Coupling virtual

Figure 2.61 The methyl region of the H NMR spectrum of mer-RhCl1(PMe2Ph)3 demonstrating the virtual coupling of the resonances as a result of the mutually tra/w-phosphines. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, published by Macmillan Press Ltd, 1975.)...

The use of this phosphine facilitates assignment of configuration as virtual coupling is observed when the phosphines are trans (section 2.9.5).) Syntheses follow established routes using methyllithium as an alkylating agent the platinum(iV) complexes can be made by direct alkylation of platinum(IV) compounds or by oxidative addition to platinum(II) species. 

The complex multiplet centred at 5.04 ppm results from the overlap of the methine and -OH protons (i.e., they are accidentally equivalent ) whilst the equally complex methyl signal is centred at 1.48 ppm. Because of this overlap, their lines are indistinguishable and so the -OH is said to be virtually coupled to the methyl group. Virtual coupling is another potential consequence of non-first order behaviour. 

It is fortunate that most applications devolve into one of two camps small molecules or proteins. In the former case, the size of these molecules has stayed fairly constant and the inexorable rise in magnetic fields has meant that the number of incidences of second-order spectra has decreased (although complications will always exist with virtual couplings). It is therefore pertinent to examine methods, which are not only designed to extract couplings from first-order spectra, but are also amenable to automation. 

Often the proton NMR spectra in cyclophosphazenes is complicated by the presence of second order effects in the form virtual coupling [3, 82]. This is manifested in the form of a broad hump that often is seen between the normal fine structure due to the first order spectrum. The criterion for the observation of... 

Virtual coupling is always a challenge for students—and for teachers and authors. We have given it appropriate emphasis. 

For platinum(II) complexes with alkylphosphine ligands there is a small but marked dependence of the values (PC) on the nature of the group tram to phosphine.1353 By analogy with the earlier method using 3H NMR, 13C 1H NMR techniques using virtual coupling have been used for phosphite complexes of platinum(II), but it does not appear that the method can be generally used to determine stereochemistry.1354... 

CONFORMATIONALLY MOBILE, OPEN-CHAIN SYSTEMS. VIRTUAL COUPLING 165... 

This molecule furnishes another example of virtual coupling (see Section 3.11.1.2). Again we can be lead astray by a structure that might be expected to give a first-order spectrum-at least at 300 MHz (see Figure 3.54a). 

The difficulty is that the COOH groups deshield the CH2 groups, thereby decreasing the Ai>IJ ratio for CH2—CH the virtual coupling results in a broadened, distorted doublet for the CH3 at 300 MHz. A similarly distorted triplet was shown in Figure 3.52. With some experience, such distortions are tolerable. But the overlapping peaks of the CH2—CH—CH2 moiety is still beyond interpretation by inspection at 300 MHz. 

---