HH Coupling constants

The coupling constants of ortho Jhh = 7 Hz), meta Jhh = 1-5 Hz) and para protons CJhh I Hz) in benzene and naphthalene ring systems are especially useful in structure elucidation (Table 2.5). With naphthalene and other condensed (hetero-) aromatics, a knowledge of zig zag coupling = 0.8 Hz) is helpful in deducing substitution patterns. 

In the case of alkenes and aromatic and heteroaromatic compounds, analysis of a single multiplet will often clarify the complete substitution pattern. A few examples will illustrate the procedure. 

for example, four signals are found in regions appropriate for benzene ring protons ( 5h =6-9, four protons on the basis of the height of the integrals), then the sample may be a disubstituted benzene (Fig. 2.6). The most effective approach is to analyse a multiplet with a clear fine structure 

A meta disubstituted benzene (Fig. 2.6 b) shows only two ortho couplings ( Jhh =7.5 Hz) for one signal ( 5h =7.5) whereas another signal ( =8.74) exhibits only two meta couplings ( Jhh = 3.0 and 2.5 Hz). In both cases one observes either a triplet (Sh =7.8) or a doublet of doublets Sh = 8.74) depending on whether the couplings ( Jhh or Jhh) are equal or different. 

In Fig. 2.7 the H signal with a t5rpical aromatic proton shift of Sh =7.1 shows a doublet of doublets with J-values of 8.5 Hz ortho coupling, Jhh) and -3 Hz meta coupling, Jhh)- The ring proton in question therefore has two protons as coupling partners, one in the ortho position 8.5 

3Jam = 8.5 Hz (ortho) 4Jmx = 2.5 Hz (meta) 5Jax - 0.5 Hz (para) 

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Table 2.4. Typical HH coupling constants (Hz) of some units in alicycles, alkenes and alkynes ...

Figure 2.18. Vicinal HH coupling constants Jhh as a function of the dihedral angle 9 of the CH bonds concerned (Karplus-Conroy relationship)...

For the stable conformers 13a-c of a substituted ethane the vicinal HH coupling constants 3Hz for syn-protons and 15 Hz for anti-protons can be derived from Fig. 2.18. If there is rotation around the C-C single bond, the coupling protons pass through the syn configuration twice and the anti configuration once. 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

Since fragment A was clearly assigned with the help of HH coupling constants, all of the C atoms not included in A, which, according to the CH COLOC plot, are two or three bonds apart from the equivalent protons at Sh = 6.72 (Table 43.1), belong to the benzene ring B. 

Table 47.3. Relative configurations of the protons between Sh = 7.23 and 3.42 from the HH coupling constants of the expanded proton multiplets. Chemical shift values (Sh) of the proton multiplets are given as large numerals in boldface and coupling constants (Hz) are as small numerals...

Because of signal overcrowding in the aliphatic range between Sh= 1.3 and 2.0, the HH coupling constants cannot be analysed accurately. Only the deshielded 3-/7 at Sh = 3.39 shows a clearly... 

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. 

O. W. Spin-state-selective exdtation. Application for E.COSY-type measurement of /hh coupling constants. 

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