, J. Am. Chem. Soc. 116, 12055-12056 (1994). 

Dega-Szafran, Z., and Dulewicz, E., Infrared and H nuclear magnetic resonance studies of hydrogen bonds in some pyridine trifluoroacetates and their deuteriated analogues in dichloro-methane, J. Chem. Soc. Perkin Trans. II 345-351 (1983). 

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