Geometry, distortion

The simple harmonie motion of a diatomie moleeule was treated in Chapter 1, and will not be repeated here. Instead, emphasis is plaeed on polyatomie moleeules whose eleetronie energy s dependenee on the 3N Cartesian eoordinates of its N atoms ean be written (approximately) in terms of a Taylor series expansion about a stable loeal minimum. We therefore assume that the moleeule of interest exists in an eleetronie state for whieh the geometry being eonsidered is stable (i.e., not subjeet to spontaneous geometrieal distortion). 

During the course of modeling copper dioxygen chemistry, Kitajima et al. reported the synthesis of a yu-peroxo dinuclear complex with a 3,5-dimethyl-substituted tris(pyrazolyl)borate ligand, which showed remarkable physicochemical similarities to oxy-Hc and oxy-Tyr. Using a 3,5-di-isopropyl-substituted terminal ligand, they provided the first structural proof of the existence of peroxo dicopper(II) core (108) (copper geometry distorted square pyrami-... 

Sargeson and co-workers have structurally characterized encapsulated zinc in hexaaza cryptands.742 743 Related cryptands (l-methyl-8-amino-3,13-dithia-6,10,16,19-tetraazabicyclo[6.6.6]-icosane and l-methyl-8-amino-3-thia-6,10,13,16,19-pentaazabicyclo[6.6.6]icosane) incorporating thioether donors also formed complexes with zinc which were structurally characterized. In both cases the zinc ion was encapsulated in the macrobicyclic cavity and the octahedral coordination geometry distorted to the mixed nitrogen and thioether donor atoms.744... 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

The presence of these higher cross-terms tend to improve the ability of the force field to predict the properties of unusual systems (such as those which are highly strained) and also to enhance its ability to reproduce vibrational spectra. It must be noticed, however, that any of the cross terms listed above have been proven to be truly of the form in which they are written. No attempts have been reported to derive that or any other form of the coupling between different geometry distortions and to estimate the corresponding constants from some independent point of view. The class III force field will also take into account further features such as electronegativity and hyperconjugation. We shall turn to these problems later. 

Subsequently, a number of authors have considered non-planar distortions of olefins and carbonyl groups as a source of diastereofacial selectivity, often emphasizing non-equivalent FMO extension (polarization of the frontier orbitals with respect to the a plane) or asymmetry of the electrostatic potential, rather than the actual geometry distortion [198, 199]. Interest in the latter, however, was revived by the end of the 70 s, stimulated by the early papers on structure correlation as well as several other contributions at that time. One of them was the seminal study by Eschenmoser and Dunitz and their coworkers in 1976, who examined conformations and nonplanarity of enamine fragments in a number of crystal structures in search for the origin of face selection in condensations of chiral enamines [200]. Another crucial development appears to be Bartlett and Watson s discovery that large pyramidaliza-... 

Other advantages are as follows the use of a 40-Hz scanning frequency results in a very small amplitude of motion which reduces geometry distortions to negligible proportions for most nuclides and minimises errors from non-linearities in the voltage/velocity correspondence of the pick-up coil. Half-life corrections with short-lived sources are also unnecessary, and high velocities can be achieved at relatively small amplitudes. One particular system accumulates two spectra simultaneously using a common input to the analyser [28]. 

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