RULES FOR SPECTRAL ANALYSIS

7 RULES FOR SPECTRAL ANALYSIS OF FIRST ORDER SPECTRA 

Rule 1 A group of n magnetically equivalent protons will split a resonance of an interacting group of protons into +l lines. For example, the resonance due to the A protons in an A X system will be split into m+1 lines, while the resonance due to the X protons will be split into +l lines. More generally, splitting by n nuclei of spin quantum number I, results in 2nl+l lines. This simply reduces to +l for protons where I = V2. 

Rule 2 The spacing (measured in Hz) of the lines in the multiplet will be equal to the coupling constant. In the above example all spacings in both parts of the spectrum will be equal to 

The true chemical shift of each group of interacting protons lies in the centre of the (always symmetrical) multiplet. 

The relative intensities of the lines within each multiplet will be in the ratio of the binomial coefficients (Table 5.9). Note that, in the case of higher multiplets, the outside components of multiplets are relatively weak and may be lost in the instrumental noise, e.g. a septet may appear as a quintet if the outer lines are not elearly visible. The intensity relationship is the first to be significantly distorted in non-ideal cases, but this does not lead to serious errors in speetral analysis. 

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