Measuring coupling constants proton chemical shift values

The ultraviolet spectrum of benzo[g]quinoxaline has been measured in ethanol and in heptane. In ethanol the principal absorption maxima are at 227, 264, and 364 nm. Proton chemical shifts and coupling constants are tabulated in Figure 1. Values calculated by HMO theory are found to be close to the experimental ones. ... 

Computer programs for simulation of proton NMR spectra are available. If accurate measurements of chemical shifts and coupling constants for all of the protons can be obtained, the simulated spectrum will be congruent with the actual spectrum. In many cases, at least some of the spin systems will be first order. If not, reasonable estimates of shifts and coupling constants may be made, and the iterative computer program will adjust the values until the simulation matches the actual spectrum—assuming, of course, that the identification is valid. 

In Fig. 2 protons and resonate at quite different chemical shifts and are both split into doublets with the spacings between the two elements of both doublets being equal. This spacing is called the coupling constant. Coupling constants are measured in Hz, their values are characteristic of certain... 

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