Chemistry 0310 Organic Chemistry 1 Nuclear Magnetic Resonance Dr Peter Wipf The nuclei of hydrogen atoms behave like small magnets If placed in a strong magnetic field their nuclear spins assume two spin states with different energies Absorption of electromagnetic radiation of the resonance frequency induces a transition between these spins states the nuclear spin flips At the external magnetic field strength of 2 4 Tesla the resonance frequency of 1H nuclei is 100 MHz The exact resonance frequency for a set of chemically equivalent nuclei is determined by the level of deshielding of the nuclei by the electronic configuration The resonance frequency correlates with the electron density surrounding the nucleus in question and the chemical shift d correlates with the molecular structure The chemical shift d is usually expressed in ppm relative to TMS tetramethylsilane Inductive and anisotropic shifts influence d An NMR spectrum gives four different kinds of information The d values tell how many different kinds of hydrogen or carbon atoms are present in the compound and are indicative of their electronic environment The integration reveals the relative numbers of the different hydrogen or carbon atoms in the compound The splitting patterns tell which atoms are closely enough connected that their nuclei can interact n 1 rule The magnitudes of the coupling constants J Hz give information about the relative positions of the hydrogen atoms Typical 1H chemical shifts Type of Hydrogen Atom d ppm Type of Hydrogen Atom d ppm RCH 3 0 9 R2C CH 2 5 0 acyclic 1 3 RCH CR2 5 3 cyclic 1 5 ArH 7 3 R3CH 1 5 2 0 RC O H 9 7 R2C C R CH 3 1 8 RNH2 1 3 RC O CH 3 2 0 2 3 RC O NHR 5 9 RC CH 2 5 ROH 1 5 RNHCH 3 2 3 ArOH 4 7 RCH 2X X Cl Br I 3 5 RC O OH 10 13 ROCH 3 RC O OCH 3 3 8 RCH 2R Typical 13C chemical shifts Type of Carbon Atom d ppm Type of Carbon Atom d ppm RCH 3 15 RC CR2 aromatic 125 acyclic 25 R2C CH 2 135 cyclic 30 RC O OH 170 R3CH 40 RC O H 190 R2C C R CH 3 30 R2C O 210 RCH 2Cl 40 RNHCH 3 50 RCH 2OH 70 RCH 2OR 70 RC CH 80 RCH 2R 2