Protons NMR spectra

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

The infrared carbonyl stretching frequencies of n- and isobutyraldehyde in the condensed phase occur at 1727.6 and 1738.0 cm , respectively (38). The proton nmr spectra of both aldehydes are weU-known (39). 

There is an admirable summary of the stereochemistry of barbiturates and di- to hexahydropyrimidines Further information on reduced pyrimidines is collected <70HC 16-81)322) and some examples of the use of proton NMR spectra in elucidating the conformations of hydropyrimidines is given elsewhere (Section 2.13.1.3.1), based on the general principles of such work <65QR426). 

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of this splitting is closely related to the factors that cause spin-spin splitting in proton NMR spectra. Certain nuclei have a magnetic moment. Those which are of particular interest in organic chemistry include H, " N, F, and P. 

FIGURE 4.16 Proton NMR spectra of several amino acids. Zero on the chemical shift scale is defined by the resonance of tetramethylsilane (TMS). (Adaptedfrom Atelrkh Library of NMR Spectra. ... 

Besides the thiocyanates, just mentioned, other 5-donor complexes which are of interest are the dialkyl sulfides, [MCl3(SR2)3], produced by the action of SR2 on ethanolic RhCl3 or on [IrClg] ". Phosphorus and arsenic compounds are obtained in similar fashion, and the best known are the yellow to orange complexes, [ML3X3], (M = Rh, Ir X = Cl, Br, I L = trialkyl or triaryl phosphine or arsine). These compounds may exist as either mer or fac isomers, and these are normally distinguished by their proton nmr spectra (a distinction previously made by the measurement of dipole moments). An especially... 

Table 1. The 200 MHz proton NMR Spectra of Peaks Isolated by Preparative Chromatography... 

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

The earliest NMR technique to gain importance in chemistry was that of proton NMR. Spectra could be obtained for compounds containing the nucleus by continuously sweeping the field at constant frequency. This... 

S. Sasaki, Handbook of Proton-NMR Spectra and Data, Academic Press, New York, NY (1996), Vols 1-5. 

The hydrolysis of the cyclic acetal, which was used as the connecting group between the polymer chain and the lipid, was confirmed both by the IR and the proton NMR spectra of the lipid recovered from the vesicular system after standing for 3 weeks at room temperature. The lactone absorption at 1805 cm-1 disappeared from the IR spectrum (Figure 6) as the result of hydrolysis. Furthermore, a new aldehyde absorption band at 1705 cm 1 was observed in the spectrum, which is related to the substituted benzaldehyde group of the hydrolyzed product. The proton NMR spectrum (Figure 10) also clearly showed the formation of the benzaldehyde, as indicated by the peak at 810.20 ppm. 

An on-flow experiment is now carried out. 50 pi of a solution of the product mixture (5 mg in 5 mL solvent) are injected and the NMR proton signal accumulation started simultaneously. The time taken for the chromatogram is 17 min. During this time a total of 128 proton NMR spectra are recorded, each with eight scans, i.e. an FID is accumulated approximately every 7 sec. After the Fourier transformation we obtain a two-dimensional representation (Fig. 33) of the on-flow experiment. 

The two axes represent the chromatogram (retention time) and the chemical shift information. The individual NMR spectra can be extracted by the software and viewed individually in the form of normal ID proton NMR spectra. 

Fig. 34a-c Proton NMR spectra from the on-flow experiment. FIDs recorded after a 1 min, b 3 min, c 7.5 min... 

The combination of cross polarization (basically a pulse sequence) and MAS is sufficient to drastically reduce the linewidths of spin-Vi nuclei. Liquid-state proton NMR spectra, as we have seen, are characterized by extremely narrow lines and complex multiplets due to spin-spin coupling in addition, the normal chemical shift range is only around 10 ppm. 

Magnetic nonequivalence is not uncommon, often deriving from the constraints of a ring, as in pentafluorophenyl derivatives or other symmetrically fluorine substituted ring systems such as those shown in Scheme 2.10. The fluorine and proton NMR spectra of 1,2-difluoroben-zene are both representative of the appearance of second order spectra of polyfluoroaromatics. They can be found in Chapter 3, Section 3.9.3. 

Thus, when fluorine and/or proton NMR spectra do not appear as simple as you might think they should, it is generally because of a second order phenomenon resulting from one of those factors described above. 

Carbon and Proton NMR Spectra of Thiophenes. Some examples that provide proton and carbon data for thiophenes and benzthiophenes that bear a trifluoromethyl group are given in Scheme 5.50. 

Carbon and Proton NMR Spectra of Trifluorovinyl Compounds. Scheme 6.31 provides the carbon and proton NMR data for a few trifluorovinyl compounds. 

Proton nmr spectra of fractions A, B and C and all bottoms products were recorded on a Varian HA lOOnmr spectrometer using a solution of the sample dissolved in pyridine-d5. Spectra were run at room temperature with tetra methyl silane (TMS) as an internal standard, with a sweep width of 0 to 1000 cps from TMS. Fraction D and the whole coal were only partly soluble in pyridine and it was therefore not possible to get representative spectra from them. 

The tacticity or distribution of asymmetric units in a polymer chain can be directly determined using NMR spectroscopy and infrared (IR) spectroscopy and has been studied for a variety of polymers. Figure 5(a) and 5(b) show the proton NMR spectra [26,27] and IR spectra [28,29], respectively, for the two stereoisomers of poly(methyl methacrylate) (PMMA), syndiotactic and isotactic PMMA. These two structures in a polymer like PMMA give rise to different signatures in both the techniques. In the case of the NMR spectra [26,27], the... 

Figure 5 (a) Proton NMR spectra for syndiotactic (upper) and isotactic (lower) poly(methyl... 

The enyne was employed in 20 % molar excess based on active hydrogen. After work-up, proton NMR spectra revealed that, despite steric hindrance at the triple bond, the product ratio Si-C=CH-/Si-... 

Fig. 10. 220 MHz proton NMR spectra of solutions of polypropylene, (a) Isotactic polypropylene broad bands between 8.62 and 8.8 r, others hidden by CH3 resonances in the region of 9.0 and 9.25 r (85). (b) Syndiotactic polypropylene a single set of peaks between 8.8 and 9.0 r (85). (c) Soluble polypropylene obtained by polymerization with Zr(benzyl)4 (38).

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