Distortions in QCPMG Spectra from Pulse Miscalibrations

A standard QCPMG NMR pulse sequence consists of a 90^o pulse followed by a train of 180^o pulses. Ideally, the resulting spikelet envelope should outline the static lineshape from a conventiaonal Hahn-echo experiment. If the first pulse deviates from 90^o 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 180^o pulse misset by as little as 20^o-30^o, 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 180^o 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), C_Q=10 MHz, η_Q=0.7, CS anisotropy= -200 ppm, coincidental EFG and CSA tensors, ω_RF/2π= 200 kHz.

Many thanks to Eric Ye of the National Ultra-high Field NMR Facility for Solids for contributing this post.

13C NMR with 1H and 31P Decoupling

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.

31P - 13C HMQC

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 ¹J_CP = 142 Hz whereas the methoxy carbons have ²J_CP = 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.