Rigidity conformational

As will be shown, even-order bonds are conformationally rigid. Enhanced activity at higher-order sites relates to the increased number of odd electrons available for bonding. 

Optical activity is one of the best known, but least understood, phenomena of organic chemistry. It is observed as the ability of certain substances to interact with linearly polarized light by rotating the plane of polarization. Linear polarization means that the electromagnetic radiation vectors oscillate in fixed orthogonal planes that intersect along the propagation vector. 

It is known from experience that only chiral molecules have the ability to rotate the plane of polarization. This observation suggests that molecular chirality is a manifestation of residual orbital angular momentum, and vice versa. When an achiral molecule is placed in a magnetic field the molecular magnetic vectors are aligned in the diection of the field and not along the local molecular symmetry directions. The molecule therefore acquires a 

The relationship between chirality and optical activity is conveniently demonstrated by a series of substituted methanes. 

An alternative modern explanation of optical activity, without mention of orbital angular momentum [83] states  

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An analysis of the stereochemistry of deamination has also been done using the conformationally rigid 2-decalylamines ... 

The /rans-fiised decalin system is conformationally rigid, and the stereochemistry of the product indicates that the initial addition of the trichloromethyl radical is from the axial direction. This would be expected on stereoelectronic grounds, because the radical should initially interact with the n orbital. The axial trichloromethyl group then shields the adjacent radical position enough to direct the bromine abstraction in the trans sense. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

This is actually observed, except when either potentially aromatic molecules such as thiepin dioxide (358) or when (relatively) sterically/conformationally rigid systems are involved. 

Recendy, Ronova and Pavlova54 suggested a set of relations between the conformational rigidity and some physical properties of the aromatic polymers. The conformational rigidity is correlated to the Kuhn statistical segment A calculated by die Monte Carlo method ... 

Diheterolevulosans, 209-211, 240 Dihexulose dianhydrides, 207 -266, see also Caramels Di-D-fructose dianhydrides 13C NMR spectra, 245-246 conformation, electronic control, 224-228 conformational rigidity, energetic outcomes, 228 hexulopyranose rings, 226 historical overview, 210-213 H NMR spectra, 248 -249 intramolecular hydrogen-bonds, 227 isomerization, 231 -232 1,2-linked, ero-anomeric effect, 224-225 listing, 240-241 nomenclature, 208-210 optical rotations and melting points, 242-243 protonic activation... 

Several reports have indicated that enzymes are more thermostable in organic solvents than in water. The high thermal stability of enzymes in organic solvents, especially in hydrophobic ones and at low water content, was attributed to increased conformational rigidity and to the absence of nearly all the covalent reactions causing irreversible thermoinactivation in water [23]. 

The stereoselectivity is enhanced if there is an alkyl substituent at C(l). The factors operating in this case are similar to those described for 4-r-butylcyclohexanone. The tnms-decalone framework is conformationally rigid. Axial attack from the lower face leads directly to the chair conformation of the product. The 1-alkyl group enhances this stereoselectivity because a steric interaction with the solvated enolate oxygen distorts the enolate to favor the axial attack.57 The placement of an axial methyl group at C(10) in a 2(l)-decalone enolate introduces a 1,3-diaxial interaction with the approaching electrophile. The preferred alkylation product results from approach on the opposite side of the enolate. 

Strapped tetramines show conformationally rigid coordination environments that can lead to novel stability properties. The coordination properties of bowl-shaped cavities or adamanzanes such as the ligand l,5,9,13-tetraazabicyclo-[7.7.3]nonadecane (95) were investigated.745 The... 

Molecules aim at being packed as closely as possible in the crystal 26 28). Bulkiness of a molecule, however, is an impediment. It presupposes a molecular constitution rather unbalanced in its spatial dimensions and a certain extent of conformational rigidity. Here we encounter a parallel to an everyday occurrence trunks can easily be arranged to a compact ordered stack, unlike bulky root stocks which cannot. The... 

The 7 g of elastomers synthesized at AWE was generally found to be within the range 30° to 40°C (see Table 2), which is significantly higher than that for standard poly(dimethylsiloxane). The introduction of the bulky m-carborane unit into the siloxane backbone has clearly elevated the Tg. However, although the carborane unit introduces conformational rigidity, the polymer chains retain sufficient flexibility and mobility to have a T% < -30°C. 

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