Refocused INEPT sequence

Figure 4.27. Natural abundance N INEPT spectra of adenosine-5 -monosulphate 4.5. Lower traces show results with the INEPT sequence and upper traces with the refocused-INEPT sequence. Delays were calculated assuming (a) J = 90 Hz and (b) J = 10 Hz. For (a) the lower trace displays the 1 0 -1 pattern of the NH2 group whilst in (b) all resonances display antiphase two-bond H- N couplings. Spectra are referenced to nitromethane.

The basic INEPT sequence does not allow IR decoupling during data acquisition and the IR refocused INEPT sequence incorporating an reverse INEPT unit has been developed. Essentially the sequence has two functional parts The first INEPT unit creates antiphase coherence (I lyS ) and polarization transfer (lyS Iz y). The overall signal enhancement depending upon the difference in value between D(C, R) and the optimum coupling constant Jnominal (d2 = 1 / (4 Jnominal))- The second, reverse... 

Refocusing period of the INEPT+ experiment, the arrow denotes the additional pulse (purge pulse) in comparison to the refocused INEPT sequence. 

The PENDANT sequence is an extended version of the INEPT sequence for the detection of quaternary carbon atoms in a similar way that the DEPTQ sequence is an extension of the standard DEPT sequences. Initially the only difference between PENDANT and the refocused INEPT sequence is an extra 90° l C pulse executed simultaneously with the first 90° H pulse. Upon closer inspection a second difference is apparent, the refocusing period delay d3 in the INEPT sequence is 3/(8 H(C, H)) and 5/(8 U(C, H)) for PENDANT. Compared to the DEPT sequence, the modifications necessary to detect quaternary carbon atoms are easier to implement in the refocused INEPT sequence. The refocused INEPT sequence has two periods for refocusing chemical shift evolution so that the quaternary carbons atoms coherences which are generated by the initial 90° pulse evolve under the chemical shift evolution during the free precession periods and are refocused immediately before the acquisition period. In contrast the DEPTQ sequence requires an additional coherence evolution delay... 

The decoupled HSQC experiment employs the refocused INEPT sequence to generate an in-phase SQ coherence that is desirable e.g. for measurements of heteronuclear relaxation rates. The transfer function of the refocused INEPT depends on the proton multiphcity. i.e. the number of protons bound to the heteronucleus. This can be used to edit the spectrum or, when an overall maximal transfer efficiency is required, 2r = l/(3y) makes a good compromise. The constant time... 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

Both experiments are based on polarization transfer from sensitive nuclei to insensitive nuclei, and therefore the mjyor portions of their pulse sequences are common. The INEPT experiment, without refocusing and decoupling, however, yields spectra with distorted" multiplets. For instance, the two lines of a doublet appear in antiphase with respect one another. Similarly, the central line of a triplet may be too small to be visible, while the outer two lines of the triplet will be antiphase to one another. Introducing a variable refocusing delay A and broadband decoupling in the INEPT sequence can convert this experiment into a more useful one. 

Besides measurements of proton relaxation in crowded 3H NMR spectra indirectly through measurements of the much less crowded 13C or 29Si spectra356, INEPT can be used to speed up the direct measurements of 29Si spin-lattice relaxation357. The pulse sequence used for INEPT-enhanced measurement of spin-lattice relaxation times is just a minor modification of decoupled refocused INEPT (Section V.B) ... 

The width of the final1H pulse in the DEPT sequence ( ) affects the intensity and phase (positive or negative) of the 13C peaks in a way very similar to the length of the refocusing delay A in refocused INEPT, according to the relation = nJA. The intensities are shown below for the three types of carbon in DEPT ... 

Figure 12.13. Pulse sequences and resulting spectra for (a) simple INEPT, (b) INEPT with elimination of intensity differences due to natural component (2Af-) of Mc, (c) refocused INEPT (the evolution time A must be adjusted for multiplicities other than doublet), and (d) refocused INEPT with decoupling.

The sequence can easily be modified to allow the two separate components to combine—refocused INEPT. After the final 90° pulses but before data acquisition, a period 2A is added, with A = 1/4J, during which the two vectors combine. H and 13C 180° pulses are placed in the middle of this precession period to refocus chemical shifts. If decoupling is now applied during data acquisition, a single line of intensity 8 is obtained, that is, four times as large as would be obtained... 

An alternative pulse sequence that provides the same multiplicities as INEPT but with intensity ratios that follow the binomial theorem is DEPT (distortionless enhancement by polarization transfer).The pulse sequence, depicted in Fig. 12.3a, can be used, like refocused INEPT, for sensitivity enhancement but is usually employed as an editing technique. The three evolution periods T are chosen to approximate 1/2/, but the length of the pulse labeled 0 can be varied. As we show in the following, CH has maximum intensity at 0 = 90° CH2 has zero... 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

Figures Pt H NMR spectra of [Pt(C5H5 N)(PPh3)Cl2] recorded in THF (bottom) normal spectrum (middle) with INEPT enhancement from P, yijys = 40.48/21.50 2, note the antiphase multiplet (top) with refocused INEPT enhancement from P, note slight loss of intensity due to signal dephasing during the longer pnlse sequence...

Figure 6 111.9MHz Sn NMR spectrum of l,l-dimethyl-2,2-bis-(trimcthyl-stannyl)hydrazine (2S°C 2S% in C () recorded by the refocused INEPT pulse sequence with H decoupling and Hahn-echo (HEED) extension [20] (Hahn-echo delay 0.16 s), showing the reduced intensity of the parent line, allowing the straightforward assignment of the N satellites (marked by asterisks /(" Sn, N)=4S.SHz) and the measurement of the isotope induced shift A / N(" Sn) = —0.0365 ppm...

Figure 2.1.1 186.5 MHz Sn NMR spectra (recorded by the refocused INEPT pulse sequence with CPD...

Figure 2.1.8 186.5 MHz 5n /-/ NMR spectrum (refocused INEPT pulse sequence with CPD H decou-...

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