The Importance of a Proper Hartmann-Hahn Match in CPMAS NMR

Cross polarization (CP) occurs when the Hartmann -Hahn condition is met by way of the dipolar coupling between the abundant isotope and the dilute isotope. The Hartmann -Hahn match is very sensitive to power levels when the dipolar coupling between the abundant and dilute isotopes is comparable to the MAS spinning speed. The matching curve in this case is separated into a series of minima and maxima. In some materials, the dipolar coupling between the dilute nuclei and the protons can be very dependent on the chemical environment of the dilute nucleus. In such materials, the appearance of the spectrum is very dependent on the quality of the Hartmann-Hahn match. Differences in the spectra as a function of the Hartmann-Hahn matching condition can be used to learn something about the heteronuclear dipolar coupling for different chemical sites. In the figure below, the ²⁹Si CPMAS spectrum is shown for the mineral sepiolite with and without a proper Hartmann-Hahn match. There are three resolved sites with intensity ratio 1:1:1. Once can see that when the Hartmann-Hahn condition is slightly off, the intensity of one of the sites is drastically reduced with respect to the other two.

1H Chemical Shift Referencing in Aqueous Solutions

Typically one uses either dissolved TMS or the residual protons of commercially available deuterated solvents as convenient chemical shift references for solutions in organic solvents. This is not possible for aqueous solutions as TMS is not soluble in water and the 1H chemical shift of water is notoriously dependant on temperature, pH, chemical exchange etc... A common replacement for TMS for aqueous solutions is sodium 2,2-dimethyl-2-sila-pentane-5-sulfonate (DSS). The chemical shift of the three equivalent methyl groups of this compound can be set at 0 ppm.

What is a Magnet Quench?

Superconducting NMR magnets contain a large solenoid coil of superconducting wire in a closed loop. The wire is superconducting (i.e. passes current without resistance) only when cryogenically cooled by liquid helium. On installation, the coil is cooled below its critical point, a current is introduced by way of an external power supply until the specified magnetic field is reached. A superconducting switch is then closed forming a closed loop through which current perpetually flows without the need for an external power supply. Should any part of the wire increase in temperature beyond its critical point, the magnet will quench. During a quench, the wire becomes resistive and therefore generates heat. The magnetic field is lost. The heat boils off the liquid helium very quickly. Magnet quenches can be very dramatic.

To see a 900 MHz magnet quench, follow this link.

Measuring 2H NMR Spectra

Occasionally students will come to the NMR lab with the need to measure the ²H NMR spectrum of a synthetic product they are attempting to deuterate. Every so often, one will come with their product dissolved in a deuterium labelled solvent. This is not a good idea for the same reason it is not a good idea to run the ¹H NMR spectrum of a product dissolved in a protonated solvent. The solvent resonance will be orders of magnitude larger than the solute signal. When measuring the ²H NMR spectrum of a synthetic product, dissolve the product in a regular protonated solvent. Since there is no deuterated solvent, DO NOT ATTEMPT TO LOCK. You will have to shim the magnet as described here or use proton gradient shimming if possible. It is usually possible to observe the natural abundance ²H resonance of the solvent. This signal can be conveniently used as the chemical shift reference. The chemical shift scales are the same for ²H and ¹H, therefore the natural abundance ²H chemical shift of the solvent is (to a very good approximation) equal to the ¹H chemical shift of the same solvent. The figure below shows the ²H NMR spectra of two partially deuterated synthetic products run in chloroform and acetone. The natural abundance ²H signals of the solvents are highlighted in yellow.

Hartmann-Hahn Match as a Function of MAS Spinning Speed

Cross polarization with magic angle spinning (CPMAS) is a means of obtaining high resolution, high sensitivity NMR spectra of dilute isotopes in solids. The dipolar coupling between the protons and the dilute isotope to be observed ( typically ¹³C, ¹⁵N, ²⁹Si etc....) is exploited as a means for magnetization transfer from the abundant protons to the dilute isotope. The transfer is only possible when the product of the gyromagnetic ratio and applied power for the dilute isotope equals the product of the gyromagnetic ratio and applied power for the protons. For static or slow spinning samples, one can vary either the X or ¹H power level during the contact time and one will observe a maximum signal when the Hartmann-Hahn condition is met. The situation is more complicated when a sample is spinning at a rate comparable to or faster than the magnitude of the heteronuclear dipolar coupling used for the magnetization transfer. In such a case the MAS interferes with the dipolar coupling. The effect is that the Harmann Hahn matching curve (intensity vs X or ¹H power) is split into a series of maxima and minima separated by the spinning speed. This is illustrated in the figure below. When fast MAS CP experiments are to be used, it is important to set up the Hartmann-Hahn condition on a maximum for a standard sample at the same spinning speed to be used for the sample of interest.

NOESY vs ROESY for Large Molecules.

NOESY experiments work well for molecules of very low and very high molecular weight. They do not work well for molecules with molecular weights of approximately 1000 - 2000 g/mol at typical field strengths, where the NOE's are very close to zero. A ROESY experiment can be used to get NOE information for molecule in this intermediate molecular mass regime. For high molecular weight molecules a NOESY and a ROESY experiment will give very similar results with the exception that the cross peaks will be in phase with respect to the diagonals for a NOESY and 180 degrees out of phase with the diagonals in the case of a ROESY. The figure below shows both a NOESY and a ROESY for gramacidin at 300 MHz The red contours are negative and the black contours are positive.

Setting the Magic Angle

High resolution solid state NMR employs a technique called "Magic Angle Spinning" where the sample is spun at an angle of 54.736 degrees with respect to the magnetic field at a rate fast with respect to the interactions being averaged. In order to achieve high resolution, the angle must be set very precisely. This is commonly done by looking at the ⁷⁹Br signal of KBr. The intensity of the spinning sidebands for the satellite transition of the I = 3/2 ⁷⁹Br is strongly related to the precision of the magic angle. The sidebands appear as rotational echos in the ⁷⁹Br FID. The more rotational echoes - the stronger the sidebands and therefore the more precise the angle setting. Although any quadrupolar isotope with strong satellite transition sidebands can be used for this purpose, the ⁷⁹Br of KBr is particularly convenient as the resonance is very close to ¹³C, has a short T₁, and can be seen easily in one scan. The spectrometer is set up to scan without adding the signals and the angle adjustment is made while observing the FID until a maximum number or rotational echos is observed.

How to Assess a Used Superconducting Magnet

The superconducting magnet is the single most expensive component of an NMR spectrometer. There is no reason why a properly maintained superconducting magnet should not have a lifetime spanning decades. The console of an NMR spectrometer, on the other hand, begins to fail or becomes obsolete long before the end of the useful life of a superconducting magnet. As a result many laboratories find themselves buying new consoles for existing magnets. Also, when spectrometers are decommissioned, their perfectly good magnets may be available at a very much reduced price as "used". Acquiring a used magnet can represent a huge financial saving when obtaining an NMR instrument. It is useful therefore to address the points to consider when assessing a used magnet. These are outlined here.

1. Cryogen consumption - All NMR laboratories maintain detailed logbooks for the maintenance of their magnets. If you are considering a used magnet it is essential that you have access to these logs. They will detail the date of liquid helium and liquid nitrogen fills as well as quantities of cryogens used and regularly measured cryogen levels. From this information, you will know the boil off rates for both helium and nitrogen. A comparison of these boil off rates to the published boil off rates for new magnets will allow you to do a financial assessment of cryogen costs for the used magnet vs. the capital cost of purchasing a new magnet. It is very common for cryogen boil off rates to slowly increase over time. This is often the result of helium leaking into the vacuum through the magnet seals. The seals of magnets should be replaced periodically (every 10 to 20 years). This should also be considered when evaluating the cryogen boil off for a used magnet. The seals should always be replaced when recommissioning a used magnet.

2. Drift rate - Superconducting magnets are remarkably stable, however they do have measurable drift rates. It is important to know whether or not the drift rate is acceptable for you applications. Many laboratories will periodically measure the drift rate and keep a record of it.

3. Homogeneity - Superconducing magnets have cryoshims which are used to make the magnet homogeneous. This is done by the service engineer during installation. He/she will have noted the linewidth and lineshape (typically of water) the last time the magnet was charged up. The currents in the cryoshim coils will have also been noted. These currents should not be near their maximum values. Magnets are further shimmed by the user with a set of room temperature shim coils which are inserted into the magnet. NMR laboratories will routinely evaluate the lineshape and resolution using the globally accepted lineshape sample. If the same room temperature shim set will be used, this lineshape data should be compared to that for a new magnet. If the magnet is to be used for solids NMR, the homogeneity requirements are not as critical as for liquids NMR.

4. How was the magnet decommissioned? - The magnet should be decommissioned by a qualified service engineer. Magnets that are decommissioned by quenching may have suffered damage as a result of the quench.

5. Shipping - Superconducting magnets are very sensitive pieces of equipment and must be shipped only with the proper shipping restraints. Any magnet moved without the proper restraints should be avoided.

6. Stray magnetic fields - Most new NMR magnets are actively shielded and have very small stray fields. Many older magnets are unshielded and have much larger stray fields. It is essential that you know whether these stray fields are acceptable for your workspace. Unshielded magnets require significantly more space than shielded magnets.

Ultra-high Resolution NMR in the Earth's Magnetic Field

Many NMR spectroscopists take great pride in their ability to shim their very expensive magnets well and acquire NMR spectra with beautiful sharp lines. Despite the skill of the spectroscopist, the line widths are still limited by the homogeneity of the magnetic field. There are NMR instruments designed to measure NMR spectra in the earth's magnetic field. If these instruments are set up outside of any external magnetic interference, then the magnetic field at the sample (the earth's field) is extremely homogeneous and one can obtain very sharp NMR lines - much sharper than those easily obtained in an expensive superconducting magnet. In the figure below, the ¹H NMR spectrum of TMS is shown at 9.4T and in the earths magnetic field. The vertical scale of the spectrum has been increased to show the ²⁹Si satellites. Note that the spectrum obtained in the earth's magnetic field (Stephan Appelt, Holger Kühn, F. Wolfgang Häsing and Bernhard Blümich, Nature Physics 2, 105-109 (2006)) has far superior resolution compared to the one acquired in a well shimmed 400 MHz magnet. Of course, this resolution comes at a price - there is essentially no chemical shift dispersion at such low fields and the sample must be polarized by an external magnet before data are collected.

Field Dependence of 13C Line Shapes for Nitrogen Bearing Carbons in the Solid State

The frequency separation between the two peaks in the 1:2 doublets typically observed for ¹³C nuclei bonded to ¹⁴N in ¹³C CPMAS spectra is inversely dependant on the strength of the magnetic field. As a result narrower resonances are observed for nitrogen bearing carbons at higher field. The figure below shows the ¹³C CPMAS spectrum of D,L-alanine acquired on both a 200 MHz and a 500 MHz instrument.

High Resolution CPMAS NMR of 13C Bonded to 14N

In ¹³C CPMAS spectra, the resonances of ¹³C bonded to ¹⁴N can give unusual line shapes. They often show up as 1:2 doublets which should not be interpreted as seperate resonances. This effect is observed because the dipolar coupling between the spin I=1/2 ¹³C and the quadrupolar, spin I=1 ¹⁴N is not fully averaged by the magic angle spinning as is the case for a pair of spin I=1/2 nuclei. The appearance of the spectrum is a function of the quadrupolar coupling constant for the ¹⁴N, the orientation of the electric field gradient tensor with respect to the internuclear vector, the distance between the ¹³C and the ¹⁴N, the strength of the magnetic field and the ¹⁴N T₁ relaxation time. This effect is not specific to ¹³C and ¹⁴N, but a general effect between and spin I=1/2 and quadrupolar neulei. In the figure below, the 50 MHz ¹³C CPMAS spectrum of D,L-alanine is presented as an example. The resonance shaded and expanded in yellow is the carbon directly bonded to nitrogen.

Homonuclear Decoupling

Decoupling is the application of radiation at one frequency while observing other frequencies. In the heteronuclear case, the decoupling frequency and the observation frequency differ by many tens of MHz and the decoupling radiation can be applied while the points of the FID are being sampled. In the homonuclear case however, the decoupling frequency lies within the spectral width for observation and the decoupling radiation can only be applied during the time between sampling points in the FID (the dwell time). If the radiation is applied while sampling points then the receiver would be overwhelmed by the decoupler, thus obliterating the spectrum. Homonuclear decoupling is an available option on most NMR spectrometers and used to be used very commonly as a means to assign proton spectra. It has largely been replaced by the 2D COSY experiment. The scheme for homonuclear decoupling is depicted in the figure below. The blue spectrum is a standard ¹H spectrum of 3-heptanone. the red spectrum is of the same compound with the protons of carbon 1 irradiated. In the case of the standard spectrum, there is overlap between the protons of carbons 2 and 4. In the spectrum where homonuclear decoupling has been applied, one can clearly see that the protons from carbon 2 are a singlet whereas those from carbon 4 are a triplet.

1D TOCSY for Mixture Analysis

Many two dimensional techniques such as the 2D TOCSY, have similar 1D analogs where selective pulses are used to get effectively only one slice of the comparable 2D experiment. If you are only interested in a few correlations, then the 1D experiments can represent a tremendous time saving ( i.e. money saving). Below is a series of 1D TOCSY spectra run on a mixture of toluene, p-dichlorobenzene and pyridine. In this case, the technique can be used to separate out the components of the mixture. In the figure, the lower trace is the standard ¹H spectrum. In the other traces red arrows indicate the resonance selectively irradiated with the shaped pulse.

Spinning Sideband Suppression in Solid State MAS NMR

In the MAS or CPMAS NMR spectra of solids, it may be difficult to distinguish between an isotropic peak and a spinning sideband or the spinning sidebands from one resonance may overlap with the isotropic peak of others. This tends to complicate the analysis of the data. One simple approach to alleviate these problems is to collect two spectra at different spinning speeds. The sidebands will occur at different frequencies whereas, the isotropic peaks will remain at the same frequency. An alternative to this approach is to use a pulse sequence to suppress the spinning sidebands leaving only the isotropic peaks. The most common of these sequences is the TOSS ( TOtal Suppression of Spinning sidebands) sequence (Dixon et. al., J. Magn. Reson. 49, 341 (1982)). In this approach four properly timed 180 degree pulses are applied before the acquisition of the FID. These pulses have the effect of randomizing the phases of the spinning sidebands while preserving the phase of the isotropic resonances. The figure below illustrates the use of this sequence. The lower trace is the 50 MHz ¹³C CPMAS spectrum of the ethanol inclusion of Dianin's compound with a spinning speed of 2500 Hz. The upper trace is the comparable ¹³C CPMAS-TOSS spectrum.

NOESY: Small Molecules vs Large Molecules

2D NOESY (Nuclear Overhauser Effect SpectroscopY) experiments are used extensively to study the conformation of molecules both small and large. In such experiments correlations exist between protons that are close to one another in space. The intensity of the correlations also depends on the molecular weight of the compound being studied. Fast moving (small) molecules have positive NOE's for protons in close proximity, whereas slow moving (large) molecules have negative NOE's. For molecules of an intermediate mass (1000 g/mol - 1500 g/mol), the NOE's are very close to zero at commonly used magnetic field strengths. The figure below illustrates that in the NOESY experiment, small molecules will exhibit cross peaks of opposite sign to the diagonal (positive NOE's) while large molecules will exhibit cross peaks of the same sign as the diagonal (negative NOE's).

Importing Bruker Data into MestReC 2.3

Many students are still running the old freeware version of MestReC (version 2.3) to process their NMR data. This version of MestReC has a problem importing data collected on Bruker NMR spectrometers with digital receivers (DRU). When the FID is imported and Fourier transformed in MestReC 2.3, the spectrum has an enormous first order phase error as seen, for example, in the left hand panel of the figure below. A huge first order phase correction can be applied within MestReC 2.3 to get a phased spectrum. This is not a problem in later versions of the software.

Finding "Invisible" Protons with 27Al Decoupling

Sometimes when protons are coupled to quadrupolar nuclei, Q, the ¹H signals can range from being sharp singlets, to very broad or "invisible" signals, to sharp multiplets. The appearance of the proton signal depends on the relaxation properties of the quadrupolar isotope coupled to the proton and can also be broadened by chemical exchange. When the relaxation rate between the energy states of the quadrupolar isotope is fast with respect to the coupling constant, J_H-Q, the ¹H signal will be a sharp singlet. When the relaxation rate between the energy states of the quadrupolar isotope is slow with respect to J_H-Q, the ¹H signal will be a sharp multiplet governed by the spin of Q. If the relaxation rate is of the same order as J_H-Q, the ¹H signal will be broad and may even be "invisible". Such is the case for the complex below where a hydride proton is coupled to ²⁷Al. In this case the hydride signal can be "found" by applying ²⁷Al decoupling while observing protons.

1H TOCSY

The TOCSY (TOtal Correlation SpectroscopY) sequence is a homonuclear experiment which produces a COSY-like plot. A COSY will give correlations between protons that are coupled to one another. A TOCSY, on the other hand, will give correlations between all protons in a given spin system. Below is the TOCSY spectrum of 3-heptanone. There are two ¹H spin systems; the protons on carbons 1 and 2 and the protons on carbons 4, 5, 6 and 7. The protons on carbons 1 and 2 have a correlation as they would in a COSY spectrum. The protons on carbons 4, 5, 6 and 7 are all correlated to one another whereas in a COSY spectrum, the main correlations would be between 4-5, 5-6 and 6-7 but not, for example, between the protons on carbons 4 and 7.

Dipolar Dephasing as an Assignment Tool

The dipolar dephasing sequence is routinely used in ¹³C CPMAS NMR as described previously. The lower trace of the figure below is the ¹³C CPMAS spectrum of Dianin's compound which forms an inclusion compound with ethanol. The upper trace shows the ¹³C CPMAS spectrum with a 40 microsecond dephasing delay. All rigid CH and CH₂ carbon signals disappear leaving the quaternary carbons. The methyl signals and the CH₂ signal for ethanol survive the delay as the carbon - proton dipolar coupling is very much reduced due to methyl group rotation and molecular motion of the ethanol, respectively.

13C Cross Polarization with Dipolar Dephasing

A very simple modification to the standard cross polarization sequence is to put a delay at the end of the contact time just before the acquisition where the proton amplifier is turned off. During this delay, the signals for carbons with strong dipolar coupling to protons will decay quickly while those with weak diploar coupling to protons will decay very slowly.The duration of the delay can be chosen such that CH and CH₂ signals will decay entirely while those from quaternary or carbonyl carbons will decay very little. Methyl carbons will behave like carbonyl or quaternary carbons as the dipolar coupling is averaged by the fast rotation of the methyl group. The delay is approximately 30 -50 microseconds when spinning speeds of 3-6 kHz are used. When faster spinning speeds are necessary, the heteronuclear dipolar coupling is more efficiently averaged by the magic angle spinning and the delay must be made longer. For long dephasing delays, a 180 degree ¹³C pulse may have to be inserted at the midpoint of the delay to enable proper phasing. Running a ¹³C CPMAS spectrum with dipolar dephasing will produce a spectrum with only quaternary, carbonyl or methyl carbons present. This method can also be used to show if there is a high degree of molecular motion causing the dipolar coupling to protons to be averaged.

13C Showing J Coupling to Both 31P and 14N

Despite my efforts, some novice users still believe that all ¹³C resonances are sharp singlets. This is indeed the case in proton decoupled ¹³C spectra where there is no additional heteronuclear coupling, exchange or fast relaxation. In the BLOG entries for December 11, 2007 and October 9, 2007 , I showed ¹³C NMR spectra where the carbon was coupled to ¹⁴N and ¹⁹F, respectively. The spectrum below is a ¹³C spectrum where ¹³C couples with ³¹P and ¹⁴N. The yellow region of the full spectrum is expanded in the inset. Carbon 2 is particularly interesting as it shows coupling to both ³¹P and ¹⁴ N.

NMR Books

Stanislav Sykora, creator of Stan's NMR BLOG has compiled a very useful list of almost all the NMR books published to date, complete with the publisher's information and ISBN numbers. If you are looking for a particular book on NMR .... look no further.

National Ultra-high Field NMR Facility for Solids

Ottawa boasts the National Ultra-high Field NMR Facility for Solids, an accessible, well equipped NMR facility housing a Bruker AVANCE 900 MHz NMR spectrometer for solids. The facility is located on the Montreal Road campus of the National Research Council of Canada in building M40. For information on specific probes, the time schedule, the procedures for applying for time, the cost, the annual reports etc... visit the Web site below.

http://nmr900.ca/index_e.html

Proper Data Processing Can Be Very Rewarding

Shimming Without a Lock Signal

Many people who do liquids NMR are at a loss when asked to shim a magnet without the benefit of a deuterium lock signal. Solids NMR people must do this routinely. To shim a magnet without a lock signal, you must set up the spectrometer to observe a strong FID (usually the proton signal is used but a strong narrow signal from any nucleus will do). The instrument must be setup to acquire scans without adding up the signals ("gs" on a Bruker instrument and "fid" on a Varian instrument). While observing the FID adjust the shims until the FID is as long lived as possible while retaining an exponentially decaying envelope. If multiple signals are present, the carrier frequency may have to be changed to get an FID with the "best appearance".

Searching for Small Peaks in 1H Spectra - 13C Decoupling

It is not usually necessary to decouple ¹³C while observing protons as ¹³C is only 1.1% naturally abundant and ¹²C has no spin. The only thing ¹³C decoupling will do for you is collapse the very small ¹³C satellites into single resonances which (aside from a very small isotope effect) will be coincident with the proton resonances for the protons bonded to ¹²C. In certain instances however, ¹³C decoupling is desirable. Such is the case when one is looking for very small peaks in a ¹H spectrum. Below is an example of how such small peaks can be observed more easily when the ¹³C satellites are removed with decoupling. The lower trace is a conventional proton spectrum while the upper trace is the same spectrum with ¹³C decoupling. This is very useful if one is searching for small amounts of impurities underneath the ¹³C satellites of much larger resonances.

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How Can I Get a Quantitative 13C NMR Spectrum?

With conventional acquisition parameters, ¹³C NMR spectra of liquids are rarely quantitative and usually not integrated. There are two main reasons for this. Proton decoupling is normally used to simplify the spectrum and as a way to build up sensitivity as a result of the nuclear Overhauser effect. Also, typical T₁'s for ¹³C are very long. One can still get quantitative data and a proton decoupled spectrum if a very long recycle delay (perhaps 5 minutes) is used and the ¹H decoupling is applied only during the acquisition time which is very short with respect to the recycle delay. Using such parameters however makes the collection of data prohibitively time consuming. To avoid this problem, you can add a paramagnetic relaxation agent to the sample to reduce the relaxation times.

In the spectra below the data were collected with a 10 second recycle delay. In the left hand spectrum, the duty cycle of the decoupler was 100 % and the spectrum exhibits an NOE. In the middle spectrum, the NOE is greatly reduced by decoupling only during the acquisition time (decoupling duty cycle = 10 %), however the long T₁'s prevent the spectrum from being quantitative. The spectrum on the right was acquired similar to that of the one in the middle except some Cr(acac)₃ was added to the sample to reduce the relaxation times for the carbons. This spectrum is quantitative.

Improving Your Baseline Correction

Spectral baselines are often corrected in Bruker's software by using the "abs" command after Fourier transformation. This correction is not always well suited to the data. Another method of getting flatter baselines is to define specific frequency limits "absf1" and "absf2" over which the baseline will be corrected and then use the "absf" command rather than the "abs" command. The smaller the difference between "absf1" and "absf2", the better the correction will be. Below is an example where "absf1" and "absf2" are defined by the region in pink.

31P - 109Ag HMQC

Silver has two spin I = 1/2 isotopes (¹⁰⁹Ag and ¹⁰⁷Ag) with natural abundances of 48.18% and 51.82%, respectively. Although ¹⁰⁹Ag has a lower natural abundance, it is the preferred isotope to observe as its gyromagnetic ratio is slightly higher than that of ¹⁰⁷Ag. Direct observation of ¹⁰⁹Ag is problematic in that the Larmor frequency is low, the NOE's to protons are negative and most importantly, the T₁'s are very long (minutes to hours). In some silver phosphine complexes with ¹⁰⁹Ag - ³¹P coupling, the problems of observing silver directly can be circumvented by using indirect detection via a ³¹P detected ³¹P - ¹⁰⁹Ag HMQC. The advantages of using this approach are that the directly observed ³¹P frequency is 8.7 times higher than that of ¹⁰⁹Ag and the relaxation delay in the experiment depends on the ³¹P T₁'s which tend to be orders of magnitude shorter than those of ¹⁰⁹Ag.

Below is an example of a ³¹P detected ³¹P - ¹⁰⁹Ag HMQC for a silver phosphine complex. These data were acquired without ¹⁰⁹Ag decoupling. In the ³¹P trace on the top of the spectrum one can see a single ³¹P resonance with coupling to both ¹⁰⁹Ag and ¹⁰⁷Ag. The ¹⁰⁹Ag is indirectly observed in the F1 domain.

Better 180 Degree Pulses

Many pulse sequences use 180 degree pulses. Standard hard pulses are not perfect at inverting all of the lines in a spectrum with a wide spectra width. In such cases a better choice may be a shaped adiabatic pulse. A comparison is shown below for the ¹³C spectrum of ethyl acetate. Each spectrum was acquired with the same number of scans. The trace on the left was acquired with a hard 180 degree pulse and the spectrum on the right was acquired with the CRP60comp.4 pulse from the Bruker pulse shape library. Note the better performance resulting in improved nulls.