X-Y correlations

It is perhaps not surprising that more recent developments for identifying X-Y eonelations have incorporated proton detection and pulsed field gradients for signal selection, with many of the techniques being derived from the triple-resonance techniques originally 

A triple-resonance sequence for recording 2D H-(X)-Y heteronuclear correlations. The delays are set to Ai = 1/2Vhx and A2=1/2 7xy with gradient strengths set as G2=G,(7y/7h). 

One such approach that illustrates this principle is shown in Fig. 6.53, which may be viewed as an extended HSQC experiment now employing two transfer steps between nuclei [115,116]. In this case, the transfers are made with INEPT stages sequentially from 

The 2D spectrum therefore correlates with the Y spin via a pathway involving both /xH and /xy couplings, where n is 1 or greater according to the available 

HMBC-based experiments). From the 2D spectrum it was possible to directly assign the platinum resonances of the different isomers and hence determine the isomer ratios. These data were collected on a triple-channel instrument equipped with a Broadband probe for which the broadbanded channel had been tuned to Pt. 

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Applications of Heteronuclear X/Y-Correlation Spectroscopy in Organometallic and Organoelement Chemistry Recent Developments... 

Considering that the literature on the development of experimental methods and important fields of application of X/Y correlations in inorganic, organoelement and organometallic chemistry up to 1997 has been covered in earlier reviews,11 we will focus here on recent improvements of experimental techniques and novel applications for compound characterisation. Despite the recently increasing interest in the application of X/Y correlation spectroscopy in solids,12,13 this review will cover only solution NMR techniques. Likewise, a survey of specialised triple-resonance NMR experiments devoted to the characterisation of bio-molecules, and their application, is considered beyond the scope of this article. 

In contrast to applications in structural biology where X/Y correlations are nowadays normally executed as H detected, three-dimensional experiments because of sensitivity reasons,14 many studies on inorganic or organometallic compounds are still performed as two-dimensional experiments with direct detection of one heteronucleus and under -decoupling. As compared to these two categories, one-dimensional polarisation transfer methods such as (semi) selective X/Y-INEPT or INDOR-type techniques, which had in the past been shown to be particularly useful for the characterisation of substrates with only one or two heteronuclei,11 have recently received less attention.15 NOE-based correlations, which are frequently employed for the structure elucidation of bio-molecules, remain rare, and apart from an earlier report of a 13C/6Li HOESY experiment,16 have not been further investigated. 

Reports on the application of pulsed field gradient (PFG) assisted pulse schemes for two-dimensional X/Y correlation spectroscopy focused mainly on the adaptation of HSQC sequences which seemed to perform better than HMQC experiments under these conditions.21 Although the generalisation of standard pulse sequences for / C correlation spectroscopy should in principle be straightforward, large spectral ranges and short relaxation... 

The task of generating a display of heteronuclear X/Y-connectivities with optimum sensitivity can in principle be performed by recording a three-dimensional proton detected shift correlation in which the chemical shifts of both heteronuclei X and Y are each sampled in a separate indirect dimension. Three-dimensional fourier transformation of the data then generates a cube which is defined by three orthogonal axes representing the chemical shifts of the three nuclei 1H, X, Y, and the desired two-dimensional X/Y-correlation is readily obtained as a two-dimensional projection parallel to the axis... 

Fig. 7. Schematic representation showing how X/Y correlation maps can be obtained either from the appropriate 2D projection of a three-dimensional 1H/X/Y correlation spectrum (a), or alternatively from a sequence of two-dimensional 1H/Y correlations acquired with relayed 1H/(Y)/X coherence transfer via a selected Y-nucleus as relay.

HSQC rather than HMQC-based transfer schemes have recently in particular been employed in various indirectly detected two- and three-dimensional 111/X/Y correlation experiments involving multi-step coherence-transfer in either direction.38 40 43 44 The application of PFG s appears to be essential to obtain a sufficiently clean spectrum that is free of artefacts, and in many cases the pulse sequence shows only a satisfactory performance if composite pulses, with a larger excitation bandwidth than normal ones, are employed.21,38,39,43 The pulse schemes yield generally phase-sensitive spectra with pure absorptive lines and do not suffer from splitting or broadening of the cross peaks as a consequence of the undesired evolution... 

Recent applications of X/Y correlation techniques to measure chemical shifts of low- , spin-1/2 transition metal nuclei other than rhodium focused mainly on the analysis of silver and tungsten complexes in addition, the use of 13C-relayed H/195Pt correlations for the stereochemical analysis of some platinum complexes was explored.44 A report on the application of 31P/51V correlation spectroscopy demonstrated further the possibility of indirect detection of rapidly relaxing quadrupolar metal nuclei and the feasibility of the... 

Although the different applications discussed in this section are closely related and follow similar conceptions, we will discuss applications of two-dimensional X/Y and three-dimensional H/X/Y correlations in two separate... 

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