Proton (1H) NMR spectroscopy gives information about the hydrogen atoms in an organic molecule. Hydrogen atoms that are in the same position relative to the rest of the molecule are in the same chemical environment. They absorb at the same frequency and give one peak.The position of each peak is its chemical shift, δ, measured in ppm relative to the reference compound tetramethylsilane (TMS), which is given δ = 0 ppm. Most 1H peaks lie between 0 and 12 ppm.A 1H NMR spectrum tells you two basic things:
• the number of peaks = the number of different proton environments
• the chemical shift of each peak = the type of proton environment, found by comparing with the ranges on the data sheet.Protons near an electronegative atom (such as O, N or a halogen) or a C=O group are deshielded and appear at a higher chemical shift. Approximate ranges: alkyl C-H 0.5-2.0 ppm; H-C next to C=O 2.0-3.0 ppm; H-C-O 3.0-4.3 ppm; aromatic C-H 6.5-8.0 ppm; aldehyde CHO 9.0-10.0 ppm; carboxylic acid COOH 10.0-12.0 ppm.Worked example: ethanal, CH3CHO, has two proton environments, so its spectrum has two peaks. The CH3 protons are next to a C=O group and appear at about 2.2 ppm. The aldehyde CHO proton appears at about 9.8 ppm.
• the number of peaks = the number of different proton environments
• the chemical shift of each peak = the type of proton environment, found by comparing with the ranges on the data sheet.Protons near an electronegative atom (such as O, N or a halogen) or a C=O group are deshielded and appear at a higher chemical shift. Approximate ranges: alkyl C-H 0.5-2.0 ppm; H-C next to C=O 2.0-3.0 ppm; H-C-O 3.0-4.3 ppm; aromatic C-H 6.5-8.0 ppm; aldehyde CHO 9.0-10.0 ppm; carboxylic acid COOH 10.0-12.0 ppm.Worked example: ethanal, CH3CHO, has two proton environments, so its spectrum has two peaks. The CH3 protons are next to a C=O group and appear at about 2.2 ppm. The aldehyde CHO proton appears at about 9.8 ppm.