ABX spin system

An important consequence of the often relatively large y( P, P) value is that spectra of bis-phosphine complexes are often second order, e. g., the =CH signals for the two isomers of [Pd(NCCH=CHCN)(Me-Duphos)], 77, see Figure 1.15 [3]. The figure shows that these absorptions appear as complicated multiplets (i.e., the X-part of an ABX spin system). 

The signal at 4.45 is proper for the benzyl group, but the splitting pattern is problematic until it is recognized that because there is a chiral center at C-2, the benzyl protons are diastereotopic and thus nonequivalent. They are part of an ABX spin system and thus give the complex splitting pattern seen—actually a two-proton multiplet that looks like a doublet or a very close AB quartet. 

Fig. 15.55 Observed (a) and simulated (b) variable temperature proton NMR spectra of 0si(C01k)(m.<-> -CH,CH2C=CCHjCH,). The spectrum at -65 °C. characteristic of two equivalent ABX, spin systems, is produced when the movement of the alkync on the metal surface is slowed sulliciently that it is fixed in one position on the NMR time scale. At -82 C the alkyne molecule is moving rapidly on the metal. surface giving an averaged spectrum in which all methylene protons arc equivalent. (From Rosenberg, E. Bracker-Novak. J. Gellert, R. W. Aimc, S. Gobelto. R. Osella, D. J. Organomet. Client. 1989.

The class of alkaloids represented by 4-hydroxycularines is readily distinguished by H-NMR spectroscopy. Beside the characteristic quartet for H-1 (as the X part of an ABX spin system) between 4.0 and 4.6 ppm, a second low-field aliphatic proton (at 4.6 ppm) appears as a quartet (X part of a second ABX spin system) with two small coupling constants to indicate the C-4 position of the hydroxy group. 

Fig. 4.10 (A) and (B) show H spectra recorded at 500 MHz of a solution of 4-methoxy indole in 50 50 (v/v) D2O (containing 0.1% trifluoroacetic acid-dj) and methanol. Spectrum (A) shows the spectrum very soon after dissolution - the indole NH and 3 protons have already exchanged with solvent. Spectrum (B), obtained after a weekend in solution, shows the resonance of proton 5 has been completely exchanged. The 5 proton signal in (A) forms the X part of a tightly coupled ABX spin system.

Tryptoquivaline E (FTE) and the five tryptoquivalines F—have structures which are closely related to (104) and (105). FTE is dextrorotatory and exhibits a positive Cotton effect in the ord spectrum like (104) and (105). In the nmr spectrum of FTE, an ABX spin system assigned to the protons of the five-membered spirolactone ring closely resembles similar systems in the spectra of (104) and (105). These observations strongly suggest that all three metabolites have the same stereochemistry. 

As shown by multinuclear NMR studies, compounds 3a/3b-5a/5b also exist in solution (3a-5a, 4b [DslDMSO 3b, 5b CDCI3). Due to the chiral nature of these zwitterions, their SiCH2N protons are diastereotopic (3a-5a ABX spin systems in the H-NMR spectra, with F as the X nucleus 3b-5b ABMX spin systems in the H-NMR spectra, with N// as the M nucleus and F as the X nucleus). 

Figure 28.5 NMR spectrum (500.13 MHz, CDCI3, 298 K) of a low molecular weight poly(lactic acid) (PLLA) sample prepared with d/HC CCH OH ([l-LA]o/[4]o/[HC=CCH20H]o = 5000 1 100 = 4700 g/mol, MJM = 1.13). Note the ABX spin system...

The 3 p CP MAS NMR spectrum of [Cu(PPh3)2(NCMe)2]X is an ABX spin system with /( Cu P) = 1350 Hz and V(3 p3ip) = 75 Hz. /( Cu ip) has also been observed in some bis(tribenzylphosphine)cuprate salts.Broad line NMR spectroscopy has been used to study Cu soaps.NMR data have also been reported for [CuCN-MesSnCN-O.Sbipy], Cu2(C5Hi2N2)2CU, K-(ET)2Cu N(CN)2 Br, (BEDT-... 

The 300 MHz H NMR spectrum of very pure humulone shows three distinct signals for hydroxyl protons in addition to resonances for the protons of six methyl groups and the three ABX-spin systems for the side chain protons. These characteristics are not so pronounced for less pure humulone. Furthermore, the chemical shift values are concentration dependent. The hydrogen atom of the tertiary... 

The 300 MHz H NMR spectrum (26) displays two independent ABX-spin systems, which can not be accounted for by structure 39. On the other hand, the ABXY-spin system of 39 is not traced back in the spectrum. Only five methyl resonances are observed, indicating that one of the original six methyl groups is converted upon formation of TCD. The spin systems have been analyzed completely. 

The epimer of 41 with the 2-methylethenyl side chain at C-9 in endo-position can be excluded due to steric hindrance. It is clear that 40 has two independent ABX-spin systems, while 41 should display a vicinal coupling between the protons at C-8 and C-9. Such features are not observed in the spectrum. If the conformation, derived from the coupling constants of the two ABX-spin systems, is correct, it follows from the inspection of Dreiding models that the torsion angle between the protons at C-8 and C-9 is about 90°. The resulting value will in this case be minimal, which can account... 

The nature of the oxidized side chain can be determined from the NMR spectrum, which displays an ABX-spin system and two singlets for geminal methyl groups next to a carbon-oxygen bond (45). All signals for the 3-methylbutanoyl side chain are present, but no absorptions for a 3-methyl-2-butenyl side chain are found. It follows that this chain has been cleaved oxidatively. A similar mechanism is operative in the formation of the most important oxidation product of the hop beta acids, hulupone (56, Fig. 28) (see 13.1.1.1.2.). The reaction proceeds with ring contraction... 

---