1 The Use of NMR Spectroscopy 13.1 Nuclear Magnetic Resonance ...
The Use of NMR Spectroscopy Nuclear Magnetic Resonance Spectroscopy
Used to determine relative location of atoms
within a molecule Most helpful spectroscopic technique in
organic chemistry Related to MRI in medicine (Magnetic
Chapter 13 Part 1
Resonance Imaging) yhdrogen framework of molecules Depends on very strong magnetic fields Maps carbon -
13.1 Nuclear Magnetic Resonance Spectroscopy ______ 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, ______, the proton or nucleus will orient ______ or ______ to the orientation of the external field.
The parallel orientation of the proton or nucleus is ______ in energy than the anti-parallel orientation.
Radio energy of exactly correct frequency (______)
causes nuclei to flip into anti-parallel state
Energy needed is related to molecular environment
(proportional to field strength, Bo ) – see Figure 13.2
Figure 13.1
Figure 13.2
1
Nuclear Magnetic Resonance Spectroscopy (1H)
13.2 The Nature of NMR Absorptions
The energy of the radiation required is within the
Electrons in bonds shield nuclei from magnetic field
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 ______ requires a E.M. radiation of 60 ______ to resonate.
Different signals appear for nuclei in different
environments
E = 2.4 x 10-5 kJ/mol I.R. energies ≅ 48 kJ/mol
A proton in a magnetic field of 1.41 telsa requires a
E.M. radiation of 60 MHz to resonate.
1H
Calculate the energy.
The NMR Measurement The sample is dissolved in a solvent that
13C
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
13.3 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 is upfield Nuclei that absorb on upfield side are strongly shielded. Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]
2
Chemical Shifts
Measuring Chemical Shift
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
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 ______ so the shift is in parts per million (______) Absorptions normally occur ______ of TMS, to the left on the chart
Increasing magnetic field strength Increased shielding of nucleus Downfield
Upfield
Measuring Chemical Shift Calibrated on relative scale in ______ scale
δ is the number of parts per million (ppm) of the magnetic field expressed as the spectrometer’s operating frequency (used ahead of value as it is a ratio and not a unit) Independent of instrument’s field strength 1 ppm = 1 delta (δ) unit. For a 60 MHz spectrum, 1 δ = ______ Note: NMR spectra are recorded on chart paper that is calibrated in “ppm” and in “Hz”. If data is reported in Hz, the field strength of the instrument must be specified. (60 or 300 MHz).
Measuring Chemical Shift
Chemical Shift Calculate the chemical shift in ppm for the
following: Bromoform (CHBr3), 413 Hz (at 60MHz) Iodoform (CHI3), 322 Hz (at 60MHz) Methylchloride (CH3Cl), 184 Hz (at 60MHz)
Remember: the chemical shift is in ppm.
3
13.8 1H NMR Spectroscopy and Proton Equivalence
Nonequivalent H’s
Proton NMR is much more sensitive than
Replacement of each H with “X” gives a different
13C and the active nucleus (1H) is nearly 100 % of the natural abundance Shows how many kinds of nonequivalent hydrogens are in a compound Theoretical equivalence can be predicted by seeing if replacing each H with “X” gives the same or different outcome Equivalent H’s have the same signal while nonequivalent are different
constitutional isomer Then the H’s are in constitutionally heterotopic
environments and will have different chemical shifts – they are nonequivalent under all circumstances
There are degrees of nonequivalence
Equivalent H’s Two H’s that are in identical environments
(homotopic) have the same NMR signal Test by replacing each with X if they give the identical result, they are equivalent
Enantiotopic Distinctions If H’s are in environments that are mirror images of
each other, they are enantiotopic Replacement of each H with X produces a set of
enantiomers The H’s have the same NMR signal (in the absence
of chiral materials)
4
Diastereotopic Distinctions In a chiral molecule, paired hydrogens can have
different environments and different shifts
Replacement of a pro-R hydrogen with X gives a
different diastereomer than replacement of the pro-S hydrogen Diastereotopic hydrogens are distinct chemically and spectrocopically
Consider: CH3CH2CH3
Are the protons equivalent?
Consider:
CH3
H
CH3
CH3
Are the protons equivalent?
How about?
1-Bromobutane Methylbenzene 2-Methyl-1-butene
5
13.9 Chemical Shifts in 1H NMR Spectroscopy Proton signals range from δ 0 to δ 10 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
CHCl3
Increased shielding of methyl protons ¸ Decreasing electronegativity of attached atom ¸
CH3F
CH3OCH3
(CH3)3N
CH3CH3
methyl fluoride
Dimethyl ether
Trimethylamine
Ethane
* shift, ppm
* shift, ppm
4.3
3.2
benzene 7.3
2.2
ethylene 5.3
CH2Cl2
CH3Cl
trichloromethane dichloromethane * shift, ppm
7.3
5.3
chloromethane 3.1
0.9
ethane 0.9 * shift, ppm
hexamethyl benzene
2,3-dimethyl-2butene
2.2
1.7
6
Last Slide Chapter 13 Part 1
7
Used to determine relative location of atoms
within a molecule Most helpful spectroscopic technique in
organic chemistry Related to MRI in medicine (Magnetic
Chapter 13 Part 1
Resonance Imaging) yhdrogen framework of molecules Depends on very strong magnetic fields Maps carbon -
13.1 Nuclear Magnetic Resonance Spectroscopy ______ 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, ______, the proton or nucleus will orient ______ or ______ to the orientation of the external field.
The parallel orientation of the proton or nucleus is ______ in energy than the anti-parallel orientation.
Radio energy of exactly correct frequency (______)
causes nuclei to flip into anti-parallel state
Energy needed is related to molecular environment
(proportional to field strength, Bo ) – see Figure 13.2
Figure 13.1
Figure 13.2
1
Nuclear Magnetic Resonance Spectroscopy (1H)
13.2 The Nature of NMR Absorptions
The energy of the radiation required is within the
Electrons in bonds shield nuclei from magnetic field
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 ______ requires a E.M. radiation of 60 ______ to resonate.
Different signals appear for nuclei in different
environments
E = 2.4 x 10-5 kJ/mol I.R. energies ≅ 48 kJ/mol
A proton in a magnetic field of 1.41 telsa requires a
E.M. radiation of 60 MHz to resonate.
1H
Calculate the energy.
The NMR Measurement The sample is dissolved in a solvent that
13C
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
13.3 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 is upfield Nuclei that absorb on upfield side are strongly shielded. Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]
2
Chemical Shifts
Measuring Chemical Shift
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
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 ______ so the shift is in parts per million (______) Absorptions normally occur ______ of TMS, to the left on the chart
Increasing magnetic field strength Increased shielding of nucleus Downfield
Upfield
Measuring Chemical Shift Calibrated on relative scale in ______ scale
δ is the number of parts per million (ppm) of the magnetic field expressed as the spectrometer’s operating frequency (used ahead of value as it is a ratio and not a unit) Independent of instrument’s field strength 1 ppm = 1 delta (δ) unit. For a 60 MHz spectrum, 1 δ = ______ Note: NMR spectra are recorded on chart paper that is calibrated in “ppm” and in “Hz”. If data is reported in Hz, the field strength of the instrument must be specified. (60 or 300 MHz).
Measuring Chemical Shift
Chemical Shift Calculate the chemical shift in ppm for the
following: Bromoform (CHBr3), 413 Hz (at 60MHz) Iodoform (CHI3), 322 Hz (at 60MHz) Methylchloride (CH3Cl), 184 Hz (at 60MHz)
Remember: the chemical shift is in ppm.
3
13.8 1H NMR Spectroscopy and Proton Equivalence
Nonequivalent H’s
Proton NMR is much more sensitive than
Replacement of each H with “X” gives a different
13C and the active nucleus (1H) is nearly 100 % of the natural abundance Shows how many kinds of nonequivalent hydrogens are in a compound Theoretical equivalence can be predicted by seeing if replacing each H with “X” gives the same or different outcome Equivalent H’s have the same signal while nonequivalent are different
constitutional isomer Then the H’s are in constitutionally heterotopic
environments and will have different chemical shifts – they are nonequivalent under all circumstances
There are degrees of nonequivalence
Equivalent H’s Two H’s that are in identical environments
(homotopic) have the same NMR signal Test by replacing each with X if they give the identical result, they are equivalent
Enantiotopic Distinctions If H’s are in environments that are mirror images of
each other, they are enantiotopic Replacement of each H with X produces a set of
enantiomers The H’s have the same NMR signal (in the absence
of chiral materials)
4
Diastereotopic Distinctions In a chiral molecule, paired hydrogens can have
different environments and different shifts
Replacement of a pro-R hydrogen with X gives a
different diastereomer than replacement of the pro-S hydrogen Diastereotopic hydrogens are distinct chemically and spectrocopically
Consider: CH3CH2CH3
Are the protons equivalent?
Consider:
CH3
H
CH3
CH3
Are the protons equivalent?
How about?
1-Bromobutane Methylbenzene 2-Methyl-1-butene
5
13.9 Chemical Shifts in 1H NMR Spectroscopy Proton signals range from δ 0 to δ 10 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
CHCl3
Increased shielding of methyl protons ¸ Decreasing electronegativity of attached atom ¸
CH3F
CH3OCH3
(CH3)3N
CH3CH3
methyl fluoride
Dimethyl ether
Trimethylamine
Ethane
* shift, ppm
* shift, ppm
4.3
3.2
benzene 7.3
2.2
ethylene 5.3
CH2Cl2
CH3Cl
trichloromethane dichloromethane * shift, ppm
7.3
5.3
chloromethane 3.1
0.9
ethane 0.9 * shift, ppm
hexamethyl benzene
2,3-dimethyl-2butene
2.2
1.7
6
Last Slide Chapter 13 Part 1
7
