216+W11-NMR-I - Chem 216 W11 Topic: _NMR-I_ page 1 of 5....

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Chem 216 W11 Topic : _NMR-I___ page 1 of 5. N uclear M agnetic R esonance Spectroscopy I. Introduction Each nucleus in a molecule has a nuclear spin that shows characteristic properties in a magnetic field. nuclear spin: I ( I 0 to be detectable by NMR spectroscopy) Nuclear spins types # of protons (Z) # of neutrons (N) nuclear spin # ( I ) examples (1) even even 0 4 He, 12 C, 16 O, 32 S (2) odd even 1/2, 3/2, 5/2, etc. 1 H ( I = 1/2) (3) even odd 1/2, 3/2, 5/2. etc. 13 C ( I = 1/2), 15 N ( I = 1/2) (4) odd odd 1, 2, 3, etc. 2 H (=D, I =1), 14 N ( I = 1) Natural abundance Hydrogen: 1 H 99.9844% ( I = 1/2) Carbon: 12 C 98.892% ( I = 0) 2 H (=D) 0.016% ( I = 1) 13 C 1.108% ( I = 1/2) 3 H (=T) 0% ( I = 1/2) (radioactive) 14 C ≈ 0% ( Ι = 0) (radioactive) Relative magnetic sensitivity 1 H 1.00 (combination of natural abundance and 2 H 0.00965 magnetic susceptibility of each 3 H 1.21 (assuming 100% abundance) nucleus) 13 C 0.0159 II. NMR Phenomenon 1 H nuclei nuclear spins lining up against the direction of H p _____________ I = -1/2 ____ nuclear spins ________________ I = +1/2 ____ γ H : magneto- nuclear spins lining up gyric ratio in the direction of H p random orientation of nuclear spins w/o external magnetic field When an external magnetic power ( H p) is applied. Δ E = h γ Η 2 π H p When a nucleus has the spin number I , (2 I + 1) energy levels are created in the magnetic field. e.g., 1 H has I = 1/2, thus 2 x (1/2) + 1 = 2 two energy levels when placed in the magnetic field 2 H has I = 1, thus 2 x 1 + 1 = 3 three energy levels when placed in the magnetic field
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216+W11-NMR-I - Chem 216 W11 Topic: _NMR-I_ page 1 of 5....

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