Indirect covariance spectroscopy

Indirect covariance spectroscopy was proposed by Zhang and Briischweiler [85] soon after publication of their pioneering work on covariance spectroscopy [58]. Its apphcation to soHd-state NMR was reported a few years later by Hu et al. [86]. The procedure of the indirect covariance spectroscopy starts with a 2D Fourier-transformed FIETCOR spectrum. This is in contrast to the original direct covariance spectroscopy, in which covariance processing is appHed to an array of one-dimensional spectra so that Fourier transformation is performed only with respect to the direct dimension. From a single I—S 2D FiETCOR spectrum, a pair of HOMCOR spectra are... 

Until recently, the scope of covariance NMR spectroscopy had been limited to HOMCORs. (Note that, in indirect covariance spectroscopy, HETCORs are used to obtain HOMCORs.) Extension of covariance spectroscopy to include HETCORs was reported by Takeda et al, who showed that the covariance processing can be used to obtain HETCORs together with HOMCORs in H- H dipolar coupling-mediated chemical... 

Zhang EL, Bruschweiler-Li L, Bruschweiler R (2010) Simultaneous de novo identification of molecules in chemical mixtures by doubly indirect covariance NMR spectroscopy. J Am Chem Soc 132(47) 16922-16927... 

According to Eq. (5.14), covariance transformations can be applied to any type of spectral data sets that are connected to each other by a common history or domain. The spectra thus generated represent heterospectral correlation maps. In NMR spectroscopy, this concept was taken up as unsymmetrical indirect covariance (UIC) NMR, relating, for example. 

B. Hu, J.-P. Amoureux, J. Trebosc, Indirect covariance NMR spectroscopy of through-bond homo-nuclear correlations for quadmpolar nuclei in solids imder high-resolution, Solid State Nucl. Magn. Reson. 31 (2007) 163. 

In liquid solutions, nuclear spins are correlated through J-couplings, whereas in solids dipolar interactions come into play as well. The first report on the application of covariance to soHd-state NMR was published by Hu et al. in which an indirect covariance homonuclear correlation (HOMCOR) spectrum was obtained from a 2D-FT heteronuclear correlation (HETCOR) spectrum. The indirect covariance scheme, which will be discussed in Section 5.1, contrasts with the original direct covariance spectroscopy, where the covariance processing is performed over an array of one-dimensional spectra. 

In covariance NMR spectroscopy, pulse sequences to be used are the same as those in 2D-FT, but much fewer amounts of data sets along the indirect dimension suffice to produce a 2D spectrum similar to that obtained with 2D-FT. 

P— P correlations in aluminophosphates, and Si— oi correlations in sodium borosihcate glass and silicate-1. Interestingly, they demonstrated covariance processing in double-quantum correlation experiments employing POST-C7 [71], BABA [72], and INADEQUATE [73]. In double-quantum 2D spectroscopy, time evolution along the indirect dimension is characterized by the frequencies given by the sum of the chemical shifts between the correlated spins so that horizontal pairs of correlation peaks appear as shown in Fig. 3. There, 125 data sets were acquired with a step of 12.5 ps. Thus, the coherence profile during the evolution time was truncated at 1.6 ms. 

Even though the same data set can be used to obtain a pair of the conventional 2D-FT spectra (Figs. 23A and 25C and D), the covariance-processed HOMCOR/HETCOR spectrum retain the merits that have already been confirmed for the original covariance NMR spectroscopy the spectral resolution along the indirect dimension is determined by that... 

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