The 1H NMR Spectrum. Absorbance of energy by a proton via NMR results in flipping of its nuclear spin state. Both 1H and 13C have spin numbers of I = ½. The number of allowed spin states is = 2(I) + 1. Both 16O,12C and 2H (deuterium) have spin numbers of zero and are NMR inactive. The spinning nucleus generates a small magnetic field. When placed in an external magnetic field (Bo) it will align either with or against Bo. The energy needed to flip spin states is in the radiofrequency region (resonance energy) and is proportional to the field strength, Bo.
The 1H NMR Spectrum. The resonance energy of a given H is affected by the electron density around that H. The applied field causes es to circulate around H which induces a field opposing Bo thus the nucleus does not feel the full effect of Bo. The nuclei of those Hs having a greater electron density are said to be shielded from Bo and those with less e-density are deshielded. Resonance energy is a frequency and is reported in cps = Hz or ppm = .
Terms used in an NMR spectrum. Downfield Upfield Less shielding More shielding Low induced field High induced field Larger Smaller 300 Hz ppm = = -------------- = 5 or 5 ppm 60 MHz values
Information from the 1H NMR Spectrum. Four different features of a 1H NMR spectrum provide information about the structure of a compound: 1. Number of signals. 2. Position of signals. 3. Intensity of signals. 4. Splitting of signals. Standard = tetramethylsilane: (CH3)4Si These methyl groups are highly shielded.
Chemical Shift Values H Environment δ H Environment δ (CH3)4Si 0 (defined) R-COOCH ~3.5-4.5 Alkane -CH3 ~0.9 C=C-H ~4.5-6 -CH2- ~1.3 -CH- ~1.6 Ar-H ~6.5-8 (C, N, O)=C-CH ~1.5-2.5 R-CHO ~9-10 -C≡C-H ~2.5 R-COOH ~10-12 (N, O)-CH ~2.5-4 R-OH ; R-NH ~1-5
Spin-Spin Splitting Not all NMR signals are singlets. Some exist as doublets, triplets, or other multiplicities. The spin state of a proton within a three bond distance of another proton(s) feels the effect of the spin states of the other Hs. This effect of nonequivalent Hs on adjacent Cs is transmitted through the sigma bonds. The result of this interaction is that the NMR signal is split into (N + 1) peaks where N = the number of equivalent Hs. The distance between peaks of a multiplet are called J values (coupling constants) and are measured in Hz.