Thursday, April 21, 2011
Double quantum filters are used to filter out single quantum magnetization and allow the passage of double quantum magnetization. In the proton observe heteronuclear case, the double quantum filter (like the BIRD filter) allows the selective observation of the weak satellite signals from protons coupled to dilute spin I = 1/2 X nuclei (e.g. X = 13C, 15N, 29Si ....) but rejects the strong singlets from the uncoupled protons in the vicinity of 12C, self decoupled 14N, and 28Si . The figure below illustrates the heteronuclear double quantum filter described by Stefan Berger and Siegmar Braun in 200 and More NMR Experiments applied to the 1H NMR spectrum of tetramethylsilane. The bottom trace of the figure shows a conventional 1H NMR spectrum. The middle trace was collected using a 1H-29Si double quantum filter and shows only the 1H-29Si doublet. The top trace was collected using a 1H-13C double quantum filter and show only the 1H-13C doublet. In both cases there is excellent suppression of the 1H singlet signal.
Monday, April 18, 2011
An FT NMR spectrum is obtained by applying a pulse at the Larmor frequency to a sample in a magnetic field. The precession of the spins induces a voltage in the receiver coil which is recorded as a function of time. The Fourier transform of the time dependent signal is the NMR spectrum. What happens if you do not provide any pulses? You might think that you would not observe a signal - but this is not the case. Even without any pulses there is sufficient noise present to allow incoherent precession of the nuclear spins. This precession can be measured and indeed produce an NMR spectrum. This is demonstrated in the figure below. The bottom trace shows a conventional 300 MHz 1H NMR spectrum of ethyl acetate collected with one scan using a 30° pulse. The top trace was collected on the same sample by adding 256 single scan magnitude spectra using no pulses whatsoever. Although very weak, one can clearly see the NMR spectrum of ethyl acetate.
Friday, April 8, 2011
QCPMG has made a tremendous impact on the field of solid state NMR in that it has enabled the collection of data for very broad resonances for unreceptive nuclei. This technique is based on the collection of a train of echoes, the Fourier transform of which produces a "spikelet" spectrum whose intensity envelope mimics the static line shape. The figure below compares the 2H quadrupolar echo spectrum and the QCPMG spectrum of perdeuterated poly-methyl methacrylate. The spectrum contains two overlapping powder patterns; a narrow one from the rotating methyl groups (QCC ~ 56 kHz) and a much less intense broad one from the rigid methlyene deuterons (QCC ~ 170 kHz). It is clear from the figure that the envelope of spikelets in the QCPMG spectrum reproduces quite well the lineshape in the quadrupolar echo spectrum.