Dynamic polarizabilities of polyaromatic hydrocarbons using coupled-cluster linear response theory

Coupled-cluster theory with single and double excitations is applied to the calculation of optical properties of large polyaromatic hydrocarbons. Dipole polarizabilities are reported for benzene, pyrene, and the oligoacenes sequence n=2-6. Dynamic polarizabilities were calculated on polyacences as large as pentacene for a single frequency and for benzene and pyrene at many frequencies. The basis set effect was studied for benzene using a variety of basis sets in the Pople [Theor. Chim. Acta 28, 213 (1973)] and Dunning [J. Chem. Phys. 90, 1007 (1989)] families up to aug-cc-pVQZ and the Sadlej pVTZ basis [Collect. Czech. Chem. Commun. 53, 1995 (1998)], which was used exclusively for the largest molecules. Geometries were optimized using HF, B3LYP, PBE0, and MP2 and compared to experiment to measure method dependence and the possible role of bond-length alternation. Finally, the polarizability results were compared to four common density functionals (B3LYP, BLYP, PBE0, PBE).     

C and H nuclear magnetic shielding and spin–spin coupling constants of C-enriched bromomethane in the gas phase

¹³C and ¹H NMR spectral parameters are investigated for ¹³CH₃Br in gaseous matrices. It is found that both the ¹³C and ¹H chemical shifts of ¹³CH₃Br are linearly dependent on solvent density. Similar dependence is also detected for one-bond spin–spin coupling, ¹J(CH). For the first time the ¹³C and ¹H magnetic shielding constants and ¹J(CH) spin–spin coupling are obtained for an isolated ¹³CH₃Br molecule together with the coefficients responsible for solute–solvent molecular interactions in gaseous matrices. The present experimental results confirm the accuracy of some recent ab initio calculations of nuclear magnetic shielding performed for bromomethane.     

Vibrational frequencies and force constants of N2O4 in complexes with molecules of different solvents determined by Raman spectra andAb initio calculation

The Raman spectra of N₂O₄ solutions in organic solvents have been recorded. The frequencies ofv ₁,v ₂, andv ₃ bands of N₂O₄ increase with increasing solvent electron-donor properties. Especially large changes ofv ₃ N-N stretching band have been observed (254.5 cm^–1 in n-hexane, 276.5 cm^–1 in 1,4-dioxane). The ab initio calculations have shown that the interaction between N₂O₄ and electron-donor molecules causes an increase of N-N and N-O stretching and O-N-O bending force constants of N₂O₄ in agreement with the results of Raman study.     

2‐Methylphenyl 2‐methoxyacridine‐9‐carboxylate

The title compound, C₂₂H₁₇NO₃, crystallizes in the monoclinic space group P2₁/c with four molecules per unit cell. The mol­ecules are arranged in centrosymmetric pairs, joined via the C and attached H atoms in the meta position relative to the methoxy group. These pairs are bonded in the crystalline phase as a result of non‐specific dispersive interactions, and through a network of C—H?O interactions involving the non‐bonded O atom of the carboxy group and, to some extent, the O atom of the methoxy group. The methoxy substituent lies in the plane of the almost planar acridine moiety and is directed towards the phenyl ester group. The phenyl ester group itself is twisted by 35.9 (5)° relative to the mean plane of the acridine moiety.     

Application of Brønsted-type LFER in the study of the phospholipase C mechanism

Phosphatidylinositol-specific phospholipase C cleaves the phosphodiester bond of phosphatidylinositol to form inositol 1,2-cyclic phosphate and diacylglycerol. This enzyme also accepts a variety of alkyl and aryl inositol phosphates as substrates, making it a suitable model enzyme for studying mechanism of phosphoryl transfer by probing the linear free-energy relationship (LFER). In this work, we conducted a study of Br?nsted-type relationship (log k = beta(lg) pK(a) + C) to compare mechanisms of enzymatic and nonenzymatic reactions, confirm the earlier proposed mechanism, and assess further the role of hydrophobicity in the leaving group as a general acid-enabling factor. The observation of the high negative Br?nsted coefficients for both nonenzymatic (beta(lg) = -0.65 to -0.73) and enzymatic cleavage of aryl and nonhydrophobic alkyl inositol phosphates (beta(lg) = -0.58) indicates that these reactions involve only weak general acid catalysis. In contrast, the enzymatic cleavage of hydrophobic alkyl inositol phosphates showed low negative Br?nsted coefficient (beta(lg) = -0.12), indicating a small amount of the negative charge on the leaving group and efficient general acid catalysis. Overall, our results firmly support the previously postulated mechanism where hydrophobic interactions between the enzyme and remote parts of the leaving group induce an unprecedented negative-charge stabilization on the leaving group in the transition state.     

Synthesis and isomer distribution of 2-alkyltropinones and 2-alkylgranatanones

Tropinone (8-methyl-8-azabicyclo[3.2.1]octan-3-one) metal (Li, Na, K, Mg) enolates were used to achieve α-alkylation. The reactions, regardless of the metal or conditions used, were low-yielding. N,N-Dimethylhydrazones of tropinone and granatanone (pseudopelletierine, 9-methyl-9-azabicyclo[3.3.1]nonan-3-one) were prepared and α-alkylated using n-butyllithium as the lithiating reagent. Lithium amides, including a polymer-supported lithium amide, were less effective. The reactions were modelled using DFT calculations at the B3LYP 6-31G(d) level and the CPCM solvent model, revealing that the face-selective alkylations of tropinone and granatanone hydrazones favoured the exo-isomers. Granatanone and α-isopropyl tropinone derivatives resisted typical mild hydrolytic hydrazone cleavage (aqueous trifluoroacetic acid) and required more forceful conditions (p-TsOH, boiling dioxane). Using the hydrazone alkylation strategy, 16 α-alkyl derivatives (benzyl, methyl, propyl, isopropyl, allyl, pentyl, heptyl, p-methoxybenzyl) were prepared in 52–90% yields overall. For the α-alkylated tropinones and granatanones (10 examples), the DFT calculations and experimental thermodynamic distributions in base catalysed equilibrations showed that except for the α-isopropyl derivatives, the endo-isomers were more stable than the exo-isomers and were the major products. For 2-isopropyltropinone, the bulky substituent favoured the exo (axial) position in the bicyclic skeleton. The thermodynamic distribution for the α,α′-dibenzyl tropinone isomers was also evaluated.     

Chelating Ruthenium Phenolate Complexes: Synthesis,General Catalytic Activity,and Applications in Olefin Metathesis Polymerization

Cyclic Ru‐phenolates were synthesized, and these compounds were used as olefin metathesis catalysts. Investigation of their catalytic activity pointed out that, after activation with chemical agents, these catalysts promote ring‐closing metathesis (RCM), enyne and cross‐metathesis (CM) reactions, including butenolysis, with good results. Importantly, these latent catalysts are soluble in neat dicyclopentadiene (DCPD) and show good applicability in ring‐opening metathesis polymeriyation (ROMP) of this monomer.     

A facile route to electronically conductive polyelectrolyte brushes as platforms of molecular wires

A facile strategy for the synthesis of conjugated polyelectrolyte brushes grafted from a conductive surface is presented. Such brushes form a platform of molecular wires oriented perpendicularly to the surface, enabling efficient directional transport of charge carriers. As the synthesis of conjugated polymer brushes using chain-growth polymerization via a direct “grafting from” approach is very challenging, we developed a self-templating surface-initiated method. It is based on the formation of multimonomer template chains in the first surface-initiated polymerization step, followed by the second polymerization leading to conjugated chains in an overall ladder-like architecture. This strategy exploits the extended conformation of the surface-grafted brushes, thereby enabling alignment of the pendant polymerizable groups along the template chains. We synthesized a new bifunctional monomer and used the developed approach to obtain quaternized poly(ethynylpyridine) chains on a conductive indium tin oxide surface. A catalyst-free quaternization polymerization was for the first time used here for surface grafting. The presence of charged groups makes the obtained brushes both ionically and electronically conductive. After doping with iodine, the brushes exhibited electronic conductivity, in the direction perpendicular to the surface, as high as 10^–1–10⁰ S m^–1. Tunneling AFM was used for mapping the surface conductivity and measuring the conductivity in the spectroscopic mode. The proposed synthetic strategy is very versatile as a variety of monomers with pendant polymerizable groups and various polymerization techniques may be applied, leading to platforms of molecular wires with the desired characteristics.     

On the electronically excited states of uracil

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation.Our best estimates for the vertical excitation energies for the lowest singlet n --> pi* and pi --> pi* are 5.0 +/- 0.1 eV and 5.3 +/- 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/- 0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A" and A' states leading to intensity borrowing by the forbidden transition.