Chemical shift ranges, nuclear magnetic resonance

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture 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 (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

Fig. 3.44 Approximate ranges of proton chemical shifts (R = H or alkyl Y = SR, — NR2 X = OR, — NHCO-R, — 0 C0 R, halogen). Data reproduced from L. M. Jackman and S. Stemhell (1969). Applications of Nuclear Magnetic Resonance in Organic Chemistry. 2nd edn. London Pergamon Press, p. 161.

Nuclear magnetic resonance (NMR) spectra of 1,2,3-selenadiazoles have been described previously <1996CHEC-11(4)743, 1981ZNB1017>. The characteristic 111 chemical shifts of H-4 and H-5 lie in the range S 8.2-8.4 and S 8.8-9.4ppm, respectively. The newly reported data also show similar chemical shifts (Table 19). 1J H-Se for 4-substituted-l,2,3-selenadiazoles and V and 2J C-Se coupling constants for 4,5-disubstituted-l,2,3-selenadiazoles were also reported (Tables 19 and 20). 

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. 

Misra et al. [113] have reported the synthesis and optical/electrical properties of new 5-coordinated Al-complexes designed as Alq(l) and Alq(2). The complexes are vacuum evaporable as well as soluble in many organic solvents. EL peaks of these new complexes emit in the range 522-523 nm, which is nearly 8 nm blue shifted compared to that of Alq3. The chemical structures of the complexes were determined with the help of the Hydrogen Nuclear Magnetic Resonance (HNMR) and Fourier Transform Infrared (FTIR) spectroscopy techniques. The structure of these complexes is shown in Fig. 4.13. 

The structurally important nuclear magnetic resonance (NMR) data was thoroughly established in the previous chapter of CHECH <1996CHEC-II(1)333>. Since most of the structural evidence is based on "B NMR data, the values previously reported are given for comparison purposes. The "B chemical shifts for structures of type 1 are in the range from 70 to 73 ppm, for type 2 from -9 to -27 ppm, the type 3 compounds have values of 34-52 ppm, for 4 between 27-49 ppm, and for 5 between -10 and -20 ppm. Compound 6 was only determined theoretically. 

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