Shaped Pulses

Shaped rf pulses are used frequently in modern NMR experiments for selective excitation and more efficient inversion. The figure below shows some of the pulse shapes in the Bruker shape library measured with an oscilloscope on an AVANCE III console. Each 50.3 MHz rf pulse was 1 msec in duration and was measured at the output of the signal generation unit.

"Absolute" Water Suppression

Collecting ¹H NMR spectra of aqueous samples is complicated by the presence of the enormous, broad water signal which is often many orders of magnitude more intense than the signals from the solute of interest. The water signal can be suppressed by presaturation or multiple pulse techniques employing gradients (such as WATERGATE). These techniques are compared here and do a very good job, but neither is able to completely suppress the water signal in very dilute challenging samples. Recently, a technique introduced by Buuan Lam and Andre Simpson* which uses both presaturation and W5-WATERGATE has been used to suppress the water signal. In this case, the presaturation consists of many low power shaped 180° pulses. The combination of these two techniques provides incredible water suppression. The authors are able to measure the ¹H NMR spectra of dissolved organic matter in natural waters without any preconcentration of the samples. Their spectra are completely free of the water signal! At the 2011 ENC the same group presented a poster** highlighting the water-free ¹H NMR spectrum of the dissolved organic matter in melted glacial ice. The water suppression is so remarkable that this technique should be called "The Simpson Sledgehammer". The figure below illustrates further examples of the implementation of this suppression method. The bottom trace shows the 32 scan ¹H NMR spectrum of a dilute unlabelled protein (which includes the NH resonances, albeit attenuated due to chemical exchange). The middle trace shows the 32 scan ¹H NMR spectrum of supernatant human saliva. The upper trace shows the ¹H NMR spectrum of rain water collected from an asphalt tile roof. The spectrum was collected overnight and shows the presence of long chain hydrocarbons from the asphalt as well as formaldehyde. In all cases the water signal is completely absent in the spectrum.

* Buuan Lam and Andre J. Simpson. Direct ¹H NMR Spectroscopy of Dissolved Organic Matter in Natural Waters. The Analyst 113 263 (2008).

** Brent Pautler, Andre Simpson, Li-Hong Tseng, Manfred Spraul, Ashley Dubnick, Martin Sharp and Myrna Simpson. Trace Level Analysis of Dissolved Organic Matter in Glacial Ice Using SPR-W5-WATERGATE. POSTER 264, ENC (2011).

Heteronuclear Double Quantum Filters

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.

FT NMR Spectra Without Pulses

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.

QCPMG for 2H NMR

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.

MAS Rotor Crashes

There is no sound more pleasing to a solids NMR spectroscopist than that of an MAS rotor spinning stably. What happens though when the spinning is abruptly interrupted? This is called a rotor "crash" and when it occurs not only is the pleasing sound replaced by the horrible sound of rushing air, but one often finds damage to both the MAS rotor and the NMR probe. The picture below shows what used to be the solenoid coil of a 4 mm MAS probe after an unfortunate rotor crash. The rotor and sample were reduced to dust. Fortunately in this case the stator was not destroyed.

Here are a few tips to avoid expensive rotor crashes:

1. Spin only as fast as needed for your experiment. Just because your car can go 200 km/h does not mean that it should be driven at 200 km/h. Likewise, just because your probe is rated to spin samples at 15 kHz does not mean that all samples should be spun at 15 kHz.

2. Check the integrity of the rotor and the cap before use. Damaged rotors are weakened and should NEVER be used. Damaged caps can cause instability which may lead to a rotor crash.

3. Make sure the cap fits snugly on the rotor. A cap that lifts or comes off the rotor while spinning will cause a rotor crash.

4. Be aware of sample heating due to spinning. The rotor and sample heat up during spinning due to friction. If the temperature increases such that your sample melts or emits a gas, the rotor may become unbalanced or the cap may be forced off causing a rotor crash.

5. Make sure the rotor is marked properly so the speed can be monitored and regulated. Failure to do this may result in the spin counter receiving a bogus signal and it is possible that the rotor may spin faster than its rated speed causing a crash.

6. Pack your sample evenly to ensure that the rotor is properly balanced during MAS. Rotors that do not spin smoothly and stably should be repacked until they do.

7. Start and stop the rotor gradually to ensure stability while speeding up or slowing down.

Two New Twitter Feeds to Follow

Two new and very useful Twitter feeds have recently appeared online. The first is by Luke O'Dell called solidstateNMR. It posts links to publications related to solid state NMR as soon as they appear online. The second is by Victor Terskikh called nmr900 which posts news and information from Canada's showpiece high field solids NMR facility, The National Ultrahigh-Field NMR Facility for Solids.

Stay up-to-date and check them out!

Excitation Profiles for Shaped Pulses

Shaped pulses are very commonly used for selective excitation and nonselective inversion in a large number of NMR pulse sequences. The frequency domain excitation profile of a radio frequency pulse is the Fourier transform of the time dependent pulse shape and determines the width, uniformity and phase of the frequency spectrum excited. Since time and frequency are reciprocals of one another, short rf pulses have very wide excitation profiles and long rf pulses have very narrow selective excitation profiles. In a previous BLOG post the excitation profiles of rectangular pulses of varying duration were determined experimentally. The Fourier transform of a rectangular pulse is a sinc function which is observed to be the experimentally determined excitation profile. Short high power rectangular pulses are very desirable for uniform excitation over wide frequency ranges as the entire NMR spectrum of interest usually occupies only a very small central portion of the central lobe of the sinc shaped excitation profile which, to a first approximation, is flat over the observed spectral width. Rectangular pulses are often not desirable when narrow excitation profiles are required as the excitation is not uniform over the desired region and the ripples of the sinc excitation profile cause nodes of excitation and negative peaks.

The Fourier transform of a sinc function is a box function, so if a long low power sinc shaped rf pulse is used one obtains a narrow flat box shaped excitation profile. This is indeed the case as can be seen in the bottom trace of the figure below. The trace is composed of a series of ¹H NMR spectra of H₂O/D₂O where a truncated, 200 msec, 10 cycle sinc shaped monochromatic rf pulse was applied with varying rf frequency offsets. One can see that the excitation profile is, to a good approximation, a narrow flat box function. The deviation from a flat box function is the result of the truncation of the sinc pulse. It should be noted that the phase of the resonances is not constant across the excitation profile. Sinc pulses are not frequently used for selective excitation because of the phase problem and the fact that very long, minimally truncated pulses must be used. A frequently used alternative to the sinc pulse for selective excitation is the Gaussian shaped pulse. The Fourier transform of a Gaussian is a Gaussian and one therefore will obtain a narrow Gaussian shaped excitation profile when a long low power Gaussian shaped pulse is used. This is shown in the top trace of the figure below. This trace is similar to the bottom trace except a 20 msec Gaussian shaped pulse (with truncation at 1 % of the total height) was used rather than a 200 msec sinc pulse. Although the excitation is not flat, the phase is constant across the excitation profile and the total duration of the pulse is 10 times shorter than the sinc pulse.

First-Order Phase Errors

The phase of a signal in an NMR spectrum is described here and is determined by the axis on which the magnetization vector resides after the observe pulse relative to the receiver. The phase of the spectrum is typically corrected such that the peak in the real spectrum is entirely in absorption mode while that in the imaginary spectrum is entirely in dispersion mode. The correction in phase is referred to as the zero-order phase correction. A zero-order phase correction applies to all peaks in the spectrum regardless of their offset, Ω, from resonance. There are also first-order phase errors where the phase error for a resonance is linearly dependant on Ω and the duration of the pulse, t_p. The further a peak is away from the center of the spectrum for a given pulse duration, the larger the first-order phase error. Similarly, the longer the duration of the pulse for a peak of given offset, the larger the first-order phase error. These errors arise from the fact that a 90° pulse of fixed duration is only a true 90° pulse for a peak on-resonance. For off-resonance peaks, the pulse will not only be slightly less than 90° but also produce a small amount of magnetization along the axis of the pulse. For example, a 90°_x pulse applied to an equilibrium magnetization vector will produce only –y magnetization for an on-resonance signal but for an off-resonance signal, it will also leave a small amount of residual z magnetization as well as produce some x magnetization. It is the x magnetization that gives rise to the first order phase error. The effect of the pulse duration for a peak of given offset frequency is demonstrated in the figure below. The sample was a mixture of chloroform and acetone. The transmitter was set such that the chloroform signal was on-resonance. Six 90° pulses were calibrated at different power levels based on the on-resonance chloroform signal, ranging from 10 µsec in the lower trace to 320 µsec in the upper trace. For all spectra, the same zero-order phase correction was applied such that the chloroform signal was in-phase. No first-order phase correction was applied. One can see that the phase error in the off-resonance acetone signal increases as a function of the pulse duration. It should also be noted that the overall intensity of the acetone signal decreases with respect to the chloroform signal as a function of the pulse duration due to the width of the excitation profile decreasing as a function of the pulse duration.

Agilent's MR BLOG - SPINSIGHTS

Recently Agilent Technologies has started a MR BLOG called "SPINSIGHTS". The posts to this BLOG are written by Agilent employees and discuss the advances of Agilent's MR products as well as educational posts on both MRI and NMR spectroscopy. It is a very interesting read and promises to be a very useful resource. Thanks Agilent - keep it up!

"The MRI Photocopier"

A very interesting note* has recently been published in Concepts in Magnetic Resonance Part A by Joseph Hornak (generous provider of the two excellent free online text books, The Basics of NMR and The Basics of MRI ). In this work, it is demonstrated that an MRI scanner can be used to reproduce an image of printed text. The technique is based on the fact that the toner used to print text contains ferromagnetic particles and when placed in the uniform magnetic field of an MRI magnet, the magnetic field near the ferromagnetic toner is distorted. The "sample" is prepared by placing the printed page face up on a polycarbonate plate and covering it with a very thin sheet of polyethylene. The polyethylene is covered with water (doped with CuSO4 as a relaxation agent). The "sample" is placed in the magnet such that the plane of the page is perpendicular to the magnetic field. The text is "scanned" by taking a 2D proton MR image of a slice of the water near the surface of the polyethylene. The distortion in the magnetic field due to the ferromagnetic toner renders the text visible in the 2D magnetic resonance image. This is demonstrated in the figure below (from the reference) where a 2.5 mm thick slice above the polyethylene was imaged. The text was printed in #36 Arial font. The author speculates that this technique may be applied to the analysis of paintings.
* Joseph P. Hornak, "Magnetic Resonance Imaging of Printed Text", Concepts in Magnetic Resonance Part A. 36A, 347 (2010).

Sample Slice Selection in NMR Spectroscopy

In MRI, field gradients are used routinely for slice selection while imaging a sample. Gradients are also finding applications in high resolution NMR spectroscopy employing fast data collection techniques using parallel data acquisition for multiple slices of the same sample. A good example of this can be found in Carlos' BLOG (and the references therein). The slice selection is accomplished by turning on a linear field gradient across the sample while applying an excitation pulse. While the gradient is on, the frequencies of each of the NMR resonances is spread out according to length of the sample and the strength of the field gradient across the sample. A particular individual slice of the sample can be selected by either modifying the strength of the gradient while using a semi-selective pulse of a given excitation frequency or by modifying the offset frequency of the semi-selective pulse for a constant gradient strength. Both the gradient strength and the excitation profile of the pulse determine the thickness of the slice selected. When the gradient is turned off and the receiver turned on, the FID representing only the spectrum of the selected slice of the sample is collected. An educational example is shown in the figure below. A sample of toluene and H₂O/D₂O was prepared. As these two liquids are immiscible, the sample is layered with the less dense toluene on top and the more dense water on the bottom. The bottom trace in the figure shows a conventional ¹H NMR spectrum. Since the pulse used to collect the spectrum was a hard 90° pulse with a wide excitation profile, one can see both the toluene and the water in the spectrum. For the middle and upper traces, a field gradient of 48 G/cm was turned on while a 100 µs 90° Gaussian excitation pulse was applied. The only difference between the middle and upper traces is the offset frequency used in the excitation pulse. The middle trace represents the spectrum of a slice of the water in the bottom layer of the sample and the upper trace represents the spectrum of a slice of the toluene in the top layer of the sample.

NMR and Candy Canes

The Phase of an NMR Spectrum

Most students know that the phase of an NMR spectrum has to do with the degree to which the NMR resonances are above or below the baseline of the spectrum (i.e. the amount of absorption and dispersion character). Most students have also learned that the phase of a periodic time domain function depends only on the value of the function at time zero. Thus, the only difference between a cosine and a sine function is where the function starts at time zero. A sine is said to be 90° out of phase with respect to a cosine. Many students do not understand the connection between the phase of their NMR spectrum and the phase of the periodic time domain function giving rise to their spectrum by way of a Fourier transform. The figure below is an attempt to make the connection.

The left-hand portion of the figure shows equilibrium magnetization vectors being rotated by radio frequency pulses. 90° pulses along the x, y, -x and -y axes rotate the z magnetization vector to the -y, x, y and -x axes of the rotating frame, respectively according to the right-hand screw rule. After the pulse, the magnetization vector rotates in the rotating frame of reference at a frequency equal to the difference between the transmitter frequency and the frequency of the NMR resonance. In the figure, the magnetization is assumed to be rotating anti-clockwise representing an NMR resonance with a positive frequency with respect to the transmitter frequency. The NMR spectrometer measures the time dependent voltages on two of the four orthogonal axes of the rotating frame separated by 90° (quadrature detection). The time dependent voltages are proportional to the amount of magnetization on the axis as a function of time. One of the two time dependent voltages is called the "real" signal and the other is called the "imaginary" signal. Together these two signals make up the complex free induction decay (FID) which is Fourier transformed to produce the NMR spectrum. In the figure, the -y(t) voltage is the real FID and the x(t) voltage is the imaginary FID. The figure shows representations of the real and imaginary FIDs after the delivery of pulses along each of the orthogonal axes. Note that the phases of the real and imaginary FIDs depend on the pulse delivered. For example, after a 90°_x pulse, the magnetization resides on the -y axis. The real FID (along the -y axis) starts at a maximum (cosine) and the imaginary FID (along the x axis) starts at zero (sine) and increases as the magnetization vector rotates anti-clockwise. The Fourier transform of the complex FID produces a real spectrum (typically the one displayed to the user) and an imaginary spectrum (typically not displayed to the user). Note that the degree of absorption vs. dispersion character in the spectrum depends on the phase of the FID signals. If an NMR spectrum is not in phase, perhaps due to a receiver dead time problem, it can be corrected after the collection of the data by calculating the phase angles needed to put the real spectrum enitirely in absorption mode and the imaginary spectrum entirely in dispersion mode.

New Bench Top 1H NMR Spectrometer

A very interesting product has just been put on the market from a company called picoSpin. - a 7 lb bench top 45 MHz ¹H NMR spectrometer at a very tempting price. From the information available on their website (including a YouTube video), it looks as if this instrument would be an excellent teaching tool in a university laboratory.








This is not meant as a product endorsement.

NMR and Food Chemistry - Coffee

Many of us enjoy a hot delicious cup of coffee. Many (not including myself) will even spend a great deal of money and a great deal of their time in a long queue to get it! The coffee snobs will preach about the differences between a freshly brewed cup of java and the lowly cup of instant coffee. Is there really a difference? Spectrum A in the figure below shows the ¹³C CPMAS NMR spectrum of ground coffee. Among other things, the spectrum consists of resonances from the woody fibers, polysaccharide gums, tannins, alkaloids and aromatic oils. Spectrum B is the spectrum of used coffee grounds which were subsequently dried. The spectrum is due to the woody fibers and other insoluble material. Spectrum C is the difference between spectrum A and B and represents all the "goodness" of a fresh brewed cup of coffee. Among other things, it represents all of the alkaloids, tannins and aromatic oils. Spectrum D is the ¹³C CPMAS NMR spectrum of instant coffee which is manufactured by freeze drying brewed coffee. The spectrum is very similar to the difference spectrum, C. This indicates either that the difference between a freshly brewed cup of coffee and a cup of instant coffee lies in very low concentration constituents or that the coffee snobs are wasting their money and time in long queues. I think both are true.

13C NMR of "Perdeuterated" Solvents

When one acquires a ¹³C NMR spectrum of a sample, the deuterated solvent is observed with resonances characterized by the J coupling pattern of the deuterons attached to the carbon atoms. (CD = 1:1:1 triplet, CD₂ = 1:2:3:2:1 quintet, CD₃ = 1:3:6:7:6:3:1 septet). Often, however; small peaks are observed near the main solvent resonances. These are due to other isotopomers of the solvent. An example of this is shown in the bottom spectrum of the figure below for the high frequency resonance of "THF-d₈". The spectrum consists mainly of the expected 1:2:3:2:1 quintet however, there is a small peak present on the high frequency side of the quintet marked by the arrow. It is due to one of the components of the C1 resonance from THF-1,2,2,3,3,4,4-d₇. The supplier of the solvent claims that the isotopic purity is 99.5 atom % D. If the deuteration is uniform, approximately 0.5 % of the molecules will contain a single proton. Half of these molecules will be mono-protonated on the high frequency carbons. If a DEPT spectrum is acquired on the same sample, the non-protonated carbons are suppressed leaving only the mono-protonated carbons. This is shown in the top spectrum of the figure, which shows a 1:1:1 triplet from CHD. The isotope shift between CHD and CD₂ is 0.354 ppm. Other examples can be found here and here.

Deuterium Decoupling

Bruker NMR spectrometers equipped with a ²H TX board and ²H amplifier can be setup to carry out ²H observation and decoupling experiments while still maintaining a ²H lock. Two channel systems can use the ²H transmitter for ²H, ¹H[²H], ²H[¹H] and X[²H] measurements. In addition to these, a three channel system can be set up for X[²H, ¹H] measurements. The first figure below shows the ¹³C and ¹³C[²H] spectra collected for a mixture of deuterated solvents using two channels of an AVANCE spectrometer. The second figure shows the ¹³C, ¹³C[¹H], ¹³C[²H] and ¹³C[¹H, ²H] spectra for the standard ¹³C sensitivity sample (40% para-dioxane and 60% deuterobenzene) collected on a three channel Bruker AVANCE spectrometer.

HSQC - TOCSY

There are many different NMR techniques used to probe molecular structure. Two very useful methods are the HSQC (or HMQC) sequence used to obtain heteronuclear coupling correlations and the TOCSY sequence used to look at extended homonuclear coupling correlations. These are among the most widely used pulse sequences by chemists to help elucidate the molecular structure of organic molecules. The sequences can also be combined into a single HSQC-TOCSY sequence which is simply an HSQC followed by a TOCSY spin lock. For the case of ¹H and ¹³C in organic molecules, the HSQC-TOCSY spectrum provides ¹H TOCSY subspectra of the isolated spin systems in the rows at each of the ¹³C frequencies. Overlap of ¹H resonances which may have appeared in a simple ¹H TOCSY spectrum can be resolved in the HSQC-TOCSY spectrum provided there is enough shift dispersion in the ¹³C spectrum. An example of this is shown in the figure below. The top panel shows the HSQC-TOCSY spectrum for 3-heptanone collected on a 300 MHz instrument. One can see the two spin systems on either side of the carbonyl group, color coded in yellow and pink. The second panel shows an expansion of the region within the red square of the first panel. Here we can separate the overlapping quartet and triplet for the methylene protons on either side of the carbonyl group.

Temperature Calibration - An Alternative Method

It is well known that the actual temperature of a sample in an NMR probe is not necessarily the same as that read from the variable temperature unit on the spectrometer. This is because the thermocouple used by the variable temperature unit is below the sample tube and not in the center of the rf coil where the NMR measurements are made. One normally must make a calibration plot for the actual temperature vs. the set temperature. For temperatures above room temperature this can be done by employing the known temperature dependent chemical shift difference between the two proton resonances of ethylene glycol (see this link). At temperatures below room temperature, the same measurement can be made for the known temperature dependent chemical shift difference between the two proton resonances of methanol. The actual temperature is determined from the chemical shift difference and plotted against the temperature read from the variable temperature unit. One potential problem with this method is that the resistance of the magnet shim coils change slightly with temperature affecting the shim currents and the NMR line shapes of the resonances. This makes it difficult to measure a precise chemical shift difference. In order to obtain reliable results, the magnet must be reshimmed at each temperature.

I have used is a very simple alternative method for calibrating the temperature of the sample compared to that of the variable temperature unit. This is illustrated in the figure below.A "sample" is prepared by pushing a NONMAGNETIC thermocouple through an NMR tube cap. The depth of the thermocouple is adjusted such that when the cap is put on the NMR tube, the tip of the thermocouple sits in the center of the rf coil. The NMR tube should contain a suitable liquid filled to the correct depth. The tube is placed in the spinner and set to the proper depth with a depth gauge. While holding onto the thermocouple, the sample is lowered into the magnet until it sits correctly in the NMR probe. The thermocouple is connected to a digital thermometer (some of these devices can use a second thermocouple in an ice water bath as a reference). The desired temperature is set on the variable temperature unit. When the temperature on both the variable temperature unit and digital thermometer have stabilized (~ 10 minutes), the values from each are recorded. This is repeated for temperatures over the desired temperature range and a calibration plot is constructed. Shimming is not an issue. Note that no NMR measurements are made and that the sample tube is not spinning.

Solid State NMR of Half Integer Quadrupolar Nuclei

Many students who do liquid state NMR or solid state NMR of spin I=1/2 nuclei have very little appreciation for the information content and complexity of the solid state NMR spectra of spin I = n/2 quadrupolar nuclei (n= 3, 5, 7....). In part, I think this may be due to the mathematics involved with explaining the important effects. With this post, I attempt to describe the NMR spectrum of an I = 5/2 nucleus in the soild state without resorting to mathematics. I hope that this post helps to boost the understanding and appreciation for the NMR spectra of these important nuclei.

When one collects the NMR spectrum a spin I = n/2 nucleus in solution, one normally observes a single NMR resonance line for each species as the (n+1) Zeeman energy levels are equally spaced and the nucleus is undergoing fast isotropic motion. The width of the resonance depends upon the efficiency of the relaxation (usually dominated by the quadrupolar interaction). The more efficient the relaxation, the broader the resonance. In many cases, the width of the resonance may be comparable to the complete chemical shift range for the nucleus and in some cases it may be so broad as to make its observation impractical or impossible. For these reasons I = n/2 quadrupolar nuclei generate less than their share of interest in liquid state NMR compared to spin I = 1/2 nuclei like ¹H, ¹³C, ³¹P ..... etc.

In the solid state, the situation is much different as there is rarely fast isotropic motion. When the quadrupolar interaction is much less than the Zeeman interaction, the Zeeman energy level diagram is affected by both a first order and a second order quadrupolar perturbation as shown in the figure below for an I = 5/2 nucleus. The second order perturbation is much smaller than the first order.The energy levels are no longer equally spaced. Furthermore, the position of the resonances resulting from the transitions depends on the orientation of the nucleus with respect to the magnetic field. Since all orientations are represented equally in a powder sample, each transition gives a powder spectrum. This is illustrated on the right-hand side of the figure. Note that the m = 1/2 and m = -1/2 energy levels are affected equally to first order and that the central (m = 1/2 - m = -1/2) transition is affected only by the second order perturbation. As a result the line width of the resonance from the central transition is much narrower than the other transitions. (In the figure, the central transition is clipped significantly.). Unlike the first order interaction, the second order interaction is field dependent. The width of the central transition depends inversely on the strength of the magnetic field.

Magic angle spinning is a technique used by solid state NMR spectroscopists to obtain high resolution NMR spectra of solids. Magic angle spinning at infinite speed completely averages the first order quadrupolar interaction but only partially averages out the second order interaction. The energy level diagram for a spin I = 5/2 nucleus spinning infinately fast at the magic angle is shown in the figure below along with a simulated spectrum. The spectrum consists of a central transition, CT, and two satellite transitions, ST1 and ST2. Note that along with a lineshape due to the orientational dependence of the nucleus in the magnetic field, there is also an isotropic quadrupolar shift. The central and satallite transitions are not at the same frequency. This effect is completely separate from the chemical shift.
In practice, we cannot spin at infinite speed, however, we can often spin at a rate fast with respect to the width of the central transition. If the quadrupolar coupling constant is small enough, the central transition will be observed and affected only by the second order interaction. The satellite transitions, affected by both the first and second order interaction, are observed as a manifold of spinning sidebands. The intensity of the centerbands for the satellite transitions is greatly attenuated as the overall intensity is spread among all of the sidebands. Often the centerbands for the satellite transitions are so small in comparison to that of the central transition that they are not observed. This is illustrated with simulations in the lower portion of the figure below. For comparison, the upper portion shows similar simulated spectra without magic angle spinning. The spectra highlighted in yellow are expansions of the central portion of the spectrum. These simulations are also supported by observations.

Heat Dissipation in Bruker AVANCE Spectrometers

It is very important that an NMR spectrometer operate within a fixed temperature range. To control the temperature, many units within the console are equipped with cooling fans which must be kept in good working order. Failure to do so will shorten the life of the spectrometer and cause instability or malfunctions. In the Bruker AVANCE series of spectrometers, the SGU (Signal Generation Unit) boards are particularly sensitive to temperature. Each SGU houses a DDS (Direct Digital Synthesizer) containing numerically controlled oscillators which regulate rf frequencies and amplitudes. The DDS is normally temperature regulated at 55° C to insure stability. Both the SGU board temperature and DDS temperature can be measured either with the "UniTool" (AVANCE and AVANCE II) or web based tools (AVANCE III).

After years of service, two of our Bruker AVANCE spectrometers were running very warm. Using "UniTool" to check the temperature of the SGUs revealed that the board temperatures averaged 58° C! The DDS temperatures were all >55° C and not regulated. The air filters in the console doors were removed and cleaned and all eight fans of the AQS unit, housing the SGU's, were replaced on each spectrometer. After a day for the instruments to come to a thermal steady state, the SGU and DDS temperatures were again measured. The SGU board temperatures averaged 49°C. The DDS units in three of the five SGU's were now regulated properly at 55°C. The other two were still > 55°C and unregulated. In an attempt to lower the temperature further, the backs of the spectrometers were removed and again the instruments were allowed to reach a thermal steady state over a 24 hour period. The SGU and DDS temperatures were measured again. This time the SGU board temperatures averaged 38°C and all five DDS units were regulated properly at 55°C. The SGU temperatures in our AVANCE II and III spectrometers (with backs on) did not exceed 40°C and the DDS temperatures were regulated properly.

Although this may not be recommended by Bruker, I now run our AVANCE spectrometers with the back panels removed.

19F - 13C HMQC

If one has an NMR spectrometer with hardware capable of synthesizing and amplifying the frequency of ¹⁹F from the ¹H channel and a broadband NMR probe whose ¹H channel can tune down to ¹⁹F, then one is able to do ¹⁹F - ¹³C HMQC experiments. The figure below shows an example of a ¹⁹F - ¹³C HMQC spectrum collected on a Bruker AVANCE 500 NMR spectrometer using a 5 mm broadband probe. The fluorine spectrum is plotted on the top and the ¹³C[¹⁹F] spectrum is plotted on the side. The panel on the left shows the spectrum optimized for one-bond coupling, while that on the right shows the spectrum optimized for two-bond coupling. Note that the protonated carbons are doublets as proton decoupling is not possible in this configuration. The large signal, off scale in the ¹³C[¹⁹F] spectrum is due to the solvent (benzene-d₆).

E.COSY and the Relative Signs of Coupling Constants

Spin-spin coupling constants can have values greater than or less than zero. The absolute sign of the coupling constants cannot be discerned from the simple examination of a ¹H NMR spectrum. The E.COSY¹ (Exclusive COrrelation SpectroscopY) technique is one method which can be used to determine the relative signs of coupling constants. E.COSY is a phase sensitive COSY variant which produces off-diagonal signals showing only the active coupling (i.e. the coupling directly responsible for the cross-peak) as 2x2 antiphase square tetrads displaced in both the F2 and F1 domains by an amount equal to the passive coupling constants (i.e. the couplings not directly responsible for the cross peak). The slope of a line drawn through the cross-peaks is used to determine the relative signs of the passive coupling constants. The sign of the slope depends on whether the signs of the passive couplings are the same or whether they differ. The figure below shows the gradient E.COSY spectrum² (using the Bruker pulse program,"ecosygpph" ) for the ABX three-spin system in phenylalanine.The cross-peaks highlighted in yellow in the top panel of the figure are expanded in the bottom panel. Black contours are positive and red contours are negative. The panel on the bottom left shows the cross-peaks for AX and BX. In the case of the AX cross-peak, the line drawn through the cross peak has a positive slope indicating that the passive couplings J_AB and J_BX are of opposite sign. The line drawn through the BX cross peak also has a positive slope indicating that the passive couplings J_AB and J_AX are of opposite sign. From this we can deduce that J_AX and J_BX are of the same sign. The panel on the bottom right shows the cross-peak for AB. In this case, the line drawn through the cross peak has a negative slope confirming that the passive couplings J_AX and J_BX are of the same sign. In conclusion, the geminal and vicinal coupling constants are of opposite sign.

1. C. Griesinger, O.W. Sorensen & R.R. Ernst, J. Magn. Reson. 75, ; 474 - 492 (1987).

2. The ecosygpph pulse program produces spectra similar to those described in reference 1 as "complimentary" E.COSY spectra. The slope of the lines through the cross peaks in "complementary" E.COSY spectra are of opposite sign to those obtained from the E.COSY spectra described in reference 1.

Job Opening: NMR Technician, University of Ottawa

The Faculty of Science of the University of Ottawa is seeking a technician for its very active Nuclear Magnetic Resonance (NMR) facility. This rapidly growing NMR facility serves more than 25 active research groups and consists of 7 NMR instruments ranging in field from 200 to 500 MHz. The facility conducts a wide range of modern NMR experiments on both solids and liquids as well as MRI. As the NMR technician, you must hold at least a B.Sc. degree in chemistry or a related field (M.Sc. preferred) with at least two years experience in either one or both high resolution or solid state NMR spectroscopy. You must be enthusiastic, highly motivated and work well both independently and in groups. Extensive computer experience is mandatory. As you will be interacting daily with students and faculty, you must have excellent interpersonal skills as well as excellent oral and written communication skills. Reporting to the NMR facility manager, you will collect routine NMR data as a service, assist students and answer questions, train new users of the equipment, help with equipment maintenance (including cryogen fills), maintain a data archive, keep accurate time and usage records, and assist the NMR facility manager in the general operation of the facility. French/ English bilingualism is a definite asset. The position is open for one year starting August 15, 2010 and will be renewable, availability of funds permitting. The annual salary range for this position is $38,862 - $51,504. For consideration, please email your CV and cover letter to the Human Resources Service mycareer@uottawa.ca and refer to competition number TSCI9583NEP. Please also have three letters of recommendation emailed or sent to:

Dr. Glenn A. Facey
NMR Facility Manager,
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa, Ontario, K1N 6N5
CANADA
ph. (613) 562-5800 ext 6077
fax. (613) 562-5170
email. gafacey@uottawa.ca

The deadline for applications is Friday June 25 at 5:00 pm.

More information is available at the following web site:

http://www.hr.uottawa.ca/careers/support/competition/cont/sci/TSCI9583NEP.php

Retrieving Empty Spinners From Magnets

Despite my best efforts in teaching students how to properly use NMR spectrometers, invariably a new user will, on occasion, drop a sample spinner into a magnet without a sample. This is usually done in our undergraduate laboratory by inexperienced students who have finished collecting their data and feel a need to return the empty spinner to the magnet. The problem is that ejecting the empty spinner with the eject air is not possible as the air passes through the spinner. In the past I have removed empty rotors from magnets on Varian spectrometers by removing the upper barrel of the magnet. On Bruker spectrometers, I have removed the probe and inserted a semi-rigid NONMAGNETIC plastic hose in the bottom of the magnet and pushed the spinner to the top of the upper barrel where it can be removed by a helper. In either case, the entire operation took between 5 minutes (Varian) and 30 minutes (Bruker) depending on how much reshimming was required. To avoid this tedious chore, I had our machine shop make a plastic tool for empty rotor removal shown below.

This tool consists of a pole with a diameter slightly less than that of the spinner. On the end of the pole is a tapered prong with a diameter of 4 mm at the tip and 6 mm at the base. This tool is inserted in the top of the magnet and gently lowered until the prong gets wedged into the empty spinner. The spinner is removed with the tool. The entire operation takes less than 10 seconds and no reshimming is required.

Gradient Spin Echoes for Selective Excitation

Shaped excitation pulses can replace the non-selective hard pulses typically used in a one-pulse measurement to achieve selective excitation. Another method of achieving selective excitation is the gradient spin echo using a selective 180° pulse. This technique is demonstrated in the figure below. A non-selective hard 90°_x pulse is first given followed by a pair of identical pulsed field gradients sandwiching a soft selective 180° pulse about the y axis. The hard 90° pulse rotates all spin vectors onto the -y axis. During the first gradient pulse the spin vectors dephase and evolve according to their offset frequencies. The soft 180°_y pulse flips a single resonance 180° about the y axis leaving all other resonances untouched. During the second gradient pulse, the "selected" resonance is rephased and its offset frequency evolution is refocused. The unselected resonances dephase more and continue to evolve according to their offset frequencies. The receiver is then turned on to collect the FID of the "selected" resonance, all others are dephased and therefore suppressed. This is demonstrated in the figure below which shows ¹H NMR spectra for a mixture of methylence chloride and acetone. The bottom trace shows a standard one-pulse measurement. The middle and top traces show results from a selective gradient spin echo measurement with the selective 180° pulse set for methylene chloride and acetone, respectively.

Background Suppression in Liquids

High resolution NMR probes for liquids may contain parts near the coil consiting of the nuclei being observed. The parts give rise to background signals which can severely affect the NMR data. When observing¹¹B, there is a background signal from boron containing parts near the coil and also the borosilicate glass in the NMR tube containing the sample.

Cory and Ritchey* introduced a very simple, clever method to suppress background signals in 1988. Their method uses a composite pulse, consisting of a 90° and two 180° pulses with appropriate phase cycling, in place of a conventional 90° pulse. The phase cycled composite pulse is essentially a 90° pulse for all spins inside the coil and 0° for all spins outside of the coil. An example of its implementation is shown in the figure below. The bottom traces show the ¹¹B [¹H] NMR spectra for a dilute sample of NaBH₄ and a "real" synthetic sample on the left and right, respectively. One can see an enormous background signal from both the NMR probe and the NMR tube. In the case of the "real" synthetic sample, the information from the spectrum is difficult or impossible to recover. The top traces show similar spectra acquired using the composite pulse. The only background signal remaining is that from the portion of the NMR tube inside the coil. This pulse sequence (without proton decoupling) is in the Bruker pulse program library called "zgbs". It is not exclusive to ¹¹B.

D.G. Cory and W.M. Ritchey. Journal of Magnetic Resonance, 80, 128 (1988).

Hahn Echo for 11B Background Suppression in Solids

Solids NMR probes often contain boron rich parts near the coil in which the sample resides. Boron nitride, in particular, is a material very commonly used. This can be very problematic if one wishes to collect ¹¹B NMR data, in that a strong background signal may be observed. Even though these parts are not directly inside the coil with the sample, they do experience a small amount of rf from the coil and the coil does detect a ¹¹B NMR signal from them. One simple way to avoid this problem is to use a Hahn echo to observe the ¹¹B spectrum. Unlike the sample inside the coil, which experiences the 90° and 180° pulses required for the Hahn echo, the boron rich parts outside of the coil experience pulses of much smaller flip angles and therefore the echo signal from them is much reduced. This is illustrated in the figure below, which shows ¹¹B NMR spectra collected in a 4 mm MAS probe without magic angle spinning. In the lower trace a simple one-pulse measurement was made with high power ¹H decoupling. The spectrum contains a very large background signal and it is very difficult to extract any useful information from the data. The spectrum in the upper trace was collected with a Hahn echo with high power ¹H decoupling during the acquisition. The background signal is completely removed revealing a beautiful, information-rich line shape for the central transition resulting from the second order quadrupolar and chemical shielding anisotropy interactions.

Thank you to Joseph Weiss of David Bryce's laboratory for providing the sample and collecting the data. See more of Joseph's beautiful ¹¹B spectra here:

Joseph W.E. Weiss and David L. Bryce J. Phys. Chem. 114 (2010) ASAP.
http://dx.doi.org/10.1021/jp101416k

Dummy Scans

Dummy scans (DS, Bruker) or steady state scans (SS, Varian) are scans taken in an NMR acquisition before the receiver is turned on and data are collected. Each dummy scan contains all of the rf pulses, delays and gradients used in the pulse program; the only difference is that the receiver is not turned on to collect data. Dummy scans are typically used to ensure that a spin system is in a steady state before data are collected. For example, One may collect a spectrum using a relaxation delay short with respect to the T₁'s of some of the resonances in the spectrum. The first scan may find the system at equilibrium, as the sample may have sat in the magnet for a few minutes while the probe was being tuned and matched or the magnet was being shimmed. The second and subsequent scans will find the system not at equilibrium, as the system has been perturbed by the rf pulses of the preceding scan(s) and not allowed to fully relax. The data from the first and seccond scan are therefore not the same. After several scans, although not at equilibrium, the system will be in a steady state before each additional scan and therefore each subsequent scan will collect similar data. An example of the use of dummy scans is shown in the figure below. The bottom trace shows a single scan ¹³C NMR spectrum of a concentrated solution of menthol in CDCl3 after the sample had sat in the magnet for a minute or so. The top trace shows a similar single scan acquisition preceded by 16 dummy scans. The relaxation delay was set to 1 second and the acquisition time for the FID was 1 second. 90° pulses were used in both spectra. One can see that some of the resonances in the top spectrum are attenuated in comparison to the bottom spectrum. The ¹³C signal from the CDCl₃ (which has a long T₁) is missing in the spectrum acquired with dummy scans. The dummy scans have selectively presaturated the solvent.