Out-of-plane deviations

A quadratic term is used to calculate the energy for this deformation, but the function differs from those above because the ideal value is zero (corresponding to planarity) and therefore does not appear in the equation. 

Here too, ks is a force constant that determines the amount of energy that must be expended to achieve a certain distortion. 

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For INOR1, INOR2 and SNOR a significant pyramidalization of the endocyclic double bonds can be observed by all methods. The out-of-plane deviations appear to be around 1.9° to 3.1° (see Table 14). 

Similar small but significant deviations of the carbon atoms from a planar conformation have been found in chrysene (10) and in 20-methylcholanthrene (12) (Iball and MacDonald, 1960). The deformation energies for these molecules (0-025-0-040 and 0-155-0-260 kcal mole-1 respectively Ali and Coulson, 1959) are of the order of one-hundredth of the sublimation energy and serve to demonstrate that the small out-of-plane deviations found by Iball and his associates could easily arise in the process of packing the separate molecules into the crystal. 

The carbon frameworks in the two molecules were held planar during the analysis the progress of the refinement showed that to a good approximation this was a justifiable assumption. However, as the standard deviation in positional parameter in a direction normal to the benzene planes is at least 0-024 A, small out-of-plane deviations of the exocyclic carbon atoms from the benzene planes cannot be precluded. Similar uncertainties in conformation have already been pointed out in hexachloro- and hexamethyl-benzene. 

Out-of-plane deviation functions are used when a group of four atoms is most stable when they are coplanar. This is the case for groups involving multiple bonds such as alkenes or conjugated systems such as carboxylate groups. The deviation is usually defined as the angle between the vector from the central atom to the fourth atom and the plane defined by the other three atoms. 

Out-of-plane deviations are critical in square-planar metal complexes. Important example are the anti-cancer active Pt(II) complexes such as ds-(PtCl2(NH3)2j. In modeling the interaction of these compounds with DNA, it has been found that the substantial stresses present cause deviations from planarity, and therefore it is essential that a good estimate of the energy cost of this deformation be available. One set of force constants has been developed empirically by modeling simple bis-nucleotide complexes of the type cis-[Pt(NH3)2(Nu)2]. In a number of these complexes significant deviations from planarity have been observed experimentally. By empirical adjustment of the force constant it was possible to obtain a value that reproduces adequately the deviations in these complexes [193]. Other methods for modeling square-planar metal complexes are discussed in Section 3.6. 

It is somehow strange that the spin state changes from S = 3/2 to S = 5/2 as the temperature is lowered because lowering the temperature usually stabilizes the lower spin state. The novel spin transition described above should be ascribed to the unique molecular and crystal structures of [Fe(OMTArP)(H20)]. In the case of five-coordinate iron(lll) complexes, the out-of-plane deviations of the iron(lll) ion signified as AFe are quite different between high-spin and intermediate-spin complexes. While the AFe values of high-spin Fe(OETPP)CI and Fe(OMTPP)CI are 0.46-0.47 A, they are only 0.23-0.29 Ain the intermediate-spin complexes as revealed from the data in Table 10.10. When the temperature is extremely lowered, the N4 cavity should be reduced. As a result, the iron(lll) ion is extruded away from the N4 plane. 

These changes in the degree of pyramidalization of the exoamino group must be taken with caution, since the specific interactions of this group with water molecules in the first hydration shell likely dominate the out-of-plane deviation. Nevertheless, the results clearly illustrate how solvation can induce non-negligible effects in the structural arrangement of these groups. 

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