Coupled Interaction

This difference in carbonyl absorption frequencies displayed by the carbon dioxide molecule results from strong mechanical coupling or interaction. In contrast, two ketonic carbonyl groups separated by one or more carbon atoms show normal carbonyl absorption near 1715 cm1 because appreciable coupling is prevented by the intervening carbon atom(s). 

Useful characteristic group frequency bands often involve coupled vibrations. The spectra of alcohols have a strong band in the region between 1260 and 1000 cm-1, which is usually designated as the C—O stretching band. In the spectrum of methanol this band 

The requirements for effective coupling interaction may be summarized as follows  

The vibrations must be of the same symmetry species if interaction is to occur. 

Strong coupling between stretching vibrations requires a common atom between the groups. 

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The simplest theoretical orbital-based estimate of the coupling interaction, is provided by the McConnell relation ... 

Figure C3.2.9. Both nearest neighbour and nonnearest neighbour coupling interactions mediate superexchange between tire temrinal pi-electron groups of rigid dienes witlr saturated bridging units. From [31],...

The cross relation has proven valuable to estimate ET rates of interest from data tliat might be more readily available for individual reaction partners. Simple application of tire cross-relation is, of course, limited if tire electronic coupling interactions associated with tire self exchange processes are drastically different from tliose for tire cross reaction. This is a particular concern in protein/protein ET reactions where tire coupling may vary drastically as a function of docking geometry. 

Various data sources (44) on plasma parameters can be used to calculate conditions for plasma excitation and resulting properties for microwave coupling. Interactions ia a d-c magnetic field are more compHcated and offer a rich array of means for microwave power transfer (45). The Hterature offers many data sources for dielectric or magnetic permittivities or permeabiHty of materials (30,31,46). Because these properties vary considerably with frequency and temperature, available experimental data are iasufficient to satisfy all proposed appHcations. In these cases, available theories can be appHed or the dielectric parameters can be determined experimentally (47). 

Figure 5.28 A schematic drawing of an AMX spin system representing coupling interactions recorded at low digital resolution so that no fine splittings are visible. Note the symmetrical appearance of the cross-peaks on either side of the diagonal.

Figure 5.34 Schematic representation of the coupling interactions of the H, and Hf, protons of Klibromobenzene. The H and protons are split into double doublets due to their couplings with //j and is split into a triplet by the two...

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

The HMBC spectrum of podophyllotoxin is shown. The cross-peaks in the HMBC spectrum represent long-range heteronuclear H/ C interactions within the same substructure or between different substructures. Interpretation should start with a readily assignable carbon (or proton), and then you identify the proton/s (or carbon/s) with which it has coupling interactions. Then proceed from these protons, and look for the carbon two, three, or, occasionally, four bonds away. One-bond heteronuclear interactions may also appear in HMBC spectrum. 

The H-NMR and C-NMR chemical shifts have been assigned and substractures have been deduced on the basis of COSY-45° (Problem 5.13) and other spectroscopic observations. Interpret the HMBC spectrum and identify the heteronuclear long-range coupling interactions between the H and C nuclei. 

The 2D INADEQUATE spectrum contains satellite-peaks representing direct coupling interactions between adjacent C nuclei. The 2D INADEQUATE spectrum and C-NMR data of methyl tetrahydrofuran are shown. Assign the carbon-carbon connectivities using the 2D INADEQUATE plot. 

Fi and F. The off-diagonal peaks (cross-peaks) represent the direct coupling interactions between protons. Working through cross-peaks, one can easily correlate protons that are coupled to each other. Several versions of the COSY experiment have been designed to get optimum performance in a variety of situations (such as DQF COSY, COSY-45°, and COSY-60°). 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

The COSY-45° spectrum of vasicinone displays two distinct spin systems, as indicated by square brackets. Cross-peaks A-H represent coupling between five relatively upfield protons cross-peaks I-K are due to couplings between the aromatic protons. We start from the most down-field proton of the upfield spin system, resonating at 8 5.10, and trace all the other coupling interactions. For instance, the peak at 8 5.10... 

Dipolar coupling The direct through-space coupling interaction between two nuclei. It is responsible for nOe and relaxation, and represents the... 

Scalar coupling Coupling interaction between nuclei transmitted through chemical bonds (vicinal and geminal couplings). 

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