Carbons, quaternary/methine methylene/methyl

DEPT Spectra Identifying Quaternary, Methine, Methylene and Methyl Carbons... 

For quaternary carbons (C) For methine carbons (CH) For methylene carbons (CH2) For methyl carbons (CHj) ... 

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

Fig. 10.9. Multiplicity edited DEPT traces for the methine, methylene and methyl resonances of santonin (4). Quaternary carbons are excluded in the DEPT experiment and must be observed in the 13C reference spectrum or through the use of another multiplicity editing experiment such as APT.

The structure was determined by NMR spectral analysis including a variety of two-dimensional NMR techniques. The 500-MHz XH NMR spectrum of 77 taken in CDCI3 (Figure 26) revealed the presence of 5 aromatic protons, 15 olefinic protons, a methoxy (63.65), an allylic methyl (62.14) and a tertiary methyl group (61.33). The 13C NMR spectrum showed signals due to all 34 carbons, which were assigned to 7 quaternary carbons, 23 methines, 1 methylene and 3 methyls by DEPT experiments. The 13C and XH NMR spectral data are summarized in Table 27. 

A second difficulty of fully decoupled 13C NMR spectra is that die connectivity in the molecule is difficult to establish (except by chemical shift correlation) because coupling patterns are absent. This dilemma is partially resolved by die use of a technique called off-resonance decoupling. In off-resonance decoupled 13C spectra, the carbons are coupled only to diose protons directly attached to diem and die coupling is first order. Thus quaternary carbons are singlets, methine carbons are doublets, methylene carbons are triplets, and methyl carbons are quartets. It is possible to use diis information to establish proton-carbon connectivity,... 

Mohan et al. [8] used an NMR spectroscopic method to characterize impurity D in samples of clopidogrel bisulfate. The method entailed XH NMR (at 400.13 MHz) and 13C NMR (at 100.62 MHz), with a sample concentration of 1 mg/ml in DMSO-t/f, (this solvent also served as an internal chemical shift standard). It was found that the 1H NMR spectrum of impurity D exhibited one hydrogen band than did that of clopidogrel, while the 13C NMR and DEPT135 NMR spectra indicated the presence of one methyl carbon, two methylene carbons, eight methine carbons, and five quaternary carbons. This corresponded to a similar structure as for clopidogrel, but with one less methylene carbon and one more methine carbon. 

A methyl group (Cby is a primary carbon centre (attached to one carbon), a methylene group (CH2) is a secondary carbon centre (attached to the other carbons), a methine group (CH) is a tertiary carbon centre (attached to three other carbons) and a carbon centre with four alkyl substituents (C) is a quaternary carbon centre (attached to four other carbons) ... 

The other experiment worth mentioning, which, by the way, is also obsolete, is the attached proton test or APT. This experiment is based on the different magnitudes of Tl—13C coupling for methine, methylene, and methyl groups. By adjusting certain delays in the pulse sequence (not given), quaternary and methylene carbons could be phased up, and methine and methyl carbons could be phased down. Since phase is arbitrary, this order could be reversed. This ability of distinguishing... 

If infrared and ultraviolet spectra are available, they should be inspected for preliminary clues about the functional groups and conjugation that might be present. A fairly superficial survey of the JH and 13C spectra should enable you to assess such factors as the degree and kind of symmetry in the molecule the ratio of aromatic to aliphatic carbons the number of methyl, methylene, methine and quaternary carbons and the number of exchangeable protons. The next stage is a detailed study of the one-dimensional or COSY spectrum ... 

Interrupted-Proton-Decoupling Experiments. Interrupted-pro-ton-decoupling experiments were carried out with a 50- xs interruption. This technique selects quaternary carbons (those lacking an attached proton) and rapidly moving methyl carbons. Resonances from methylene and methine carbons are suppressed unless, for some reason, their motion within the solid occurs at a rate similar to that of methyl groups. In essence, the interrupted-proton-decoupling experiment, which has not been previously reported for amber samples, provides an alternative fingerprint for the samples. 

The distortionless enhanced polarization transfer (DEPT) experiment is a carbon selectivity experiment.HO-116 Depending on the pulse length selected, one can selectively observe different types of carbon entities. We recommend setting the DEPT proton pulse length to 135°. In this case, quaternary carbons are suppressed, methylenes are inverted, and methine and methyl carbons... 

The J-coupling patterns in spectra can help to elucidate chemical structure, but they also comphcate the spectrum. Spectral-editing experiments allow such interactions to be observed in a controlled way. One such technique, DEPT (ifistortionless enhancement by polarization transfer), edits the spectrum based on scalar coupUng the pulse sequence is depicted in Fig. 15. DEPT is actually a set of three experiments, with tip angles, /, of 45°, 90°, and 135°. In NMR, appropriate addition and subtraction of the resulting subspectra separate the contributions from methyl, methylene, and methine carbons quaternaries are not observed. 

VI) Finally, any alkyl group attached to only one other carbon is called primary if attached to two other carbons, then it is secondary to three other carbons, tertiary and attached to four other carbons, quaternary. While these designations are not part of formal lUPAC names, they are part of the systematic description of compounds. Thus, in the earlier example of the isomers of C4FI10, butane and 2-methylpropane, the former may be said to have two primary carbons (in the methyl groups) and two secondary carbons (in the methylene groups), while the latter has three primary carbons (in the methyl groups) and one tertiary carbon (in the methine). 

Several other techniques now allow for the easy determination of the number of hydrogens attached to a carbon. One of the most convenient techniques goes by the acronym DEPT (distortionless enhancement with polarization transfer). In this technique, several separate NMR spectra are determined for a molecule under conditions that allow for the appearance of only methine (CH), methylene (CH2), or methyl (CH3) carbons. Quaternary carbons can be determined by difference Any signal in the full spectrum that does not appear in the separate spectra for CH, CH2, and CH3 carbons must belong to a quaternary carbon. 

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