Why does proton nmr splitting produce different sized peaks within the split?

[Hoogie183]

Why does proton nmr splitting produce different sized peaks within the split?

[Fryemall]

The chemical shift is not the only indicator used to assign a molecule. Because nuclei themselves are little magnets they influence each other, changing the energy and hence frequency of nearby nuclei as they resonate—this is known as spin-spin coupling. The most important type in basic NMR is scalar coupling. This interaction between two nuclei occurs through chemical bonds, and can typically be seen up to three bonds away.The effect of scalar coupling can be understood by examination of a proton which has a signal at 1ppm. This proton is in a hypothetical molecule where three bonds away exists another proton (in a CH-CH group for instance), the neighbouring group (a magnetic field) causes the signal at 1 ppm to split into two, with one peak being a few hertz higher than 1 ppm and the other peak being the same number of hertz lower than 1 ppm. These peaks have half the area of the former singlet peak. The magnitude of this splitting (difference in frequency between peaks) is known as the coupling constant. A typical coupling constant value would be 7 Hz.The coupling constant is independent of magnetic field strength because it is caused by the magnetic field of another nucleus, not the spectrometer magnet. Therefore it is quoted in hertz (frequency) and not ppm (chemical shift).In another molecule a proton resonates at 2.5 ppm and that proton would also be split into two by the proton at 1 ppm. Because the magnitude of interaction is the same the splitting would have the same coupling constant 7 Hz apart. The spectrum would have two signals, each being a doublet. The area of the doublets will be the same as another, because they're both produced by one proton each.The two doublets at 1 ppm and 2.5 ppm from the fictional molecule CH-CH are now changed into CH2-CH:The total area of the 1 ppm CH2 peak will be twice that of the 2.5 ppm CH peak. The CH2 peak will be split into a doublet by the CH peak—with one peak at 1 ppm + 3.5 Hz and one at 1 ppm - 3.5 Hz (total splitting or coupling constant is 7 Hz). In consequence the CH peak at 2.5 ppm will be split twice by each proton from the CH2. The first proton will split the peak into two equal intensities and will go from one peak at 2.5 ppm two peaks, one at 2.5 ppm + 3.5 Hz and the other at 2.5 ppm - 3.5 Hz—each having equal intensities. However these will be split again by the second proton. The frequencies will change accordingly:The 2.5 ppm + 3.5 Hz signal will be split into 2.5 ppm + 7 Hz and 2.5 ppm The 2.5 ppm - 3.5 Hz signal will be split into 2.5 ppm and 2.5 ppm - 7 Hz The net result is not a signal consisting of 4 peaks but three: one signal at 7 Hz above 2.5 ppm, two signals occur at 2.5 ppm, and a final one at 7 Hz below 2.5 ppm. The ratio of height between them is 1:2:1. This is known as a triplet and is an indicator that the proton is three-bonds from a CH2 group.This can be extended to any CHn group. When the CH2-CH group is changed to CH3-CH2 keeping the chemical shift and coupling constants identical.The relative areas between the CH3 and CH2 subunits will be 3:2. The CH3 is coupled to two protons into a 1:2:1 triplet around 1 ppm. The CH3 is coupled to three protons. Something split by three identical protons takes a shape known as a quartet, each peak having relative intensities of 1:3:3:1.

[Fai]

I think because it's due to the hydrogens, which can be singlet, doublet, triplet, etc. So, each compounds will have different peaks due to the proton.

[twinsfan]

pascal's triangle... look up the mechanism of splitting and you'll see why (it has to do with how many possibilities of each state there are)

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