Christian B. Nielsen

Density functional self-consistent quantum mechanics/molecular mechanics theory for linear and nonlinear molecular properties: Applications to solvated water and formaldehyde.

A combined quantum mechanics/molecular mechanics (QM/MM) method is described, where the polarization between the solvent and solute is accounted for using a self-consistent scheme linear in the solvent polarization. The QM/MM method is implemented for calculation of energies and molecular response properties including the calculation of linear and quadratic response functions using the density-functional theory (DFT) and the Hartree-Fock (HF) theory. Sample calculations presented for ground-state energies, first-order ground-state properties, excitation energies, first-order excited state properties, polarizabilities, first-hyperpolarizabilities, and two-photon absorptions strengths of formaldehyde suggests that DFT may in some cases be a sufficiently reliable alternative to high-level theory, such as coupled-cluster (CC) theory, in modeling solvent shifts, whereas results obtained with the HF wave function deviate significantly from the CC results. Calculations carried out on water gives results that also are comparable with CC calculations in accuracy for ground-state and first-order properties. However, to obtain such accuracy an exchange-correlation functional capable of describing the diffuse Rydberg states must be chosen.

Organic Light‐Emitting Diodes from Symmetrical and Unsymmetrical π‐Extended Tetraoxa[8]circulenes

The understanding and development of molecular electronics depends on the discovery and exploitation of new pextended organic materials. One area of interest is the design of small molecules that are suitable as the fluorescent component in blue organic light-emitting diodes (OLEDs). OLEDs based on small organic molecules or conjugated polymers have been explored for applications in full-colour flat-panel displays, since the seminal work by the group at Kodak. Some classes of low-mass blue-emitting materials have been exploited, such as spirobifluorenes, oligofluorenes, siloles and distyrylarylenes, but the introduction of completely new structural motifs remain scarce. To this end, we report the acid-mediated condensation of a 2,3-diACHTUNGTRENNUNGalkyl-1,4-benzoquinone with 1,4-naphthoquinone to produce a series of highly soluble p-extended tetraoxa[8]circulenes (Figure 1) as well as the application of these materials in blue OLEDs. Tetraoxa[8]circulene (4 B) is a planar heteroaromatic compound formed in trace amounts by treating 1,4-benzoquinone with strong acids. The structure of the tetraoxa[8]circulene framework was deduced by means of mass spectrometry of the all-naphthalene tetraoxa[8]circulene (4 N) by Erdtman and Hçgberg, and was later confirmed by single-crystal X-ray crystallography. It became clear that when one side of the 1,4-benzoquinone was substituted, either by using 1,4-naphthoquinone or a 2,3-disubstituted1,4-benzoquinone, the tetraoxa[8]circulene was a major product upon acid treatment. More recently a series of liquid-crystalline tetraoxa[8]circulenes were synthesised by attachment of linear alkyl chains to the tetraoxa[8]circulene framework. That work also introduced a relatively mild method for the cyclisation reaction: treatment of the quinone with BF3·OEt2 in boiling CH2Cl2. The tetraoxa[8]circulene has been suggested as an intercalator for DNA. Rathore and co-workers used this synthetic protocol to prepare new stable radical cation salts based on tetraoxa[8]circulenes. From a synthetic and a materials point of view, the exploration of p-extended tetraoxa[8]circulenes is of interest. As the processing ability of large aromatic materials and polymers relies on solubility in organic solvents, the development of tetraoxa[8]circulenes based on 2,3-dialkyl-1,4-ben[a] Dr. C. B. Nielsen, T. Brock-Nannestad, Dr. T. K. Reenberg, Dr. P. Hammershøj, Dr. J. B. Christensen, Dr. M. Pittelkow Department of Chemistry, University of Copenhagen Universitetsparken 5, 2100 Copenhagen Ø (Denmark) E-mail : pittel@kiku.dk [b] Dr. J. W. Stouwdam Laboratory of Macromolecular and Organic Chemistry Eindhoven University of Technology, P. O. Box 513 5600 MB Eindhoven (The Netherlands) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002261. Figure 1. Structure of symmetrical and unsymmetrical p-extended tetraACHTUNGTRENNUNGoxa[8]circulenes.

Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

Diazadioxa(8)circulenes: Planar Antiaromatic Cyclooctatetraenes

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one-pot transformation in which two C-C bonds and two C-O bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight-membered ring has antiaromatic character.

Nuclear magnetic shielding constants of liquid water: Insights from hybrid quantum mechanics/molecular mechanics models

We present a gauge-origin independent method for the calculation of nuclear magnetic shielding tensors of molecules in a structured and polarizable environment. The method is based on a combination of density functional theory (DFT) or Hartree-Fock wave functions with molecular mechanics. The method is unique in the sense that it includes three important properties that need to be fulfilled in accurate calculations of nuclear magnetic shielding constants: (i) the model includes electron correlation effects, (ii) the model uses gauge-including atomic orbitals to give gauge-origin independent results, and (iii) the effect of the environment is treated self-consistently using a discrete reaction-field methodology. The authors present sample calculations of the isotropic nuclear magnetic shielding constants of liquid water based on a large number of solute-solvent configurations derived from molecular dynamics simulations employing potentials which treat solvent polarization either explicitly or implicitly. For both the (17)O and (1)H isotropic shielding constants the best predicted results compare fairly well with the experimental data, i.e., they reproduce the experimental solvent shifts to within 4 ppm for the (17)O shielding and 1 ppm for the (1)H shielding.

Nucleus-independent chemical shift criterion for aromaticity in π-extended tetraoxa[8]circulenes

AbstractRecently synthesized π-extended symmetrical tetraoxa[8]circulenes that exhibit electroluminescent properties were calculated at the density functional theory (DFT) level using the quantum theory of atoms in molecules (QTAIM) approach to electron density distribution analysis. Nucleus-independent chemical shift (NICS) indices were used to characterize the aromaticity of the studied molecules. The tetraoxa[8]circulene molecules were found to consist of two antiaromatic perimeters (according to the Hückel “4n” antiaromaticity rule) that include 8 and 24 π-electrons. Conversely, NICS calculations demonstrated the existence of a common π-extended system (distributed like a flat ribbon) in the studied tetraoxa[8]circulene molecules. Thus, these symmetrical tetraoxa[8]circulene molecules provide examples of diatropic systems characterized by the presence of induced diatropic ring currents. FigureSpecial aromaticity of the tetraoxa[8]circulenes

Two-photon absorption cross sections: An investigation of solvent effects. Theoretical studies on formaldehyde and water

The effects of a solvent on the two-photon absorption of microsolvated formaldehyde and liquid water have been studied using hybrid coupled-cluster/molecular mechanics (CC/MM) response theory. Both water and formaldehyde were considered solvated in water, where the solvent water molecules were described within the framework of molecular mechanics. Prior to the CC/MM calculations, molecular dynamics simulations were performed on the water/formaldehyde and water/water aggregates and many configurations were generated. By carrying out CC/MM response calculations on the individual configurations, it was possible to obtain statistically averaged results for both the excitation energies and two-photon absorption cross sections. For liquid water, the comparison between one- and two-photon absorption spectra is in good agreement with the experimental data available in the literature. In particular, the lowest energy transition occurring in the one-photon absorption spectrum of water only occurs with a relatively small strength in the two-photon absorption spectrum. This result is important for the interpretation of two-photon absorption data as these results show that in the absence of selection rules that determine which transitions are forbidden, the spectral profile of the two-photon absorption spectrum can be significantly different from the spectral profile of the one-photon absorption spectrum.

Two-photon absorption cross sections: An investigation of the accuracy of calculated absolute and relative values

We have studied the basis set and electron correlation effects on the ab initio calculations of two-photon absorption cross sections of water. Various series of correlation consistent basis sets up to triply augmented basis sets of valence pentuple zeta level as well as the popular 6-31G(d) basis set have been employed in combination with several coupled cluster, configuration interaction, and density functional theory methods. We find that it is very difficult to obtain converged values of the cross sections for even a small molecule such as water. Acknowledging these difficulties in obtaining a fully converged cross section for a given state, we also investigated the possibility of determining relative cross sections for a series of organic molecules. However, we did not find consistency between the relative cross sections calculated at the Hartree-Fock level and several coupled-cluster methods using the 6-31G(d) and aug-cc-pVDZ basis sets. However, we could reproduce the relative ordering of the two-photon absorption cross sections of the molecules studied at the Hartree-Fock level.

Response theory in the multipole reaction field model for equilibrium and nonequilibrium solvation: Exact theory and the second order polarization propagator approximation

Exact response functions are derived for a multipole solventreaction field model of equilibrium and nonequilibrium solvation using the generalized Ehrenfest theorem and assuming a spherical cavity surrounding the solute. The starting point is the Schrodinger equation and we shortly review how the reaction field is introduced into the Schrodinger equation in order to clearly identify the limitations of describing solute–solvent interactions with a reaction field model. The solvent is described as an isotropic homogeneous linear dielectric medium characterized by a static and an optical dielectric constant. From the exact response functions we derive linear response functions within the higher random phase and the second order polarization propagator approximation. Excitation energies,oscillator strengths, and polarizabilities are then calculated for solvated H 2 S and furan using the augmented correlation consistent triple-ζ (aug-cc-pVTZ) and double-ζ (aug-cc-pVDZ) basis sets for H 2 S and furan, respectively. We have also calculated excitation energies and oscillator strengths for H 2 S with standard (vacuum) ab initio methods using a variety of basis set, as there has been no previously reported values of these quantities calculated with the second order polarization propagator approximation. The second order polarization propagator approximation gives excitation energies and oscillator strengths close to values obtained by coupled cluster methods for a solvated H 2 S molecule, whereas the higher random phase approximation tends to overestimate the value of these quantities. The solventeffect of the excitation energies follow the same trends for all of the reaction field ab initio methods used in the present study, but some oscillator strengths show different solventeffects whether they are calculated with correlated or with noncorrelated ab initio methods. The calculated polarizabilities show the same solventeffect independent of any inclusion of dynamical electron correlation. It is also shown that the equilibrium solvation model is not appropriate for high-frequency perturbations.

A safflower oil based high-fat/high-sucrose diet modulates the gut microbiota and liver phospholipid profiles associated with early glucose intolerance in the absence of tissue inflammation

Scope: Omega‐6 (n‐6) PUFA‐rich diets are generally considered obesogenic in rodents. Here, we examined how long‐term intake of a high‐fat/high‐sucrose (HF/HS) diet based on safflower oil affected metabolism, inflammation, and gut microbiota composition. Methods and results: We fed male C57BL/6J mice a HF/HS diet based on safflower oil—rich in n‐6 PUFAs—or a low‐fat/low‐sucrose diet for 40 wk. Compared to the low‐fat/low‐sucrose diet, intake of the safflower‐based HF/HS diet only led to moderate weight gain, while glucose intolerance developed at week 5 prior to signs of inflammation, but concurrent with increased levels of linoleic acid and arachidonic acid in hepatic phospholipids. Intake of the HF/HS diet resulted in early changes in the gut microbiota, including an increased abundance of Blautia, while late changes coincided with altered inflammatory profiles and increased fasting plasma insulin. Analysis of immune cells in visceral fat and liver revealed no differences between diets before week 40, where the number of immune cells decreased in the liver of HF/HS‐fed mice. Conclusion: We suggest that a diet‐dependent increase in the n‐6 to omega‐3 (n‐3) PUFA ratio in hepatic phospholipids together with gut microbiota changes contributed to early development of glucose intolerance without signs of inflammation.