The fundamentals L1 Flashcards
What do protons, neutrons and electrons all possess
Protons neutrons and electrons have intrinsic spin, angular momentum of 1/2 and orbital angular momentum
How does the nucleus behave
The nucleus behaves as a single particle with spin angular momentum
Define total angular momentum of the nucleus
Total angular momentum of nucleus is the sum of spin and orbital angular momenta of all protons and neutrons in the nucleus
How if at all are spin and orbital angular momenta quantised
Spin and orbital angular momenta are quantised in both magnitude and direction
How do we calculate magnitude
Magnitude = square root of [I(I+1)ħ]
Where I is nuclear spin
h is (h/2pi) and h is planks constant
How do we calculate direction
Iz = mIħ
Iz = Z component of angular moment (with respect to an arbitrary Z axis
Mi - magnetic quantum number
For a given nucleus how many possible spin states are there
For a given nucleus with spin I there are 2I+1
How do we know the value of I for a particular nucleus
Spins of protons or neutrons can pair up and spins cancel
Protons do not pair up with neutrons. It is not possible to predict the value of I only whether it is zero, half integer or integer
Describe nuclei with odd number of protons and or neutrons
Nuclei with odd number of protons and neutrons have a total angular momentum that is not zero therefore they have spin
They are therefore nmr active
Describe nuclei with even number of protons and or neutrons
Nuclei with even number of protons and neutrons do not possess spin and are nmr inactive therefore cannot be detected
Define isotope specific
Different isotopes of the same elements have different spins
What does spin of a nucleus produce
Spin of nucleus creates movement of electrical charge which produces a magnetic field
What does a nucleus with non zero spin have
A nucleus with non zero spin has a magnetic moment (mu)
Magnetic moment is a vector quantity quantised in the same way as I
How do you calculate magnetic moment (mu)
Magnetic moment = magnetogyric ratio * I
Where I is angular momentum
What is the magnetogyric ratio
Proportionality constant specific to each nucleus
Describe nuclear spins in absence of an applied field
In absence of an applied magnetic field, nuclear spins are randomised
2I+1 spin states are degenerate
Describe nuclear spins in presence of an external magnetic field
In the presence of an external magnetic field the 2I+1 spin states have different energies. The spins either align or oppose withe the magnetic field
What is B0
B0 is the applied external magnetic field
What are quantum restrictions
Quantum restrictions prevent spins aligning exactly with B0
What do energy difference between spin states depend on
External magnetic field and magnetogyric ratio
How do we calculate the energy difference between spin states
E=hv=(h/2pi)magnetogyric ratioB0
Define resonant frequency
Resonant frequency is the frequency corresponding to energy of transition
How do you calculate resonant frequency
(Magnetogyric ratio*B0)/2pi
What are the selection rules in terms of energy levels
For a given B0 the spacing of E-levels are equal.
Change in magnetic quantum number must be plus or minus 1
What happens to the nucleus in an applied magnetic field
In an applied magnetic field the nucleus processes about it own axis of spin, the frequency of precession is equal to the resonant frequency
What happens upon application of radio frequency radiation of the same frequency to the nucleus
Causes resonance in the nucleus and energy can be absorbed and transitions between the spin states can occur
What leads to population of higher energy states
Thermal motion
What does the Boltzmann distributions tell us
Population difference between energy levels
What is the equations for Boltzmann distribution
N upper/ N lower =e^deltaE/kt
Where k is the Boltzmann constant
What induces transitions between spin states
Irradiation of sample in external magnetic field at larmor frequency induces transitions between spin states
Describe the probability of excitation
The probability of excitation (lower to upper) is the same as the probability of emission (upper to lower)
What does detection of transitions rely on
Detection of transitions relies on very small population difference between energy levels.
Ie the intensity of NMR signals depends on difference between the number of absorptions and emissions
What does sensitivity for NMR depend on
Sensitivity greatest for nuclei with large magnetogyric ration
Sensitivity increases with increasing B0 and increases with decreasing temp
What is relaxation in terms of NMR
After excitation spins relax to re establish Boltzmann distributions. In absence of relaxation populations of spin states equal so no NMR signals detected so spins become saturated
What are the two distinct relaxation processes
Spin lattice relaxation and spin spin relaxation
Outline the spin lattice relaxation process
Loss of energy to surrounding eg solvent molecules, wall of nmr tube via tumbling and vibrations
Tumbling at rate similar to larmor frequency creates oscillating field with enables relaxation
Outline spin spin relaxation
Energy exchange between nuclear spins leads to randomisation of spins but no net loss of energy
Describe the effect of relaxation on linewidths
Relaxation of nuclear spins are generally slow so linewidths are narrow, but in some cases relaxation of nuclear spins are very fast and peaks so broad they are difficult to observe
How do we measure linewidths
Measurement of linewidths. Distance between half height on peak
Why is spectroscopy at higher frequencies much more sensitive
Higher energy photons are easier to detect
How do we optimise signal strengths in nmr
By using strong magnetic field to maximise delta E
