Quaternary carbons, attached proton

APT is a technique for -decoupled 13C spectra, which uses the phase (normal or upside down) of the 13C peaks as a way to encode information about the number of protons attached to a carbon Cq (quaternary carbon, no protons), CH (methine, one proton), CH2 (methylene, two), or CH3 (methyl, three). These spectra are called edited because the phase (positive absorptive or negative absorptive) is modified relative to a normal 13C spectrum in order to encode additional information. APT gives all of the information of a normal carbon spectrum with somewhat reduced sensitivity, and it tells you whether the number of attached protons is odd (CH3 or CH) or even (CH2 or quaternary). 

One disadvantage of the APT experiment is that it does not readily allow us to disdnguish between carbon signals with the same phases, i.e., between CH3 and CH carbons or between CH2 and quaternary carbons, although the chemical shifts may provide some discriminatory information. The signal strengths also provide some useful information, since CH3 carbons tend to be more intense than CH carbons, and the CH2 carbons are usually more intense than quaternary carbons due to the greater nuclear Overhauser enhancements on account of the attached protons. 

Obviously we cannot however simply correlate the signal intensities with the presence of attached protons. So relaxation must also play a very important role. Relaxation times T, for carbon atoms also depend on whether these are protonated or not, and while T, for methyl or methylene groups may only be a few seconds, it may be as long as around 2 min for quaternary carbons Now the choice of an ideal relaxation delay becomes impossible, and so we have to make compromizes, which result in the large variations in signal intensity. 

APT, you will perhaps remember, stands for Attached Proton Test, meaning that this spectrum tells you the multiphcity of the signals (Me, CH2, CH or quaternary C). These two spectra tell you how many magnetically non-equivalent types of carbon are present in the molecule, but (for the reasons we discussed earlier) we do not use integration to try to find out relative numbers. We shall present APT spectra as follows CH, CH3 in negative phase (down), CH2 and quaternary C in positive phase (up). 

Another good reason for fully decoupling protons from 13C is that the 13C sensitivity, to some extent benefits from Overhauser enhancement (from proton to 13C which comes about as a result of decoupling the protons). This explains why quaternary carbons appear less intense than those attached to protons -they lack the Overhauser enhancement of the directly bonded proton. 

The J-MODulated (JMOD) C experiment, also known as Attached Proton Test (APT) was the first and simplest way to determine "C multiplicities. In contrast to DEPT no polarization transfer is included in the pulse sequence (Fig. 3.16) and as a consequence the signals of quaternary carbons are visible in the spectrum, but the sequence is far less sensitive than DEPT or INEPT. The value of D2 is used to differentiate between the different carbon multiplicities. The signal intensities of quaternary carbons are not influenced by the value of D2 for D2 equal to 1 CH and CH, groups have maximum negative intensity and CHj has maximum positive intensity. For D2 equal to 1 /C2 J, ) only the signals of quaternary carbons are visible. 

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,... 

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... 

An important point about quaternary carbons requires comment. Until now, we have had no direct correlations for carbons without protons, nor have we been able to see through heteroatoms such as oxygen, nitrogen, sulfur, etc. Both the two- and three-bond coupling correlations of HMBC provide us with both types of critical information. For example, C-4 of caryophyllene oxide at 59.1 ppm has no attached protons, and so far it has only appeared in the l3C spectrum of the compound, and we know that it is quaternary... 

With the complete proton assignments and the direct correlations between carbons and attached protons from the HMQC, we are able to assign all of the carbon resonances, except for the quaternary carbon, which is a trivial assignment in this case. An interesting example is found in the inset of the HMQC spectrum, which shows the correlations of the two overlapped protons, H-4 and H-5. Even though they are overlapped in the proton spectrum, they are well resolved in the HMQC spectrum because the carbon resonances are not overlapped. 

At this point, we have assigned all of the protons but still cannot differentiate between the methyl groups at H-8 and H-9. Since we know all the H assignments, it is a trivial task to transfer assignments to the 13C signals through the HMQC spectrum. The quaternary carbons C-3 and C-7 have no attached protons and cannot be correlated in the HMQC spectrum. An HMBC spectrum could be used to corre-... 

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