Singlet assignments

Three types of radical can be detected in irradiated polyethylene [4 — 65]. The methylene radical, —CH2—CH—CH2 — (I), possesses a sextet electron spin resonance spectrum. It is formed exclusively during irradiation at liquid nitrogen temperature. At or near room temperature, the ESR spectrum is the superposition of this sextet, which progressively disappears, and a more stable septet assigned to the allyl radical —CH—CH=CH2 — (II), Fig. 11. At very high doses, a singlet assigned to the polyenyl radical is observed. 

K. consists of a singlet superimposed on a triplet (1, 32), the triplet spectrum being assigned to the acetonyl radical. Blandamer et al. (7) and Nitta et al. (31) also observed a singlet assigned to trapped electrons in y-irradiated acetone at 77 °K. 

The NMR spectra of enmein-type diterpenoids in which 6a-H is part of a hemiacetal contain only a singlet assignable to this proton because of its ca. 90° dihedral angle with 5-H [e.g. isodocarpin (64), carpalasionin (77) in Table VIII, and enmein (62), isodotricin (65) etc. in Table II]. In spirolactone-type diterpenoids having a hemiacetal function 6-H is a doublet [e. g. trichorabdal E (96) in Table VIII and trichorabdals F (97) and G (98) in Table II]. In the enmein-type diterpenoids of this group, C-6 is R except for rabdosin A (73), while the spirolactone-type diterpenoids occur as a mixture of 6R- and 6S-isomers 81). Shikodonin has been reported to have structure (88) on the basis of X-ray analyses of its 6-O-methyl and 6-0-ethyl derivatives, but our experimental results are inconsistent with this proposal, and thus the stereochemistries of C-5 and C-6 in (88) seem questionable. 

HC-NMR spectrum of Neu5Ac is shown in Fig. 13. This proton noise-decoupled spectrum shows 11 intense singlets. Assignment of the resonances was made by comparison with spectral data of carbon atoms in similar chemical environments of model compounds and by various proton decoupling techniques (Bhattacharjee et al. 1975). The relatively low intensities for the carbonyl carbons and for C2 are due to the restricted nuclear Overhauser enhancement as a result of the absence of proton substituents at these carbon atoms. In addition, the spectrum shows several small resonances (see Table 11) belonging to the aNeuSAc anomer which is present for 9% in the equilibrium mixture (Jaques et al 1977). 

Proton magnetic resonance (chloroform-d) S (multiplicity, number of protons, assignment) 3.30 (singlet, 12, OC/fg), 6.10 (singlet, 4, ring protons.)... 

Using these molecular states, the weak absorption observed between 490 and 640 nm for Cbo in solution (Fig. 6) [67] is assigned to transitions between the singlet ground state So and the lowest excited singlet state 5i (associated with the tiu orbital and activated by vibronic coupling). 

The salts of some enamines crystallize as hydrates. In such cases it is possible that they are derived from either the tautomeric carbinolamine or the amino ketone forms. Amino ketone salts (93) ( = 5, 11) can serve as examples. The proton resonance spectra of 93 show that these salts exist in the open-chain forms in trifluoroacetic acid solution, rather than in the ring-closed forms (94, n = 5, 11). The spectrum of the 6-methylamino-l-phenylhexanone cation shows a multiplet at about 2.15 ppm for phenyl, a triplet for the N-methyl centered at 7.0 ppm and overlapped by signals for the methylene protons at about 8.2 ppm. The spectrum of 93 ( = 11) was similar. These assignments were confirmed by determination of the spectrum in deuterium oxide. Here the N-methyl group of 93 showed a sharp singlet at about 7.4 ppm since the splitting in —NDjMe was much reduced from that of the undeuterated compound. 

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