Solid-State NMR Studies of Molecular Motion
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Transcript of Solid-State NMR Studies of Molecular Motion
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Solid-State NMR Studies ofMolecular Motion
Sharon AshbrookSchool of Chemistry, University of St Andrews
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Solution Solid
13C NMR of solid alanine
13C NMR of solid alanine
13C NMR of alanine in solution
13C NMR of alanine in solution
CO
CH
CH3
Motion in NMR
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Motion in NMR
• C60 is a “plastic crystal”
• At low temperature lineshape broadenedby CSA
• At higher temperatures molecules tumblerapidly and isotropically on their latticesites
• A single sharp, narrow line is observed
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Motion in NMR
• At low temperature CSA tensor with three principal components• At higher temperature C6 jump averages two components of the tensor
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Timescales
10–12 10–610–9 110–3 103 timescale(τc = k–1) / s
Fast Intermediate Slow
Spin-lattice(T1) relaxation
(~ inverse ofLarmor frequency)
Quadrupolar CSA;dipolar
Lineshapes (“T2 relaxation”)
(~ inverse ofanisotropic broadening)
Magnetization“storage”
(2D exchange)
(~ T1)
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Timescales
10–12 10–610–9 110–3 103 timescale(τc = k–1) / s
SlowIntermediateFast
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Rapidly fluctuating Hamiltonians
B0 Hint • Orientationally-dependent (anisotropic)interactions, Hint
• In liquids, rapid tumbling motion results inresonance frequency being determined onlyby isotropic or average value of Hint
• Also Δm = ±1, ±2 off-diagonal elements ofHint that cause Δm = ±1, ±2 transitions ifmotion is comparable with ω0 or 2ω0
• These random transitions give rise to thespin-lattice (T1) relaxation and part of thespin-spin (T2) relaxation behaviour
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Rapidly fluctuating Hamiltonians
B0 Hint • Orientationally-dependent (anisotropic)interactions, Hint
• In liquids, rapid tumbling motion results inresonance frequency being determined onlyby isotropic or average value of Hint
• Also Δm = ±1, ±2 off-diagonal elements ofHint that cause Δm = ±1, ±2 transitions ifmotion is comparable with ω0 or 2ω0
• These random transitions give rise to thespin-lattice (T1) relaxation and part of thespin-spin (T2) relaxation behaviour
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Spin-lattice (T1) relaxation
S(t) = Seq (1 – exp(–t/T1))
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Spin-lattice (T1) relaxation
DiamondT1 ~ 24 hours
AdamantaneT1 ~ 5 s
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Anisotropic T1 in 2H NMR
180 x pulse
delay time
!
90 x pulse
acquire
Mo
Mo
–M
z(!)
z z z z
x x x x
Result:
increasing !
!
0
!null
!null = T ln 21
S(?)
(!)S
Fourier transform wrt τ
spinning methyl groupin d3-alanine
Schleicher et al., J. Chem.Phys. 92, 6432 (1990)
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NOE measurements
Ashbrook et al., J. Am. Chem.Soc. 128, 6782 (2006)
0% 11B NOE
53% 31P NOE
• NOE common in the liquid state for I = 1/2 nuclei• Not usually observed for quadrupolar nuclei (efficient quadrupolar relaxation)• Rare in solids as motion usually too slow
steady-stateNOE in CDCl3
BH3PPh3
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NOE measurements
Ashbrook et al., J. Am. Chem.Soc. 128, 6782 (2006)
transient NOE in BH3PPh3 solid
152% 11B NOE
• Rapid BH3 rotation modulates the dipolarinteraction (to produce NOE) but not thequadrupolar interaction (so no rapid relaxation)
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Slowly fluctuating Hamiltonians
• To calculate resonance frequencieswe truncate Hint (“make the secularapproximation”, “retain only the termsthat commute with HZ”)
• Example: the truncated shielding canbe represented by an ellipsoid orshielding tensor, σ, that has threeprincipal axes: σ11, σ22, and σ33
• In a solid, observed shielding is givenby σzz
B0
!11!
22
!33
σzz
Isotropic shielding: σiso = σzz = (1/3) (σ11 + σ22 + σ33)
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Slowly fluctuating Hamiltonians
B0
!11!
22
!33
σzz
!11
!22
!33
σzz'k
k
σzz'σzz
Spectrum:
symmetrical two-site “exchange”
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Exchange broadening
Me
MeN–N
–
O=–
k
k
Me
MeN–N
–
O=–
A
B
Δν
A B
k k ≈ ΔνDimethylnitrosamine –
the textbook example inliquids NMR
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Powder averaging and dynamics
B0• A powdered solid consists of
microcrystals with a random,spherical distribution oforientations
• Therefore, exchange-broadened“pairs” must be averaged over allorientations to obtain powderspectrum
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Powder averaging and dynamics
k
100 kHz • Example: 2H (spin I = 1) powderNMR spectrum as function of k
• Changes in powder lineshape andunderlying linewidth (“T2relaxation”) are best observedusing echo methods (e.g.,quadrupolar echo)
• Very powerful method of testingmechanism of reorientation anddetermining k
• “Exchange broadening” and“T2 relaxation caused by slowmotions” can be thought of asthe same thing
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Powder averaging and dynamics
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Powder averaging and dynamics
109° 2-site 120° 2-site 109° 3-site
Effective CSA interactiontensor is the AVERAGEover all the sites visited
during the motion
Θ
Axial motion:
η = 0axially symmetric
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Powder averaging and dynamics
109° 2-site 120° 2-site 109° 3-site
Effective CSA interactiontensor is the AVERAGEover all the sites visited
during the motion
180° flip
η = 1
NB- Change to handout
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Powder averaging and dynamics
2H two-site exchange withhopping angle = 120º, with
τc = 10–5 s
Duer, Introduction to NMRSpectroscopy (2004)
• In addition to lineshape changeswith temperature motion canalso be probed by lineshapechanges with echo delay
• Anisotropic “T2” relaxationdepending upon type of motionand orientation with respect toB0
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Dynamics and MAS
t
1 / νR
t = 0
no motion
motion
“Jump” in νQ value during rotor period 1/νR resultsin decreased intensity of next rotational echo
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Dynamics and MAS2H MAS of Na2S4O6.2D2O
293 K
325 K
345 K
νR = 10 kHz~
~
~
50 kHz
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Dynamics and MAS
Typical CSA lineshapesunder two-site exchange
with hopping angle of 120º
Duer, Introduction to NMRSpectroscopy (2004)
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Dynamics and MAS
Duer, Introduction to NMRSpectroscopy (2004)
2H lineshapes under two-site exchange with
hopping angle of 120º
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Double-quantum MAS
SQ
DQν0+νQ ν0–νQ
ν0
Zeeman + Quadrupolar
ν0 ν0+νQ
ν0 ν0–νQ
2ν0
mI = –1
mI = 0
mI = +1
k
SQ
100 kHz
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Double-quantum MAS
SQ
DQν0+νQ ν0–νQ
ν0
Zeeman + Quadrupolar
ν0 ν0+νQ
ν0 ν0–νQ
2ν0
mI = –1
mI = 0
mI = +1
k
DQ
10 kHz
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Double-quantum MAS
τex t1 t2
90° 90° 90° 90° 90°τex τz
+2+1 0–1–2
p =
DQ
SQ
Cutajar et al., Chem. Phys.Lett. 423, 276 (2006)
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Double-quantum MAS2H DQMAS oxalic acid dihydrate
SQ
DQ
SQrotor
synchronized in t2
D2O
CO2D
CO2D
D2O
not rotorsynchronized in t2
Cutajar et al., Chem. Phys.Lett. 423, 276 (2006)
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Typical two-dimensionalexchange experiment
Exchange experiments
For a single site, staticexperiments can provideinformation on the type of
reorientation
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Exchange experiments
experimental simulation
polymer main chain axis
Schmidt Rohr et al.,Macromol. 27, 4733 (1994)
side chain flips of 180°
13C NMR of PMMA
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Exchange experiments
Spiess, NMR: Basic Principles andProgress 15, 55 (1978)
2H exchange NMR of DMS
0° 20°
45° 90°
jumps around the C2 axis of 106°
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Slow MAS exchange experiments
slowMAS
Use MAS to improve resolution and sensitivity
For slow MAS the interaction anisotropy is retained in spinning sidebands
Exchange peaks between different sites
Exchange peaks between sidebands of one site
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Slow MAS exchange experiments
Titman et al. J. Am. Chem.Soc. 114, 3765 (1992)
13C MAS NMR of bullvalene
Two sets of cross peaks
1. Sidebands of different sites(Cope rearrangement, 15 kcal mol–1)
2. Sidebands of same site(3 fold jumps, 21 kcal mol–1)
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MAS exchange experiments• Under fast MAS high resolution but no anisotropy information• Exchange between different sites produces cross peaks
τm = 0
ω2
ω1
1 2 3
12
3
τm > 0
ω2
ω1
1 2 3
12
3
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MAS exchange experiments
tt tg
τm = 5 ms τm = 500 ms
tt
tg
tttg
13C MAS NMR atatic polypropylene
rotational motion of polymer backbone justbelow glass transition temperature
interchanges different conformations
Zemke et al. Macromol.24, 6874 (1991)
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Timescales
10–12 10–610–9 110–3 103 timescale(τc = k–1) / s
Fast Intermediate Slow
Spin-lattice(T1) relaxation
(~ inverse ofLarmor frequency)
Quadrupolar CSA;dipolar
Lineshapes (“T2 relaxation”)
(~ inverse ofanisotropic broadening)
Magnetization“storage”(2D exchange)
(~ T1)
Slow