Chapter 16 Nuclear Magnetic Resonance Spectroscopy

Transcription

1 hapter 16 Nuclear Magnetic Resonance Spectroscopy

2 The Spinning Proton A spinning proton generates a magnetic field, resembling that of a small bar magnet. An odd number of protons in the nucleus creates a net nuclear spin. The nuclear spin will generates a small magnetic field called the magnetic moment.

3 Applying an External Field An external magnetic field (B0) applies a force to a small bar magnet. The bar magnet aligns with the external field.

4 Nuclear Spins in a Magnetic Field In the absence of an applied magnetic field nuclei do not align their nuclear spins. In an applied magnetic field nuclei orient their nuclear spins either parallel or antiparallel to the applied field. The parallel orientation is more stable.

5 Nuclear Spins in a Magnetic Field

6 Influence of a Radio frequency Pulse The principle behind NMR is that many nuclei have spin. If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level. The energy transfer takes place at a wavelength (the resonance frequency) that corresponds to the radio frequency region. When the spin returns to its base level, energy is emitted at the same frequency. The signal that matches this transfer is measured in many ways and processed in order to yield an NMR spectrum for the nucleus concerned.

7 ommon NMR Solvents tetramethylsilane (internal standard)

8 Proton NMR Spectrum of Methanol In an NMR spectrum the magnetic field increases from left to right. Signals on the right side of the spectrum are said to be upfield and those on the left are said to be downfield. The more shielded methyl protons appear toward the right of the spectrum. The less shielded hydroxyl proton appears toward the left (lower field).

9 The Induced Magnetic Field Resonance Frequency Protons are shielded by valence electrons surrounding them which circulate in an applied magnetic field producing a local diamagnetic current in the opposite direction. This diamagnetic shielding will affect the frequency of radiation necessary to cause a nucleus to spin flip (the resonance frequency). Nuclei will absorb radiation of slightly different frequency depending upon their local magnetic environments.

10 The energy difference between the parallel orientation of a nuclear spin in a magnetic field and the antiparallel orientation of the spin in the field is proportional to the strength of the applied magnetic field. Energy vs Field Strength Energy 60 Mz 300 Mz 600 Mz 1.41 tesla 7.04 tesla tesla Applied Magnetic Field (B0)

11 Diagram of an NMR Spectrometer An NMR spectrometer is an instrument which measures the applied magnetic field strengths required to produce a certain energy difference between nuclear spins of various atoms in a molecule oriented parallel and antiparallel to the applied magnetic field.

12 ontinuous Wave (W) NMR Instruments ontinuous wave NMR spectrometers are similar in principle to optical spectrometers. The sample is held in a strong magnetic field, and the frequency of the source is slowly scanned or The source frequency is held constant, and the field is scanned.

13 Fourier Transform (FT) NMR instruments In FT-NMR, all frequencies in a spectrum are irradiated simultaneously with a radio frequency pulse. A single oscillator (transmitter) is used to generate a pulse of electromagnetic radiation of frequency. Following the pulse, the nuclei return to equilibrium. A time domain emission signal (called a free induction decay (FID)) is recorded by the instrument as the nuclei relax back to equilibrium. A frequency domain spectrum is then obtained by Fourier transformation of the FID.

14 haracteristics of NMR Spectra Location of Signals ( hemical Shift ) Area of Signals (Integration) Shape of Signals (Splitting)

15 Identifying Magnetically Equivalent Protons (which resonate at identical frequencies)

16 omotopic Protons Protons that are interchangeable by rotational symmetry Replace with another group X Replace with another group? X Identical!! The protons are homotopic!!

17 Enantiotopic Protons Protons that are not interchangeable by rotational symmetry, but are interchangeable by reflectional symmetry. Replace with another group 3 X Replace with another group 3? Enantiomers!! 3 X The protons are enantiotopic!!

18 omotopic and Enantiotopic Protons are Magnetically Equivalent

19 omotopic Protons methyl acetate

20 omotopic vs Enantiotopic N 2 glycine N 2 There are three magnetically distinct types of protons in the molecule. a b b The a protons are magnetically N equivalent and homotopic. a c The b protons are magnetically equivalent and enantiotopic.

21 omotopic vs Enantiotopic ethyl acetate There are three magnetically distinct types of protons in the molecule. a a b b c a c c The a protons are magnetically equivalent and homotopic. The b protons are magnetically equivalent and enantiotopic. The c protons are magnetically equivalent and homotopic.

22 Diastereotopic Protons Nonequivalent protons which produce diastereomers through the replacement test. Replace with another group 3 * X 3 * 3 3 Replace with another group 3 *? 3 X Diastereomers!! The protons are diastereotopic!!

23 Diastereotopic Protons Nonequivalent protons which produce diastereomers through the replacement test. Replace with another group X 3 3? Diastereomers!! Replace with another group 3 X The protons are diastereotopic!!

24 Diastereotopic Protons are Magnetically NNequivalent Diastereotopic protons are usually single protons in magnetically and chemically distinct environments in molecules which already possess a chiral center.

25 Are all the protons in cyclohexane magnetically really equivalent?

26

27 ow many magnetically different sets of protons does the following molecule contain? 2 2 enantiotopic N N 3 pyridoxal 3 homotopic

28 ow many magnetically different sets of protons does the following molecule contain? isocitrate diastereotopic

29 ow many magnetically different sets of protons does the following molecule contain? DEET homotopic homotopic homotopic

30 ow many magnetically different sets of protons does the following molecule contain? N F N paroxetine F

31 hemical Shifts

32 hemical Shifts DWNFIELD DESIELDED ELETRN-PR IGER FREQUENY UPFIELD SIELDED ELETRN-RI LWER FREQUENY

33 NMR Delta (δ) Scale

34 Typical hemical Shift Values

35 Inductive Effects on hemical Shift I Br l F δ ppm l l l l l l δ ppm

36 Anisotropic Effects

37 Magnetic Fields Around Aromatic Rings δ7.2 The induced magnetic field of the circulating aromatic electrons opposes the applied magnetic field along the axis of the ring. The aromatic hydrogens are on the equator of the ring, where induced field lines curve around and reinforce the applied field. Protons in the region where the induced field reinforces the applied field are deshielded and will appear at lower fields in the spectrum (to the left).

38 Magnetic Fields Around Aromatic Rings δ 0.3

39 Magnetic Field of Alkenes δ5.5 Vinyl protons are positioned on the periphery of the induced magnetic field of pi electrons. In this position, they are deshielded by the induced magnetic field. The pi electrons of the double bond generate a magnetic field that opposes the applied magnetic field in the middle of the molecule but reinforces the applied field on the outside where the vinylic protons are. This reinforcement will deshield the vinylic protons making them shift downfield in the spectrum to the range of 5-6 ppm.

40 Magnetic Fields of Alkynes δ2.5 When the acetylenic triple bond is aligned with the magnetic field, the cylinder of electrons circulates to create an induced magnetic field. The acetylenic proton lies along the axis of this field, which opposes the external field. The acetylenic protons are in the axis of the generated field. Since the generated magnetic field opposes the applied field, the acetylenic protons are shielded and will be found at higher fields than vinylic protons.

41 Typical hemical Shift Values

42 Integration

43 Methyl t-butyl Ether

44 4-ydroxy-4-methyl-2-pentanone