9 research outputs found

    Conformational Control in [22]- and [24]Pentaphyrins( by Meso Substituents and their N‚ÄĎFusion Reaction

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    <i>meso</i>-Substituted pentaphyrins( were unexpectedly isolated as N-fused species under Rothemund-type conditions. The reaction mechanism is unknown at present, but the first example of a nonfused [22]¬≠pentaphyrin was reported in 2012. Here, the conformational preferences and N-fusion reaction of [22]- and [24]¬≠pentaphyrins have been investigated using density functional calculations, together with their aromaticity-molecular topology relationships. Two global minima are found for the unsubstituted [22]¬≠pentaphyrin corresponding to <i>T0</i> and <i>T0</i><sup>4,D</sup> HuŐąckel structures. MoŐąbius transition states are located in the interconversion pathways with activation barriers of 27 kcal mol<sup>‚Äď1</sup>. Conversely, [24]¬≠pentaphyrin is able to switch between HuŐąckel and MoŐąbius conformers with very low activation barriers. However, nonfused [24]¬≠pentaphyrins are unstable and spontaneously undergo an N-fusion reaction driven by the strain release. On the contrary, nonfused [22]¬≠pentaphyrins could be isolated if a <i>T0</i><sup>4,D</sup> conformation is adopted. Importantly, conformational control of pentaphyrins can be achieved by <i>meso</i>-substituents. Two stable conformations (<i>T0</i><sup>4,D</sup> and <i>T0</i><sup>A,D</sup>) are found for the nonfused [22]¬≠pentaphyrin, which are delicately balanced by the number of substituents. The <i>T0</i><sup>A,D</sup> conformation is preferred by fully <i>meso</i>-aryl pentaphyrins, which is converted to the N-fused species. Interestingly, the removal of one aryl group prevents the N-fusion reaction, providing stable aromatic nonfused [22]¬≠pentaphyrins in excellent agreement with the experimental results

    The Linear Response Kernel: Inductive and Resonance Effects Quantified

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    Calculations of conceptual density functional theory (DFT) reactivity indices are mainly restricted to global quantities and local functions, whereas values for the nonlocal kernels are rarely presented. We used a molecular orbital-based expression to calculate the atom-condensed linear response kernel. The results are the first published values of this quantity that have been obtained through a direct and generally applicable methodology. This letter focuses on the off-diagonal elements, which provide insight into the nonlocal contributions to chemical reactivity. A detailed study of a set of eight functionalized alkane and polyalkene derivatives enabled us to quantify inductive and resonance effects

    Correlation part of the interaction energy (őĒ) computed for the 10 stacked DNA/RNA base dimers (kcal/mol) versus the product of the polarizabilities of each base over (see ) (a

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    <p><b>Copyright information:</b></p><p>Taken from "Influence of the ŌÄ‚ÄďŌÄ interaction on the hydrogen bonding capacity of stacked DNA/RNA bases"</p><p>Nucleic Acids Research 2005;33(6):1779-1789.</p><p>Published online 23 Mar 2005</p><p>PMCID:PMC1069514.</p><p>¬© The Author 2005. Published by Oxford University Press. All rights reserved</p>u.)

    Qualitative Insights into the Transport Properties of HuŐąckel/MoŐąbius (Anti)Aromatic Compounds: Application to Expanded Porphyrins

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    Expanded porphyrins have been recently identified as promising candidates for conductance switching based on aromaticity and molecular topology changes. However, the factors that control electron transport switching across the metal‚Äďmolecule‚Äďmetal junction still need to be elucidated. For this reason, the transport properties of HuŐąckel/MoŐąbius (anti)¬≠aromatic compounds are investigated thoroughly in this work to gain qualitative understanding into the conductivity of these unique macrocycles. Starting from a polyene model, a simple counting rule is developed to predict the occurrence of quantum interference around the Fermi level at the HuŐąckel level of theory. Next, the different approximations of HuŐąckel theory are lifted, enabling the exploration of the influence of each of these approximations on the transport properties of expanded porphyrins. Along the way, a detailed study on the relationship between the conductance and aromaticity/topology has been undertaken. Even though it has been proposed that the ŌÄ-conjugated systems of expanded porphyrins can be approximated as polyene macrocycles based on the ‚Äúannulene model‚ÄĚ, it turns out that the distortion induced by the pyrrole rings to the electronic structure of the expanded porphyrins causes the simple counting rule for the prediction of quantum interference developed for polyenes to fail in some specific situations. Nevertheless, our back-of-the-envelope approach enables an intuitive rationalization of most of the transport properties of expanded porphyrins. Our conclusions cast further doubt on the proposed negative relationship between conductance and aromaticity and highlight the importance of the connectivity on determining the shape of the transmission functions of the different states. We hope that the new insights provided here will offer experimentalists a road map toward the design of functional, multidimensional electronic switches based on expanded porphyrins

    Captodative Substitution: A Strategy for Enhancing the Conductivity of Molecular Electronic Devices

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    We explore a new strategy to tune the conductivity of molecular electronic devices: captodative substitution. We demonstrate that a careful design of such substitution schemes on a benzene parental structure can enhance the conductivity by almost an order of magnitude under small bias. Once this new strategy has been established, we apply it to molecular wires and demonstrate that it enables the unprecedented anti-Ohmic design of wires whose conductivity increases with the length. Overall, the captodative substitution approach provides a very promising pathway toward full chemical control of the conductivity of molecules which opens up the possibility to design molecular switches with an improved on/off ratio among others