Proton carbon dipolar coupling

C NMR has been used to investigate the phenyl ring motions occurring at room temperature in BPA-PC, by using the dipolar rotational spin echo technique [40], Indeed, the reduction in dipolar coupling between carbons and directly attached protons arising from molecular motion (of frequency... 

Any pairs of interactions may be separated in this way provided suitable signals are obtainable. To date, such pairs include chemical shift/spin-spin coupling, proton/carbon chemical shifts, as well as dipolar/chemical shifts discussed above. A most promising 2-D plot represents the correlation between proton and carbon shifts. It is also possible to look for polarization transfer only between interacting carbon-proton pairs. This last experiment is otherwise done with fairly complex methods such as spin tickling or off-resonance decoupling, in many cases 2-D NMR can do it in a straightforward manner. Finally, it is possible to detect forbidden transitions by 2-D NMR, but we will not go into it in this book. 

Figure 1. Aromatic part of the phase-sensitive 2D resolved dipolar spectrum of the liquid crystal 4 -methoxybenzylidene-4-n-butylaniline (MBBA) in the nematic phase. The spread in the ardirection is governed by the proton-carbon dipolar couplings C+ carbon chemical shifts) while the spread along the horizontal axis is determined by the carbon chemical shifts, exclusively. The corresponding proton-decoupled ID spectrum is sketched at the top of the 2D spectrum for clari ation. Projection of the 2D spectrum onto the

In spite of the apparent simplicity of the method, its drawback comes from the fact that a two-spin system has been assumed. It provides merely global information spanning all protons prone two interact by dipolar coupling with the considered carbon. Selective information requires pulsed experiments stemming from the general solution of Equation (14) given below. 

The NOE factor provides a global information, that is the sum of crossrelaxation rates of all protons which can interact by dipolar coupling with the considered carbon. It is therefore very useful when the considered... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

Fig. 10.22. Diagram showing the cross-polarization from protons, H, to a heteronucleus, X, such as carbons. Heteronuclear dipolar coupling enables the transfer of magnetization from H to X, such as protons to carbons. Homonuclear dipolar coupling between the abundant protons enables the redistribution of proton spin energy through spin diffusion.

However, if side-chain carbon assignments are wanted, C(CC)(CO)NH experiments [33] that start directly with carbon magnetization and transfer it further to the amide proton for detection are available. If protonated substituents, for example methyl groups, have been introduced into the otherwise perdeuterated protein, the usual HC(C)(CO)NH-TOCSY pulse sequence can be used to obtain the proton chemical shifts. These protons can provide a small number of NOEs that, together with residual dipolar couplings and the secondary structure identification from chemical shifts, make the determination of the global fold of large proteins possible. 

Fig. 11.16 The pulse sequence used to monitor the evolution of carboncarbon double-quantum coherence over a single rotor period in the presence of the proton-carbon heteronuclear dipolar coupling (a). The evolution of the double-quantum coherence between the Cl 4 and Cl 5 carbons in the retinal of bacteriorhodopsin in the ground state (b). The observed evolution is consistent with a C14-C15 torsion angle of 164° (reproduced with permission from Ref. [172]).

Since the intensities of the resonances in the CPMAS experiment depend upon strengths of the dipolar couplings and molecular motions in the solid, it is not straightforward to obtain quantitative spectra. However, one may adjust the time during which magnetization is transferred from the proton to the carbon resonances, the CP time, and determine the time for which the solid state and solution integrals are equal. One can thereby obtain measurements of the aromatic to ethynyl ratio during the course of the cure. 

Figure 6. The proton(I)-carbon(S) dipolar coupling during a C-13 T,p and decoupled Tj experiment are compared. The relaxation rate is determined by the molecular fluctuation at the spin lock frequency u>,c or decoupling frequency a,a-...

---