EPSRC UK National Solid-state NMR Service at Durham Solid-state NMR: Basic Principles, Practice, Interpretation and Applications 2015 Practice

EPSRC UK National Solid-state NMR Service at Durham The Spectrometer Naming of the parts …..

EPSRC UK National Solid-state NMR Service at Durham The Experiment

EPSRC UK National Solid-state NMR Service at Durham Routine set up of a CPMAS measurement Assumptions: Spin-½ nuclei A standard sample (with known behaviour) is available CaHPO 4 ·2H 2 O hexamethylbenzene (HMB) glycine adamantane tetrakis(trimethylsilyl)silane (TTMSS) brushite The spectrometer is operating correctly and is in a state such that only fine tuning (checking/calibration) is required

EPSRC UK National Solid-state NMR Service at Durham The magic-angle Advantages: 79 Br resonance is close to 13 C T MHz vs for 13 C) No decoupling required Shimming not critical Good after repair or maintenance when angle badly set Method 1 – 79 Br, maximising the rotary echoes/sidebands from KBr

EPSRC UK National Solid-state NMR Service at Durham The magic-angle Method 1 – maximising the rotary echoes/sidebands from KBr time / ms centreband on resonance  rotary echo (separation = rotor period) AcquireAdjust FID (Free Induction Decay)

EPSRC UK National Solid-state NMR Service at Durham Glossary Convention: high frequency low frequency (low field) (high field) Fourier transform (FT)

EPSRC UK National Solid-state NMR Service at Durham Glossary Repetitions transients “scans” NMR is not a sensitive technique so we repeat our experiment many times, adding the results. 1 repetition 40,000 repetitions Signal-to-noise ratio S/N

EPSRC UK National Solid-state NMR Service at Durham The magic-angle Method 1 – maximising the rotary echoes/sidebands from KBr Acquire – adjust – acquire …….  time / ms

EPSRC UK National Solid-state NMR Service at Durham (Acquire) n – adjust – (acquire) n ….. gives more precision n = 0.2 s, 45 o 4 kHz (maybe a problem) The magic-angle Good for most samples … … unless they contain highly anisotropic environments (>C=O, C-D, >M-)

EPSRC UK National Solid-state NMR Service at Durham

Excitation methods Direct excitation: Pulse on X, observe on X Cross polarisation: optional

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation Transfer of magnetisation (polarisation) Usually 1 H to n X 1H1H nXnX

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation Almost all measurements are …. pulse(s) – acquire – wait – pulse(s) – acquire – wait ….. RD depends on T 1 X DE RD recycle delay recycle pulse delay d1

EPSRC UK National Solid-state NMR Service at Durham Spin-lattice relaxation T 1 : Spin-lattice relaxation time constant Equilibrium magnetisation at time t after excitation (which will contribute to the signal after the next excitation) T 1 : ultimately depends on molecular motion and is different (in principle) for each nuclide and each type of environment

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation RD depends on T 1 X DE RD

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation Repetition rate depends on T 1 H CP Repetition rate depends on T 1 X DE

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation T 1 C : different for each carbon. Average value (excluding CH 3 ) 190 s. T 1 C (CH 3 ) is approximately 1 s. T 1 H : a single, molecule-wide average is measured 1.7 s

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation Another advantage for CP over DE: For efficient (100%) magnetisation transfer there is a signal enhancement of : magnetogyric ratio (physical constant for every nuclide) nuclide  / 10 7 radT -1 s -1 enhancementtime saving factor 1H1H P C Si N

EPSRC UK National Solid-state NMR Service at Durham Cross polarisation vs. direct excitation CP: 448 repetitions, 3 s recycle 22 minutes DE: 448 repetitions, 3 s recycle 22 minutes DE: 448 repetitions, 120 s recycle 15 hours ×2

EPSRC UK National Solid-state NMR Service at Durham

Pulse angles, RF field calibration Even complicated pulse sequences are built up from pulses with well defined tip angle (30°, 45°, 90°, 180° ….) 90° 180° low power element modulated decoupling high power element

EPSRC UK National Solid-state NMR Service at Durham Terminology and relationships RF is the frequency at which it is applied (MHz) 1 is the nutation rate produced by the pulse (kHz) 1 =  B 1 /2  = 1/(4  90 ) V pp = 2√(2P × 50  ) or V pp = 20√P V pp : volts (peak-to-peak), P: power P(dB) = 10 × log 10 (P 1 /P 2 ) for dBm P 2 = 1 mW 90 o pulse 1 B 1 ( 13 C) 3  s 83.3 kHz7.8 mT 400 W50 dBm53 dBm 400 V100 W200 W 56 dBm200 V283 V (Bulk magnetisation, rotating frame) Pulse angles, RF field calibration

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration (H-channel) 1. Set amplitude 2. Vary duration 2. Observe signal amplitude

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration Build up a nutation “curve”

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration Nutation “curve”  90 = (19-9.6)/2 = 4.7  s absolute value

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration The sample is not excited uniformly through the rotor (coil) – rf inhomogeneity

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration Nutation “curve”  90 = (19-9.6)/2 = 4.7  s

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration It is important for the system to fully relax between successive increments of the pulse duration. The nutation curve should approximate to a damped sine wave.

EPSRC UK National Solid-state NMR Service at Durham Pulse angles, RF field calibration (X-channel) X relaxation can be slow (much slower than 1 H), so calibrating X by direct excitation can take time …. but there is a short cut

EPSRC UK National Solid-state NMR Service at Durham Shimming For solid-state NMR, once a good set of shims have been obtained for a probe they do not need much adjustment. [In practice, load the shims appropriate to the probe before starting any calibration.]

EPSRC UK National Solid-state NMR Service at Durham Shimming Z1 Z2 Z3 Z4 X Y ZX ZY XY X 2 -Y 2 Z 2 X Z 2 Y ZXY.

EPSRC UK National Solid-state NMR Service at Durham Referencing “Spectral referencing is with respect to an external sample of neat tetramethylsilane (carried out by setting the high-frequency signal from adamantane to 38.5 ppm).”

EPSRC UK National Solid-state NMR Service at Durham Referencing Nuclide Reference (chemical shift / ppm) Nuclide Reference (chemical shift / ppm) 1H1HAdamantane (1.9) 7 LiLiCl (0) 13 C Adamantane (38.5, CH 2 ) Glycine (176.5, COO ) 11 BBF 3 /O(CH 2 CH 3 ) 2 (0) 15 N NH 4 15 NO 3 (–5.1, NO 3 ) Glycine (–346.8) 17 OH 2 O (0) 19 FC 6 F 6 ( –164.9) 23 NaNaCl (0) 29 Si tetrakis(trimethylsilyl) silane (–9.8,–135.4) 27 AlAl(NO 3 ) 3 (0) 31 PCaHPO 4.2H 2 O (1.0) 45 ScSc(NO 3 ) 3 (0) 77 Se(NH 4 ) 2 SeO 4 (1040) 51 V  -NaVO 3 (–519) (solid) 119 SnSn(C 6 H 12 ) (–97.4) 59 CoK 3 Co(CN) 6 (0) 199 Hg [Hg(dmso) 6 ][O 3 SCF 3 ] 2 (–2313)

EPSRC UK National Solid-state NMR Service at Durham Interaction between nuclear quadrupole moment and an electric field gradient

EPSRC UK National Solid-state NMR Service at Durham Quadrupolar nuclei energy levels

EPSRC UK National Solid-state NMR Service at Durham Quadrupolar nuclei The quadrupole coupling (χ or C q ) may be large (relative to the Zeeman interaction) and this complicates the response of the system to an RF pulse. To further complicate things, different environments may have significantly different χ values and therefore behave differently with respect to a pulse.

EPSRC UK National Solid-state NMR Service at Durham Quadrupolar nuclei Response expected for a spin-½ nuclide Response obtained for a quadrupolar nuclide Calibrate on a solution (except for relaxation, unaffected by quadrupole coupling )

EPSRC UK National Solid-state NMR Service at Durham Quadrupolar nuclei For 27 Al (spin-5/2) a 30 o pulse (as calibrated on a solution) would give maximum signal (central transition only fully affected) Response will behave like a spin-½ system Intermediate – complex response. Use a small angle (short pulse) [1  s < 25 o ] If:

EPSRC UK National Solid-state NMR Service at Durham

Matching Hartmann-Hahn match condition: 1H1H nXnX

EPSRC UK National Solid-state NMR Service at Durham Matching In practice do this first – it has more impact on signal than minor mis-set of 1 H 90 o duration. For adamantane the match is very dependent on spin-rate.

EPSRC UK National Solid-state NMR Service at Durham Matching At low spin-rate a ramp about “0” position is adequate

EPSRC UK National Solid-state NMR Service at Durham Matching Hexamethylbenzene (HMB) is more tolerant of spin rate (up to about 10 kHz) Typically the ramp is:

EPSRC UK National Solid-state NMR Service at Durham Matching At high spin-rate the match profile can break up into a “sideband” pattern The match condition no longer holds and is modified to: The upshot is that the centre of the ramp needs to be repositioned. Revary

EPSRC UK National Solid-state NMR Service at Durham Matching

EPSRC UK National Solid-state NMR Service at Durham System (signal-to-noise ratio) test Using HMB with well-defined acquisition parameters:

EPSRC UK National Solid-state NMR Service at Durham Starting from scratch (David’s method) For systems in an unknown state, probes after repair, installation of new equipment. Start with low RF powers and work upwards. Can be useful to observe output with an oscilloscope or power meter to understand how the instrument power control maps onto real output. For example, we have two power controls: (a)Goes from -16 to 63 (nominally dB) (b)Goes from 0 to 4095 (linear) So, 63(4000) is approximately full power (say 1 kW) 60(4000) = ½ power, 60(2000) = ¼ power …… 48(2000) = approximately 2% of full power – should be very safe. [Test this with an oscilloscope/power meter. Note high powers may need to be attenuated to protect the measuring equipment.]

EPSRC UK National Solid-state NMR Service at Durham Starting from scratch (1) Using adamantane: Observe 1 H. Calibrate 90 o pulse. Work upwards towards specification (e.g. 2.5  s – see your manual). Line will be very broad (with sidebands). (2) Observe 13 C (directly) with decoupling (now you know a safe 1 H setting). Calibrate 90 o pulse – as above. Recycle T. Broaden lines to mask poor line shape. (3) Using KBr: Set angle (use 13 C power settings). (4) Using HMB (or admantane): Fine tune the match (remember ). (5) Using adamantane: Shim and reference. (6) Using HMB: Test S/N (note for future reference).

EPSRC UK National Solid-state NMR Service at Durham

The magic-angle (2) The resonance from any species in an anisotropic environment is potentially sensitive to the angle of the spinning axis, e.g. the nitrate signal from ammonium nitrate.

EPSRC UK National Solid-state NMR Service at Durham The magic-angle (2)  NH 4 15 NO MHz. CP: 1 repetition [30 s], contact 8 ms, spin rate 5 kHz, 6 mm rotor, acquisition time 100 ms. Shims, cross polarisation, decoupling required

EPSRC UK National Solid-state NMR Service at Durham The magic-angle (3) Deuterium (spin-1) spectra can be extremely sensitive to the angle.

EPSRC UK National Solid-state NMR Service at Durham The magic-angle (3)  Alanine-2-d MHz CP: 16 repetitions, 2 s recycle delay, contact 1 ms, spin rate 8 kHz, 4 mm rotor

EPSRC UK National Solid-state NMR Service at Durham The magic-angle Recap: o For most measurements setting the angle with KBr is sufficient. o Be wary of odd line shapes for resonances from highly anisotropic species. o For some experiments the angle must be set with very high precision, e.g. 2 H, STMAS (satellite transition MAS) for quadrupolar nuclei. If a suitable set up compound cannot be found it may be necessary to set the angle on the sample/experiment itself. KBr is a cubic crystal, the bromine environment is isotropic If we need anisotropy to observe rotary echoes/sidebands where does this come from?

EPSRC UK National Solid-state NMR Service at Durham Nuclear Magnetic Resonance The precession of magnetisation induced by the static field and manipulated by a radiofrequency pulse produces a voltage detected by the coil.

EPSRC UK National Solid-state NMR Service at Durham The sample itself (recycle) QuantitativeBest S/N

EPSRC UK National Solid-state NMR Service at Durham The sample itself (contact time) Best S/N

EPSRC UK National Solid-state NMR Service at Durham The sample itself Recycle optimised Contact time optimised Optimise decoupling? Check spin-rate/sideband positions Check acquisition time GO!

EPSRC UK National Solid-state NMR Service at Durham Nitrogen-15 (from an organic compound) Optimisation of the acquisition conditions rarely feasible CP is only viable method (at natural abundance). Repetition rate depends on T 1 H (from carbon). Good spectra are often those where a long contact time can be used.

EPSRC UK National Solid-state NMR Service at Durham