Molecular mechanics calculations, cation

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

For many polycyclic carbocations there is a good correlation between the changes in strain energies on carbocation formation, predicted by molecular mechanics calculations, and the solvolysis rates. However, at 70° cubyl cation is formed 10 times faster than expected from this correlation." " " What is responsible for this huge rate acceleration ... 

Another significant deviation from known rules has been observed in the rather low affinity of K+ with the 1,10-diaza crown 18-C-6 with only AG=10 kJ/mol (in methanol). It has been shown that the free energy of binding AG in crown ether and ciyptand complexes usually is an additive function of number and electron donicity of the host donor atoms which are in contact with the metal ion.[17] Molecular mechanics calculations suggest the reduced affinity with the diazacrown to be due to the N-lone pairs in pseudoaxial position, pointing away from the metal ion (Figure 4). This has led to experiments with the N-methyl substituted crown here the N-alkyl substituents would clash which each other inside the macrocycle, therefore a pseudoequatorial lone pair orientation towards the cation is enforced, and the stability of the complex indeed returns to the normal scale with an increase to 29.5 kJ/mol.[ 18]... 

Thus the low reactivity of 87 certainly shows the absence of the strong conjugative stabilization found for the bisected cyclopropylcarbinyl arrangement, but does not provide a quantitative assessment as to the respective roles of ring strain and the inductive effect of cyclopropyl in giving the observed rate of deceleration relative to dimethyl substituents. Accurate molecular mechanics calculations for 87 and the derived cation would provide an assessment of the former, and a measure of o-j for c-Pr free of resonance effects is much to be desired. 

Dynamic two-dimensional and NMR studies of the conformationally restricted amide iV-(l-naphthoyl)-d5-2,6-dimethylpiperidine reveal correlated rotation about the aryl-CO and amide bonds. 9-(l-Adamantylamino-methyl)-9,10-dihydroanthracene was prepared via condensation of 1-adaman-tylnitrile with 9-anthryllithium and its hydride reduction. An NMR study confirmed a boat conformation for the central ring, with the substituent of C9 in a pseudo-axial position. The structure of bis[(l,3)2-oxaadamantano]-18-crown-6 (I) and its potassium picrato complex were studied using NMR, X-ray structure analysis and molecular mechanics calculations. In the gas phase and in CDCI3 solution, in both I and the corresponding complex cation the most stable conformation of C2h symmetry is found. 

Thus, the detection of the direct and selective influence of the paramagnetic lipophilic electron relay on the H-NMR spectra of the surrounding lipid molecules, which constitute the bilayer lipid membranes, appears to be direct evidence for the localization of the paramagnetic moiety of lipophilic cetylviologen cation radicals just near the surface of the vesicle membrane. Evidently, this coincides with the above data from molecular mechanics calculations. 

The first molecular mechanics studies of alkali metals were of the Na" ", Kr ", Rb and Cs complexes of 18-crown-6 (Figure 15.1) [606]. The AMBER software system was used and the M. .. O interactions were modeled using a combination of van der Waals and electrostatic terms. The solvation energies were found to make a larger contribution than did the intrinsic, molecular-mechanics-calculated, complexation energies. Consequently, in a later study, molecular mechanics and dynamics studies were carried out with the complexes and cations surrounded by clusters of solvent molecules [607]. 

Calixarenes may also be linked through covalent interactions to form ditopic receptors, such as in the symmetric linking of two p-rerr-butylcalix[4] arene molecules via ethylene linkages to form a calix[4]tube. The calix[4]tube shows remarkable selectivity toward cations, with bound within the central ethylether cage of the calix[4]tube. Molecular mechanics calculations suggest that the uptake of cations requires an... 

Studies on alkali metal ion interaction with carbon nanotubes also gets moment [243, 244]. Mpourmpakis et al. [243] used ab initio and molecular mechanics calculations to study the interaction between different types of carbon nanotubes with alkali metal cations (LT, Na, and K+). The results showed that ions are located on top of a phenyl group of the nanotube, resulting in a strong cation- r interactioa Umadevi and Sastry studied the same alkali metal ions binding with carbon nanotubes and identified some interacting stractures [244]. 

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