NMR coupling

Calculated rr-bond orders are summarized in Table 1. These calculations are supported by chemical evidence that the S—N bond is the one most easily cleaved. Attempts have been made to relate bond orders and electron densities to NMR coupling constants (74CJC833, 77aC6i9> and CNMR (75CJC596, 75CJC1677> and NNMR (78JOC4693> chemical shifts, with limited success. 

J. L. Marshall, Carbon-Carbon and Carbon-Proton NMR Couplings Applications to Organic Stereochemistry and Conformational Analysis, Verlag Chemie International, Deerfield Beaeh, FL, 1983. 

Their properties (dipole moment, polarizability, NMR coupling constants etc.). 

The first derivative is the hyperfine coupling constant g (as measured by ESR), the second derivative with respect to two different nuclear spins is the NMR coupling constant, J (Planck s constant appears owing to the convention of reporting coupling constants in Hertz, and the factor of 1/2 disappears since we implicitly only consider distinct pairs of nuclei). 

A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platinum(II and IV) complexes. The trans-influence had hitherto been explained in terms of d7r-p7r bonding, in other words, such a mechanism dominated with trans-effect... 

Table 3.23 NMR coupling constants for platinum phosphine complexes [155]...

Eq. (1) has potential application to other types of measurements of substituent effects besides those specifically considered in this paper e.g., nmr coupling constants and shifts for other nuclei, ir and uv spectral shifts and intensities. We caution (with emphasis) in these applications the needed use of data sets of high quality, both with respect to the precision of the measurement and substituents considered (i.e., a full complement of substituent o/ and Or properties must be encompassed for a meaningful correlation to be obtained). There is, of course, no requirement that all data sets will be uniquely fitted by eq. (1) using one of the four or scales of Table V. For example, the data for the ionization of the conjugate acids of pyridine-N-oxides (30), HjO, 25° is found to fit equally well the or(ba.) or Or scales (SD=. 14 /=. 072). The data (31) for the rates of alkaline ("OMe) cleavage of ArSnMea are not fitted to acceptable precision (fs >. 23) by any of the Or parameters. This data set is nevertheless indicated... 

Both MS and NMR coupling to HPLC have been employed for the analysis of p-carotene isomers and determination of lutein and zeaxanthin isomers in spinach, sweet com, and in retina. Capillary high performance hquid chromatography with stop flow connected to NMR (600 MHz) was used for stracture elucidation of all-trans deoxylutein 11 and its isomers.Efforts are in progress to eliminate the remaining major drawbacks such as obligatory use of deuterated solvents in the mobile phase, poor sensitivity, and low throughput of HPLC-NMR analyses. 

Haasnoot, C. A. G., De Leeuw, F. A. A. M., De Leeuw, H. P. M., Altona, C. Relationship between proton-proton NMR coupling constants and substituent electronegativities. III. Conformational analysis of proline rings in solution using a generalized Karplus equation. Biopolymers 1981, 20,1211-1245. 

On-line SFE-NMR coupling was also reported [151,152], SFE provides some degree of separation by means of solubility and affinity to the matrix. This offers the possibility of transferring analytes directly from the extraction into the NMR probe. Drawbacks in the acquisition of SFE-NMR and SFC-NMR spectra are the elongated spin-lattice relaxation times 7) of protons and the pressure dependence of H NMR chemical shifts [153]. 

SFC-NMR is available from 200 to 800 MHz, and is suitable for all common NMR-detected nuclei. SFC/SFE-NMR requires dedicated probe-heads for high pressure (up to 350 bar) and elevated temperature (up to 100 °C). SFC-NMR is carried out with conventional packed columns, using modifier, pressure and temperature gradients. The resolution of 1H NMR spectra obtained in SFE-NMR and SFC-NMR coupling under continuous-flow conditions approaches that of conventionally recorded NMR spectra. However, due to the supercritical measuring conditions, the 111 spin-lattice relaxation times 7) are doubled. 

Applications Albert et al. [455] have shown continuous-flow SFC-NMR spectra of five plasticisers (DEP, DNPP, DPP, BBP, DNBP). On-flow and stopped-flow pSFC-NMR of synthetic mixtures of phthalates were reported [457]. The feasibility of SFC-NMR coupling has been demonstrated with real-life applications [458]. Figure 7.20 shows a reconstruction of an extraction profile from a PVC tube [152]. The profiles of the integral aromatic proton signals between 7.2 and 8.2 ppm and the ester protons at 4.42 ppm display the relative concentration of the extracted phthalate as a function of the proceeding extraction. The structure of the extracted phthalate could be assigned to DEHP (Figure 7.21). 

On-line SFC- H NMR has also been applied for the investigation of monomeric acrylates [456] SFC-NMR and HPFC-NMR were compared for both these acrylates [456] and phthalates [459]. Direct SFC-NMR coupling offers the advantage that the recorded continuous-flow H NMR spectrum is not obscured by solvent signals. 

Our standard molecule is however not ideally suited for certain experiments (e.g. magnetic non-equivalence, NOE, HPLC-NMR coupling). In such cases other simple compounds of the same type, compounds 2-7, will be used ... 

The instrumental setup for capillary HPLC-NMR coupling is shown in Figure 4.6. The capillary pump is connected via 50 pm capillaries between the capillary HPLC pump, the UV detector, and the NMR flow probe. 

FIGURE 4.6 Instrumental setup for capillary HPLC-NMR coupling. (From Hentschel, P. et al., J. Chromatogr. A, 285, 2006. With permission.)... 

In the first, quantum mechanics can be used to calculate the NMR coupling constant between two nuclear spins in the molecular environment or the NMR shielding constant. Jensen [8] in Chapter 10 of the reference provides a comprehensive introduction to the methods for calculating these molecular properties. The NMR coupling constant, KAB, is related to the second derivative of the energy with respect to the internal magnetic moments, /u, arising from the nuclear spin of the two atoms, A and B. 

Subsequently, cytotoxic fractions from a Fijian sponge, Xestospongia sp., were demonstrated to contain the related unsaturated-1,2-dioxanes, xestin A (3) and xestin B (4) [25]. A weakly cytotoxic but-3-enolide (5) was also isolated. The relative stereochemistries of the ring substituents were established from a combination of chiroptic analysis of reduction products and H NMR coupling... 

---