11B Double Quantum - Single Quantum Correlation Spectroscopy

The 2D ¹³C INADEQUATE method provides double quantum - single quantum (DQ/SQ) correlations and enables one to determine the carbon - carbon skeleton of small organic molecules.  The method is quite insensitive for ¹³C since the natural abundance of ¹³C is only 1.1% and the chance of having two adjacent ¹³C nuclei is only 1 in 8264.  For spins other than ¹³C, for which the natural abundance is high, one expects the sensitivity of DQ/SQ correlation spectroscopy to be much higher.  ¹¹B has a natural abundance of 80.42% and the chance of having two adjacent ¹¹B nuclei in compounds with boron-boron bonds is 1 in 1.55.  ¹¹B NMR spectra are often sufficiently broad due to efficient quadrupolar relaxation such that homonuclear ¹¹B - ¹¹B J coupling is unresolved however the T₂'s are long enough to allow the collection of  2D ¹¹ B COSY and 2D DQ/SQ data.  The figure below shows the 2D ¹¹B DQ/SQ correlation spectrum of ortho-carborane.  The ¹¹B bonding connectivity can be determined easily from the spectrum.

Boron Isotope Effects in Fluorine NMR Spectra

In previous posts on this BLOG, examples of  ¹H/²H and ¹²C/¹³C isotope effects were discussed.  The figure below shows an example of a ¹⁰B/¹¹B isotope effect observed in the ¹⁹F NMR spectrum of tetrabutyl ammonium tetrafluorobarate.

The spectrum clearly shows two resonances separated by 0.05 ppm with an intensity ratio of approximately 20:80 corresponding to the natural abundances of  ¹⁰B and ¹¹B, respectively.  The low frequency resonance is due to ¹¹BF₄^-.  Since  ¹¹B is a spin I = 3/2 nuclide we observe a 1:1:1:1 quartet with J = 1.25 Hz corresponding to the one bond ¹⁹F - ¹¹B coupling.  The high frequency resonance is due to ¹⁰BF₄^-.  Since  ¹⁰B is a spin I = 3 nuclide we observe a very poorly resolved 1:1:1:1:1:1:1 septet with J ~ 0.4 Hz corresponding to the one bond ¹⁹F - ¹⁰B coupling.  

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

11B Background Signals

Unfortunately NMR probes and NMR tubes cannot be "transparent" for all of the isotopes one may want to observe. Depending on the NMR probe, it is very common to have background signals for ¹⁹F, ²³Na, ²⁷Al, ²⁹Si, ⁶⁵Cu, ¹⁰B and ¹¹B. These background signals must be taken into account when interpreting NMR spectra. The background signal for ¹¹B on a Bruker AVANCE 300 with a 5 mm broadband probe is shown in the figure below with several different types of NMR tubes commonly used for routine work in our laboratory. The probe was tuned before running each spectrum and the spectra were collected with proton decoupling. The left hand panel shows the background signal for the NMR probe and the other spectra show the combined background of the probe and the indicated NMR tubes. It is obvious that the magnitude and shape of the ¹¹B background depends on the type of NMR tube used. It should be noted that all of the major NMR tube manufactures offer quartz NMR tubes which have little (if any) ¹¹B background signal.