Pure analytical thinking

The purely analytical approach we have used throughout this book can be supplemented by forwards and backwards thinking from a key reaction. If a reaction is good enough—the Diels-Alder reaction is an obvious example— a synthesis may come from thinking forwards from a possible Diels-Alder product as well as backwards from the TM. 

We have already established that the carbene carbon is an electrophilic center and, hence, it should be very easily attacked by nucleophiles. In most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines (49) and tertiary amines (50), such addition products are isolable analytically pure under certain conditions (1 in Fig. 3). For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbene ligand (2 in Fig. 3). One can also very formally think of the carbene complex as an ester type of system [X=C(R )OR with X = M(CO)j instead of X = 0], because the oxygen atom as well as the metal atom in the M (CO) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, it is not surprising that the... 

The first and most obvious is to make sure that the starting material is pure, dry, and free from solvents, and that it is indeed what you think it is. A critical perusal of all the analytical and spectroscopic data will usually be sufficient. If you have used several batches of starting material then do make certain that the spectra which you check are from the batch which you used in the failed reaction. If necessary, re-run the spectrum to ensure that the starting material has not partially decomposed, or picked up moisture. 

All these correspondences are approximate and we observe that to each model correspond its own parameters. Thus, we note that the excluded volume only has a meaning for the discrete link model in continuous space. This model is not particularly useful in polymer theory. In fact, the best models are the lattice model, which is simple and well-adapted to numerical simulations, and the purely continuous model, which is very useful for analytic calculations. Thus, the famous excluded volume is not as interesting in polymer theory as many people think (in 1988). 

You might think that a pellet made from pure ground KBr should be used for the background spectrum. Remember from Chapter 2 that the only difference between the measurement of the background and sample spectra should be the absence or presence of the sample, and that for contributions to ratio out they need to be identical in both measurements. It is difficult to make two KBr pellets with the same optical properties, such as thickness and shape. Thus an analyte-free pellet will not provide a good background because its contributions will not ratio out. However, the optical properties of the pellet holder are reproducible, which is why it should be used to run the background spectrum for KBr pellets. 

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