Other NMR Spectra

You can see from this example how NMR and NMR spectroscopy complement each other. NMR spectra provide an estimate of the electronic environment (i.e., electron rich versus electron poor) of a hydrogen nucleus under observation (S), a measure of its relative abundance (integration), and an indication of how many neighbors (and their number of types) it has (spin-spin splitting). Proton-decoupled NMR provides the total number of chemically distinct carbons, their electronic environment (5), and, in the DEPT mode, even the quantity of their attached hydrogens. Application of both techniques to the solution of a structural problem is not unlike the methods used to solve a crossword puzzle. The horizontal entries (such as the data provided by NMR spectroscopy) have to fit the vertical ones (i.e., the corresponding NMR information) to provide the correct answer. 

In principle, every nucleus in a molecule, with spm quantum number /, splits every other resonance in the molecule into 2/ -t 1 equal peaks, i.e. one for each of its allowed values of m. This could make the NMR spectra of most molecules very complex indeed. Fortunately, many simplifications exist. 

A number of other software packages are available to predict NMR spectra. The use of large NMR spectral databases is the most popular approach it utilizes assigned chemical structures. In an advanced approach, parameters such as solvent information can be used to refine the accuracy of the prediction. A typical application works with tables of experimental chemical shifts from experimental NMR spectra. Each shift value is assigned to a specific structural fragment. The query structure is dissected into fragments that are compared with the fragments in the database. For each coincidence, the experimental chemical shift from the database is used to compose the final set of chemical shifts for the... 

A 2-methylthio substituent decreases the basicity of thiazole pK = 2.52) by 0.6 pK unit (269). The usual bathochromic shift associated with this substituent in other heterocycles is also found for the thiazole ring (41 nm) (56). The ring protons of thiazole are shielded by this substituent the NMR spectrum of 2-methylthiothiazole is (internal TMS, solvent acetone) 3.32 (S-Me) 7.3 (C -H) 6.95 (Cj-H) (56, 270). Typical NMR spectra of 2-thioalkylthiazoles are given in Ref. 266. 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

The polarographic properties of the halogenothiazoles in comparison with other thiazole compounds have also been investigated (73, 74). Infrared, Raman, ultraviolet, and NMR spectra of mono-halogenothiazoles have been measured (2, 3, 6, 10, 15, 17, 24, 29) (Table V-4). 

Figure 13 23 compared the appearance of the H and NMR spectra of 1 chloropentane and drew attention to the fact each carbon gave a separate peak well separated from the others Let s now take a closer look at the NMR spectrum of 1 chloropentane with respect to assigning these peaks to individual carbons... 

A variety of experimental techniques have been employed to research the material of this chapter, many of which we shall not even mention. For example, pressure as well as temperature has been used as an experimental variable to study volume effects. Dielectric constants, indices of refraction, and nuclear magnetic resonsance (NMR) spectra are used, as well as mechanical relaxations, to monitor the onset of the glassy state. X-ray, electron, and neutron diffraction are used to elucidate structure along with electron microscopy. It would take us too far afield to trace all these different techniques and the results obtained from each, so we restrict ourselves to discussing only a few types of experimental data. Our failure to mention all sources of data does not imply that these other techniques have not been employed to good advantage in the study of the topics contained herein. 

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

Nuclear Magnetic Resonance Spectroscopy. Bmker s database, designed for use with its spectrophotometers, contains 20,000 C-nmr and H-nmr, as weU as a combined nmr-ms database (66). Sadder Laboratories markets a PC-based system that can search its coUection of 30,000 C-nmr spectra by substmcture as weU as by peak assignments and by fiiU spectmm (64). Other databases include one by Varian and a CD-ROM system containing polymer spectra produced by Tsukuba University, Japan. CSEARCH, a system developed at the University of Vieima by Robien, searches a database of almost 16,000 C-nmr. Molecular Design Limited (MDL) has adapted the Robien database to be searched in the MACCS and ISIS graphical display and search environment (63). Projects are under way to link the MDL system with the Sadder Hbrary and its unique search capabiHties. 

Infrared, uv, nmr spectra (66), and photoelectron spectra have been reviewed (67). Physical properties of siHcon peroxides are summarized in Reference 43. Other physical properties, eg, dipole moments, dihedral angles, and heats of combustion ate Hsted in Reference 68. The oxygen—oxygen bond strengths of various diaLkyl peroxides have been reported (69). 

The NMR spectra of both the parent [2,3-f ] and [3,4-f ] pyridopyrazine systems have been analyzed (66JCS(C)999). Shift values are given in Table 3. These studies were extended to the phenomenon of covalent hydration in both systems (66JCS(C)999,79JHC301) (see Section 2.15.13.2), as well as the addition of other nucleophiles such as amide ion (79JHC301, 79JHC305). 

A number of studies on the NMR spectra of isoxazole has been compiled and this list includes the coupling constants in various solvents as well as the neat liquid. The N signal for isoxazole appears at 339.6 p.p.m. relative to TTAI and is at much lower field than in other azoles. Reports of spectra of substituted isoxazoles also abound (79AhC(25)147, p. 201). 

NMR spectra of oxaziridine enantiomers may be different from each other in chiral media. In the presence of chiral arylperfluoroalkylcarbinols, shift differences of up to 0.35 p.p.m. are observed, which may be used for discrimination of enantiomers (77JOC3217). 

A gene encoding this sequence was synthesized and the corresponding protein, called Janus, was expressed, purified, and characterized. The atomic structure of this protein has not been determined at the time of writing but circular dichroic and NMR spectra show very clear differences from B1 and equally clear similarities to Rop. The protein is a dimer in solution like Rop and thermodynamic data indicate that it is a stably folded protein and not a molten globule fold like several other designed proteins. 

Figure 18.16 One-dlmenslonal NMR spectra, (a) H-NMR spectrum of ethanol. The NMR signals (chemical shifts) for all the hydrogen atoms In this small molecule are clearly separated from each other. In this spectrum the signal from the CH3 protons Is split Into three peaks and that from the CH2 protons Into four peaks close to each other, due to the experimental conditions, (b) H-NMR spectrum of a small protein, the C-terminal domain of a cellulase, comprising 36 amino acid residues. The NMR signals from many individual hydrogen atoms overlap and peaks are obtained that comprise signals from many hydrogen atoms. (Courtesy of Per Kraulis, Uppsala, from data published in Kraulis et al.. Biochemistry 28 7241-7257, 1989.)...

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

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