Classical NMR (7) Flashcards
What is the physical basis for nuclear precession?
force applied by an external RF field to dipole moment along the axis of the rotation causes dipole movement in the plane perpendicular to B0 field direction. The axis of the rotating particle moves in a circular motion
Why do nuclear spins precess in the presence of an external magnetic field?
dipole moment movement along the axis
What is the Larmor precessional frequency?
The frequency of the precession of a spinning nucleus in an applied B0 magnetic field
How can you predict the direction of precession in Larmor?
the sign of γ; γ<0 = counterclockwise, γ>0 = clockwise
Where does the resonance in the term “nuclear magnetic resonance” come from?
if RF radiation has the same frequency as that of the precessing particle, it can interact coherently with the particle and energy can be exchanged…this is resonance
What is the relationship between ∆E and Larmor frequency?
∆E = hv
How is the Larmor frequency related to magnetic field strength?
directly proportional
What is M0, the bulk or net magnetization vector?
addition of the z components of all of the nuclear magnetic dipole moments pointed along the +z direction
Is there net magnetization in the transverse plane?
nope
What is the relationship of the angle of precession to the energy of a precessing nucleus?
when energy is absorbed, angle changes
How can the angle of precession be changed?
addition of small secondary magnetic fields applied at right angles
Why is B1 applied to the x-y plane?
supplies energy for rotation
What happens to the net magnetization in the z axis as a function of time?
varies
What is a saturated state?
ground and excited states equally populated by absorption, so absorption signal decreases to 0
Does a saturated state produce an NMR signal?
nope
How is the lifetime of an excited state related to saturation?
if excited = ground population, it becomes saturated
What is spin-lattice relaxation?
excess spin energy in excited nuclei dissipated throughout the sample by equilibration with surroundings; absorbed energy dissipated as heat
Why is it also called longitudinal relaxation?
longitudinal component is the one relaxing
How is excess spin energy dissipated in spin-lattice relaxation?
equilibration with the surroundings
What is T1?
relaxation time
How is T1 related to the average lifetime of nuclei in the excited energy state?
T1 is the measurement of the average lifetime
How is T1 affected by the mobility of the sample lattice?
low mobility = large, increasing mobility decreasing T1
What is spin-spin relaxation?
excess spin energy in excited nuclei is shared with other nuclei; interchange of 2 quantum states (nucleus in lower spin state excited while nucleus in higher state relaxes)
Why is spin-spin also called transverse relaxation?
transverse vector component is the one relaxing
How is excess spin energy dissipated in spin-spin relaxation?
shared with other nuclei
What is T2?
relaxation parameter
Why are NMR spectra very broad in solids?
T2 is very short
How is the heisenberg uncertainty principle applied to NMR spectral line widths?
long relaxation times result in minor uncertainty in excited state energy levels
What is the effect of long relaxation times on NMR bandwidth?
narrow line width
What is the effect of short relaxation times on NMR bandwidth?
widen line width
