Rigid-molecule test

Some criteria have been suggested in order to decide when a thermal motion analysis can be expected to yield meaningful results. Two of them, the rigid-bond test and the rigid-molecule test, investigate whether or not bonds or portions of molecules are considered to move in a physically reasonable way. They will be described in turn. 

Bally and Rablen ° followed up their important study of the appropriate basis sets and density functional needed to compute NMR chemical shifts with an examination of procedures for computing proton-proton coupling constants." They performed a comparison of 165 experimental with computed proton-proton coupling constants from 66 small, rigid molecules. They tested a variety of basis sets and functionals, along with questioning whether all four components that lead to nuclear-nuclear spin coupling constants are required, or if just the Fermi contact term would suffice. 

Another very rigid molecule which unexpectedly showed the same high afiSnity for the a, and the other receptors tested. 

Regardless of the method used, it is essential to demonstrate the correcmess of calculated charge densities against experimental dipole moments. As dipole moments are conformation dependent, this test should only be performed for rigid molecules or small and conformation-ally undemanding model compounds. 

A few things can be said about the overall results of the four blind tests (Table 2.2) there has been some success for rigid molecules, although the predictability of the different category 1 and 2 crystal structures is variable. Molecular flexibility is a serious obstacle for current methods of crystal structure prediction only one category 3 success was achieved in the first three blind tests and, while more success was achieved for the flexible molecule in the latest test, this was partly due to the more restricted molecular flexibility of the molecule chosen that year [19]. 

The success of crystal structure generation methods for rigid molecules has been quite high across the blind tests, with most of the participants locating the observed structure somewhere in their lists for most molecules in categories 1 and 2. The notable exceptions are molecules XI, IX, and xm (Table 1). 

When it comes to the final choice of structures, the best that could be said after the first three blind tests was that crystal structures were occasionally successfully predicted. Some rigid molecules were predicted successfully by several groups flexible molecules were a significant problem (only one successful prediction in category 3 during the first three... 

---