We use a variety of specialized pulse sequences and methodologies to conduct our solid state NMR experiments.
If you are interested in obtaining copies of any of these sequences, we now have them available for Bruker (Topspin, Avance II and III consoles) and Agilent/Varian (VNMRj and SpinSight) NMR spectrometers. Contact me to obtain the most recent versions of these sequences… we would be more than happy to help you set them up and get your experiments going!
If you are interested in obtaining copies of any of these sequences, we now have them available for Bruker (Topspin, Avance II and III consoles) and Agilent/Varian (VNMRj and SpinSight) NMR spectrometers. Contact me to obtain the most recent versions of these sequences… we would be more than happy to help you set them up and get your experiments going!
QCPMG - Quadrupolar Carr-Purcell Meiboom-Gill
The QCPMG pulse sequence is utilized to enhance signal to noise for nuclei possessing broad NMR patterns and suitably long transverse relaxation time constants (T2). The sequence features a 90o pulse to get things started, followed by a series of alternating 180o pulses and acquisition periods.
Larsen, F.H.; Jakobsen, H.J.; Ellis, P.D.; Nielsen, N.C.: Sensitivity-enhanced quadrupolar-echo NMR of half-integer quadrupolar nuclei. J. Phys. Chem. A 1997, 101, 8597-8606.
WURST-CPMG
The WURST-CPMG pulse sequence [1] features two elements: WURST (wideband uniform-rate smooth truncation) pulses [2] are used for broadband excitation and refocusing, and the CPMG echo train (similar to the one in the QCPMG sequence above) is used for continuous refocusing and signal capture. The WURST pulses are both phase and amplitude modulated - in particular, we use so-called WURST-80 pulses. This sequence is ideal for efficient excitation of broad patterns for both spin-1/2 and quadrupolar nuclei, and acquisition of ultra-wideline (UW) NMR spectra.[3,4]
[1] O'Dell, L.A.; Schurko, R.W.: QCPMG using adiabatic pulses for faster acquisition of ultra-wideline NMR spectra. Chem. Phys. Lett. 2008, 464, 97-102.
[2] (a) Kupce, E.; Freeman, R.: Adiabatic pulses for wideband inversion and broadband decoupling. J. Magn. Reson., Ser. A 1995, 115, 273-6. (b) Bhattacharyya, R.; Frydman, L.: Quadrupolar NMR spectroscopy in solids using frequency-swept echoing pulses. J. Chem. Phys. 2007, 127.
[3] Schurko, R.W.: Acquisition of Wideline Solid-State NMR Spectra of Quadrupolar Nuclei. In Encyclopedia of Magnetic Resonance; Wasylishen, R.E., Ashbrook, S.E., Wimperis, S., Eds.; John Wiley & Sons, Ltd., 2012.
[4] O'Dell, L.A.; Rossini, A.J.; Schurko, R.W.: Acquisition of ultra-wideline NMR spectra from quadrupolar nuclei by frequency stepped WURST-QCPMG. Chem. Phys. Lett. 2009, 468, 330-335.
BRAIN-CP (BRoadband Adiabatic INversion - Cross Polarization)
The BRAIN-CP sequence is used for broadband cross polarization. The 1H channel operates just like a conventional CP experiment…the interesting stuff happens on the X channel. First, during contact pulse, the WURST A swept pulse serves two purposes: (i) to allows for transfer of polarization from the abundant 1H nuclei, and (ii) to store this polarization along the effective field, and by the end of the pulse, store this polarization along the -z axis. The WURST B pulse is broadband, and rotates the stored spin polarization into the transverse plane for detection (at this point, signal can be observed!). However, if you want further signal enhancement, the WURST-CPMG sequence can be tacked on at the end, and works just as described above! In fact, the whole sequence pictured below is the BRAIN-CP/WURST-CPMG pulse sequence (which is quite a mouthful). We just call it BRAIN for short.
Harris, K.J.; Lupulescu, A.; Lucier, B.E.G.; Frydman, L.; Schurko, R.W.: Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy. J. Magn. Reson. 2012, 224, 38-47.
Frequency-Stepped Spectral Acquisitions
Even with the broadband pulses utilized in the sequences above, we sometimes have spectra that are so broad, that they must be acquired "in pieces". This was first suggested by Massiot et al. [1], and expanded upon by Frydman et al.[2], Ellis and co-workers,[3] and our group.[4] When utilized with QCPMG type experiments, the offsets between transmitter settings must be optimized so that the spectra are acquired efficiently, and there are no gaps in spectral intensity. Furthermore, they transmitter spacings must be equal to integer multiples of the spikelet spacings that arise from FT of the CPMG echo train FID.
Below, a beautiful example of a 35Cl WURST-CPMG NMR spectrum of dichlorobis(acetonitrile) palladium, a transition-metal chlorine complex, acquired at 21.1 T (NRC, Ottawa) is shown (thanks to Karen Johnston for acquiring this spectrum!). Amazingly, this powder pattern represents a single Cl site, and is over 2.5 MHz in breadth!!
Below, a beautiful example of a 35Cl WURST-CPMG NMR spectrum of dichlorobis(acetonitrile) palladium, a transition-metal chlorine complex, acquired at 21.1 T (NRC, Ottawa) is shown (thanks to Karen Johnston for acquiring this spectrum!). Amazingly, this powder pattern represents a single Cl site, and is over 2.5 MHz in breadth!!
[1] Massiot, D.; Farnan, I.; Gautier, N.; Trumeau, D.; Trokiner, A.; Coutures, J.P.: 71Ga and 69Ga NMR of Beta-Ga2O3. Solid State Nucl. Magn. Reson. 1995, 4, 241-248.
[2] Medek, A.; Frydman, V.; Frydman, L.: Central transition NMR in the presence of large quadrupole couplings: 59Co NMR of cobaltophthalocyanines. J. Phys. Chem. A 1999, 103, 4830-4835.
[3] Lipton, A.S.; Smith, M.D.; Adams, R.D.; Ellis, P.D.: 67Zn solid-state and single-crystal NMR spectroscopy and X-ray crystal structure of zinc formate dihydrate. J. Am. Chem. Soc. 2002, 124, 410-414.
[4] (a) Hung, I.; Schurko, R.W.: Solid-state Zr-91 NMR of bis(cyclopentadienyl)-dichlorozirconium(IV). J. Phys. Chem. B 2004, 108, 9060-9069. (b) Tang, J.A.; Masuda, J.D.; Boyle, T.J.; Schurko, R.W.: Ultra-wideline 27Al NMR investigation of three- and five-coordinate aluminum environments. ChemPhysChem 2006, 7, 117-130.