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 It has largely been replaced by the 2D COSY experiment.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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