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Proton NMR Spectroscopy. The NMR Phenomenon. Most nuclei possess an intrinsic angular momentum , P . Any spinning charged particle generates a magnetic field. P = [I(I+1)] 1/2 h/2 p where I = spin quantum # I = 0, 1/2, 1, 3/2, 2, …. Which nuclei have a “spin”?.
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The NMR Phenomenon • Most nuclei possess an intrinsic angular momentum, P. • Any spinning charged particle generates a magnetic field. P = [I(I+1)]1/2 h/2p where I = spin quantum # I= 0, 1/2, 1, 3/2, 2, …
Which nuclei have a “spin”? • If mass # and atomic # are both even, I = 0 and the nucleus has no spin. e.g. Carbon-12, Oxygen-16 • For each nucleus with a spin, the # of allowed spin states can be quantized: • For a nucleus with I, there are 2I + 1 allowed spin states. 1H, 13C, 19F, 31Pall have I = 1/2 DE = g(h/2p)Bo
When a nucleus aligned with a magnetic field, B0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence, NMR
NMR instruments typically have a constant Rf and a variable B0. A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T). Each unique probe nucleus (1H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf. All protons come into resonance between0 and 12/1,000,000 (0 – 12 ppm) of the Rf.
Energy Difference (E) Between Two Different Spin States of a Nucleus With I=1/2
What Does an NMR Spectrum Tell You? • # of chemically unique H’s in the molecule # of signals • The types of H’s that are present e.g. aromatic, vinyl, aldehyde … chemical shift • The number of each chemically unique H integration • The H’s proximity to eachother spin-spin splitting
Shielded Protons • A naked proton will absorb at 70,459 gauss. • A shielded proton will not absorb at 70,459 gauss so the magnetic field must be increased slightly to achieve resonance.
Chemical Shift • The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called chemical shifts. • The chemical shift (in ppm) is independent of the spectrometer used. • Most common scale is the d (delta) scale.
Electronegative Atoms • More electronegative atoms deshield more and give larger shift values. • Additional electronegative atoms cause an increase in chemical shift.
Location of Electronegative Atoms • The deshielding effect of an electronegative substituent drops off rapidly with distance.
Chemical Shift is Affected by Electron Density Around Nucleus
Number of Signals • Methyl tert-butyl ether has two types of protons, giving two NMR signals. • Chemically equivalent hydrogens have the same chemical shift. All the methyl groups of the tert-butyl group are equivalent, and they produce only one signal.
Intensity of Signals: Integration • The amount the integral trace rises is proportional to the area of that peak. • The integration will have a trace for the tert-butyl hydrogens that is three times as large as the trace for the methyl hydrogens. The relative area for methyl and tert-butyl hydrogens is 1:3.
tert-Butyl Acetoacetate • The spectrum of tert-butyl acetoacetate has only three signals. The most shielded protons are the methyl groups of the tert-butyl. The most deshielded signal is the methylene (CH2) because it is in between two carbonyl groups.
The N + 1 Rule If a proton is coupling to N equivalent protons, (on adjacent atoms) it is split into N + 1 peaks.
Spin-Spin Splitting Distance • Equivalent protons do not split each other. • Protons bonded to the same carbon will split each other if they are nonequivalent. • Protons on adjacent carbons normally will split each other. • Protons separated by four or more bonds will rarely split each other.
Hint Ethyl, propyl and isopropyl groups are common. Learn to recognize them from their splittingpatterns.
