Transcript Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance
Spectroscopy
Renee Y. Becker
Valencia Community College
CHM 2011C
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The Use of NMR Spectroscopy
• Used to determine relative location of atoms
within a molecule
• Most helpful spectroscopic technique in organic
chemistry
• Related to MRI in medicine (Magnetic Resonance
Imaging)
• Maps carbon-hydrogen framework of molecules
• Depends on very strong magnetic fields
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Nuclear Magnetic Resonance Spectroscopy
•
1H
or 13C nucleus spins and the internal magnetic field
aligns parallel to or against an aligned external magnetic
field (See Figure 13.1)
• Applying an external magnetic field, Bo, the proton or
nucleus will orient parallel or anti-parallel to the
orientation of the external field.
– The parallel orientation of the proton or nucleus is lower in energy
than the anti-parallel orientation.
• Radio energy of exactly correct frequency (resonance)
causes nuclei to flip into anti-parallel state
•
Energy needed is related to molecular environment
(proportional to field strength, Bo ) – see Figure 13.2
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Nuclear Magnetic Resonance Spectroscopy (1H)
• The energy of the radiation required is within the
radio frequency range.
• The energy required is dependent upon the nucleus
and the strength of the magnetic field.
• A proton in a magnetic field of 1.41 telsa requires
a E.M. radiation of 60 MHz to resonate.
– E = 2.4 x 10-5 kJ/mol
– I.R. energies  48 kJ/mol
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The Nature of NMR Absorptions
• Electrons in bonds shield nuclei from magnetic field
• Different signals appear for nuclei in different
environments
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1H
13C
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The NMR Measurement
• The sample is dissolved in a solvent that does not
have a signal itself and placed in a long thin tube
• The tube is placed within the gap of a magnet and
spun
• Radiofrequency energy is transmitted and
absorption is detected
• Species that interconvert give an averaged signal
that can be analyzed to find the rate of conversion
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Chemical Shifts
• The relative energy of resonance of a particular nucleus
resulting from its local environment is called chemical
shift
• NMR spectra show applied field strength increasing from
left to right
• Left part is downfield right part is upfield
• Nuclei that absorb on upfield side are strongly shielded.
• Chart calibrated versus a reference point, set as 0,
tetramethylsilane [TMS]
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Chemical Shifts
• Let’s consider the just the proton (1H) NMR.
• 60 MHz NMR experiments are carried out with a constant
RF of 60 MHz and the magnetic field is varied. When a
spin-flip occurs (resonance), it is detected by an R.F.
receiver.
Bare proton
Proton in organic molecule
Ho
Ho’ > Ho
Increasing magnetic field strength
Increased shielding of nucleus
Downfield
Upfield
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Measuring Chemical Shift
• Numeric value of chemical shift: difference between
strength of magnetic field at which the observed
nucleus resonates and field strength for resonance of
a reference
– Difference is very small but can be accurately
measured
– Taken as a ratio to the total field and multiplied by
106 so the shift is in parts per million (ppm)
• Absorptions normally occur downfield of TMS, to
the left on the chart
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Measuring Chemical Shift
observed shift ( Hz )
chemical shift( ) 
spectrometer frequency( MHz )
Remember: the chemical shift is in ppm.
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1H
NMR Spectroscopy and Proton Equivalence
• Proton NMR is much more sensitive than 13C and
the active nucleus (1H) is nearly 100 % of the
natural abundance
• Shows how many kinds of nonequivalent
hydrogens are in a compound
• Equivalent H’s have the same signal while
nonequivalent are different
– There are degrees of nonequivalence
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Chemical Shifts in 1H NMR Spectroscopy
• Lower field signals are H’s attached to sp2 C
• Higher field signals are H’s attached to sp3 C
• Electronegative atoms attached to adjacent C cause
downfield shift
• See Tables 13-2 and 13-3 for a complete list
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Integration of 1H NMR Absorptions: Proton Counting
• The relative intensity of a signal (integrated area) is proportional to the
number of protons causing the signal
• This information is used to deduce the structure
• For example in ethanol (CH3CH2OH), the signals have the integrated
ratio 3:2:1
• For narrow peaks, the heights are the same as the areas and can be
measured with a ruler
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Integration of 1H NMR Absorptions: Proton Counting
• This is proportional to the relative number of protons
causing each signal.
– An integration ratio of 1.5:1 is consistent with a 6:4
ratio of protons as with a 3:2 ratio of protons.
– How many signals would you expect from the 1H NMR
spectrum of chloromethyl methyl ether, ClCH2OCH3,
and what would you expect the signal area ratios to be?
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Spin-Spin Splitting in 1H NMR Spectra
• Peaks are often split into multiple peaks due to
interactions between nonequivalent protons on
adjacent carbons, called spin-spin splitting
• The splitting is into one more peak than the
number of H’s on the adjacent carbon (“n+1 rule”)
• The set of peaks is a multiplet (2 = doublet, 3 =
triplet, 4 = quartet)
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Rules for Spin-Spin Splitting
• Equivalent protons do not split each other
• The signal of a proton with n equivalent
neighboring H’s is split into n + 1 peaks
• Protons that are farther than two carbon
atoms apart do not split each other
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13.12 More Complex Spin-Spin Splitting Patterns
• Spectra can be more complex due to overlapping
signals, multiple nonequivalence
• Example: trans-cinnamaldehyde
H
O
H
H
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p-bromotoluene
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Analysis of NMR Spectra
• The NMR spectra provides the following information that
can assist in the determination of chemical structure
– The number of signals
– The chemical shift
– The intensity of the signal (area under each peak)
– The splitting of each signal
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