Second-Order Splitting

SECOND-ORDER SPLITTING

Our discussions of spin – spin splitting and multiplicity have been based on first-order or weakly coupled spectra which are spin systems where Δv/J ≥ 10. The difference in chemical shift in hertz of coupled protons divided by the coupling constant is 10 or more. In such a case clean 1 : 1 doublets, 1 : 2 : 1 triplets, and so on, are observed and coupling constants and chemical shifts can be read directly from line positions in the spectrum.

As Δv/J decreases, the simple multiplets observed in weakly coupled spectra become increasingly distorted; new lines can appear and others merge or disap-pear. Such spectra are termed second-order or strongly coupled spectra. In these cases the chemical shift does not lie in the center of the multiplet and coupling constants are not always obvious. A simple example of such a change is seen when the chemical shifts of a first-order AX system become much closer and the spectrum becomes a second-order AB system (Figure 11.23). 

This is not a 1 : 3 : 3 : 1 quartet but an AB quartet in which the intensities of the inner and outer lines depend on the difference in chemical shifts.

The treatment of such systems is outside the scope of this book, but it is possible to calculate the chemical shifts and coupling constants from line positions and intensities. There are also experimental methods by which chemical shifts and coupling constants can be determined in complex spectra. These include isotope exchange, decoupling techniques, lanthanide shift reagents, and the use of higher field NMR spectrometers. Since increases with the strength of the magnetic field while J values do not change with magnetic field strength, the ratio Δv/J increases as the field strength increases. Thus the higher the field strength, the larger is the ratio Δv/J and the greater is the chance to observe first-order coupling. In recent times spectrometers of 300 – 500 MHz are routinely accessible so that the problems of second-order spectra are becoming much less common. With the advent of even higher field instruments, first-order spectra will be available for most compounds. (The first commercial 750-MHz spectrometer was delivered in 1994.)