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Thursday, June 21, 2018

Glycine as a 13C CPMAS Setup Sample

Glycine is an excellent setup compound for 13C CPMAS NMR measurements.  Its utility in this regard has been described in detail.1,2  It can easily be observed in one scan and has reasonably short 1H T1's, allowing it to be used for 1H 90° pulse calibration and to setup the Hartmann Hahn matching condition.  The width of the methylene carbon signal can be conveniently used to evaluate the proton decoupling efficiency.  The width and shape of the carbonyl signal are very sensitive to the angle at which the sample is spun and can be used to set the magic angle with a reasonably high degree of precision.  In addition the carbonyl resonance is sharp and can be used as a secondary standard for chemical shift calibration.  If used as a secondary chemical shift standard, one must be aware that glycine has three polymorphic forms, each with different carbonyl chemical shifts.  The polymorphic form is not generally displayed on the reagent bottle and different suppliers may provide different polymorphs or mixtures of polymorphs.  It is therefore important to know which polymorph is being used to calibrate the chemical shift scale.   The α and γ polymorphs are the most common and stable, while the β polymorph is less stable and easily converted over time to the α polymorph.  Furthermore, the β polymorph has a very short 1H T at room temperature and therefore difficult to observe with typical millisecond CP contact times.  The γ polymorph can be converted to the α polymorph at 165°C.  The chemical shifts for the carbonyl resonances for the α and γ polymorphs are 176.5 ppm and 174.6 ppm, respectively.1 The chemical shift of the β polymorph is between that of the α and γ polymorphs however, it is not usually observed.  The figure below shows the carbonyl region of the 13C CPMAS spectrum of three samples of glycine: pure α, pure γ and a mixture of the α and γ polymorphs.
If a single carbonyl resonance is observed for a sample of glycine using typical millisecond CP contact times, one can determine if it is the α or γ polymorph by measuring its chemical shift with respect to another chemical shift standard.  Alternatively, since the 1H T characteristics for the α and γ polymorphs are quite different from one another at room temperature, the authors of reference 1 report that a CPMAS spectrum collected with a 20 msec contact time will show almost no signal for the carbonyl carbon of the γ polymorph.  The carbonyl signal of the α polymorph, on the other hand, will be only slightly attenuated compared to a CPMAS spectrum measured with a 1 msec contact time.

1.  M.J. Potrzebowski, P. Tekely, Y. Dusausoy. Solid State Nuclear Magnetic Resonance. 11, 253 (1998).
2. R. E. Taylor. Concepts in Magnetic Resonance. 22A, 1 (2004).

Wednesday, June 20, 2018

Information-Rich 13C Satellites

Seemingly simple NMR spectra often contain much more information than one might think.  For example, the 1H NMR spectrum of 1,4-dioxane is primarily a singlet from which one obtains only an isotropic 1H chemical shift value.  There is however much more information available in the spectrum which is often not recognized or used.  The 1H NMR spectrum of a naturally occurring sample of 1,4-dioxane is the weighted sum of the 1H spectra of all possible isotopomers.  It is the dominant tetra-12C isotopomer that gives rise to the singlet but since 13C (spin I = 1/2) is 1.1% naturally abundant, one expects to observe also the mono-13C isotopomer.  The di-, tri- and tetra-13C isotopomers are very rare and can be neglected.  The symmetry in the mono-13C isotopomer is lost compared to the tetra-12C isotopomer and one obtains a complex second-order spectrum, part of which can be represented by an AA'BB'X spin system.  The spectrum of the AA'BB'X spin system depends on many more parameters than just the isotropic 1H chemical shift.   This is illustrated in the figure below.

The bottom panel of the figure is the measured 300 MHz 1H NMR spectrum of 1,4-dioxane with an exaggerated vertical scale to accentuate the 13C satellites resulting from the protons color coded in pink in the mono-13C isotopomer.  The large central region of the spectrum is the result of all the protons color coded in yellow from both the tetra-12C and mono-13C isotopomers.   A simulation of this second-order spectrum was calculated from the parameters below and is shown in the top panel of the figure. 
Any isotope shifts in the 1H frequencies due to 13C vs 12C bonding were neglected in the simulation.  The fit of the simulation to the 13C satellites is particularly sensitive to 1JC-Ha1JC-Hb3JHa-Hc3JHa-Hd, 3JHb-Hc and 3JHb-Hd and much less sensitive to  2JC-Hc2JC-Hd2JHa-Hb and  2JHc-Hd.  A fit of the simulation to the experimental spectrum produces estimates for all of the coupling constants in the AA'BB'X spin system - much more information than a single 1H isotropic chemical shift!

Wednesday, June 13, 2018

Distortions due to Lock Saturation

The amplitude of the 2H lock signal provides information for an electronic feedback circuit which continuously corrects the magnetic field strength (by way of a B0 shim) to compensate for environmental instability.   A poor 2H lock signal will provide unreliable input for the feedback circuit and B0 compensation will be erratic.  This leads to undesirable effects in NMR spectra.  For example, noisy lock signals will lead to undesirable noise at the base of the observed NMR peaks.  If one uses too much lock power, the 2H lock signal gets saturated and the lock amplitude is unstable.  A saturated 2H lock will lead to problems in the NMR spectrum since the input to the B0 compensation feedback circuit is unstable.  This is demonstrated in the figure below.
When one scan is collected, there are spectral distortions at the base of the NMR resonances.  When 16 scans are collected these artifacts average to produce a general broadening at the base of the NMR resonances.  Be careful not to saturate the 2H lock.

Friday, June 1, 2018

The limitations of 19F GARP Decoupling

In a previous post, it was shown that distorted line shapes are obtained for resonances in broadband decoupled NMR spectra when the resonances of the decoupled nuclide are outside of the effective decoupling bandwidth.  This can be a particularly difficult problem when observing 1H NMR spectra with 19F decoupling.  19F has a large chemicals shift range so, if there are multiple widely spaced 19F resonances, it will be difficult or impossible to decouple all 19F sites at once, particularly at higher magnetic field strengths.  If one is not aware of this problem, data misinterpretation may be an issue as distorted line shapes will  lead incorrect splittings used to measure coupling constants.  The figure below illustrates this problem.  The top three panels of the figure show the 300 MHz 1H[19F] NMR spectra for the three 1H resonances of 1,2-difluoropyridine as a function of the 19F decoupler offset.  The GARP decoupling scheme was used with 90° pulses of 80 µsec.  The decoupler offsets, depicted in the bottom panel of the figure, were varied in 5 ppm increments.
Of the 11 decoupler offsets used, only offset 6 (at -116 ppm) effectively decoupled both 19F sites.  Varying the decoupler offset by only ± 5 ppm leads to distorted line shapes, which are particularly pronounced for the H3 resonance.  These distorted line shapes could easily lead to data misinterpretation and erroneous coupling constants.  In this case, the 19F decoupling bandwidth is 55 ppm.  Since the chemical shift difference between the two  19F resonances is 52 ppm, one is able to obtain a fully 19F decoupled 1H spectrum with the careful choice of the decoupler offset frequency however, there will be cases where the decoupling bandwidth would not be sufficient to decouple all 19F resonances in some molecules.  How then can one generally evaluate all of the coupling constants in fluorine containing molecules?  The 19F-19F couplings can be evaluated in a 19F[1H] spectrum (not shown).  Specific 1H-19F coupling constants can be determined by measuring a 1H PSYCHE spectrum or be collecting 1H spectra with selective 19F continuous wave (CW) decoupling  for each of the19F resonances.  The latter is shown in the figure below.  The bottom panel shows a standard 1H spectrum.  The middle two panels show the 1H spectra for each of the 19F sites decoupled separately using CW decoupling.  The top panel shows the fully 19F decoupled spectrum.
Using these data, all of the coupling constants can be evaluated and are shown in the figure below.
In conclusion, one must be careful in interpreting 1H[19F] spectra and understand the limits of the 19F decoupling scheme used.