Pulse sequences couplings

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

The ID homonuclear Hartmann-Hahn (HOHAHA) experiment is an excellent way to determine complete coupled spin networks (18). The following pulse sequence is used ... 

HSQC Heteronuclear single quantum coherence, e.g. inverse CH correlation via one-bond coupling providing the same result as HMQC but using an alternative pulse sequence... 

To lessen experimental time, the null-point method may be employed by locating the pulse spacing, tnun, for which no magnetization is observed after the 180°-1-90° pulse-sequence. The relaxation rate is then obtained directly by using the relationship / , = 0.69/t n. In this way, a considerable diminution of measuring time is achieved, which is especially desirable in measurements of very low relaxation-rates, or for samples that are not very stable. In addition, estimates of relaxation rates for overlapping resonances can often be achieved. However, as the recovery curves for coupled spin-systems are, more often than not, nonexponential, observation of the null point may violate the initial-slope approximation. Hence, this method is best reserved for preliminary experiments that serve to establish the time scale for spin-lattice relaxation, and for qualitative conclusions. 

Jeener s idea was to introduce an incremented time ti into the basic ID NMR pulse sequence and to record a series of experiments at different values of second dimension to NMR spectroscopy. Jeener described a novel experiment in which a coupled spin system is excited by a sequence of two pulses separated by a variable time interval <]. During these variable intervals, the spin system is allowed to evolve to different extents. This variable time is therefore termed the evolution time. The insertion of a variable time period between two pulses represents the prime feature distinguishing 2D NMR experiments from ID NMR experiments. 

Figure 5.7 (A) Pulse sequence for gated decoupled /-resolved spectroscopy. It involves decoupling only during the first half of the evolution period

Figure 5.10 (A) Selective spin-flip pulse sequence for recording heteronuclear 2D / resolved spectra. (B) Its effect on magnetization vectors. The selective 180° pulse in the middle of the evolution period eliminates the large one-bond coupling constants, /< ...

The pulse sequence used in homonuclear 2D y-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton, A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors (faster... 

Figure 5.18 (A) Pulse sequence for homonuclear 2D y-resolved spectroscopy. (B) Effect of 90° H and 180° H pulses on an H doublet. (C) In the absence of coupling, the vectors are refocused by the 180° H pulse after t. This serves to remove any field inhomogeneities or chemical shift differences.

Fiffire 5.38 Pulse sequence for delayed COSY—a modification of the COSY experiment. The fixed delays at the end of the evolution period t and before the acquisition period <2 allow the detection of long-range couplings between protons. 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

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