Download Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy NMR:
Based on the measurement of absorption of electromagnetic
radiation in the radio frequency region (4 - 900 MHz).
Nuclei of atoms are involved in the absorption process.
Intense external magnetic field is required.
(to develop the energy states required for absorption to occur)
Importance:
 The most powerful tool available for organic structure
determination.
 Useful for quantitative analysis.
Theoretical basis:
 Certain atomic nuclei should have the properties of spin and
magnetic moment.
 As a consequence, exposure to magnetic field would lead to
splitting of their energy levels.
 Nuclei absorb electromagnetic radiation in a strong magnetic
field as a consequence of energy level splitting induced by
the magnetic field.
 Molecular environment influences the absorption of RF
radiation by a nucleus in a magnetic field.
( can be correlated with molecular structure)
Theory of NMR:
Explained by both
• Classical mechanics…
Yields a clear physical picture of the absorption process and how its
measured
• Quantum mechanics…
Provides a useful relationship between Absorption freq. and nuclear
energy states
The two treatments yield Identical relationships.
Quantum Description of NMR:
Assume that nuclei rotate about an axis and thus have a property of
spin.
Nuclei with spin have angular momentum, p.
Max value of p is quantized and must be integral or half integral
multiple of h/2π .
Max number of spin state for a particular nucleus is its spin
quantum number, I.
Nucleus will have 2I+1 discrete states.
The component of angular momentum for these states in any
chosen direction will have values of I, I-1, I-2,…., -I.
In the absence of external field, the various states have identical
energies.
1H, 13C, 15N, 19F and 31P are nuclei of the greatest use in NMR.
(all have I = ½ )
So, No. of spin states = 2 x(1/2) +1= 2 spin states corresponding to I =
+1/2, and I = -1/2.
Heavier nuclei have spin numbers ranges from 0– 9/2 .
Nuclear Spin:
 A nucleus with an odd atomic number or an odd mass number has
a nuclear spin.
 The spinning charged nucleus generates a magnetic field.
=>
External Magnetic Field:
When placed in an external field, spinning protons act like bar
magnets.
Two Energy States:
• The magnetic fields of the spinning nuclei will align either with the
external field, or against the field.
• A photon with the right amount of energy can be absorbed and
cause the spinning proton to flip.
E and Magnet Strength:
 Energy difference is proportional to the magnetic field
strength B0.
 E = h =  h B0
2
 Gyro magnetic ratio, , is a constant for each nucleus
(26,753 s-1gauss-1 for H).
 In a 14,092 gauss field, a 60 MHz photon is required to flip a
proton.
 Low energy, radio frequency.
NMR Instrumentation:
There are two general types of NMR instruments
1. Continuous Wave NMR
Similar in principle to optical spectrometers
• An absorption signal is monitored as the frequency of the source is
slowly scanned.
• In some inst., the freq. of the source is held constant while the
strength of the field is scanned.
• CW spectroscopy is inefficient in comparison to FT NMR, as it
probes the NMR response at individual frequencies in succession.
As the NMR signal is intrinsically weak, the observed spectra
suffer from a poor S/N ratio.
• Dissolve the sample in a solvent (with no proton), like CCl4.
• Addition of internal standard TetramethylSilane TMS.
• The sample cell (cylindrical glass) located betn. The two poles of
magnet with continuous spanning. (homogenous magnetic moment)
• Source: Radio frequency oscillator betn. the two poles (gives radio
freq. magnetic radiation).
• Detector: Coil perpendicular on the RF oscillator. (Detect the RF
Signal)
CW NMR, Methods:
1. Constant applied magnetic is exerted and changeable RF radiation
to bring absorption of all nuclei.
2. Const. RF radiation and variable magnetic field by sec. are applied
Fourier Transform NMR:
Nuclei in a magnetic field are given a radio-frequency pulse close
to their resonance frequency.
The nuclei absorb energy and precess (spin) like little tops.
A complex signal is produced, then decays as the nuclei lose
energy.
Free induction decay is converted to spectrum.
Magnetic Shielding:
 If all protons absorbed the same amount of energy in a given
magnetic field, not much information could be obtained.
 But protons are surrounded by electrons that shield them from the
external field.
 Circulating electrons create an induced magnetic field that opposes
the external magnetic field.
Shielded Protons:
Magnetic field strength must be increased for a shielded proton to flip at
the same frequency.
Protons in a Molecule:
Depending on their chemical environment, protons in a molecule are
shielded by different amounts.
NMR Signals:
 The number of signals shows how many different kinds of protons
are present.
 The location of the signals shows how shielded or deshielded the
proton is.
 The intensity of the signal shows the number of protons of that
type.
 Signal splitting shows the number of protons on adjacent atoms.
The NMR Spectrometer:
The NMR Graph:
Tetramethylsilane:
CH3
H3C Si CH3
CH3
• TMS is added to the sample.
• Since silicon is less electronegative than carbon, TMS protons are
highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal.
Chemical Shift
 Measured in parts per million.
 Ratio of shift downfield from TMS (Hz) to total spectrometer
frequency (Hz).
 Same value for 60, 100, or 300 MHz machine.
 Called the delta scale.
Delta Scale:
Location of Signals:
• More electronegative atoms deshield more and give larger shift
values.
• Effect decreases with distance.
• Additional electronegative atoms cause increase in chemical shift.
Typical Values:
O-H and N-H Signals:
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions deshield the protons,
so signal is around 3.5 for N-H and 4.5 for O-H.
• Proton exchanges between the molecules broaden the peak.
Carboxylic Acid Proton, 10+:
Number of Signals:
Equivalent hydrogens have the same chemical shift.
Intensity of Signals:
• The area under each peak is proportional to the number of protons.
• Shown by integral trace.
How Many Hydrogens?
When the molecular formula is known, each integral rise can be assigned
to a particular number of hydrogens.