Scalar .7 Coupling Evolution

Providing the echo interval in WATERGATE is kept short to minimize /-modulation, T2 relaxation and molecular diffusion effects (i.e. note the similarity with the Hahn spin-echo-based PGSE sequence in Fig. 9B), the desired resonances are retained in the spectrum with near-full intensity while the water peak should be suppressed by a factor of at least 1(1. Scalar coupling evolution can be refocused if the ir pulse in the WATERGATE sequence excites only some of the solute resonances but not the scalar coupled partners (for example amide resonances). ... 

The heteronuclear polarization transfer step The simplest sequence unit to create coherence transfer from a sensitive nucleus I to a insensitive nucleus S is a 90°(I) pulse and a 90°(S) pulse enclosing a scalar coupling evolution delay. This unit forms part of many pulse sequences including the HMQC and the HMBC experiment. 

Homonuclear correlation of nuclei of with low natural abundance, spin-echo sequence for scalar coupling evolution, DQF to suppress unwanted coherences, a double quantum frequency scaled dimension in 2D spectra... 

Scalar coupling evolution results in the interconversion of components of in-phase and antiphase magnetization. In the case of a component of in-phase -spin magnetization 4)/ evolution due to a scalar coupling to a spin / during a period T can be... 

Figure 2 Sign conventions for product operator evolution under radiofrequency pulse, chemical shift and scalar coupling evolution.

Figure 3 Vector model representations of the evolution of /c-spin magnetization. (A) The effect of a p/ pulse on z magnetization. (B) The effect of chemical shift evolution for a time t on y magnetization. (C) The effect of scalar coupling evolution for a time t on y magnetization.

It can be seen from the above that the product operators for even very simple systems can proliferate rapidly. Consolidation reduces the 16 terms present at the end of the calculation to two. It can be seen from the result that overall there is no evolution due to chemical shift (and by implication the effects of magnetic field inhomogeneities), which makes the pulse sequence useful for measuring transverse relaxation. However, scalar coupling evolution continues unaffected. In reality, it is rarely necessary to undertake a calculation of such complexity the results of standard pulse sequences such as the spin echo are well known, so it is only necessary to write... 

As noted above, all chemical shift evolution cancels out and neither coherence will exhibit significant scalar coupling evolution during the subsequent I1Jqy I evolution period. The sin0 dependence of the coherence that is ultimately observed on the angle of the subsequent pulse is important, as will become evident below. This pulse converts part of the coherence into antiphase single-quantum coherence which is observed after it has become in-phase. 

Evolution of the Hr magnetization under the heteronuclear long-range scalar coupling nJCH... 

Evolution of the Hd magnetization under the one-bond scalar coupling Vch... 

Scalar coupled experiments COSY and TOCSY The correlated spectroscopy (COSY) experiment is one of the most simple 2D-NMR pulse sequences in terms of the number of RF pulses it requires [32]. The basic sequence consists of a 90-C-90-acquire. The sequence starts with an excitation pulse followed by an evolution period and then an additional 90° pulse prior to acquisition. Once the time domain data are Fourier transformed, the data appear as a diagonal in... 

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

Experimental approaches to direct characterization of the conformational exchange motions in proteins have been suggested earlier [67-69]. The most recent methods [66, 70-73] are based on a relaxation-compensated version of CPMG that alleviates the previous restriction on the duration of the refocusing delay due to evolution of magnetization from scalar couplings and dipole-dipole cross-correlations. 

The effects of chemical shift and scalar coupling on the time evolution of one spin can be accounted for independently, regardless of the order (the two effects are said to commute). For example ... 

Figure A6-3 Evolution of the two vectors from scalar coupling (7) in the xy plane from two perspectives (a) and (b) and resolved into components along the x and y axes (c).

Figure 2.13. Scalar couplings in the rotating frame. Multiplet components evolve according to their coupling constants. The vectors have an antiphase disposition after an evolution period of 1/2J and 1/4J s for doublets and triplets respectively.

---