The most important instrumental technique used by organic chemists to determine the structure of organic compounds. NMR spectroscopy helps to identify functional groups, and the C-H framework of an organic compound. The technique is based on the spinning nuclei of atoms behaving as if they were tiny magnets. These nuclei include: 1 H, 13 C, 15 N, 19 F, 31 P. CH 9-3: Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclear Magnetic Resonance (NMR) In the absence of an external magnetic field the N-S poles of hydrogen nuclei are randomly oriented.

Nuclear Magnetic Resonance (NMR) In the presence of a very strong magnetic field, the hydrogen nuclei may align themselves with or opposite to the external field, at different energy levels. EnergyEnergy

Nuclear Magnetic Resonance (NMR) EnergyEnergy If the sample is irradiated with electromagnetic energy (radio frequency energy), the nuclei absorb energy, the poles “flip,” and the energy of the nuclei are at the same level.

Nuclear Magnetic Resonance (NMR) EnergyEnergy When the excited nuclei “relax”, the absorbed energy is released and measured, giving a signal on the NMR spectrum.

BSU 400 MHz NMR Spectrometer Modern “high field” NMR spectrometers have superconducting magnets. BSU 400MHz NMR obtained with a $350,000 from the NSF. 1 MHz = 106 Hz (measure of frequency of EMR) AM radio ~ 1 MHz FM radio ~ 100 MHz

Information obtained from NMR (1) Chemical equivalence of H’s and C’s (4) Identification of acidic H’s by acid-base exchange with D 2 O. (2) Types of functional groups – “Chemical Shift” (ppm) scale. (3) “Signal splitting” gives us structural fragments: puzzle pieces.