Geometric isotope effect

There are two geometric isotope effects A/ (D — H), which refers to the change in bond length / . .. g of a hydrogen bond AHB on deuteration, and 8/ , which is defined as the change in the distance between the two minima of the potential energy well of the hydrogen bond when it is deuterated. The isotope effect, A/ (D—H), was dealt with previously (see p. 263). 

However, in the 1980s some doubts arose about the validity of this model. Results of several experiments indicated that a model based on order-disorder motion of PO4 groups could be more appropriate to describe the phase transition [3,4]. Within such a type of a model, the isotope effect on Tc could be explained in terms of a geometric isotope effect. More precisely. 

However, Ichikawa et al. [12] found a linear relationship between Tc and Ro-o, which is common to several KDP-type crystals, irrespective of whether they are deuterated or not. This finding suggests that the difference in Ro-o between KDP and DKDP (J o-o = 2.50 A for KDP and Rq-q = 2.52 A for DKDP at room temperature [10]) or the geometric isotope effect, rather than the difference in tunneling frequencies, causes the isotope effect on Tc. 

Ichikawa M (1978) The O-H vs O- 0 distance correlation, the geometric isotope effect in OHO bonds, and its application to symmetric bonds. Acta Cryst B 34 2074-2080... 

Structural investigations of the NH-deuteriated salts 44 DBr87 and 102 DBr78 revealed no geometrical isotope effect within the experimental error. 

Very recently, Singleton has suggested a new way of calculating isotope effects illustrated by the bromonium ion [61]. This method could possibly also be used for tautomeric systems. Another new approach is the multicomponent molecular orbital method for direct treatment of nuclear quantum effects [62]. The basic idea is to incorporate the nuclear wave function and in particular the proton wave function directly into the electronic structure calculation. This approach has great potential but has so far been tested only for secondary isotope effects on chemical shifts [63]. The geometric isotope effect has also been looked into based on Pauling valence-bond orders [20]. 

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