1 Diffusion Ordered Spectroscopy Provides a way to separate different compounds in a mixture based on the differing translational diffusion coefficients differences in the size and shape of a molecule ID: 264851
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Slide1
Diffusion Ordered Spectroscopy
1Slide2
Diffusion Ordered Spectroscopy
Provides a way to separate different compounds in a mixture based on the differing translational diffusion coefficients (differences in the size and shape of a molecule)Achieved by radio-frequency pulses as used in routine NMR spectroscopy and magnetic field gradients that encode spatial information
2Slide3
Self-Diffusion
Random translational motion of molecules or ions through the surrounding media driven by thermal energy (Brownian motion)
NO
thermal gradient (convection)
NO
concentration gradient (mutual diffusion)
3Slide4
Diffusion Coefficient (D)
Quantifies this motion as a measure of the rate of mean square displacement of the molecule (Units of m2s-1
)
We
can measure diffusion by NMR if we can map the l
ocation of
a molecule in solution and how this varies
as
a function of
time
4Slide5
Diffusion and Mass
Diffusion relates to molecular size!
5Slide6
Study of Self-Diffusion
Two steps:Spatially label the nuclear spins using gradients of magnetic field
Monitor their displacement by measuring their spatial positions at 2 distinct times
6Slide7
Refresher: NMR Basics
larmour
frequency,T
2
, rotating frame of reference
7Slide8
How to measure diffusion coefficients?
Short period (~1ms) in which magnetic field experienced by the NMR sample is made inhomogeneous!
8Slide9
Pulse Sequence – Pulsed Field Gradient Echo
9Slide10
DOSY uses two PFG pulses separated by a diffusion time Δ
First PFG destroys (
dephases
) all signals
Second PFG acts in opposition to first & may recover (
rephase
) signals
IF NO MOVEMENT during
Δ
– FULL signal recovered
IF MOVEMENT OCCURS during
Δ
, signal is NOT fully
rephased
leading to loss of signal
10Slide11
Diffusion NMR
Movement of molecules during Δ leads to LOSS of resonance intensity
Diffusion profile is obtained by increasing magnitude of field gradient
G
z
for repeated 1D experiments
Faster molecular diffusion corresponds to faster signal
attentuation
as a function of
G
z
11Slide12
Diffusion & Magnetic Field Gradient
12Slide13
13Slide14
Attenuation of Signal as Gz
Increases
14Slide15
DOSY NMR
15Slide16
Stokes-Einstein
Stokes- Einstein relation relates the Diffusion coefficient, D, of a particle to its molecular shape via a friction coefficient f (FOR SPHERE)
16Slide17
Diffusion Spectra
17Slide18
What can we study with DOSY?
Analysis of MixturesIntra-molecular interactionsSupra and biomolecular complexesAffinity
Chemical exchange
18Slide19
Diffusion Applications
Aggregation Slower Diffusion as molecules self-aggregate
Host-guest formation
Binding of small “guest” molecules within larger host leads to slower diffusion
Supramolecular
chemistry
Assessment of molecular size
19Slide20
Complexes and Exchange
ComplexesExchange
20Slide21
Host-Guest Complexes
Cameron,K
., Fielding, L. 2001.
J. Org. Chem
. 66, 6891.
21Slide22
Solving for Ka
– for small molecule and large Host
Cameron,K
., Fielding, L. 2001.
J. Org. Chem
. 66, 6891.
22Slide23
DOSY: Ka
Approximations remove need to perform titrations, and Ka
in principle can be derived from a single experiment.
Assumption is sound for small molecules binding to macro(biological molecules)
However for smaller Host-Guest chemistry – this assumption is not always true
23Slide24
Host-Guest Complexes
Cameron,K
., Fielding, L. 2001.
J. Org. Chem
. 66, 6891.
24Slide25
Aggregation
Simplest form of oligomerization is dimerization
Two monomers come together to form a dimer
Similar to H + G HG
2A A
2
K
dimer
= [A
2
]/[A]2
25Slide26
DOSY-NMR analysis of ring-closing metathesis (RCM) products
from β-lactam precursors
Limitation of RCM for formation of
intramolecular
ring-closed products is the occurrence of side products from
intermolecular
oligomerization
!
Identification of reaction products is not straightforward:
1
H
13C NMR data may be inconclusive because of complexity. Mass spec – inconclusive.DOSY is the answer!
Sliwa
, A.,
Marchand-Brynaert
, J.,
Luhmer
, M. 2011
Magn
.
Reson
. Chem
. 49, 812.
26Slide27
Sliwa
, A.,
Marchand-Brynaert
, J.,
Luhmer
, M. 2011
Magn
.
Reson
. Chem
. 49, 812.
27Slide28
Sliwa
, A.,
Marchand-Brynaert
, J.,
Luhmer
, M. 2011
Magn
.
Reson
. Chem
. 49, 812.
28Slide29
Determination of Precursors:
29
Sliwa
, A.,
Marchand-Brynaert
, J.,
Luhmer
, M. 2011
Magn
.
Reson
. Chem
. 49, 812.Slide30
Limitations
Measuring accurate diffusion constants required a high quality gradient coil. Gradients have to be linear.Good temperature stability required
Assumptions of spherical shape often used – not always accurate
2D Transformation Errors – diffusion coefficients should differ as much as possible from one another & Standard errors should be marginal
30Slide31
Limitations
Cohen, Y.,
Avram
, L.,
Frish
, L., 2005.
Angew
. Chem.
44, 520
31Slide32
In Summary: DOSY
Powerful method for the NMR analysis of many types of mixturesMeasure diffusion coefficients which reflect size and shape of molecular speciesApplications:
association constants
, investigating
aggregation
,
encapsulation
,
intermolecular interactions
in multi-component systems and
size and structure of labile systems.
32Slide33
Questions?
33