Topic 3.1: Relaxation revision Flashcards
Relaxation review summary
Relaxation returns NMR spins back to … … (described by the … distribution)
The transitions between … states required to obtain populations of levels at thermal equilibrium are induced by … … fields at frequencies of the transitions. If a transition is wanted between 2 levels, an oscillation … such … is required
For relaxation, the source of oscillating fields are molecular motions modulating orientation of … interactions (CSA/dipolar/quadrupolar coupling). … interactions will depend on orientation of molecule with respect to … …. At different orientation of molecules (when one moves with respect to magnetic field) value of interaction changed and interactions are a form of magnetic field/corrections to magnetic field.
Relaxation is induced by … … … where there is no phase coherence between different …. Even if every spin in a sample changes in the same way, all will do so at different times
To use relaxation for quantification of motions one needs to … effects of … processes that affect all spins in the same way e.g. pulses and … processes e.g. molecular motion
Relaxation review summary
Relaxation returns NMR spins back to thermal equilibrium (described by the Boltzmann distribution)
The transitions between spin states required to obtain populations of levels at thermal equilibrium are induced by oscillating magnetic fields at frequencies of the transitions. If a transition is wanted between 2 levels, an oscillation matching such transition is required
For relaxation, the source of oscillating fields are molecular motions modulating orientation of anisotropic interactions (CSA/dipolar/quadrupolar coupling). Anisotropic interactions will depend on orientation of molecule with respect to magnetic field. At different orientation of molecules (when one moves with respect to magnetic field) value of interaction changed and interactions are a form of magnetic field/corrections to magnetic field.
Relaxation is induced by incoherent random processes where there is no phase coherence between different spins. Even if every spin in a sample changes in the same way, all will do so at different times
To use relaxation for quantification of motions one needs to separate effects of coherent processes that affect all spins in the same way e.g. pulses and incoherent processes e.g. molecular motion
- Generally magnetic field is aligned along …
- Generally magnetic field is aligned along z-axis
- What is spin-lattice relaxation?
- Longitudinal relaxation (T1) aligns parallel to BO (z-axis) and is the recovery of longitudinal relaxation
- Dictates how often an NMR experiment can be repeated
- How does spin-latticerelaxation dictate how often an experiment can be repeated?
- Signal is proportional to z magnetisation at equilibrium state a start of experiment.
- If insufficient time given for z magnetisation to recover to equilibrium value, z magnetisation will be less than its thermal equilibrium value and therefore less than what it could be.
- Has a minimum for specific value of correlation time
- How can SNR be improved in an experiment
- More repeats lead to more signal and less noise (n1/2 noise ∝ n signal ∝ repeats)
- Must give sufficient time for longitudinal magnetisation to relax between repeats
- What is spin-spin relaxation?
- Transverse relaxation (T2) aligns perpendicular to BO (xy plane) and is the loss of transverse magnetisation (decay of signal)
- Affects line width of peaks in spectra
- Increases monotonically with increasing correlation time.
- NMR relaxation is induced by … …
- NMR relaxation is induced by molecular motions
- Briefly outline how NMR is used to study dynamic processes
- Equilibrium state perturbed via a radiofrequency (rf) pulse at a certain Larmor frequency
- After this 90o/180o pulse the now non-equilibrium state has had its spins rotated, it is excited
- Relaxation occurs where motion is induced in sample
- Describe the vector and energy level pictures associated with an NMR equilibrium state
- Vector picture describes magnetisation aligned with z axis, value of which given by Boltzmann distribution
- In energy level picture, spins can be aligned with (α) or against (β) BO
- Magnetisation is the difference between populations in each state (after cancelling)
- How will vector and energy level picture change in the case of transverse magnetisation and population inversions
- Rf pulse induces transition to form non equilibrium states
- Angle in vector model attained through ensemble averages via QM and represents coefficient of how much α/β mixed in
- Vector model fails to distinguish # of states of each spin beyond the angle of which vector points
- Radiofrequency pulse is an … electromagnetic field
- To … magnetisation for specific spins/induce … between energy levels for specific spins the rf pulse must oscillate at the appropriate … frequency
- E.g. … … frequency to rotate 1H magnetisation
- Radiofrequency pulse is an oscillating electromagnetic field
- To rotate magnetisation for specific spins/induce transitions between energy levels for specific spins the rf pulse must oscillate at the appropriate Larmor frequency
- E.g. 1H Larmor frequency to rotate 1H magnetisation
- Pulses are applied intentionally, however relaxation occurs independently. What is the main source of oscillating magnetic fields that can induce transitions and therefore relaxation?
- Relaxation is induced by modulation of anisotropic NMR interactions by molecular motions
- Anisotropic interactions are those that change depending on the orientation of the molecule with respect to BO
- Briefly outline how motion leads to relaxation in NMR
- Motion modulates angle of molecule with BO
- Different magnitude of interaction at different angles may be viewed oscillating magnetic field (e.g. dipolar coupling)
- If oscillations are at right (Larmor) frequency they will rotate spins and induce transitions (like rf pulses)
- This is the main mechanism that brings the system back to thermal equilibrium
- What is the difference between incoherent and coherent motion and processes?
- Incoherent: random motion that will induce relaxation (e.g. via thermal motion until realigned with Bo)
- No phase relationship between spins as different parts of ensemble experience different fields
- Leads to loss of coherence
- Coherent: specific phases relationship between different members of ensemble (e.g. pulse)
- Motion will not induce relaxation
- With the aid of a diagram, describe the difference incoherent and coherent fields have on magnetisation
- Relaxation is induced by randomly fluctuating magnetic fields that induce transitions
- These fields can induce rotations around different axes
- Coherent fields induce rotations of the bulk magnetisation as they are the same across the sample – e.g. a pulse
- Incoherent leads to dephasing as different parts of ensemble experience different fields
- Magnetisation decays compared to initial
- Have an oscillation along a certain axis, causing rotations on magnetisation, how does this relate to transverse/longitudinal relaxation? End of lec question
- Reference system where Bo aligned in z direction
- RHG rule; thumb in direction of phase/pulse, fingers curl to direction of rotation
- If magnetisation if on y axis as above, a pulse perpendicular, with phase of x axis will rotate magnetisation about zy plane (green circle)
- Pulse with phase y does not change direction with respect to z-axis (zx plane)
- à to rotate magnetisation, need oscillating field that is perpendicular to what is being rotated
- T1 (spin-lattice/longitudinal) relaxation
- Bo along z axis, to change longitudinal magnetisation need fields that are transverse only (x/y oscillations)
- T2 (spin-spin/transverse) relaxation
- Z-field oscillation perpendicular therefore will induce relaxation for transverse component
- BUT, oscillating field on x axis will also rotate magnetisation along y direction can have both longitudinal and transverse fields contributing to relaxation
