Decoupled refocused INEPT

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) ... 

Because the choice of A2 — 1/2J for the decoupled, refocused INEPT experiment leads to completely refocused doublets, but antiphase triplets and quartets, this particular experiment with decoupling produces a subspectrum that contains only methinyl resonances. Values of A2 also can be selected to optimize the intensities of methylene and methyl resonances. The idea can be depicted graphically by defining an imaginary angle B =... 

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. 

A refocused INEPT experiment100,104, also referred to as INEPTR105, which allows proton broad-band decoupling during acquisition, is performed as the following... 

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... 

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 5-22 The spectrum of chloroform (a) without double irradiation, (b) with H irradiation to achieve the INEPT enhancement, (c) with H irradiation to achieve refocused INEPT enhancement, (d) with H irradiation to achieve refocused INEPT enhancement and decoupling, and (e) with normal decoupling to achieve only the NOE. (Reproduced with permission from A. E. Derome,...

F. 1. 186.5 MHz Sn NMR spectra (recorded by the refocused INEPT pulse sequence with CPD H decoupling) of tris(trunethyl)stannylamine in, (MejSn)3N, toluene, labelled partially ( 13%) with N (sample by courtesy of W. Storch, M. Vosteen, University of Munich). The broad lines originate from the Sn- N and Sn- N- Sn isotopomers. All sharp lines arise from the Sn- N and " Sn- N- Sn isotopomers. Note the isotope induced chemical shift i i4/isN(ii9sn) = -38.4ppb and the resolved C satellites of the sharp lines due to V( Sn,N,Sn, C) in the isotopomers containing N. 

Fig. 2. 186.5 MHz NMR spectrum (recorded by the refocused INEPT pulse sequence with CPD H decoupling) of cis-[Pt(PPh3)2(SnMe3)OC-SnMe3]. The expected splitting of the central " Sn resonance signals is observed due to Sn- P coupling, together with the Pt satellites. The expanded region of the Sn(0) resonance also shows satellites due to 7(Sn,Sn) wWch is...

The DEPT experiment [33] (Distortionless Enhancement by Polarisation Transfer) is the most widely used polarisation transfer editing experiment in carbon-13 spectroscopy, although its application is certainly not limited to the proton-carbon combination. It enables the complete determination of all carbon multiplicities, as does the refocused INEPT discussed above, but has a number of distinct advantages. One of these is that it directly produces multiplet patterns in proton-coupled carbon spectra that match those obtained from direct observation, meaning methylene carbons display the familiar 1 2 1 and methyl carbons the 1 3 3 1 intensity patterns this is the origin of the term distortionless . However, for most applications proton decoupling is applied during acquisition and multiplet structure is of no consequence, so the benefits of DEPT must lie elsewhere. 

Figure 6.31. (a) The ID refocused INEPT experiment, shown in simplified form with the refocusing pulses at the midpoints of A1 and A2 removed for clarity. The 2D shift correlation experiment (HETCOR) in (b) is derived from INEPT by the addition of the t evolution period to encode proton chemical shifts prior to polarisation transfer. The 180 carbon pulse at the midpoint of t refocuses heteronuclear coupling evolution and thus provides carbon decoupling in fi. 

To improve the sensitivity the selective J-resolved pulse sequence may be combined with a refocused INEPT polarization transfer experiment. In contrast to the original heteronuclear J-resolved experiment the first excitation pulse is executed on the F2 channel with the spin-echo sequence sandwiched between two incremental delay. Coupling evolves during the second incremental delay before the refocused INEPT unit creates in-phase coherence for the nuclei which are coupled to the selected proton nucleus allowing decoupling on the F2 channel during data acquisition. 

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... 

Fig. 1. 62.8 MHz 2°7Pb NMR spectrum of (Cp)Fe(CO)2-PbMe3 (25 °C =5% in C6D6), recorded by using the refocused INEPT pulse sequence with CPD H decoupling (200 transients with a repetition time of 8 s and an acquisition time of 4 s).94 The line width h, /2 = 2 Hz is rather small for 207Pb NMR signals and makes it possible to observe 57Fe satellites and also 13C satellites in order to measure accurately the isotope-induced chemical shifts...

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 basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

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. 

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