INEPT spectra decoupling

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 ... 

Fig.39a-c Silicon-29 and proton spectra of diphenylsilane in C6D6. a INEPT spectrum with complete proton decoupling, b proton-coupled INEPT spectrum ( h 201 Hz) the fine structure is due to coupling with thearomatic protons, c proton spectrum showing 29Si satellites for the SiH protons)... 

FIGURE 12.2 (a) H-decoupled 13C spectrum of 2-methyl-3-pentanone in CDCl3.The carbonyl resonance is not shown, (b) H coupled, 3C INEPT spectrum of the same sample. Note the CH doublet centered about 40.5 ppm, the CH2 triplet with the center line missing at 33 ppm, and the cwo CH3 quartets centered near 18 and 8 ppm. Courtesy of Herman J. C.Yeh (National Institutes of Health). 

As shown for dodecamethylcyclohexasilane (Section III,A), long-range couplings (provided they are resolvable) can be used for polarization transfer. From (MeO)4Si, for example, one can obtain well-resolved proton coupled spectra (Fig. 6) and decoupled spectra (Fig. 7). The coupled spectra vividly show the alteration of the relative intensities of the (MeO)4Si multi-plet in comparing the standard acquisition spectrum or DEPT spectrum to the INEPT spectrum. The relatively greater enhancement of the outer lines in coupled INEPT spectra is shown by the observation of 11 lines (including the zero-intensity central line) in the INEPT spectrum (Fig. 6a), compared to nine lines seen in the DEPT and standard acquisition spectra (Figs. 6b and c). Multiplet intensities in the DEPT and standard acquisition spectra are nearly identical, as expected. 

Figure 5.21. (A) INEPT spectrum of menthol with doublets and doublet (CH) signals to appear (C) off-resonance spectrum of quartets in phase and triplets out of phase (B) value of D3 in modified menthol (D) broad-band decoupled spectrum of menthol.

The population inversion of the X isotope energy levels following an INEPT sequence implies the "up-down" appearance of the coupled INEPT spectrum. A decoupled spectrum can be obtained after a suitable delay T which will bring again the X jXg components in phase. Table II gives the T values when X is coupled to n = 1,2,3 12 equivalent spin 1/2 isotopes. The refocused... 

Vicinal VnAg coupling of 8.57 Hz between the imine proton and the silver atom has been reported by Kay et for the four-coordinate Ag complex of the ligand tris[4-(2-thienyl)-3-aza-3-butenyl]amine (TTME), [Ag(TTME)](PF6) H2O. It has been determined from the selectively decoupled INEPT spectrum of this compound. 

Fig. 6.18. C-INEPT spectrum (a), and the proton-decoupled spectrum at 125.77 MHz (b) of LDPE. (Reproduced from Ref. [48, p. 119]. 1984 American Chemical Society.)...

The 13C spectrum of an organic molecule arises from 13C nuclei coupled through one bond to zero, one, two, or three protons. It can be very helpful to ascertain for each line in a proton-decoupled 13C spectrum whether it arises from a CH3, CH2, CH, or quaternary carbon. In our discussion of INEPT in Section 9.7, we noted that each of the first three moieties has different precession behavior and refocuses at a specific time, hence can serve as a means of distinguishing among these groups. We describe here three methods that are particularly useful for such... 

Other nuclei besides H or have been used to monitor enantiomeric discrimination with chiral lanthanide chelates. This includes the NMR spectra of chiral 2-thiabicyclo[4.3.0]nonane 2,2-dioxides (14) and 8,8-dioxides (15) with Pr(hfc)3 . The Si NMR spectrum of a-C-silylated amines and alcohols (38) in the presence of Eu(tfc)3 was used to monitor the optical purity of these compounds . A refocused-decoupled INEPT (insensitive nuclei enhanced by polarization transfer) pulse sequence was used to circumvent the long spin-lattice relaxation times of the silicon. 

Figure 10 The chemical shift filtered INEPT experiment. The delay is equal to ti + 2A2 (see Figure 9) and is incremented as in a 2D spectrum. Dec. = decoupling...

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,...

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...

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