Browsing by Subject "NMR CHEMICAL-SHIFTS"

Sort by: Order: Results:

Now showing items 1-6 of 6
  • Valiev, Rashid R.; Fliegl, Heike; Sundholm, Dage (2018)
    Aromatic properties of two recently synthesized dithienothiophene-bridged (DTT) [34]octaphyrins have been investigated by calculating magnetically induced current densities and vertical excitation energies. These intriguing molecules have been proposed to be the first synthesized neutral bicycloaromatic compounds. The triplet state of their dications was even suggested to be Baird-type bicycloaromatic rendering them very interesting as a new prototype of molecules possessing simultaneously the two rare types of aromaticity. Here, we investigate computationally the aromatic properties of the neutral as well as the singly and doubly charged DTT-bridged [34]octaphyrins. Our study provides unambiguous information about changes in the aromatic properties of the DTT-bridged [34]octaphyrins upon oxidation. The calculations identify two independent diatropic ring currents in the neutral DTT-bridged [34]octaphyrins, showing that they are indeed bicycloaromatic. The current-density flow of the two independent ring currents of the bicycloaromatic compounds are visualized and individual aromatic pathways are quantified by performing numerical integration. The calculations show that two independent diatropic ring currents can indeed be sustained by molecules consisting of two aromatic rings that share a common set of π electrons. The current density calculations on the singly charged DTT-bridged [34]octaphyrins show that they are weakly antiaromatic, which does not agree with the suggested aromatic character deduced from spectroscopical studies. The triplet state of the two DTT-bridged [34]octaphyrin cations with very similar molecular structures have unexpectedly different aromatic character. One of them is Baird-type bicycloaromatic, whereas the triplet state of the other dication has one aromatic and one nonaromatic ring, which could not be resolved from available spectroscopical data. Calculations of excitation energies reveal that a simple model cannot be employed for interpreting the electronic excitation spectra of the present molecules, because more than 20 excited states contribute to the spectra above 2.5 eV (500 nm) showing the importance of computations. The present work illustrates how detailed information about molecular aromaticity can nowadays be obtained by scrutinizing calculated current densities.
  • Franzke, Yannick J.; Sundholm, Dage; Weigendb, Florian (2017)
    Magnetically induced current density susceptibilities have been studied for a number of cyclic ethyne and butadiyne-bridged porphyrin and isoporphyrin arrays. The current density susceptibilities have been calculated using the gauge-including magnetically induced current (GIMIC) method, which is interfaced to the TURBOMOLE quantum chemistry code. Aromatic properties and current pathways have been analyzed and discussed by numerical integration of the current density susceptibilities passing selected chemical bonds yielding current strength susceptibilities. Despite the interrupted p-framework, zinc(II) isoporphyrin sustains a ring current of ca. 10 nA T-1. Porphyrin and isoporphyrin dimers sustain a significant current strength at the linker, whereas the larger porphyrinoid arrays sustain mainly local ring currents. Isoporphyrin dimers with saturated meso carbons have strong net diatropic ring-current strengths of 20 nA T-1 fulfilling Huckels aromaticity rule. Porphyrin trimers and tetramers exhibit almost no current strength at the linker. The porphyrin moieties maintain their strong net diatropic ring current.
  • Valiev, Rashid R.; Benkyi, Isaac; Konyshev, Yuri; Fliegl, Heike; Sundholm, Dage (2018)
    Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo [6]pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin-orbit interaction between the lowest singlet (S-0) and triplet (T-1) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin-orbit coupling vanishes. The experimentally detected fluorescence from S-2 to S-0 of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins.
  • Sundholm, Dage; Rauhalahti, Markus; Özcan, Nergiz; Mera-Adasme, Raul; Kussmann, Jorg; Luenser, Arne; Ochsenfeld, Christian (2017)
    Nuclear magnetic shieldings have been calculated at the density functional theory (DFT) level for stacks of benzene, hexadehydro[12]annulene, dodecadehydro[18]annulene, and hexabenzocoronene. The magnetic shieldings due to the ring currents in the adjacent molecules have been estimated by calculating nucleus independent molecular shieldings for the monomer in the atomic positions of neighbor molecules. The calculations show that the independent shielding model works reasonably well for the H-1 NMR shieldings of benzene and hexadehydro[12]annulene, whereas for the larger molecules and for the C-13 NMR shieldings the interaction between the molecules leads to shielding effects that are at least of the same size as the ring current contributions from the adjacent molecules. A better agreement is obtained when the nearest neighbors are also considered at full quantum mechanical (QM) level. The calculations suggest that the nearest solvent molecules must be included in the quantum mechanical system, at least when estimating solvent shifts at the molecular mechanics (MM) level. Current density calculations show that the stacking does not significantly affect the ring current strengths of the individual molecules, whereas the shape of the ring current for a single molecule differs from that of the stacked molecules.
  • Valiev, Rashid R.; Fliegl, Heike; Sundholm, Dage (2015)
    Magnetically induced current densities have been calculated for dioxaporphyrin, dithiaporphyrin, true carbaporphyrins, and N-confused porphyrins using the gauge-including magnetically induced current (GIMIC) method. The current-strength susceptibilities (current strengths) have been obtained by numerically integrating the current flow passing selected chemical bonds. The current strength calculations yield very detailed information about the electron delocalization pathways of the molecules. The strength of the ring-current that circles around the porphyrinoid macroring is used to estimate the degree of molecular aromaticity. The studied porphyrinoid structures have been obtained by replacing the NH and N groups of porphin with formally isoelectronic moieties such as O, S, CH and CH2. Replacing an NH moiety of trans-porphin with isoelectronic O and S does not significantly change the current strengths and pathways, whereas substitution of N with an isoelectronic CH group leads to significant changes in the current pathway and current strengths. CH2 groups cut the flow of diatropic currents, whereas in strongly antiaromatic molecules a significant fraction of the paratropic ring-current is able to pass the sp(3) hybridized inner carbons. N-confused porphyrinoids sustain a ring current whose strength is about half the ring-current strength of porphin with the dominating current flow along the outer pathway via the NH moiety. When no hydrogen is attached to the inner carbon of the inverted pyrrolic ring, the current prefers the inner route at that ring.
  • Joost, Maximilian; Nieger, Martin; Lutz, Martin; Ehlers, Andreas W.; Slootweg, Jacob Christiaan (Chris); Lammertsma, Koop (2020)
    O- and S-heterocyclic carbenes (OHCs, SHCs) are shown experimentally and computationally to be stronger pi acceptors than NHCs and lack, of course, substituents at the heteroatoms. These different electronic and steric characteristics make OHCs and SHCs interesting ligands for coordination chemistry. Convenient synthetic routes are presented to access their iridium(I), iridium(III), and coinage-metal(I) (Cu, Ag, Au) complexes in good yields by means of dissociation of olefins, deprotonation of precursor salts, and transmetalation from a silver carbene complex Molecular structures and detailed bonding analyses of these complexes are presented.