Intro to NMR Flashcards
Electron (as well a protons and neutrons) posses a kind of angular momentum called?
Spin
If a particle has spin angular momentum the it shows magnetic properties
What is included in the overall spin angular momentum for the nucleus (characteried by the quantum number i)
The angular momentum fron the individual protons and neutrons comes together to give an overall spin momentum
What values can the quantum number l take
can be an interger, a half interger, or zero
Will isotopes have the same of different value of angular momentum
Different isotopes of the same element have different values of i
Nuclei with odd mass number will have what values for the quantum numberi
Have a half intergral spin
e.g: ¹H, ⁷Li, ¹¹B
Nuclei with odd number of protons and odd numbers of neutrons have what values for the quantum number i
Intergal number
²H, ⁶Li, ¹⁰B
Nuclei with even numbers of protons and even numbers of neutrons have what value for quantum number i
Have zero spin
⁴He, ¹²C, ¹⁶O
A nucleus where i doesn’t equal 0, aligns in what orientation with respect to …
2i + 1 oritentation
With respect to the direction of the field, each distinguished by the quantum number mi
What values does mi take?
from -i through all the way to +i
Each of the discrete values for quantum number mi have a different …
They are known as …
energy
Spin states
For a spin where i=1/2
The two orientation are…
Parallel and Antiparallel with the magnetic field
NMR measured
The energy from transition from different spin states
What is the basic method for NMR
The sample is dissolved within a suitable solvent and placed in a powerful magnetic (up to 20 T)
A short pulse of radio-wave radiation (pulse) causes a transition between spin states
NMR signal is detected as the spin return to equilibrium
In the spectrum the NMR signal appears as a peak
They are known as
Resonance
The frequency of the peak is releated to the energy seperation between the spin states
The resonance frequency depends on:
- the stregth of the applied magnetic field
- The isotope observed
What value of i would not be shown on an NMR spectrum
i= 0
The power of NMR spectroscopy arises from
The precise difference in energy between spin states depend on what
The fact that not all nuclei of the same isotope give a peak at the same frequency
The precise difference in energy between spin states depends on the local magnetic field experied by a given nucleus - which in turn depends on its chemical environment
How does a chemical shift on a NMR comes about
- In response to the large magnetic field, the electrons in the surrounding bonds (that determine the chemical environment) flow so as to give a current
- In turn this current generates a local magnetic field, which opposes the applied magnetic field at the nucleus
- As a result, the nucleus experiences a reduced magnetic field, which results in a lower energy separation and a shift to lower resonance frequency
Values of the chemical shift are measures in Hz. What do these values depend on?
Hence how do we standardise the measurements of NMR?
The magnetic field in the NMR spectrometer
Resonance position are reported as shift from the resonance of an agreed reference compound
Shift (ppm) is expressed as a proportion of the reference frequency so that the result is independent of the size of the magnetic field
What is the reference compound in ¹H and ¹³C NMR
Si(CH₃)₄
Each nucleus in a different environment in a molecule result in…
… a seperate resonance in the NMR spectrum
Why does the chemical shift roughly correlated with the electronegativity of the substituents
The exact chemical shift depends on the electronic environment of the nucleus
Electrons are said to shield the nucleus but neighbouring functional groups can remove electron density from the nucleus deshields it
This results in a higher frequency and more positive value for chemical shift
Why is the ¹³C signal much weaker than the ¹H signal
The natural abundance of ¹³C is only 1.1% that of naturally occuring carbon
And the most abundant isotope ¹²C doesnt have a magnetic moment
