Tetramethylsilane, chemical shift coupling constants

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. 

Note The chemical shifts in parts per million are relative to the signals for tetramethylsilane. Abbreviations are defined as follows t, triplet m, multiplet dd, doublet of doublets d, doublet q, quartet /, coupling constant and OV, overlapping peaks. 

The n.m.r. spectra were obtained by using a Bruker WM-250 multinuclei Fourier transform spectrometer. Typical anion solutions were approximately 0.2 M, and the probe temperature was 25 °C. The chemical shifts are with reference to tetramethylsilane. For the carbon-13 measurements a small amount of cyclohexane was added to the carbanion solution as an internal reference using S(SiMe4) = S(cyclohexane) + 27.7 p.p.m. b One bond I3C— H coupling constants.c Huckei j>charges... 

Proton NMR spectra were obtained at ambient temperature (39°) with a Varian A-60A spectrometer. For determination of chemical shifts and coupling constants, the magnetic field sweep width was calibrated with a standard sample of chloroform and tetramethylsilane in carbon tetrachloride. Three to five spectra were recorded and the reported values of chemical shifts and coupling constants are average values. Electronic spectra were recorded with a Cary model 14 recording spectrophotometer. ORD and CD spectra were obtained with a Cary model 60 spectropolarimeter or a JASCO ORD/UV-5. 

Table 3 - Chemical shifts (ppm from internal tetramethylsilane) and coupling constants [Hz] for peracetylated lactosylceramide from T. cruzi in CDCI3 solution at 303 K as obtained by two-dimensional 1H-NMR spectroscopy.

Table 1 summarizes the 5( Si) chemical shifts referenced against tetramethylsilane as well as important coupling constants for all compounds. 

In this section a detailed explanation is made of the NMR spectra of some of the more widely studied metal 7c-complex systems. Chemical shifts are reported tau (t) values, this parameter being independent of the oscillator frequency used in the measurement and assuming tetramethylsilane (TMS) as the reference compound with t = 10. Coupling constants (J) are given in cps. 

MHz. Chemical shifts were reported in S (ppm) relative to tetramethylsilane (TMS) or residual solvent signals as the internal standard (CHCI3, S = 7.26, DMSO-ds, S = 2.50). Spectra data were presented as follows Chemical shifts (S ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constants (Hz), integration, and assignment. 

Fig. 4. Proton magnetic resonance spectral assignments for A batrachotoxinin A, B batrachotoxin, and C homobatrachotoxin. Chemical shifts in ppm for deuterochloroform with a tetramethylsilane standard s singlet d doublet t triplet q quartet br broad. Coupling constants are in parentheses in cycles per second. Spectra (100 MHz) are depicted by Tokuyama et al. (251) and in Fig. 5. Capital letters A—M refer to assignments in Fig. 5. Values for batrachotoxinin A differ somewhat from those reported later for synthetic and natural compound by Imhof et al. (144,145). It appears likely that the values of Imhof et al. correspond to the free base and that the earlier values of Tokuyama et al. were for mixtures of free base and cationic form present in the CDCI3. Certain earlier assignments (257) have been revised in light of the detailed examination of the spectrum of batrachotoxinin A by Imhof etal. (145)...

Table 8. Proton Magnetic Resonance Spectral Assignments for Histrionicotoxin (HTX) and Congeners Chemical shifts in ppm for deuterochloroform with a tetramethylsilane standard Coupling constants (J) in parentheses br, broad m, multiplet s, singlet d, doublet t, triplet d,d doublet of doublets etc. 86, 253, 255) ...

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