Method of structure determination based upon the relative positions of hydrogens and carbons in the spectra. Only nuclei with an odd number of neutrons or an odd number of protons can give rise to an NMR signal. The most common nuclei are 1H and 13C.
A compound is placed in a magnetic field. The nuclei align themselves either with or against the magnetic field. With the right combination of magnetic field and electromagnetic radiation, the nuclei flips its spin. The absorption of energy is detected by the NMR spectrometer.
Chemical shift is the difference, in ppm, between the frequency of TMS and the frequency of the observed atom. TMS, tetramethylsilane [(CH3)4Si], is an internal standard that has a chemical shift of δ 0.00. Integration is the area under a peak that is proportional to the number of hydrogens on adjacent carbons. Spin-spin coupling is the splitting of signals into multiplets when the magnetic field of the proton is affected by protons on adjacent carbons. Coupling constant is the distance between the peaks of a multiplet.
U = C + 1 - 2H + 2N. U = unsaturation number. C = number of carbons. H = number of hydrogens + halogens. N = number of nitrogens + phosphorus. The interpretation of unsaturation number follows. U = 0; no double bonds, triple bond, or rings. U = 1; 1 double bond or ring. U = 2; 2 double bonds, 2 rings, 1 triple bond, or 1 double bond + 1 ring. U = 3; 3 double bonds, 3 rings, 1 double bond + 2 rings, 2 double bonds + 1 ring, 1 triple bond + 1 double bond, or 1 triple bond + 1 ring. U = 4; benzene. U = 5 (4 + 1); benzene + 1 double bond or ring. U = 6 (4 + 2); benzene + 2 double bonds or 2 rings or 1 triple bond or 1 double bond + 1 ring.
1H NMR Spectroscopy
The number of signals is equal to the number of different types of protons. Chemical shift is the electronic structure close to a proton. Intensities of signals corresponds to the number of protons attached to that carbon. Splitting of signals is the number of protons on adjacent carbons.
1H NMR Spectroscopy B Splitting of Signals
Singlet, s (one peak) = 0 H on adjacent C. Doublet, d (two peaks, 1:1) = 1 H on adjacent C. Triplet, t (three peaks, 1:2:1) = 2 H on adjacent C. Quartet, q (four peaks, 1:3:3:1) = 3 H on adjacent C. Quintet, quint (five peaks, 1:4:6:4:1) = 4 H on adjacent C. Sextet, sext (six peaks, 1:5:10:10:5:1) = 5 H on adjacent C. Septet, sept (seven peaks, 1:5:10:10:5:1) = 5 H on adjacent C. Multiplet, m (many peaks) = cannot tell number of H.
Proton Magnetic Resonance Frequencies
The proton magnetic resonance frequencies are given for aldehyde, vinylic, aromatic, acetylenic, alcohol, carboxylic acid, phenolic, amino, and amide.
1H NMR Problem Solving
Calculate the unsaturation number and give the interpretation. List the chemical shifts. Compare to Proton Magnetic Resonance tables. List the splitting. Measure the vertical distance of integration with ruler. Add up integrations. Determine multiplication factor by dividing H by sum of integrations. Multiply each integration by multiplication factor to get H per signal. For interpretation, use calculated H that are adjacent to splitting minus 1. Put the structure together, following the formula. Examples are given.
13C NMR Spectroscopy
The number of signals is equal to the number of different types of carbons. Chemical shift is the type of functional group for that carbon. Peak areas corresponds to the number of carbons. The splitting of signals (can also be determined by DEPT, another NMR method) is interpreted as q = CH3; t = CH2; d = CH; s = C.
Carbon Magnetic Resonance Frequencies
The carbon magnetic resonance frequencies are given for the carbon in alkyl, alkyl fluoride, alkyl chloride, alkyl bromide, alkyl iodide, amine, alcohol, ether, ester, alkyne, alkene, aromatic, nitrile, amide, carboxylic acid, ester, anhydride, anhydride, aldehyde, and ketone.
13C NMR Problem Solving
Calculate the unsaturation number and give interpretation. List the chemical shifts. List the splitting. Give the interpretation of the splitting. Give the interpretation by chemical shift, by referring to the Carbon Magnetic Resonance Frequency tables. Put the structure together. Examples are given.