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Lecture 5

Lecture 5. HDI Summary NMR Theory, Chemical Shift This week in lab: Ch 5: Distillation & Boiling Point, Procedure 1 or 2 Be sure to position thermometer correctly in the distillation set-up!! Quiz 2 on Chapter 5 Melting points for Chapter 4 Next week in lab:

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Lecture 5

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  1. Lecture 5 HDI Summary NMR Theory, Chemical Shift This week in lab: • Ch 5: Distillation & Boiling Point, Procedure 1 or 2 • Be sure to position thermometer correctly in the distillation set-up!! • Quiz 2 on Chapter 5 • Melting points for Chapter 4 Next week in lab: Procedure 3 of Chapter 5 Next class: Lecture Problem 2 due

  2. HDI & Some Possibilities Each p bond has an HDI of 1. Each ring has an HDI of 1. HDI = 1: • One double bond • One ring HDI = 2: • One triple bond • One double bond, one ring HDI = 3: • One triple bond, one ring • One triple bond, one double bond • Two double bonds, one ring • Three rings HDI = 4: HDI = 5+: • Four double bonds Probably have a phenyl ring • Two triple bonds • One triple bond, two double bonds

  3. 1H NMR Spectrum of Ethanol CH3CH2OH TMS Three signals - three different types of H’s ppm

  4. Nuclear Magnetic Resonance Use: To assist in the elucidation of a molecule’s structure Information Gained: • Different chemical environments of nuclei being analyzed (1H nuclei): chemical shift • The number of nuclei with different chemical environments: number of signals in spectrum • The numbers of protons with the same chemical environment: integration • Determine how many protons are bonded to the same carbon: integration • Determine the number of protons that are adjacent to one another: splitting patterns • Determine which protons are adjacent to one another: coupling constants

  5. How does NMR work? Basic Idea: In the presence of an applied magnetic field (Bo) - the NMR instrument: • Irridate the sample with radiofrequency radiation 2. Nuclei resonance: excite magnetic transitions 3. Measure the energy absorbed/released by nuclei 4. Obtain a spectrum

  6. How does NMR work? Facts that allow NMR to work: • Nuclei have a spin (like electrons). • Nuclei that have odd mass or odd atomic number have a quantized spin angular momentum and a magnetic moment. • The allowed spin states a nucleus can adopt is quantized and is determined by its nuclear spin quantum number, I. 1H and 13C nuclei have I = 1/2. Thus, there are two allowed spin states: +1/2 and -1/2.

  7. 1H NMR Spectroscopy • 1H nuclei have magnetic spin, I = 1/2. • The nuclei can either align with (+1/2) or oppose (-1/2) the applied magnetic field, Bo (from the NMR instrument). • When the nuclei absorb the radiofrequency pulse (a specific energy is absorbed since the spin states are quantized!), the spin flips - resonance. • When the pulse is over, the spin relaxes back to its original state. The spin releases the energy that it had originally absorbed - this is the energy that is measured. This happens to each 1H nuclei in the sample, but not every 1H nuclei are the same.

  8. Higher energy state: magnetic field opposes applied field How does NMR work? Nuclei are charged and if they have spin, they are magnetic Applied Magnetic Field = Bo Energy of transition = energy of radiowaves Lower energy state: magnetic field aligned with applied field

  9. Getting a Spectrum • Pulse sample with radiofrequency radiation, spin flip - resonance. • After pulse, the excited nuclei lose their excitation energy and return to • their original state - relax. • As the nuclei relax, they emit electromagnetic radiation; results in • free-induction decay (FID) • FID contains all emitted frequencies: • Fourier transform (FT) is performed on the FID. • FT extracts the individual frequencies on the different nuclei; results in • a spectrum.

  10. An NMR Diagram: On the Inside RF transmitter RF Receiver N S Note modern NMRs use superconducting magnets to attain very strong magnetic fields + -

  11. Chemical Shifts Not all proton nuclei resonate at the same frequency. Proton nuclei are surrounded by electrons in slightly different chemical environments - nuclei are shielded by valance electrons that surround them. As a result, the nuclei are shielded from Bo to an extent that depends on the electron density around it. A shielded nucleus will feel a diminished Bo and will absorb radiofrequency radiation at a lower frequency - have a lower ppm value. A deshielded nucleus will feel a stronger Bo and will absorb radiofrequency radiation at a higher frequency - have a higher ppm value. Different nuclei will be shielded differently and, as a result, will have different resonance frequency - different ppm values - different chemical shifts.

  12. Chemical Shifts • Protons near an electronegative group will be deshielded - feel a stronger Bo - have a higher ppm value. • Electronegative groups: OH, OR, Cl, F, Br, N • Other deshielding groups: C=C, phenyl, C=O

  13. 1H NMR Spectrum of Ethanol CH3CH2OH TMS Three signals - three different types of H’s ppm

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