Data base Resonance Spectral

Three novel tricyclic alkaloids (222, 236, and 252A) were isolated from skin extracts of a Panamanian population of the poison frog Dendrobates pumilio. Amidine structures were proposed for these alkaloids in 1987 based on mass spectral and nuclear magnetic resonance spectral analyses (77). However, recent gas chromatographic-FTIR spectra showed that these three alkaloids could not be amidines, since they had no absorption around 1630 cm where amidines show an intense absorption. A reexamination of the nuclear magnetic resonance spectral data and acquisition of new data led to revised structures 153). The simplest member (222) is a spiropentanopyrrolizidine oxime, whereas 236 is the corresponding... 

The striking difference in reactivity and stability of the 4//-l,3,4-thiadiazin-5(67/)-one (15 X = O) and the 5-ol (16) have been discussed in terms of resonance stabilization based on spectral data and extended Hiickel MO calculations <87NKZ1312>. The lactam exhibits typical amide resonance (15<- 15a) and is resistant to attack by electrophiles, nucleophiles, and free radicals, whereas the reactivity of the 5-ol (16) towards displacement of the hydroxy group by nucleophiles is attributed to resonance stabilization of the thiadiazinium cation (16a) formed by heterolysis of the C—OH bond. Extended Hiickel MO calculations on the thione analogue of the thiadiazinone, coupled with spectroscopic data, indicate that this system is best represented as the thione (15 X = S) <89CE797>. 

Regarding the 0(p,n) F reaction, only one group had reported the activation cross sections (Ruth and Wolf 1979). A recent detailed study (Hess et al. 2001) established the data base firmly for this production route of the most commonly used PET radionuclide F. The results are shown in O Fig. 39.5. The yield can now be calculated up to 30 MeV. The resonances at 5.1,6.1 and 7.2 MeV are in agreement with those found in spectral measurements of emitted neutrons. 

Fig. 7. Proton magnetic resonance spectral assignments for pumiliotoxin C hydrochloride. Chemical shifts in ppm for deuterochloroform with coupling constants in parentheses (data from 87, 143, 200, 209). Chemical shifts for the free base are 2-H, 2.64 8a-H, 2.95. The spectrum of the HCl salt (100 MHz) is given by Daly etal. (57)...

A synthesis of the azaspiro-[5.5]-undecan-8-ol ring system characteristic of the histrionicotoxins was first reported in 1975 in 30% overall yield by Gossinger etal. (SchemeXXI) (11. The carbon-13 magnetic resonance spectral peaks for azaspiro-[5.5]-undecan-8-ol were as follows 67.5 (C-8), 52.0 (C-6), 40.8 (C-2), 40.0,38.5,37.4,33.7,27.5,20.0,15.9. The base peak in the mass spectra was at m/z 126. No major ion at m/z 96 was included in the mass spectral tabulation. Infrared and proton magnetic resonance data were reported. The synthetic approach was not successful for the synthesis of 7-n-butylazaspiro-[5.5]-undecan-8-ol, a desamyl-analog of perhydro-histrionicotoxin. This compound has now been synthesized by Takahashi etal. (246). 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

Now that the general considerations concerning the 13C-n.m.r.-spec-tral data for O-glycosylated model compounds have been discussed, attention may be focused on some specific cases. Table IV gives the 13C-n.m.r.-spectral data for L-serine that has been glycosylated with various carbohydrates, such as a- and / -d-G1c, a- and /9-D-Gal, a- and / -d-Xyl, a-D-Man, and a-D-GalNAc. As mentioned earlier, the specific resonance-assignments to amino acid carbon atoms were based on chemical... 

Fig. 9.2 Part of a quantitative- /NN HNN-COSY spectrum of a 1.5 mM uniformly 13C/15N-labeled intramolecular DNA triplex. This triplex consists of five Hoogsteen-Watson-Crick T A-T and three Hoogsteen C+ G-C base triplets. The spectral region corresponds to the 10 imino resonances of the Hoogsteen-Watson-Crick T A-T triplets. The data matrix consisted of 250 (q) X1024 (i2) data points (where n refers to complex points) with acquisition times of 45 ms (tn) and 85 ms...

The C NMR spectmm of 13 showed the resonances of all 35 carbon atoms. A combirration of broadband C-NMR and DEPT spectra indicated the presence of 7 methyl, 4 methylene, 16 methine and 8 quaterrrary carbon atoms in compound 13. Complete 13C-NMR chemical shift assignments of 13 are shown around structure 13. Based on these spectral data, structure 13 was established for this new compound. 

The spectral line shape in CARS spectroscopy is described by Equation (6.14). In order to investigate an unknown sample, one needs to extract the imaginary part of to be able to compare it with the known spontaneous Raman spectrum. To do so, one has to determine the phase of the resonant contribution with respect to the nonreso-nant one. This is a well-known problem of phase retrieval, which has been discussed in detail elsewhere (Lucarini et al. 2005). The basic idea is to use the whole CARS spectrum and the fact that the nonresonant background is approximately constant. The latter assumption is justihed if there are no two-photon resonances in the molecular system (Akhmanov and Koroteev 1981). There are several approaches to retrieve the unknown phase (Lucarini et al. 2005), but the majority of those techniques are based on an iterative procedure, which often converges only for simple spectra and negligible noise. When dealing with real experimental data, such iterative procedures often fail to reproduce the spectroscopic data obtained by some other means. 

The signal of the quaternary carbon atom C-4 of inosine can be easily distinguished from that of C-2 by proton off-resonance decoupling. Comparison of the spectrum of inosine with the spectral data of 8-deuterioinosine and 6-thioinosine leads to the unequivocal signal assignment of the base residue of this nucleoside [749] (see Table 5.22). 

In a spectral UV-VIS study of a series of chalcogenopyrylium dyes it has been shown that the aromaticity of the heterocycles decreases with the size of the chalcogen, whereas the bathochromic effect increases from the oxygen to tellurium [7], The aromaticity index based on data from experimentally determined bond lengths, which yield statistically evaluated bond orders, has been proposed and applied to neutral and ionic five-membered selenaheterocycles. A reasonable parallel also exists between the aromaticity index and resonance energies [37],... 

In this case, the fact that a large resonant enhancement is seen of the states at around 1 eV is consistent with the behaviour of other dominantly trivalent Ce-based systems, as it is here that the 4f° photoemission final-state spectral weight occurs for systems with a 4f1 initial-state configuration. Thus, from the resonant photoemission data, we can confirm the trivalent nature of the Ce ions in Ce2 C72 and conclude that there is relatively weak hybridisation between the Ce 5d and the C 2s/2p states of the fullerene cage in this case. A shorthand notation for the cerium dimetallofullerene could thus be (Ce2)6+(C72)6. ... 

Other chemicals (matrix) present in the sample, especially at low analyte concentration, may affect the NMR spectral parameters and how the resonances are revealed. A general requirement for a spectrum acceptable for identification is that resonances of other chemicals do not overlap with resonances of the identified chemical. Partial overlapping may be acceptable if the resonance of the identified chemical can still be credibly explained. Where insufficient data are obtained, for example owing to severe overlapping, the resonances revealed may still be useful in supporting identifications based on other analytical techniques. 

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