Aromatic compounds coupling constant

The NMR spectrum of pyridazine shows two symmetrical quartets of A2X2 type and t values and coupling constants are as follows Te = 0.76, T4 = T5 = 2.45 t/3 4 = /5,e = 4.9 (5.05 ),. Ig 5 = 7 4,6 = 2.0, Jg g = 3.5 (1.4 ), J4 5 = 8.4. The C satellites in natural abundance in the liquid state have been examined in order to obtain a complete set of proton magnetic resonance parameters for pyridazine. It has been found that chemical shifts are strongly concentration-dependent in a sense opposite to that normally found with aromatic compounds. Coupling constants are, however, virtually invariant. From the NMR spectrum, a deshielding effect on the hydrogen atoms at positions 3 and 6 is revealed. ... 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

Table 2.5. Typical HH coupling constants (Hz) of aromatic and heteroaromatic compounds...

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

Hz for the ortho, meta and/or para coupling constants of p-nitroanisole, two dinitrotoluenes and 2,4-dinitrochlorobenzene. Both reports indicate that other aromatic compounds investigated showed even less solvent dependence. 

Similarly in condensed polynuclear aromatic compounds and heterocyclic compounds, the magnitude of the coupling constants between protons in the aromatic rings reflects the relative position of the coupled protons. 

Going one stage further phospholes present distinctive features on account of the aromatic character <1967 78 1969,77) this property is clearly reflected in the values of the coupling constants. The relevant data for 1-methyl phospholes have been published (1969>77) and are shown in 98 these are obviously contrary to the trends previously reported for 2-phospholenes in which 2/(P-CH) < 3/(PCCH). However, these are in agreement with observations on l-phenyl-2,5-dimethylphosphole (99)(1967 70) and on l-phenyl-3,4-dimethylphosphole (100) (1969,20) in the latter compound both couplings mentioned have been found to be of the same sign. 

Fluoro- or perfluoro- aromatic compounds offer examples of coupling constants with higher moduli but the signs of which are only known in the para-fluorophenyl compounds the results as obtained by McFar-lane(1969,118) are shown below ... 

In general there is a good correlation between bond distances in fused aromatic compounds and bond orders. Another experimental quantity that correlates well with the bond order of a given bond in an aromatic system is the nmr coupling constant for coupling between the hydrogens on the two carbons of the bond.74... 

However, the same sequence of downfield shifts, namely NR < S < O < Se, is observed. Thus, at least the heterocyclic moiety appears to be aromatic. The ratio of the vicinal coupling constants 75 6 / 745 (0.70-0.74) is significantly less than for compounds of the Kekule series (ca. 0.9), much higher than observed for a polyenic TT-system (cyclohexa-1,3-diene displays a value of 0.52), and close to the... 

The spectrum of l,2-azaborino[l,2-a][l,2]azaborine (22) was completely solved and the parameters were determined by spectrum simulation the shifts are S 8.03, 6.72, 7.66 and 7.31 for H-l, H-2, H-3 and H-4, respectively. The shifts were found to be consistent with 7r-electron densities calculated by SCF methods. The ortho coupling constants were 6.6, 6.0 and 11.2 Hz for J12, Jn and /34, respectively, of which the last one is larger than expected for aromatic compounds (68JA2137). 

The values of 1Jch couplings for C-2 and C-3 correspond to those observed for pyran-4-one. Two-bond coupling constants for these atoms are typically 6-7 Hz, whilst 3/m couplings of 9 Hz have been observed for C-8a and of 4-5 Hz for C-4a. The couplings within the aromatic ring are similar to those observed for related compounds such as coumarins <79JST(56)87>. 

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