Obtaining and Judging the Molecular Model

An alternative stream came from the valence bond (VB) theory. Ovchinnikov judged the ground-state spin for the alternant diradicals by half the difference between the number of starred and unstarred ir-sites, i.e., S = (n -n)l2 [72]. It is the simplest way to predict the spin preference of ground states just on the basis of the molecular graph theory, and in many cases its results are parallel to those obtained from the NBMO analysis and from the sophisticated MO or DFT (density functional theory) calculations. However, this simple VB rule cannot be applied to the non-alternate diradicals. The exact solutions of semi-empirical VB, Hubbard, and PPP models shed light on the nature of spin correlation [37, 73-77]. 

Figure 5.14 shows high-pressure isobars for the self-diffusion data of the n-alkanes in a plot of log D against log where is the molecular weight. These are the first data obtained with the titanium autoclave described in Section 1.4.2. Such results are commonly described by the Rouse model or by the reptation model, which both predict a linear correlation in this type of plot at constant pressure and temperature this linear correlation is clearly established in Fig. 5.14. Judging from the chain length of the polymethylenes the Rouse model should apply. This model predicts, that D should be proportional to M while the experiments give a D correlation, which is...

The above spectral density expression has not been used to fit experimental spectral densities, although a much simplified version has been proposed to treat the molecular reorientation plus restricted 7-motion in the nematic and smectic A phases of 50.7-d4 [7.11]. Now, attention is given to some typical NMR studies, mainly deuteron, in which the above models were used to derive motional parameters. The suitabihty of each model is judged by its prediction of the temperature and frequency dependences of spectral densities or of spin relaxation rates. Furthermore, the derived model parameters must be physical in their magnitudes and temperature behaviors. Comparison with those obtained by other experimental techniques where applicable would be most desirable. 

---