Coupling constant interpretation

The determination of the conformation of 1,6-linkages requires that the conformation of the C5, C6 bond is established, in addition to conformation of the glycosidic bond. Besides the standard techniques for obtaining the coupling constants interpretation of the cross peak patterns in phase sensitive COSY spectra proved to be very valuable [29, 30]. 

INDO calculation of coupling constants, discussion of conformation. INDO calculation of coupling constant. ) Interpretation in [67Chel] evidently wrong (semidione radical see J. Am. Chem. Soc. 93 (1971) 2432). ... 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

Rather large HH coupling constants in the aliphatics range (72.5 and 15.0 Hz) indicate geminal methyl protons in rings. In order to establish clearly the relevant AB systems, it makes sense first to interpret the CH COSY diagram (Table 52.1). From this, the compound contains two methylene groups, A and B. 

The formulae H and I summarize the results with the complete assignments of all C and //chemical shifts (H) and the HH multiplets and coupling constants (I). Here the H multiplets which have been interpreted because of their clear fine structure are indicated by the multiplet abbreviation d for doublet. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

More recently a few NMR investigations on the isomerism of diazo compounds Ar — N2-X have been published, including an early one by Suhr (1963, lH NMR), another by Yustynyuk et al. (1976 a, 13C NMR), and a more recent one by Simova et al. (1983). The latter also evaluated 13C-15N coupling constants. In some cases isomers gave distinctly different NMR results. They were interpreted more recently by Elofson et al. (1990). 

Qualitative relaxation-studies have also been reported for an extensive series of derivatives of inositols, pentopyranoses, l,6-anhydro-/3-D-hexopyranoses, furanoses, and septanoses. In all instances, the experimentally determined Ri(ns) values reflect the anticipated geometry. For the furanose derivatives especially, they provide a better means for distinguishing between epimeric pairs than the relatively ambiguous interpretation of coupling-constant data. 

As mentioned earlier, heavy polar diatomic molecules, such as BaF, YbF, T1F, and PbO, are the prime experimental probes for the search of the violation of space inversion symmetry (P) and time reversal invariance (T). The experimental detection of these effects has important consequences [37, 38] for the theory of fundamental interactions or for physics beyond the standard model [39, 40]. For instance, a series of experiments on T1F [41] have already been reported, which provide the tightest limit available on the tensor coupling constant Cj, proton electric dipole moment (EDM) dp, and so on. Experiments on the YbF and BaF molecules are also of fundamental significance for the study of symmetry violation in nature, as these experiments have the potential to detect effects due to the electron EDM de. Accurate theoretical calculations are also absolutely necessary to interpret these ongoing (and perhaps forthcoming) experimental outcomes. For example, knowledge of the effective electric field E (characterized by Wd) on the unpaired electron is required to link the experimentally determined P,T-odd frequency shift with the electron s EDM de in the ground (X2X /2) state of YbF and BaF. 

And so on. Note that in a sextet the intensities of the outer lines are very small, so that they may easily be overlooked The same rule applies when the multiplet results from coupling to neighbours with different coupling constants (e.g. in an olefin), but more care is needed in its interpretation. 

The lower signal is more complicated, and before we can interpret it exactly we need some background information. The magnitude of one-bond C-C coupling constants depends on bond hybridization (ethane 35, ethene 68, benzene 56, ethyne 172 Hz), while two- and three-bond C-C couplings are very small, often around 2-5 Hz. The second thing we have to remember, and this is a new concept, is that the lines in the multiplets from INADEQUATE spectra often come from different spin systems ... 

The total chemical shift range is over 1000 ppm, so that although fluorine-element coupling constants are relatively large the spectra are generally relatively easy to interpret. 

The fact that we have three olefinic hydrogens means that our compound is a primary olefin, the fact that the other two carbons are both methylene carbons means that our substituent, bromine, is terminal. Thus the only possibility we have is that we are dealing with 4-bromo-1-butene (try to find another isomer that fits ). But this simple molecules has a highly complex proton spectrum, which can only be interpreted completely (exact chemical shift, coupling constants) by spectrum simulation. 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

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