Solid-state NMR: Basic Principles, Practice, Interpretation and Applications 2015

The spectrum The properties of solid-state spectra are more sample-dependent than solution-state ones. Questions on what is possible to say about a sample using solid-state NMR can usually only be answered with confidence once a spectrum has been obtained. However, the general appearance of a spectrum can give us lots of basic information about the sample. Examples mostly carbon – but applies elsewhere too.

CP Reiterate. Could apply to quadrupoles – particularly for spin ½.

The spectrum

The spectrum – spinning sidebands “Rigid” solid

The spectrum – spinning sidebands Identifying sidebands Look for repeating patterns … … separated by the spin rate Usually lower intensity than centreband – but not always (CN) Depends on spin rate. Second order. Confirm by changing the spin rate nr nr

The spectrum – line width Narrow lines Crystalline (if rigid) Possibly soft (poor CP, no/weak sidebands) <100 Hz

The spectrum - purity No small signals Probably pure (in SSNMR terms) That are not sidebands

The spectrum 19 clear resonances 23 carbon sites Roughly 1:1 One molecule per crystallographic asymmetric unit

The spectrum - identification How can we tell if this is the “right” spectrum?

The spectrum – solid- vs. solution-state Some abiguity but most assignments can be made.

The spectrum – assignment ppm from tetramethylsilane Correlation chart

The spectrum – assignment ppm from tetramethylsilane  ~   

The spectrum – assignment

The spectrum – assignment

The spectrum – assignment COOH 175.4 C= 169.6 CH= 125.9 COH 88.6

The spectrum – assignment tools Interrupted decoupling Dipolar dephasing NQS. Also for 15N. Double signals. One of the pulse sequences with extra X pulse.

The spectrum - polymorphism Observed spectrum Known shifts for polymorph III Omitted

The spectrum - polymorphism Known shifts for polymorph I

Multi-component systems Inevitably, the question of quantification will arise!

Multi-component systems – quantification of a mixture Spectra should be recorded so that both components are at full intensity (fully relaxed). RD = 5×T1H(longest)

Multi-component systems – quantification of a mixture Identify one (or more) pairs of resonances to represent the components Separated – directly integrate intensity. Could use height but line widths might be different (and might vary from sample to sample)

Intensity vs. contact time

T1r “Spin-lattice relaxation in the rotating frame” Where T1 is the relaxation in the intense static field, T1r is the relaxation in the magnetic field associated with an RF pulse T1 T1r Frequency of motion causing relaxation MHz kHz Timescale of relaxation seconds milliseconds

Intensity vs. contact time Different components may have different properties so we need to model the behaviour of the intensity as a function of contact to extract S0. S0 = S (t = 0) is the intensity in the absence of relaxation – and is the value we need to compare intensities from different components. 50:50 mixture. At no contact time do we see the full intensity.

Intensity vs. contact time Equal intensity Extrapolate the long-contact behaviour

Multi-component systems - quantification The alternative is to ignore differences between the components and plot a calibration graph based on samples with known composition. Another inevitability: what are the errors? Error sources: instrument (noise), processing (deconvolution, integration-baseline shape), sampling unknown

Multi-component systems - quantification Only by replicating measurements can you have real confidence in the errors. Error sources: instrument (noise), processing (deconvolution, integration-baseline shape), sampling unknown

Amorphous materials Dn½ = 28 Hz Crystalline Dn½ = 223 Hz Amorphous

Amorphous materials polyethylene polysaccharide Quantification – same issues (more extreme)

Semicrystalline polymers Rigid, ordered T1r(H) filter 2.2 ms 2.2 T1rho filter DP 1s 15us T2 filter

Intensity vs. contact time Delayed contact

Semicrystalline polymers Rigid, ordered T1r(H) filter 2.2 ms Soft DE 1s T2(H) filter 15 ms Rigid, disordered 2.2 T1rho filter DP 1s 15us T2 filter

Dynamic systems

Dynamic systems

 Dynamic systems 3 2 1 Broad – motion? 4 To be consistent with the NMR data, the proton must jump from N1 to N2. This is accompanied by a rotation of the tetrazole ring – interchanging N3 and N4 (and making the start and finish arrangement indistinguishable by X-ray) 

Elements with spin

“Easy” elements

Intensities from overlapping lines - deconvolution

Silicon-29 Direct excitation Quantitative Abundant enough. Zeolite X. Si:Al

Silicon-29 Abundant enough. Zeolite X. Si:Al (1.9).

Deconvolution

Deconvolution Line widths : 220 - 250 Hz χ2 = 129,000 -97.4 -102.4 -107.8 22% 54% 24% Line widths : 220 - 250 Hz χ2 = 129,000

Deconvolution Line widths : 220 - 370 Hz χ2 = 129,000 χ2 = 26,000 -90.3 -93.6 -97.4 -102.4 -107.8 -111.5 -99.1 22% 54% 24% 3% 23% 47% 2% Line widths : 220 - 370 Hz χ2 = 129,000 χ2 = 26,000

Deconvolution Line widths: 150 - 320 Hz, 850 Hz χ2 = 129,000 -90.3 -93.6 -97.4 -102.4 -107.8 -111.5 -99.1 22% 54% 24% 3% 23% 47% 2% 5% 34% 18% (41%) Line widths: 150 - 320 Hz, 850 Hz χ2 = 129,000 χ2 = 26,000 χ2 = 14,000 Crystalline and amorphous components.

Silicon chemical shifts OH Si XO O R OX Si R O

Boron (11B spin-3/2) High sensitivity (11).

Boron (11B spin-3/2) Field dependence.

Boron (11B spin-3/2) Bandshape fit: Cq = 1.45 MHz η = 0 iso = -23.5 ppm Cq = 1.45 MHz, eta=0 (axial symmetry), diso=-23.5 ppm

Aluminium (27Al, 100%, spin-5/2) Framework Extra-framework

Aluminium (27Al, 100%, spin-5/2) 5? Shape - distribution

Aluminium (27Al, 100%, spin-5/2) MQMAS M = 3 Shape.

Heavy metals Range. Locating signal. Centreband. Dilution. Intensity distribution.

Coupling Residual dipolar coupling (between 13C and 14N) Less obvious at higher field. Might be just a broadening. J too small relative to linewidth. Still present under MAS. Residual dipolar coupling (between 13C and 14N)

 Coupling Dipolar coupling – through space – distance measurement “Recouple” – rotor synchronised elements of a pulse sequence e.g. REDOR Labelling. Specific pairs. PAIRS – otherwise many couplings. Triple resonance.

1H Natural “resolution” H-bond Soft solids.

1H Natural “resolution” H-bond

1H 60 kHz

1H 60 kHz HRMAS Physical properties of sample through relaxation.

Solid-state NMR in the UK EPSRC UK National Solid-state NMR Service at Durham. The UK 850 MHz Solid-state Facility at Warwick. You provide the sample we take care of the NMR For the NMR expert