Homonuclear interaction

The DQF (double-quantum filtered)-COSY spectrum of an isoprenyl coumarin along with H-NMR data are shown. Determine the H/ H homonuclear interactions in the DQF-COSY spectrum. 

Among several indicators of motional state of organic solids H linewidths has been used from very beginning of solid-state NMR. However, inherently low resolution of H MAS NMR spectra caused by the very strong homonuclear interactions and relatively low MAS speed lead to a very limited application of H MAS NMR for studies of mobility.1 A 2D wideline separation (WISE) experiment correlates carbon chemical shifts recorded under MAS with broad H lines.1,24 As a result a broad ll lines are separated in the components corresponding to the 13C sites in their close proximity. For mobile domains in a sample, the H - ll coupling are relatively weak which leads to relatively narrow 11 static lines. The opposite is observed for the rigid domains. 

Dipolar interactions with protons are usually very dominating, whereas homonuclear interactions of low natural abundant nuclei, like Si and can usually be neglected due to strong intemuclear distance dependence. 

Heteronuclear NMR As discussed above, complex spectral information can be simplified greatly by resolving homonuclear interactions into... 

First-order homonuclear interactions are seen to obey the rule, = Kr x", where n correlates positively with bond order. 

Fig. 1 Covalence curves in dimensionless units. Homonuclear interactions are described by the curve BFC and heteronuclear interactions map into the crescent CFA...

For ease of reference, we tabulate atomic ionization radius (ro), bond order (b) and exponents (n) of lowest-order observed homonuclear interactions in Table 6. 

In the calculation of heteronuclear interactions, it is necessary to substitute Ro = V o(l) ro(2). A serious complication exists therein that, as for homonuclear interactions, the critical stretch to effect a change of bond order depends on atomic volume and, in addition, also on relative atomic size. The strategy to address this problem was to use known parameters, together with experimental force constants, to calculate effective values of A from the expression... 

The hydrides of /j-block elements represent the best documented set of experimental stretching force constants. The previous strategy yields the surprising result that, as for homonuclear interactions, the effective critical stretch Ad is a function of only bond order. In the first linear region, = 1.3, the value Ad = 0.115, with bond orders as identified in Table 11, predicts the following force constants ... 

F- F homonuclear interaction, which is averaged out by fast MAS, is reintroduced using the back-to-back (BABA) recoupling sequence [64]. In a 2D MAS DQ-SQ NMR spectrum, fluorine atomic proximities between inequivalent F sites are revealed by paired cross-correlation peaks appearing at the individual chemical shifts of the two dipolar-coupled nuclei in the SQ direct dimension and the sum of them in the indirect DQ dimension, while the proximity between two equivalent F sites is disclosed by a single auto-correlation peak located on the DQ diagonal (with a slope of 2) of the 2D spectrum... 

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