james keeler‘s lecture series “understanding nmr ...product operator formalism. evolution under...
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James Keeler‘s lecture series“Understanding NMR Spectroscopy”
6 ‐ Product operatorsNMR seminar
Elisabeth Lehmann
01.04.2015
Representation of magnetisation
Quantum mechanical representation of (time‐dependent) magnetisation bydensity operator
Vector model representation of magnetisation
With , , as spin angular momentum operators
With , , as three components of the magnetisation vector
Hamiltonians for pulses and delays
Delay (=no pulse): free precession
Pulse: rotation about axis, in which pulse is applied
Ω: frequency of rotation about z‐axis (offset in rotating frame)
ω: frequency of rotation during pulse
Ω
,
,
Equation of motion: effect of x‐pulseCalculation of density operator at time t, is calculated by applying the relevant Hamiltonian to the density operator at time 0, 0 :
expression in terms ofangular momentum operators
Pulse along for
Known fromquantum mechanics
0 at equilibrium
z
yx
x
z
yx
,
Standard rotations
x
z
‐z
y‐y y
z
‐z
‐xx z
x
‐x
‐yy
Rotation axis
Precession
Example
Rotation from (=old operator)to (=new operator)by (=applied Hamiltonian operator )
Iy
Iz
t
Arrow notation
Spin echo
The outcome of the spin echo block is independent of the offset Ω and the delay τ.
The evolution due to offsets is refocused by the sequence – τ – 180°(x) – τ –.
Detailed calculation:see script 6.1.6
Application to multiple spin systems
So far: description of single spinsNext: spin systems of two or more coupled spins
Product operators can be used to describe coherence transfer and multiple quantum coherence
Operators for two spins: in‐phase
1 , 2 , 1 , 2
in‐phase magnetisation
Operators for two spins: anti‐phase
2 1 2 , 2 1 2 , 2 1 2 , 2 1 2 : anti‐phase magnetisation
2 1 2 : magnetisation on spin 1, which is anti‐phase with respect to coupling to spin 2
Antiphase magnetisation is a state that is created within free evolution of two coupled spins.
http://www.chemie.uni‐hamburg.de/nmr/insensitive/tutorial/en.lproj/antiphase_magnetization.html
Antiphase peaks cannot be converted to in‐phase peaks by phase correction.
Operators for two spins:non‐observables
2 1 2 , 2 1 2 , 2 1 2 , 2 1 2 : multiple quantum coherence
2 1 2 : non‐equilibrium population distribution (two spin state)
These types of states are not observable
Evolution of a two spin systemunder pulses
Evolution of 2 1 2 under 90° y‐pulse on both spins
1 is rotated 90°about y to 1
2 is rotated 90°about y to 2Anti‐phase magnetisation of spin 1 has been transferred
to anti‐phase magnetisation of spin 2.
This process is called coherence transfer.
Example:
Evolution under couplingHamiltonian representing coupling of two spins
12: scalar coupling [Hz]
zz
x
‐x
‐yzyz zz
y
‐y
xz‐xz
In‐phase magnetisation (x) becomes anti‐phase (y)
Anti‐phase magnetisation (x) becomes in‐phase (y)
Complete interconversion at cos 12 0 → 12 →
http://www.chemie.uni‐hamburg.de/nmr/insensitive/tutorial/en.lproj/coupling.html
2
Spin echo, homonuclear coupled spins
Homonuclear spin system: 180° pulse affects both spins
Known: spin echo sequence – τ – 180°(x) – τ – refocuses evolution due to offset → only evolu on under coupling considered here
τ:
τ:
180° (x):No net effect
Complete conversion of 1 to antiphase magnetisation 2 1 2
at cos2 12 0 → 2 12 → and 2
Interconverting in‐phase andanti‐phase states
Spin echos can be used to interconvert in‐phase magnetisation and pure anti‐phase magnetisation, while refocusing evolution due to offsets.
Pulse sequence element:
––180°(x) – –
With 180° (x) applied on both spins
Spin echo, heteronuclear coupled spins
Spin echo: – τ – 180°(x) – τ –In heteronuclear spin system, 180° pulse can be applied selectively to one or both spins
Evolution of coupling 2τ refocused refocused
All combina ons possible → In heteronuclear spin systems possible to choose evolution/refocusing effects due to offset/coupling
Evolution due to offset
refocusedrefocused
refocused2τ
2τrefocused
spin 1spin 2
Coherence order
Classification by coherence order
1 (single quantum coherence) e.g. 1 , 2 1 2 observable
0 (zero‐quantum coherence) e.g. 2 1 2 non‐observable
2 (double quantum coherence) present in e.g. 2 1 2 non‐observable
(also contains 0)
Only transverse magnetisation is observable in NMR.
Classification possible for individual spins:
2 1 2 : spin 1: 0, spin 2: 1
Raising and lowering operatorsRaising operator : 1 Lowering operator : ̶1
Example
2 200
With and
It follows
1 1 (mixtures of coherences)
12
12
Systems with three or more spins
Spin systems with three or more spins can be described with the product operator formalism.
Evolution under offsets, pulses and coupling follow the same rules as for two‐spin systems.
Occurrence of triple quantum coherences possible, e.g. in 4 1 2 3
Double anti‐phase
Operators of multiple‐quantum coherences
Set of operators representing pure multiple quantum states
Evolution of multiple‐quantum terms
Evolution under offsets(analogous to and )
Evolution under couplings(passive couplings analogous to and )
Exercise 6.912
12
2 2 ⋅12 ⋅
12
12
12
2 0
12
12
12
12 2 2
12 2 2