Wednesday, August 24, 2011
Super-conducting NMR magnets are cooled with liquid helium. The boil off rate of the liquid helium is minimized by surrounding it with a high vacuum and a liquid nitrogen cryostat. The liquid nitrogen cryostat may either be vented directly to the atmosphere or it may be maintained at a pressure slightly above atmospheric pressure with the use of pressure relief valves. NMR users should be aware that the homogeneity of an NMR magnet is affected drastically by the pressure in the liquid nitrogen cryostat. The left-hand panel of the figure below shows a well resolved 400 MHz 1H NMR spectrum of the methyl triplet of ethylbenzene in a well shimmed magnet where the nitrogen cryostat was maintained slightly above atmospheric pressure with pressure relief valves for several days. The spectrum in the right-hand panel was run under identical conditions after the cap to the nitrogen fill port was removed, relieving the pressure in the nitrogen cryostat.
Thursday, August 11, 2011
When the chemical shift difference between two J coupled nuclei is of the same order as the coupling constant, second order spectra are obtained. See this and this. One, often unrecognized, second order effect is virtual coupling which is often misinterpreted as first order weak coupling. In a three-spin system, virtual coupling occurs when the observed nucleus appears to be coupled to both of the other two nuclei even though it is only coupled to one of them. This arises in AA'X and ABX spin systems when X (the observed nucleus) is coupled to only one of the other two strongly coupled spins. This is illustrated in the figure below. The figure consists of simulations of X in an AA'X spin system as a function of JAA' with JAX set at 10 Hz and no coupling between A' and X. Clearly, the spectrum of X is affected by the coupling between A and A'. When JAA' = 0, a first order doublet is observed with a coupling constant of 10 Hz. As JAA' increases, complicated second order multiplets are observed. When JAA' = 50 Hz (or more) a "virtual triplet" with a coupling constant of 5 Hz is observed. This appears to be identical to a 1:2:1 triplet in a first order spectrum with a coupling constant of 1/2 JAX. It is however a second order spectrum and should not be misinterpreted as first order weak coupling. An example of this is illustrated in the figure below. The figure shows the 13C NMR signals for the ipso and ortho aromatic carbons of 1,2-bis(diphenylphosphino)ethane (DPPE). These carbon atoms are coupled to the nearest phosphorus but not to the remote phosphorus. The two phosphorus atoms are strongly coupled to one another. The ortho carbons appear as a "virtual triplet" and the ipso carbons, a second order multiplet.
Wednesday, August 10, 2011
Over the years, I have made much use of the Bruker Almanac. My bookshelf still holds several copies from the past 20 years or so. For those who have never seen a copy, it contains a great deal of useful information for NMR, EPR, ENDOR, IR and mass spectrometry. The NMR information includes, useful NMR formulas, chemical shift tables for some of the commonly observed isotopes, reference compounds, frequency tables etc... Recently Bruker has released its almanac in an iPod / iPhone app. Not only can you use your iPod / iPhone to take a course on Fourier transforms but you can also use it as an NMR reference book. The Bruker Almanac app is a very useful tool making it convenient to have a great deal of NMR information in your pocket - and at the console where it is most often needed. How often have you awakened at 2:18 am. and wondered what the chemical shift range is for 99Ru? Well .... now you can quickly look it up and go back to sleep. Below are three screen shots from the app. Good job Bruker!
Friday, August 5, 2011
Rod Wasylishen is dear to the hearts of the entire Canadian NMR community and to many worldwide. Not only are his scientific achievements stellar, but he has mentored an entire generation of Canadian NMR spectroscopists with his contagious curiosity and positive approach to research. All who have had the privilege of working with Rod speak highly of him. In his honor, his former students and colleagues have contributed to a special issue of the Canadian Journal of Chemistry - a journal Rod has enthusiastically supported throughout his career.
Tuesday, August 2, 2011
Exchange processes that occur on the NMR time scale affect the NMR line shapes and can be studied by line shape analysis. If the exchange process is slow on the NMR time scale, one can employ EXSY or inversion transfer methods to study the exchange. An alternative to these is the saturation transfer technique. In this method, one of the slowly exchanging resonances (A) is saturated with low power CW irradiation and the effect on the intensity of the resonance of the exchange partner (B) is monitored. If there is exchange between A and B during the period of saturation some of the saturation from A will be transferred to B. The change in the intensity of B will depend on both the rate of exchange, k and the relaxation time of B, T1B . If there is no nuclear Overhauser effects between A and B, then the rate of exchange is given by: where Io is the intensity of B with no saturation of A, and I∞ , is the intensity of B when A is saturated for an infinite time. The saturation transfer effect is useful for situations where the exchange is slow on the NMR time scale but faster than (or of the same order as) T1B. An example using 31P NMR is illustrated in the figure below for a ruthenium phosphine complex which undergoes slow exchange between isomers with different modes of bonding. The 31P [1H] NMR spectrum is shown in the upper right-hand panel of the figure. In this case, the P atoms of isomer A are chemical shift equivalent and give a singlet while those for isomer B are chemical shift nonequivalent and give an AB pattern. The spectrum of isomer B is shown in the lower panel of the figure as a function of saturation time of isomer A.
Many thanks to Carolyn Higman and Prof. Deryn Fogg for kindly allowing their data to be used in this post.