Covariance NMR methods

K.A. Bhnov, A.J. WiUiams, B.D. Hilton, P.A. Irish, G.E. Martin, The use of unsymmetrical indirect covariance NMR methods to obtain the equivalent of HSQC-NOESY data, Magn. Reson. Chem. 45 (2007) 544. 

The projection-reconstruction approach is a technique unrelated to covariance processing which can provide data typically inaccessible to the natural product chemist. For example, 13C-15N correlation spectra were obtained for vitamin B12 at natural abundance.104 Compared with a conventional three-dimensional 13C-15N correlation experiment, the projection-reconstruction method provides a sensitivity enhancement of two orders of magnitude. The final 13C-15N spectrum was reconstructed from data obtained from ll l5N and H- C correlations acquired using a time-shared evolution pulse sequence that allowed all the information to be obtained in one experiment.104 Martin and co-workers also demonstrated the ability to generate 13C-15N correlation spectra using unsymmetrical indirect covariance NMR with vinblastine as an example.105 In the latter case, 13C-15N correlation spectra were obtained from - C HSQC data and H-1sN HMBC data that were acquired separately. Both methods provide access to correlations that would be inaccessible for most natural products at natural abundance. 

Keywords Covariance NMR, Synchronous and asynchronous spectra. Non-uniform sampling. Fast methods. Structure elucidation... 

D.A. Snyder, R. Bruschweiler, Multidimensional correlation spectroscopy by covariance NMR, in G. A. Morris, J.W. Emsley (Eds.), Multidimens. NMR Methods in the Solution State, John Wiley Sons Ltd., Chichester, UK, 2010, pp. 97-105... 

While the use of direct, indirect and unsymmetrical and generalized covariance is well represented in the literature, the exploitation of heterospectroscopy with NMR as one element is stiU scarce. StiU, covariance analysis as dealt with here is a processing scheme that is open to application. It cannot be excluded therefore that some of the methods are employed without disclosure to a general audience. In this respect, the mathematical or data conversion tools for covariance treatment are readily available today. 

A covariance study by Li et al. [76] dealt with the comparison of various sampling schemes and accumulation profiles with respect to their usability with covariance processing. The authors divided sparse sampling schemes into the non-uniform and the ti cut-off (CUO) schemes. While common NUS schemes employ—exponentially—increasing A(i spacing as a function of (i, a cut-off design implies the sole acquisition of signals at short <1 times up to a maximum value. The 2D NMR spectrum is then reconstructed with appropriate methods. Various standard, NUS and... 

---