Solid-State NMR Research at St AndrewsDr Sharon E. Ashbrook, School of Chemistry

The University of St Andrews is a registered charity in Scotland: number SC013532

Research facilities• Nuclear magnetic resonance (NMR): a highly senstive probe of atomic level structure, disorder and dynamics

• NMR of solids is complicated by orientation dependent nuclear spin interactions

• The upper region of the Earth’s mantle is estimated to contain up to 600 ppm of water • Between 1941-2004, more than 1400 tons of plutonium waste has

been generated throughout the world

• 236 nuclear powerplants generate 70-80 tons of new spent plutonium each year worldwide

• Nuclear waste isotopes have long half-lives - 239Pu (24,000 y), 237Np (2,100,000 y), 233U (160,000 y) - need for long term storage

• Pyrochlores (A2B2O7) are a component of the SYNROC ceramic-based wasteform which is used for encapsulation of radioactive lanthanides and actinides

• Aluminophosphates (AlPOs) are an industrially important class of solids with applications in catalysis, gas storage and medicine

• Neutral framework of alternating AlO4– and PO4

+ tetrahedra form pores and channels of molecular dimensions which are capable of accomodating small guest species • The NaNbO3-KNbO3 solid solution is of considerable interest as a potential

‘green’ replacement for PZT, a piezoelectric used widely in industry and research

• The structure of the end-member NaNbO3 has been widely debated in the literature

• NaNbO3 adopts the perovskite (ABO3) structure, with A-site cations surrounded by BO6 octahedra

• Tilting of the B-site octahedra leads to subtle structural changes which can significantly alter the physical properties

• Additional water uptake in both the presence and absence of guest molecule species is also common and needs to be understood and controlled in structural characterisation

• JDF-2 is a AlPO framework containing methylamine guest molecules

• Published crystal structure contains 2 distinct methylamine molecules, whereas 4 sites are observed by 13C solid-state NMR

• JDF-2 undergoes reversible hydration in air to form AlPO-53(A)

Hydration of JDF-2 in air to form AlPO-53(A) canbe followed by 31P solid-state NMR

Although almost indistinguishable by conventional X-ray diffraction measurements,two-dimensional 23Na solid-state NMR spectra reveal the presence of two phases insamples synthesised by the solid-state method

Two phases present in solid-state sample characterised as Pbcm and P21ma bydiffraction, solid-state NMR and DFT calculations

Scanning electron microscopy reveals differences in macroscopic structure

Synthesis methodcan havesignificant effectson the structure

solid-statesynthesismethod

molten saltsynthesismethod

13C-1H correlation experiment revealscarbon-hydrogen connectivities

Extra carbons are found to be bondedto methylamine hydrogens, confirmingthey are not due to impturities

0

2

4

14

28

71

time(days)

JDF-2

JDF-2

AlPO-53(A)

AlPO-53(A)

H2O

• Y3+ cations are of similar charge and size to lanthanides and actinides, providing a NMR-active non-radioactive probe of local structure

Yttrium stannate-titanate Y2Ti2–xSnxO7 pyrochlore solid solution

• Titanate pyrochlores show good chemical durability

• Incorporation of tin has been shown to result in increased resistance to radiation damage

Changes in number of Sn next-nearest neighbours (NNN) observed as a systematic shift in 89Y chemical shift

DFT calculations performed on a rangeof model structures containing differentSn NNN environments

Dependence of the 89Y chemical shifton Sn NNN confirmed by calculations

• The exact mechanism of water storage is not well understood

• Solid-state NMR at St Andrews has provided detailed insight into model hydrated mantle minerals

• Samples synthesised at very high pressure and temperature in multi-anvil apparatus to simulate mantle conditions

• Clinohumite provides a model for the incorporation of water within anhydrous mantle minerals

• Diffraction studies show static disorder of hydroxyl proton positions

• Experimental and computational methods aid the aquisition and interpretation of solid-state NMR spectra

Mag

netic

fiel

d

54.7°

Rapid sample rotationat the ‘magic angle’achieves high resolutionNMR spectra

Powdered samples are spunat up to 60 kHz in small rotorsusing compressed air

High pressure synthesis leads to very small sample volumes- challenges in sensitivity

2H solid-state NMR confirmsmotion of hydroxyl deuterons

Analysis of variable temperature2H solid-state NMR spectraallows activation energy to bedetermined

motion

H1 site

H2 site

Ea = 40 kJmol–1

Crystal structure of clinohumite

NMR parameters can be calculated from first-principles using state-of-the-art densityfunctional theory (DFT) codes

• > 98% of elements accessible by NMR

• Applicable to both crystalline and disordered systems

• Specific to individual chemical elements

frequency

Solid-state NMR• 600 MHz wide-bore solid-state NMR spectrometer

• Range of NMR probes designed for both routine and specialist applications

• Contributions to research shown here by Dr John Griffin, Karen Johnston, Martin Mitchell, Daniel Dawson, Valerie Seymour, Simon Reader and Andrew Miller

• Access to EaStCHEM 35 node research computing facility

• Access to wide range of facilities for synthesis and characterisation of solids including X-ray diffraction and electron microscopy

• Access to 850 MHz UK facility based at Warwick University (SEA is a member of the national management committee)

Water storage in the inner Earth

Characterising dielectric materials

Ceramic materials for waste storage

νr δiso

δaniso

ηCS

CQ

ηQ

OMgSi

OH/F

366 K358 K349 K340 K331 K323 K314 K

375 K

20 10 0 –10 (ppm)

17O solid-state NMR spectrumunresolved due to nuclear spininteractions

Multiple-quantum NMR experimentyields high-resolution allowing

crystallographic sites to be identified

Calculated NMR parametersconsistent with experimentalhigh-resolution NMR data onlywhen dynamic disorder of protonpositions is assumed

Calculated:staticdisorder

Calculated:dynamicdisorderk = 3.2 x105 s–1

O6O2

O7O4 O8

O3

O5O1

O6O2

O7O4 O8

O3

O5O1

1/(T / K–1)

3.5

4.0

5.0

4.5

5.5

0.0027 0.0028 0.0029 0.0030 0.0031 0.0032

ln(Δ

ν SQ

mot /

Hz)

200

–10 –20

30

40 35 30 25 δ1 (ppm) 40 35 30 25

29 28 27 26

–30 δ 31P (ppm)

δ 13C (ppm)

150 100 50 δ 89Y (ppm)

δ 89Y (ppm)

200 150 100 50 200 150 100 50

200 150 100 50 0

0

n S

n N

NN

1

2

3

4

5

6

200 150 100 50 200 150 100 50 200 150 100 50

89Y solid-state NMR

89Y calculated NMR parameters

Hydration of JDF-2

Template disorder in AlPO-53(A)

x = 2 x = 1.6 x = 1.2 x = 0.8 x = 0.6 x = 0

Sn6Sn5Ti1,2-Sn4Ti21,3-Sn4Ti21,4-Sn4Ti21,2,3-Sn3Ti31,2,4-Sn3Ti31,3,5-Sn3Ti31,2-Sn2Ti41,3-Sn2Ti41,4-Sn2Ti4SnTi5Ti6

1,4-Sn4Ti2Sn5Ti 1,2-Sn4Ti2 1,3-Sn4Ti2Sn6

1,3-Sn2Ti41,2,4-Sn3Ti3 1,3,5-Sn3Ti3 1,2-Sn2Ti41,2,3-Sn3Ti3

1,4-Sn2Ti4 SnTi5 Ti6

Insight into microporous solids

29

4

3

2

1

28 27 δ 13C (ppm)

δ 1 H

(ppm

)

Y Ti/Sn

−40−2020 0

−20

−10

20

10

0

−40−2020 0

−20

−10

20

10

0

δ2 (ppm) δ2 (ppm)

δ1 (ppm) δ1 (ppm)

O6O8

O2O9

O3O6

O3O5

O5O5

O7O9

O6O4

O7

O5O1

O8