Many thanks to Eric Ye of the National Ultra-high Field NMR Facility for Solids for contributing this post.
Friday, July 31, 2009
A standard QCPMG NMR pulse sequence consists of a 90o pulse followed by a train of 180o pulses. Ideally, the resulting spikelet envelope should outline the static lineshape from a conventiaonal Hahn-echo experiment. If the first pulse deviates from 90o due to incorrect calibration, the QCPMG spikelet pattern does not change significantly, the only effect is somewhat lower overall intensity (first figure). At the same time the miscalibrated subsequent pulses lead to significantly distorted spikelet patterns (second figure). The 180o pulse misset by as little as 20o-30o, could produce considerable oscillations in the spikelet intensity across the envelope. This illustrates that QCPMG NMR experiments are much more sensitive to proper setup of the 180o pulses than the Hahn-echo experiment. The QCPMG spectra shown were calculated in SIMPSON for a central transition of a spin 3/2 nucleus resonating at 295 MHz (87Rb at 21.1 T), CQ=10 MHz, ηQ=0.7, CS anisotropy= -200 ppm, coincidental EFG and CSA tensors, ωRF/2π= 200 kHz.
Tuesday, July 28, 2009
NMR users are very familiar with the advantages of proton decoupling when observing 13C. The 13C NMR spectra of phosphorus containing compounds can be made simpler by applying 31P decoupling either on its own or in addition to proton decoupling. The figure below shows the 13C NMR spectrum of dimethyl methylphosphonate with all possible combinations of proton and 31P decoupling. The data collection required a triple resonance probe with the appropriate band pass filters.
Friday, July 10, 2009
When most people think about HMQC or HSQC spectra they think about protons and 13C or protons and 15N. Although these are by far the most common spins to probe, the HMQC technique can be applied to other spin pairs as well. In earlier posts to this BLOG, this was demonstrated for protons and 11B and 31P and 109Ag. This post shows an application of the technique to 31P and 13C. Measurements made between spin pairs, where one of the spins is not a proton, require a triple resonance probe and the appropriate filters.
The figure below shows 31P detected 31P - 13C HMQC data for dimethyl methylphosphonate with 13C decoupling during the acquisition and 1H decoupling during the entire sequence. The spectra on the top and left-hand sides of the plots are separately run one dimensional 31P and 13C spectra, respectively, both with proton decoupling. The methyl carbon of dimethyl methylphosphonate has 1JCP = 142 Hz whereas the methoxy carbons have 2JCP = 6 Hz. Two HMQC spectra were run. The one on the left was optimized for 142 Hz and the one on the right was optimized for 6 Hz. Each spectrum shows a correlation according to its coupling constant. The data were collected on a 500 MHz instrument with the appropriate bandpass filters on each channel.