Question

What are the three things the NMR spectra tell you for each proton?

Answer 1

1) Position of the signal of proton on NMR spectra refers about magnetic environment of proton i.e electronic environment.

a)Shielded proton b) Deshielded proton

The principle of NMR is to apply external magnetic field and measure the frequency at which nucleus resonance takes place.

a) Shielded proton- Appears upfield (lower ppm)- Higher electron density around the proton, higher the opposing magnetic field to external magnetic field provides great shielding effect to proton. Proton resonates at lower frequency as it experiences lower external magnetic field.

b) Deshielded proton- Appears downfield (higher ppm)-Lower electron density around the nucleus, opposing magnetic field becomes small and therefore proton feel more external field. Proton resonates at higher frequency as it experiences higher external magnetic field.

2) Number of signals refers to different kind of protons present.

3) Intensity and splitting of signal

3a) Intensity of signal- Area under that signal in NMR spectra is directly proportional to the number of nuclei causing the signal.

3b)  Splitting of signal into several peaks provides information on how many hydrogen neighbors exist for a particular hydrogen or group of equivalent hydrogens. In general, an NMR resonance will be split into N + 1 peaks where N = number of hydrogens on the neighbouring atom or atoms.

If there are no hydrogens on the adjacent atoms, then the resonance will remain a single peak, a singlet.
If there is 1 hydrogen on the adjacent atoms, the resonance will be split into two peaks of equal size, a doublet.
2 hydrogens on the adjacent atoms will split the resonance into three peaks with an area in the ratio of 1:2:1, a triplet.
If there are 3 hydrogens on the adjacent atoms, the resonance will be split into four peaks with an area in the ratio of 1:3:3:1, a quartet.