Decoupling, NMR

NOEs between nuclei are often exploited to demonstrate their spatial proximity, as described in the final section. It is possible to obtain decoupled NMR spectra without the complications of the NOE, by confining the decoupling irradiation to the period of the FID alone, and then waiting for 10 x before... 

Figure 2.4. CH multiplicities of a-pinene (1) [hexadeuterioacetone, 25 C, 50 MHz], (a) h broadband decoupled NMR spectrum (b) DEPT subspectrum of CH (c) DEPT subspectrum of all C atoms which are bonded to H (CH and CH3 positive, CH2 negative) (d) an expansion of a section of (c). Signals from two quaternary C atoms, three CH units, two CH2 units and three CH3 units can be seen...

Broadband-decoupled NMR, 451 Bromine, reaction with aldehydes, 846-848... 

Scheme 8.1 Synthesis of a sialyl-Lewis tetrasaccharide employing C-enriched protecting groups for the quantitative reaction monitoring using gated decoupling NMR spectroscopy.

In addition, the C3-H coupling constant (from a gated decoupling NMR experiment) of 161.8 Hz in 106 compared with 162 Hz in allenyllithium vs 167.5 Hz in methoxyallene and 168 Hz in allene is also in agreement with an allenic structure. However, neither the C-H coupling constant nor the NMR chemical shifts distinguish between the alternatives that 106 has a nonclassical 1,3-bridged structure 108 (M = Li) or an O-coordinated allenic structure (109). Hence the 6Li, -HOESY NMR technique which can be used to detect close proximities (ca < 3.5 A) between XH and 6Li nuclei was applied. The HOESY spectrum of a-lithiomethoxyallene in THF solution (in which 106 is dimeric) is shown... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Fig. 25 Homonuclear decoupled NMR spectrum of the methine (—C//(CH3)-) region of heterotactic PLA P = 87%) obtained from ROP of rac-lactide with [L2Mg2(p-OBn)2]2 (L = (Z)-4- [2-(dimethylamino)ethylamino](phenyl)methylene -3-methyl-l-phenyl-pyrazol-5-one) at 0 °C for 48 h (600 MHz, CDCI3) [70]...

The checkers found that the diastereomeric purity of the product was much greater than 90% based upon its 300 MHz and fully decoupled NMR spectra. Based on the proton-proton coupling constant (J = 11), trans geometry has been assigned. 

Figure 4.31 75 MHz proton-decoupled NMR spectrum of nitroethane, CH3CH2NO2, and DEPT 135 spectrum (/nsef) in CDCI3...

C Broad band decoupled NMR spectrum of ketotifen base Instrument Jeol FX-100 at 25.05 MHz. 

There are two main classes di-Os(PRi )4H2 and Os(PR3)3H2Cl2 (Scheme 12 and Table 17). The cis configuration of the former is established, in the case of Os(PMePh2)4H2 at least, by phosphorus decoupling NMR experiments 276 the high-field NNMR spectra of Os(PMePh2)2H2Cl2 and of... 

The thermal stereomutations of deuterium-labeled phenylcyclopropanes (Scheme 3) were studied in a progressive manner. First, the racemic and both achiral isomers were synthesized to provide material for kinetic work and to verify analytical methodsThe isomerizations among these three isomers at 309.3 °C were followed using either decoupled NMR spectroscopy or Raman spectroscopy the two kinetic parameters (kj + 23) = 0.36 x 10 s and ( 2 + 12) = 1.07 X 10 s at 309.3 °C were measured. Published spectra of both sorts for authentic samples of syn, anti and trans isomers, and of thermal reaction mixtures, provided... 

Complex (I) also loses CO when heated at 110°C or irradiated (A = 366 nm) under Nj. This product (II) also adds H, and a tetrahydride complex (III) is formed. The H NMR spectrum ( —40°C) shows four distinct hydride resonances (1 1 1 1). Selective decoupling NMR experiments show that each of the terminal PPh, ligand couples to two inequivalent hydrides, suggesting structure (III). 

The mechanism proposed by Albertin et al. involves ptotonation of 62 to give the vinylidene complex 63, as confirmed by the highly deshielded signal at 6 382.8 ppm in the spectrum (Scheme 5). The vinyl phospho-nium complex 64 was formed by intramolecular attack of one phosphonite on of the vinylidene ligand. The proton-coupled and -decoupled NMR spectra of 17 allowed clear assignment of the characteristic carbon atoms in the molecule the carbonyl appeared as a multiplet at 8 207.7 ppm, the metallate C-Co at 8 168.4 ppm, and the two alkene carbons at 8 63.8 and 32.1 ppm. 

Figure 1.11. NMR analysis of the keto-enol tautomerism of 2,4-pentanedione [CDCI3, 50% v/v, 25 °C, 60 MHz for H, 20 MHz for C]. (a) H NMR spectrum with integrals [result keto enol = 13 87] (b) H broadband decoupled NMR spectrum (c) C NMR spectrum obtained by inverse gated H decoupling with integrals [result keto enol = 15 85 ( 1)]...

Figure 5 H decoupled NMR spectrum of [Pt5(CO)(p2-CO)5(PEt3)4] (see Table 9). (Reproduced with permission from J. Raya, Thesis of the University of Strasbourg, 1993)...

A compound CeHsFO has a broad peak in the infrared at about 3100-3400 and the following signals in its (proton decoupled) NMR spectrum. Suggest a structure for the compound and interpret the spectra. 

Figure 6. Proton-decoupled NMR spectrum at 20,000 MHz in THF-dg of the THF-soluble, pentane-insoluble products of a 2-hr reaction of subbituminous coal in tetralin...

Figure 7. Aromatic regions of proton-decoupled NMR spectra of THF-soluble, pentane-insoluble reaction products of 10- (bottom), 35- (middle), and 120-min (top) reactions in THF-df ...

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