Ottawa U Takes Delivery of a Bruker AVANCE III 400

The Bruker AVANCE III 400 NMR spectrometer for solids was delivered to Ottawa U last week and is currently being installed. It will be available for use in February. With the addition of this instrument, the Ottawa U campus boasts 7 NMR instruments (8, including the Bruker AVANCE II 900 on the NRC campus ). For some photos of the installation, follow this link.

Weak One-bond or Multiple Bond Correlations in 1H / 13C HMQC / HSQC Spectra

Many people are quite surprised to see either unusually weak one-bond correlations or weak multiple bond correlations in their ¹H / ¹³C HMQC / HSQC spectra. These people must be reminded that there is nothing "magic" about these experiments - the responses are based solely on an assigned delay proportional a reciprocal coupling constant. The large scale success of the ¹H / ¹³C HMQC / HSQC techniques can be attributed to the fact that most one-bond ¹H - ¹³C coupling constants are very similar ( ~ 145 Hz). The pulse sequences are therefore run with a delay based on a 145 Hz coupling constant. When one-bond coupling constants are significantly different than 145 Hz then the correlation will be either very weak or absent in the spectrum. Also, if multiple bond couplings are unusually large then those multiple bond correlations may be present in the spectrum. The figure below is an example. In the 500 MHz HMQC spectrum of an alkyne (optimized for 145 Hz coupling), one can see an unusually small one-bond correlation between the terminal alkyne proton and its attached carbon. There is also a weak two-bond correlation between the terminal alkyne proton and the other alkyne carbon.

The Effect of Magic Angle Spinning and High Power 1H Decoupling on 13C Cross Polarization NMR Experiments

Cross polarization (CP), magic angle spinning (MAS) and high power ¹H decoupling are all routine methods used in solid state NMR experiments. It is useful to see the effect of each of these techniques on a solid sample. The figure below shows ¹³C cross polarization NMR spectra of glycine at 4.7 Tesla collected with various combinations of magic angle spinning and high power ¹H decoupling.The bottom spectrum was collected with neither MAS nor high power ¹H decoupling. One can see two very broad overlapping lines due to the carbonyl and methylene carbons. The broadening is due to chemical shielding anisotropy and heteronuclear dipolar coupling between the ¹³C and both ¹H and ¹⁴N. The second trace from the bottom was collected with high power ¹H decoupling but no magic angle spinning. The spectrum contains two broad resonances with very informative line shapes. The high power ¹H decoupling effectively removes the ¹³C - ¹H heteronuclear dipolar interaction. The line shapes are determined from the chemical shielding anisotropy and ¹³C - ¹⁴N dipolar coupling interactions. The second trace from the top was collected with magic angle spinning at 4.5 kHz but no high power ¹H decoupling. The spectrum apparently contains only one broad resonance with spinning sidebands. The magic angle spinning effectively removes the ¹³C chemical shielding anisotropy interaction. Although MAS does help average the ¹³C - ¹H heteronuclear dipolar interaction, the averaging is not very effective at a speed of 4.5 kHz. Also, MAS only partially averages the ¹³C - ¹⁴N heteronuclear dipolar interaction. The resonances are therefore broadened out by residual heteronuclear dipolar coupling. The methylene resonance is broadened to such an extent that it does not show up in the spectrum at all. The top spectrum was collected with both MAS and high power ¹H decoupling. One can see two very sharp resonances due to the carbonyl and methylene carbons. The ¹³C chemical shielding anisotropy and ¹³C - ¹H heteronuclear dipolar coupling interactions are effectively removed by the MAS and high power ¹H decoupling, respectively. Since MAS does not average J coupling and only partially averages dipolar coupling between a spin I = 1/2 and quadrupolar nucleus, the methylene carbon shows fine structure due to both J coupling and residual ¹³C - ¹⁴N dipolar coupling (see inset in yellow).

The BIRD Filter

Many modern NMR experiments exploit coupling interactions between protons and heteronuclei (eg. ¹³C). In such sequences the goal is to selectively observe the protons bound to ¹³C and suppress those bound to ¹²C. Since ¹³C is only 1 % naturally abundant, this means that 99% of the signal must be suppressed. One particularly simple scheme to accomplish this is the BIRD (BIlinear Rotation Decoupling) filter. The BIRD filter uses a heteroneuclear spin echo with delays equal to 1/(2¹J_CH) to align the ¹H(¹²C) and ¹H(¹³C) spin vectors along the -y and y axes of the rotating frame of reference, respectively. The 180 degree phase difference between the ¹H(¹²C) and ¹H(¹³C) spin vectors allows a 90 degree pulse to align the these vectors on the -z and z axes, respectively. At this point the ¹H(¹²C) spins are allowed to relax according to their T₁ to the null point. A final 90 degree read pulse puts the ¹H(¹³C) spins in the transverse plane for observation. The first of the two figures below demonstrates the use of the BIRD filter on the lineshape sample. The second figure shows a vector diagram explaining the sequence.

 

HMQC vs HSQC

Proton detected Heteronuclear Multiple Quantum Coherence (HMQC) and Heteronuclear Single Quantum Coherence (HSQC) are both NMR techniques used to correlate the chemical shift of the protons in a sample to a heteronucleus such as ¹³C or ¹⁵N via the J coupling interaction between the nuclei. Since both techniques essentially provide the same information - a correlation map between the coupled spins - students sometimes ask which of these two methods is better and which should they use routinely. The difference between the two techniques is that during the evolution time of an HMQC both proton and X magnetization (eg: X = ¹³C ) are allowed to evolve whereas in an HSQC only X magnetization is allowed to evolve. This means that an HMQC is affected by homonuclear proton J coupling during the evolution period while an HSQC is not affected as there is no proton magnetization during the evolution time. The homonuclear proton J coupling manifests itself as broadening in the X dimension. The top panel of the figure below shows the 7.05 T ¹H /¹³C HMQC and HSQC spectra of menthol with an expansion of one of the resonances highlighted in yellow. One can see that the expanded cross peak of the HMQC is broader in the ¹³C dimension than that of the HSQC. The bottom panel of the figure shows the corresponding ¹³C projection spectra. One can see that the resolution is better in the projection of the HSQC compared to the HMQC. One might conclude that, due to the higher ¹³C resolution, it is always better to run an HSQC rather than an HMQC. This is definitely the case if all of the pulses are calibrated well, however since there are many more pulses in an HSQC compared to an HMQC, it is more susceptible to losses in signal-to-noise-ratio due to poor probe tuning or poor pulse calibration. My advice to students is that, if high ¹³C resolution is required, then make sure the pulses are calibrated well on a well tuned and matched probe and run an HSQC. If high ¹³C resolution is not critical then run an HMQC.

Bloch-Siegert Shifts

Bloch Siegert shifts are frequency differences between NMR signals observed in the presence and absence an rf field applied during the acquisition time. The shifts arise because the applied rf field changes the effective magnetic field experienced by nearby resonances. The resonances are always displaced away from the frequency of the irradiating field. The shift is inversely related to the difference in frequency between the irradiation and the resonance and therefore is generally not observed when heteronuclear decoupling is applied. When homonuclear decoupling is employed these shifts can become significant and are typically used to calibrate the strength of the homonuclear decoupling field. One must be aware of these effects when reporting chemical shifts in homonuclear decoupling experiments. The figure below shows the effect of applying homonuclear decoupling fields of varying strength in the 300 MHz ¹H NMR spectrum of dimethyl acetamide. One can see that the displacement of the resonances is away from the decoupling frequency and that the magnitude of the shift is inversely related to the frequency difference between the resonance and the irradiation frequency.

1H / 27Al TRAPDOR NMR of Kaolinite

TRAPDOR (TRAnsfer of Populations in DOuble Resonance) NMR (Grey and Vega, JACS 117, 8232 (1995)) is a solid state NMR technique where the effects of dioplar coupling between a quadrupolar nucleus and a spin I = 1/2 nucleus can be observed in the spectrum of the spin I = 1/2 nucleus. The technique relies on a rotor synchronized spin echo of the spin I = 1/2 nucleus with CW irradiation of the quadrupolar nucleus during the first echo delay period. The CW irradiation during a single rotor cycle behaves like an adiabatic frequency sweep as the quadrupolar frequencies vary over the course of the rotor cycle. The effects of dipolar coupling between the quadrupolar nucleus and the spin I = 1/2 nuclei, which are normally averaged by MAS, are reintroduced in the TRAPDOR measurement and the complete refocusing of the spin I = 1/2 NMR signal is prevented. The technique therefore can be used to determine whether or not a spin I = 1/2 nucleus is close in proximity to a quadrupolar nucleus. The figure below shows the ¹H / ²⁷Al TRAPDOR NMR spectrum of kaolinite at 11.7 Tesla. The top two traces are conventional rotor synchronized ¹H Hahn echo spectra acquired with MAS rates of 12 kHz and 2.8 kHz, respectively. The bottom trace was acquired at the same spinning speed as the middle trace with CW irradiation of the ²⁷Al during the first echo delay. One can see a very much reduced ¹H echo indicating the presence of heteronuclear ¹H - ²⁷Al dipolar coupling.
This technique can be used to "find" quadrupolar neuclei which are "invisible" by direct detection due to their very large quadrupolar coupling constants.