| 2. |
- Lindh, Roland, 1958-, et al.
(författare)
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A MCSCF study of homoaromaticity and the role of ion pairing in the stabilization of carbanions
- 1986
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 108:21, s. 6554-6561
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Tidskriftsartikel (refereegranskat)abstract
- The bicyclo[3.2.1]octa-3,6-dien-2-yl anion (I), the anion I lithium cation complex, the allyl anion (X), the allylradical, the allyl anion lithium cation complex, the ethene molecule, and the ethene lithium cation complex have been studiedby means of multiconfigurational SCF (MCSCF) and analytical gradients. The calculations have been confined to minimaland split-valence basis sets. The large distance between the C2 olefinic bridge and the C3 carbanionic bridge of anion I andthe short C6-C7 bond distance imply bishomoaromaticity to be negligible. According to these results, homoaromaticity isnot responsible for the observed stability in many potentially homoaromatic carbanions. The stability of anion I in the gasphase is instead explained in terms of a simple electrostatic model, where the quadrupole moment in the C2 olefinic bridgestabilizes the charge in the C, carbanionic bridge. This model agrees quantitatively with experiment. Calculations on theanion I lithium cation complex showed that in solution an additional attractive interaction between the C2 olefinic bridge andthe lithium cation can contribute. This additional interaction is estimated to stabilize the carbanion-lithium ion pair in thegas phase by about 16 kcal/mol. However, the stabilizing interactions of anion I in solution (quadrupole-charge andcounterion-anion interactions) will be reduced by solvent shielding. The relative ratios of the different stabilizing interactionsare therefore difficult to estimate. The geometrical findings of this paper have been verified by a recent X-ray experiment.
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| 3. |
- Wirth, Michael, 1956, et al.
(författare)
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Interactions between DNA and mono-, bis-, tris-, tetrakis-, and hexakis(aminoacridines). A linear and circular dichroism, electric orientation relaxation, viscometry, and equilibrium study
- 1988
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 110:3, s. 932-939
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between DNA and a series of mono-, bis-, tris-, tetrakis-, and hexakis-intercalating 9-aminoacridines has been studied with flow linear dichroism (LD), circular dichroism (CD), electric orientation relaxation (EOR) techniques, and with viscometry and equilibrium analyses. The orientation of the 9-aminoacridine ligand relative to the average orientation of the DNA bases, measured by LD, shows that with both 9-aminoacridine and the bis(acridines) the in-plane short axes of the acridine ligands are oriented perfectly parallel to the planes of the DNA bases, as expected for classical intercalation, whereas the long axes are found to be significantly tilted. This is supported by the DNA lengthening measured by EOR, which for 9-aminoacridine is 1.5 base-pair units, compared with 1.0 for ethidium bromide. Also in case of the tris(acridines) LD, CD, viscometry, and equilibrium data indicate that all acridine ligands are intercalated. The binding analysis shows an increasing degree of cooperativity in the sequence 9-aminoacridine < bis(acridines) < tris(acridines), and the corresponding binding densities, 4, 8, and 11–14, respectively, are in good agreement with those expected from the nearest-neighbor exclusion principle. The LD and CD measurements show that the tetrakis- and hexakis(acridines), despite long and flexible links, bind to DNA with only three of the acridine ligands intercalated.
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