The J-modulated spin-echo

The spin-echo experiment is particularly simple to set-up as it does not require proton pulses or their calibration, a desirable property when the experiment was first introduced but of little consequence nowadays. The same results can, in fact, be obtained by the use of proton 180° pulses rather than 

The poor editing accuracy of spin-echoes in the presence of a wide range of J values and the inability to fully characterise all carbon multiplicities are the major limitations of these techniques. More complex variations on the pulsed J-modulated spin-echo are to be found that do allow a complete decomposition of the carbon spectra into C, CH, CH2 and CH3 sub-spectra [21] and which also show greater tolerances to variations in Jch [22]. Likewise, 

J-compensated APT sequences have been developed for greater tolerance to a spread of J values [23], and for the direct generation of complete sub-spectra [24]. Invariably the simplest sequences still find widest use. 

---

APT Attached proton test, a modification of the J-modulated spin-echo experiment to determine C//multiplicities, a less sensitive alternative to DEPT... 

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

The J-modulated spin-echo technique " and the DEPT technique " are pulse sequences, which transform the information of the CH signal multiplicity and of spin-spin coupling into phase relationships (positive and negative amplitudes) of the C signals in the proton decoupled C NMR spectra. The DEPT technique benefits from a polarisation transfer which increases the sensitivity by up to a factor of 4. For... 

Figure 4.15. The variation of carbon signal intensities in the J-modulated spin-echo as a function of the evolution time A (0 = ISOJA ).

Figure 4.16. Carbon spectrum of camphor 4.1 edited with the J-modulated spin-echo sequence, (a) Conventional carbon spectrum (carbonyl not shown), and edited spectra with (b) A = 1/J (6 = 180°) and (c) A = 1/2J (0 = 90°) with J assumed to be 130 Hz. Some breakthrough of protonated carbons is observed in (c) due to variations in coupling constants within the molecule.

The combination of these two spectra then allows direct identification of XH,XH2 and XH3 signals in even the most complex molecules in a similar manner as in the J modulated spin echo experiments described above. 

Difficulties are encountered in establishing carbon multiplicities with the SFORD technique when signal overlap becomes severe, or when only small quan-titites of sample are available, thus dividing the signal intensity between the lines of the multiplet. New techniques that overcome the limitations of SFORD utilizing the spin echo are the J-modulated spin-echo (231) or the Attached Proton Test (312), the Insensitive Nuclei Enhanced by Polarization Transfer (56, 79, 104), and the Distortionless Enhancement by Polarization Transfer (80). 

Another technique for determining carbon multiplicities similar to the J-modulated spin echo, but with added enhancement due to a polarization transfer from the protons to nuclei, is INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) (56, 79, 104). The polarization transfer is capable of a roughly four-fold enhancement of the carbon signal intensity. 

The J-modulated spin-echo method described here is of particular advantage when the signals lie close to one another, and determination of... 

Figure 4.13. J-modulated spin-echo sequences, (a) The decoupler-gated variant and (b) the pulsed variant.

The applications of solid-state C-NMR spectra for the study of polymorphs and solvates can go beyond evaluations of resonance band positions and make use of additional spectral characteristics. For instance, studies of relaxation times of furosemide polymorphs were used to show the presence of more molecular mobility and disorder in Form II, while the structure of Form I was judged to be more rigid and uniformly ordered [158]. The analysis of the solid-state C-NMR spectra of (li ,3 j-3-/ -thioanisoyl)-l,2,2-trimethylcyclopentanecarboxylic acid was facilitated by the 7-modulated spin-echo technique, which was used to deduce the number of protons bound to each carbon atom [159]. Differences in the dipolar dephasing behavior between the two polymorphs of ( )- ra 5-3,4-dichloro-A/-methyl-7V-[ 1,2,3,4-tetrahydro-5-methoxy-2-(pyrrolidin-1 -yl)]naphth-1 -yl-benzeneacetamide were noted and ascribed to motional modulation of the carbon-hydro-... 

While the SPT experiment has obvious utility, it is cumbersome to use unless the selective nature of the experiment is specifically being exploited for structure elucidation purposes. A group of experiments that may be categorized as J-modulated spin echo experiments allows the simultaneous investigation of the entire spectrum. As the name of this group of experiments implies, they utilize a spin echo of the type shown in Eq. (1), over which a scalar coupling driven process (using jcH most commonly) is superimposed. 

Fig. 8.6 Pulse sequence for the amplitude modulated 2DJ-resolved NMR experiment. The experiment is based on a J-modulated spin echo. The first 90° pulse tips magnetization into the xy-plane where it evolves during the first half of the evolution period, t]/2. The 180° pulse is applied and the decoupler is simultaneously gated off for the second half of evolution.

It is important to note that in this description of spin-coupled nuclei, we begin to see the limitations of the vector approach to understanding the NMR processes involved. In this case it is fairly simple to graft on the quantum aspect of spin orientation to provide an intuitively satisfying picture of decoupling and of the J modulation of spin echoes. Other cases will not be so clear and will force us to adopt a more powerful approach in Chapter 11. 

As shown in Fig. 3 a, spin coherence is manifested in the optically detected transient nutation signal for [Rh(bpy)3] (0)04)3 the phosphorescent triplet state. In this experiment, one observes that the phosphorescence intensity becomes modulated as the pulse length of microwave pulses, resonant with the D - transition, is gradually increased. The modulation is evidence that the micro-wave excitation induces a spin coherence in the ensemble of molecules in the photoexcited triplet state [44]. Moreover, from the transient nutation experiment one obtains the information about the duration of the pulses needed in a spin echo experiment. In the case of the example, the n/2 pulse is 100 ns and the 71 pulse has a length of 200 ns. Similarly, transient nutation signals for the other zero-field spin resonances could be obtained. The optically detected spin echo decay as measured for the D - j j zero-field transition for [Rhlbpylj](004)3... 

D J-resolved NMR experiments are a conceptual amalgamation of two topics discussed above, the /-modulated spin echo and the two-dimensional characteristic of the spin—lattice relaxation experiments. As the name of these experiments implies, scalar coupling information, /, will be displayed in the one frequency domain chemical shift information will be presented in the second frequency domain. The simplest 2D/ experiments sort chemical shift information in the detected time domain, labeled by convention, while the heteronuclear scalar couplings of each carbon are sorted into the indirectly determined time domain, tj (do not be confuse lower case h with the spin—lattice relaxation time, Tj). 

In heteronuclear y-resolved spectra chemical shifts of an arbitrary nucleus X which couples with protons (this is mostly are presented on one axis and proton-X J couplings on the other. The information content is equivalent to that in a proton-coupled "C spectrum (Fig. 12) but without the severe overlap of multiplets which is usually encountered in the latter. In common with off-resonance proton decoupling, y-modulated spin echo, and DEPT experiments, it facilitates multiplicity determination. In addition, it enables proton-X coupling constants to be measured. 

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

The electron-spm echo envelope modulation (ESEEM) phenomenon [37, 38] is of primary interest in pulsed EPR of solids, where anisotropic hyperfme and nuclear quadnipole interactions persist. The effect can be observed as modulations of the echo intensity in two-pulse and three-pulse experiments in which x or J is varied. In liquids the modulations are averaged to zero by rapid molecular tumbling. The physical origin of ESEEM can be understood in tenns of the four-level spin energy diagram for the S = I = model system... 

Riedel A, S Fetzner, M Rampp, F Lingens, U Liebl, J-L Zrmmermann, W Nitschke (1995) EPR, electron spin echo envelope modulation, and electron nuclear double resonance studies of the 2Ee-2S centers of the 2-halobenzoate 1,2-dioxygenase from Burkholderia (Pseudomonas) cepacia 2CBS. J Biol Chem 270 30869-30873. 

---