The Coupling Constant

That is, the coupling constant between the methyl and methylene protons is 7.5 Hz. When the protons interact, the magnitude (in ethyl iodide) is always of this same value, 7.5 Hz. The amount of coupling is constant, and hence J can be called a coupling constant. 

Chemical shift difference FIGURE 5.34 The definition of the coupUng constants in the ethyl splitting pattern. 

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

Nuclear Magnetic Resonance Spectroscopy Part One Basic Concepts 

Before closing this section, we should take note of an axiom the coupling constants of the groups of protons that split one another must be identical within experimental error. This axiom is extremely useful in interpreting a spectrum that may have several multiplets, each with a different coupling constant. 

for example, the preceding spectrum, which shows three triplets and one quartet. Which triplet is associated with the quartet It is, of course, the one that has the same 7 values as are 

Notice the top of the spectrum. It is calibrated in cyeles per second (cps), which are the same as Hertz, and since there are 20 chart divisions per 100 cps, one division equals (100 cps)/20 = 5 cps == 5 Hz. Now examine the multiplets. The spacing between the component peaks is approximately 1.5 chart divisions, so 

For the interaction of most aliphatic protons in acyclic systems, the magnitudes of coupling constants are always near 7.5 Hz. Compare, for example, 1,1,2-trichloroethane (Fig. 3.25), for which J = 6 Hz, and 2-nitropropane (Fig. 3.27), for which /= 7 Hz. These couphng constants are typical for the interaction of two hydrogens on adjacent p -hybridized carbon atoms. Different types of protons have different magnitudes of J. For instance, the cis and trans protons substituted on a double bond commonly have values of approximately J s = 10 Hz and /trans = 17 Hz. In ordinary compounds, coupling constants may range anywhere from 0 to 18 Hz. 

The magnitude of J often provides stmctural clues. For instance, one can usually distinguish between a cis olefin and a trans olefin on the basis of the observed coupling constants for the vinyl protons. Table 3.9 gives the approximate values of some representative coupling constants. A more extensive list of coupling constants appears in Appendix 5. 

Resonating nucleus A detects polarized electron spin and hence detects spin of X. 

Coupling can occur between H and C, as well as between two protons. Because C is in such low natural abundance (about 1.1%), these couplings are not important in analyzing 

---

J A measure of the coupling constant in nuclear magnetic resonance. 

Information from an n.m.r. spectrum is classified into the chemical shift, <5 (the relative shift from a standard [Me Si for H, CC13F for which is rendered independent of the field), and the coupling constants, J, which are determined directly from the spectra. 

The coupling constants of the hyperfme and the electron Zeeman interactions are scalar as long as radicals in isotropic solution are considered, leading to the Hamiltonian... 

For example, the observed transitions of an AB spin system have a Liouville matrix given m equation (B2.4.35). The coupling constant is J, and it is assumed that ciig = = -5/2, so that 5 is the frequency... 

Figure 13 20 there are two other vinylic protons Assuming that the coupling constant between the two geminal protons in ArCH=CH2 is 2 Hz and the vicinal coupling constants are 12 Hz (cis) and 16 Hz (trans) describe the splitting pattern for each of these other two vinylic hydrogens... 

Spin densities help to predict the observed coupling constants in electron spin resonance (ESR) spectroscopy. From spin density plots you can predict a direct relationship between the spin density on a carbon atom and the coupling constant associated with an adjacent hydrogen. 

Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

For a rigorous determination of the coupling constants see <79M1404U). coupling constants C-3,... 

The coupling constants are very rich in structural information. For the sake of comparison, indazole and isoindazole have been numbered like pyrazoles (Figure 16). 

A number of studies on the NMR spectra of isoxazole has been compiled and this list includes the coupling constants in various solvents as well as the neat liquid. The N signal for isoxazole appears at 339.6 p.p.m. relative to TTAI and is at much lower field than in other azoles. Reports of spectra of substituted isoxazoles also abound (79AhC(25)147, p. 201). 

The role of two-phonon processes in the relaxation of tunneling systems has been analyzed by Silbey and Trommsdorf [1990]. Unlike the model of TLS coupled linearly to a harmonic bath (2.39), bilinear coupling to phonons of the form Cijqiqja was considered. In the deformation potential approximation the coupling constant Cij is proportional to (y.cUj. There are two leading two-phonon processes with different dependence of the relaxation rate on temperature and energy gap, A = (A Two-phonon emission prevails at low temperatures, and it is... 

Figure 1.4. Two-spin system of type AX with a chemical shift difference which is large compared with the coupling constants (schematic)...

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. 

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. 

---