Double-quantum experiment

Tetraneopentyl zirconium reacts in the same way as tetraneopentyl titanium to give, on a silica (soo), a tris(neopentyl) monografted species [32]. Treatment under H2 of this surface species yields silica-supported zirconium hydrides [33], which have been characterized as a mixture of mono- (65-70%) and bis- (35-30%) hydrides based on double quanta NMR experiments (Scheme 2.11) [34]. Interestingly, the double quantum experiment allows us to prove not only the presence of the two hydrides and the monohydride of zirconium by the presence or the absence of the double quanta correlation but also to detect the through space magnetic interaction between the zirconium monohydride and the silicon di-hydride, proving thus the spatial arrangement on the surface. This confirms the mechanism by which these hydrides have been formed on the surface. 

On the other hand, the digermirane (18) reacts with Cgo to afford the 1,4-adduct (19, twist form) selectively15. The structure of the 1,4-adduct was confirmed by 13C-13C INADEQUATE (Incredible Natural Abundance Double Quantum Experiment) spectroscopy (equation 5). The rate of disappearance of Cgo was suppressed by addition of diazabicyclo[2.2.2]octane and 1,2,4,5-tetramethoxybenzene. Furthermore, the reaction was completely inhibited by addition of rubrene as triplet quencher. One plausible rationale for these observations is that an exciplex intermediate may be responsible for formation of the products (Scheme 6). 

Fig. 6.1.6. Two-dimensional time-incremented double quantum experiment on polycarbonate recorded using a single cycle of multiple quantum excitation, and a spinning speed of 14.8 kHz. The f evolution was restricted to double quantum coherences by phase suitable cycling. The oji dimension is, therefore, a double quantum spectrum. The u>2 dimension is a single quantum spectrum [21].

The value of the delay A in the double-quantum experiment affects the amount of multiple-quantum generated and hence the intensity in the spectrum. All of the couplings present in the spin system affect the intensity and as couplings cover a wide range, no single optimum value for A can be given. An unfortunate choice for A will result in low intensity, and it is then possible that correlations will be missed. No such problems occur with COSY. 

In the P MAS NMR spectrum, two distinct resonances appear and are assigned to phosphonic acid groups and the phosphonic anhydride. P double-quantum experiments show that there is no phase segregation... 

Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

INADEQUATE Incredible natural abundance double quantum transfer experiment, segregates AB or AX systems due to homonuclear one-bond couplings of less abundant nuclei, e.g. 

Figure 1. Pulse sequences of some typical 2D-NMR experiments. COSY = correlation SpectroscopY, DQFCOSY = Double Quantum Filtered COSY, RELAY = RELAYed Magnetization Spectroscopy, and NOESY = Nuclear Overhauser Effect SpectroscopY.

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

The NMR techniques discussed so far provide information about proton-proton interactions (e.g., COSY, NOESY, SECSY, 2D y-resolved), or they allow the correlation of protons with carbons or other hetero atoms (e.g., hetero COSY, COLOC, hetero /resolved). The resulting information is very useful for structure elucidation, but it does not reveal the carbon framework of the organic molecule directly. One interesting 2D NMR experiment, INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment), allows the entire carbon skeleton to be deduced directly via the measurement of C- C couplings. 

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

INADEQUATE stands for Incredible Natural Abundance DoublE QUAntum Transfer Experiment. Again, we refer you to NMR textbooks for an explanation of the principles. Here we only present the result, which is shown in Fig. 20 for the diester 2. 

There is also the rather famous experiment known as 2D INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) which allows us to correlate carbon-13 with carbon-13. Potentially this experiment is very useful, since it allows us to see directly which carbon atoms are directly bonded. However, you will remember that the natural abundance of carbon-13 is only 1.1%, so a carbon-13/carbon-13 correlation requires us to detect only about 0.01% of the carbon nuclei present. Thus the experiment is very insensitive and requires large amounts of both sample and measuring... 

High on any NMR spectrocopist s wish list would be a technique that could be used to establish connectivities directly between carbon atoms. Such a technique does exist and it goes by the name of INADEQUATE (incredible natural abundance double quantum transfer experiment). Whilst this might sound fantastic in theory, everything in the garden is far from rosy. 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

Fig. 5 Symmetry-based dipolar recoupling illustrated in terms of pulse sequences for the CN (a) and RNvn (b) pulse sequences, a spin-space selection diagram for the Cl symmetry (c) (reproduced from [118] with permission). Application of POST-CVj [31] as an element in a H- H double-quantum vs 13C chemical shift correlation experiment (d) used as elements (B panel) in a study of water binding to polycrystalline proteins (reproduced from [119] with permission)...

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