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Unformatted text preview: 215-216 HH W10 Notes- Dr. Masato Koreeda Date: January 6, 2010 Topic: _NMR-I___ page 1 of 5. Nuclear Magnetic Resonance Spectroscopy I. Introduction Each nucleus in a molecule has a nuclear spin that shows characteristic properties in a magnetic field. nuclear spin: I( I0 to be detectable by NMR spectroscopy) Nuclear spins types # of protons (Z) # of neutrons (N) nuclear spin # (I) examples (1) even even 0 4He, 12C, 16O, 32S (2) odd even 1/2, 3/2, 5/2, etc. 1H (I= 1/2) (3) even odd 1/2, 3/2, 5/2. etc. 13C (I= 1/2), 15N (I= 1/2) (4) odd odd 1, 2, 3, etc. 2H (=D, I=1), 14N (I= 1) Natural abundance Hydrogen: 1H 99.9844% (I= 1/2) Carbon: 12C 98.892% (I= 0) 2H (=D) 0.016% (I= 1) 13C 1.108% (I = 1/2) 3H (=T) 0% (I= 1/2) (radioactive) 14C 0% (= 0)(radioactive) Relative magnetic sensitivity 1H 1.00(combination of natural abundance and 2H 0.00965magnetic susceptibility of each 3H 1.21 (assuming 100% abundance)nucleus)13C 0.0159II. NMR Phenomenon1H nuclei nuclear spins lining upagainst the direction of Hp_____________ I= -1/2 ____ nuclear spins ________________ I= +1/2 ____ H: magneto-nuclear spins lining up gyric ratioin the direction of Hprandom orientationof nuclear spinsw/o externalmagnetic fieldWhen an externalmagnetic power (Hp)is applied.E =h2HpWhen a nucleus has the spin number I, (2I+ 1) energy levels are created in the magnetic field. e.g.,1H has I= 1/2, thus 2 x (1/2) + 1 = 2 two energy levels when placed in the magnetic field 2H has I= 1, thus 2 x 1 + 1 = 3 three energy levels when placed in the magnetic field215-216 HH W10 Notes- Dr. Masato Koreeda Date: January 6, 2010 Topic: _NMR-I___ page 2 of 5. E = ~10-7Kcal/molPopulations of the twoenergy levels in the ratioof 106 : (106+ 1)[the lower energy state more populated] The lower energy state has an excess of about 1 out of 106in population and this excess population is the basis of the NMR absorption phenomenon....
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