C-signal

Section 13 15 C signals are more widely separated from one another than proton sig nals and C NMR spectra are relatively easy to interpret Table 13 3 gives chemical shift values for carbon in various environments... 

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

The position of metalation of 4-substituted 3,5-dimethylisoxazole was determined to occur solely at the C-5 methyl group by examination of the products (760MR226). The C NMR for C-3 in 1,2-benzisoxazole IV-oxide occurred at 118p.p.m. (80CC421) the equivalent C signal in furoxans occurred at 114p.p.m. (760MR158). 

Figure 2.4. CH multiplicities of a-pinene (1) [hexadeuterioacetone, 25 C, 50 MHz], (a) h broadband decoupled NMR spectrum (b) DEPT subspectrum of CH (c) DEPT subspectrum of all C atoms which are bonded to H (CH and CH3 positive, CH2 negative) (d) an expansion of a section of (c). Signals from two quaternary C atoms, three CH units, two CH2 units and three CH3 units can be seen...

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

Since two quaternary atoms and four CH atoms appear in the C NMR spectrum, the latter with a benzenoid coupling constant of 7-9 Hz, this is a disubstituted benzene ring, and the C signal with 5c = 162.2 fits a phenoxy C atom. The keto carbonyl (5c = 204.9) and methyl (5c = 26.6) resonances therefore point to an acetyl group as the only meaningful second substituent. Accordingly, it must be either o- or m-hydroxyacetophenone A or B the para isomer would show only four benzenoid C signals because of the molecular symmetry. 

The cross signals in the INADEQUATE plot show the CC bonds for two part structures A and B. Taking the C signal at 8c = 174.1 as the starting point the hydrocarbon skeleton A and additional C3 chain B result. 

The keto-carbonyl C signal at 5c = 200.9 would only fit the aflatoxins B, and M,. In the C NMR spectrum an enol ether-C// fragment can also be recognised from the chemical shift value of 5c = 145.8 and the typical one-bond coupling constant Jch = 196 Hz the proton involved appears at Sh = 72, as the CH COSY plot shows. The H triplet which belongs to it overlaps with a sing-... 

The H and C signal assignments of glueopyranoside ring C are derived from the HH COSY and C//COSY diagram ... 

The position of the second CC double bond in the structural fragment E follows finally from the correlation of the C signals at 5c = 37.8 and 49.8 with the //signals at 3h = 4.47 and 4.65. Note that trans protons generate larger cross-sectional areas than cis protons as a result of larger scalar couplings. 

The assignment of the umbelliferone residue in A likewise follows from interpretation of the Jqh and relationships in the CH COSY and CH COLOC plots following Table 48.1. The C signals at 5c = 112.9 and 113.1 ean be distinguished with the help of the eoupled NMR spee-... 

The sample was a lithium alloy mounted in epoxy. As the ion beam was scanned across the epoxy-metal interface, the C signal dropped and the Li signal increased. 

Figure 3.17 depicts an ultra-shallow TOF SIMS depth profile of a 100-eV B-implant in Si, capped with 17.3 nm Si. The measurement was performed with 600-eV SF5-sputtering and with 02-flooding. The original wafer surface, into which the B was implanted, is indicated by the maxima of the alkali- and C-signals. Because of these contaminants, a minimum is observed in the °Si-signal. The dynamic range of the B-profile is more than 3.5 decades and the depth resolution is <0.5 nm. 

The DEPT experiment (Doddrell elal, 1982) involves a similar polarization transfer as the INEPT experiment, except it has the advantage that all the C signals are in phase at the start of acquisition so there is no need for an extra refocusing delay as in the refocused INEPT experiment. Coupled DEPT spectra, if recorded, would therefore retain the familiar phasing and multiplet structures (1 1 for doublets, 1 2 1 for triplets, etc.). Moreover, DEPT experiments do not require as accurate a setting of delays between pulses as do INEPT experiments. 

The APT pulse sequence provides limited information about the number of hydrogens bonded to the carbons in a molecule, since it does not readily allow us to distinguish between the CH, and CH carbons or between CH and quaternary carbons. The INEPT spectrum not only can yield information about the multiplicity of all the carbons, but also affords sensitivity-enhanced C signals due to polarization transfer. 

As stated earlier, since tt]/ = yff2yr and since the gyromagnetic ratio of proton is about fourfold greater than that of carbon, then if C is observed and H is irradiated (expressed as C H ), at the extreme narrowing limit Ti, = 198.8% i.e., the C signal appears with a threefold enhancement of intensity due to the nOe effect. This is a very useful feature. For instance, in noise-decoupled C spectra in which C-H couplings are removed, the C signals appear with enhanced intensities due to nOe effects. 

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