Bond lengths distortion

Bond length distortion destabilization of a molecule that results when one or more of Its bond distances are different from the normal values... 

Molecular Dynamics and Monte Carlo Simulations. At the heart of the method of molecular dynamics is a simulation model consisting of potential energy functions, or force fields. Molecular dynamics calculations represent a deterministic method, ie, one based on the assumption that atoms move according to laws of Newtonian mechanics. Molecular dynamics simulations can be performed for short time-periods, eg, 50—100 picoseconds, to examine localized very high frequency motions, such as bond length distortions, or, over much longer periods of time, eg, 500—2000 ps, in order to derive equiUbrium properties. It is worthwhile to summarize what properties researchers can expect to evaluate by performing molecular simulations ... 

The problems of why certain theoretical models seem to work, henceforth designated the justification problem, is a constant preoccupation of theoretical chemists. The papers written on justification are numerous. We note here the most recent attempt by Buenker and Peyerimhoff to summarize arguments and data relevant to the justification of the old Mulliken-Walsh model of molecular structure which proved particularly successful in the interpretation of the dependence of bond angle and bond length distortions on the number of electrons (i.e. MO occupancy) in small molecules26S ... 

A quadratic function defines a symmetric parabola and therefore cannot exactly reproduce the true relationship between the distortion of a bond length or valence angle and the energy needed to effect that distortion. However, a central assumption in the application of simple molecular mechanics models is that distortions from ideal values are small and in such cases it is only necessary that the potential energy function be realistic in the region of the ideal value. This is shown in Fig. 17.8.1, where a quadratic curve is compared to a Morse potential that is believed to more accurately reflect the relationship between bond length distortion and energy cost. 

Another interesting perturbation at the three-membered ring is introduced by an attached 7t-system as illustrated by the structure of vinylcyclopropane (3) [48], The bond length distortions of the cyclopropane unit in 3 show the opposite behavior, with the vicinal bond tending to be lengthened to 1.514(1) A, whereas the distal bond does not differ from the distance reported for the X-ray structure of cyclopropane with 1.499(1) A [48,49]. 

Early work on the dark red compound of Mn111 with edta suggested that its structure was K[Mn(edta)H20] 1.5H20.661 However, X-ray work shows the compound to possess an octahedral structure with a sexadentate edta residue. Axial bond length distortion is observed, with two tram O atoms being 2.050 and 2.022 A from the Mn atom, while the two equatorial Mn—O bonds are 1.907 and 1.889 A and the two Mn—O bonds are 2.211 and 2.237 A.662 A very similar structural result was found in the case where edta was replaced by franj-l,2-diaminocyclohexane tetraacetic acid in K[Mn(L)]-H20.663... 

With respect to the cr and tt interactions towards the geometry in the allyl system, Frenking separated the distortions into the rotation of the methylene group and bond distances. The former turns off 7r-conjugation while bond-length distortion only changes... 

Idealized structures up to nine-coordination are summarized in Figure 4.21. These do not represent all of the shapes met, since, apart from all these idealized structures, it is necessary to remember that bond angle and bond length distortions of these structures can occur some of the shapes resulting from these effects are themselves common enough to be represented as named shapes, and we have discussed some examples of these earlier. Further, beyond nine-coordination, an array of additional shapes can be found, of which perhaps the best known are the bicapped square antiprism (for ten-coordination), the octadecahedron (for eleven-coordination) and the icosahedron (for twelve-coordination). Clearly, the options are extensive, so it may be time to find out what directs a complex to take a particular shape. 

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