Roof effect

Note how dramatically the roofing effects of the AB and BC part systems change the intensities of the doublet of doublets of proton B in spectrum 2. 

Fig. 2.14 A series of spectra of a two spin system in which the Larmor frequency of spin 1 is help constant and that of spin 2 is moved in closer to spin 1. The spectra become more and more strongly coupled showing a pronounced roof effect until in the limit that the two Larmor frequencies are equal only one line is observed. Note that as the outer lines get weaker the inner lines get proportionately stronger. The parameters used for these spectra were v0,i = -10 Hz and. /12 = 5 Hz the peak in the top most spectrum has been truncated.

These spectra illustrate the so-called roof effect in which the intensities of the lines in a strongly coupled multiplet tilt upwards towards the multiplet from the coupled spin, making a kind of roof Fig. 2.15 illustrates the idea. The spectra in Fig 2.14 also illustrate the point that when the two Larmor frequencies are identical there is only one line seen in the spectrum and this is at this Larmor frequency. In this limit lines 1-2 and 2 4 both appear at the Larmor frequency and with intensity 2 lines 1-3 and 3 4 appear elsewhere but have intensity zero. 

Fig. 2.15 The intensity distributions in multiplets from strongly-coupled spectra are such that the multiplets tilt towards one another this is called the roof effect.

In Fig. 3.3 the roof effect, a typical indication of strongly coupled spins, can be observed for the AB spin system, whereas the line intensities of the AX spin system indicate that it may be analysed on a simple first order basis. Normally a first order spin system can be assumed if the chemical shift difference in Hz for the coupled spins i and k is a lot greater than the scalar coupling constant Jj. ... 

Dach effect. Syn. roof effect. The skewing of the intensities of the individual peaks (legs) of a multiplet caused by the close proximity (in the spectrum) of another resonance to which the resonance in guestion is coupled. The Dach effect is due to nonfirst-order coupling behavior. 

Notice that the doublet for He leans towards the doublet for H - this is called the roof effect... 

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