Rules, Baur

Niobium pentachloride mentioned on page 66 is another example. The Baur rules also express these facts (cf. p. 60). 

In the IR methods the frequency shift Av between the IR band of non H-bonded OH vibration and the different H-bonded, vibration is determined. By the Badger-Baur rule which gives a proportionality between Av and the H-bond interaction energy the points of Fig. 4 are12,3S) obtained. The curve goes down symmetrically to zero at 220°. This curve is taken in the energetically most favoured plain of two molecules. In other plains there are different curves with repulsion orientation as well. 

Two additional rules, put forward by W. H. Baur, deal with the bond lengths d(MX) in ionic compounds ... 

Burdett and McLarnan (1984) went on to test the valence sum rule by assessing a further prediction implied in the rule, namely that anions of the same electronegativity should be found in sites of equal bond sum. This was confirmed by calculating that, when hypothetical anions with equal parameters midway between O and N are used, the most stable structure is the W-Pmc2, ) type, where all the sites are of equal bond-strength sum. Also investigated was the Zachariasen-Baur extension of the second rule (Baur, 1970), stating that oversaturated anions should... 

Perhaps the more important questions raised by Rule 5, as Burdett and McLarnan (1984) point out, concern the extent to which it really is borne out by observation. For example, Baur et al. (1983) have developed a numerical index for the degree of parsimony in a crystal structure and have shown that, using this measure, many crystal structures are not parsimonious but lavish in their use of different local environments. Also, the dominance of short-range forces is by no means obvious when ordered structures with extremely large unit cells are observed (e.g., a c dimension of 1500 A in some SiC polytypes Shaffer, 1969). The explanation of such structures poses problems for electrostatic as well as covalent models. 

Van Aubel found that Trouton s rule held for metals, the constant being about 20, but Baur and Brunner found values from 16-5 to 29-7 (see l.VIII L). Sonagha concluded that Trouton s rule holds best at a temperature T— Tb. It is quite generally found that for gases of low b.p. the Trouton coefficient is abnormally low, and Kleeman deduced that 4 and d4/dr are both zero at r=0. Some observed molecular heats of evaporation... 

Figure 3. Variation in -(T-0) for (PO4) groups as a function of

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