We report on the comparison of the electronic and photophysical properties of a series of related donor-acceptor-donor oligomers incorporating the previously known 2H-benzo[d][1,2,3]triazole (BTz) moiety as the acceptor and the recently reported BTzTD acceptor, a hybrid of BTz and 2,1,3-benzothiadiazole (BTD). Although often implied in the polymer literature that BTz has good acceptor character, we show that this moiety is best described as a weak acceptor. We present electrochemical, computational, and photophysical evidence supporting our assertion that BTzTD is a strong electron acceptor while maintaining the alkylation ability of the BTz moiety. Our results show that the identity of the central atom (N or S) in the benzo-fused heterocyclic ring plays an important role in both the electron-accepting and the electron-donating ability of acceptor moieties with sulfur imparting a greater electron-accepting ability and nitrogen affording greater electron-donating character. We report on the X-ray crystal structure of a BTzTD trimer, which exhibits greater local aromatic character in the region of the triazole ring and contains an electron-deficient sulfur that imparts strong electron-accepting ability. Additionally, we examine the transient absorption spectra of BTzTD and BTz oligomers and report that the BTz core promotes efficient intersystem crossing to the triplet state, while the presence of the thiadiazole moiety in BTzTD leads to a negligible triplet yield. Additionally, while BTz does not function as a good acceptor, oligomers containing this moiety do function as excellent sensitizers for the generation of singlet oxygen.

Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods, where a real catalyst, cis-Ru(H)2(PMe3)3, was employed in calculations and compared with a model catalyst, cis-Ru(H)2(PH3)3. Significant differences between the real and model systems are observed in CO2 insertion into the Ru(II)-H bond, isomerization of a ruthenium(II) eta1-formate intermediate, and metathesis of the eta1-formate intermediate with a dihydrogen molecule. All these reactions more easily occur in the real system than in the model system. The differences are interpreted in terms that PMe3 is more donating than PH3 and the trans-influence of PMe3 is stronger than that of PH3. The rate-determining step is the CO2 insertion into the Ru(II)-H bond. Its deltaG(o++) value is 16.8 (6.8) kcal/mol, where the value without parentheses is calculated with the MP4(SDQ) method and that in parentheses is calculated with the DFT method. Because this insertion is considerably endothermic, the coordination of the dihydrogen molecule with the ruthenium(II)-eta1-formate intermediate must necessarily occur to suppress the deinsertion. This means that the reaction rate increases with increase in the pressure of dihydrogen molecule, which is consistent with the experimental results. Solvent effects were investigated with the DPCM method. The activation barrier and reaction energy of the CO2 insertion reaction moderately decrease in the order gas phase > n-heptane > THF, while the activation barrier of the metathesis considerably increases in the order gas phase < n-heptane < THF. Thus, a polar solvent should be used because the insertion reaction is the rate-determining step.

By making full use of the field theoretical nature of the chiral quark soliton model, we demonstrate that the nonvanishing quark condensate as a signal of the spontaneous chiral symmetry breaking of the QCD vacuum brings about a delta-function singularity at xϭ0 in the chirally odd twist-3 distribution e(x) of the nucleon. This singularity in e(x), which would be observed as a sizable violation of the 1st moment sum rule, is then interpreted as giving a very rare case that the nontrivial vacuum structure of QCD manifests in an observable of a localized QCD excitation, i.e., the nucleon.

We propose an orbital optimized method for unitary coupled cluster theory (OO-UCC) within the variational quantum eigensolver (VQE) framework for quantum computers. OO-UCC variationally determines the coupled cluster amplitudes and also molecular orbital coefficients. Owing to its fully variational nature, first-order properties are readily available. This feature allows the optimization of molecular structures in VQE without solving any additional equations. Furthermore, the method requires smaller active space and shallower quantum circuits than UCC to achieve the same accuracy. We present numerical examples of OO-UCC using quantum simulators, which include the geometry optimization of water and ammonia molecules using analytical first derivatives of the VQE.

Transmetalation between palladium(II)-vinyl complex and vinylsilane was theoretically investigated with the DFT and MP2 to MP4 methods to clarify the reaction mechanism and the reasons why fluoride anion accelerates the Pd-catalyzed cross-coupling reaction between vinyl iodide and vinylsilane. This transmetalation occurs with a very large activation barrier (45.8 kcal/mol) and a very large endothermicity (25.6 kcal/mol) in the absence of fluoride anion, where the potential energy change resulting from the solvation effect is evident. This is consistent with the experimental fact that this cross-coupling reaction does not proceed well in the absence of fluoride anion. The effects of fluoride anion were investigated in three possible reaction courses. In the first course, fluorovinylsilicate anion is formed before the transmetalation, and it reacts with the palladium(II)-vinyl complex. In the second course, an iodo ligand is substituted for fluoride anion, and then the transmetalation occurs between the palladium(II)-fluoro-vinyl complex and vinylsilane. In the third course, fluoride anion attacks the Si center of vinylsilane in the transition state of the transmetalation between the palladium(II)-iodo-vinyl complex and vinylsilane. Our theoretical calculation suggests that fluorovinylsilicate anion is not formed in the case of trimethylvinylsilane. In the second and third cases, the transmetalation occurs with a moderate activation barrier (E(a)) and a considerably large exothermicity (E(exo)): E(a) = 25.3 kcal/mol and E(exo) = 5.7 kcal/mol in the second course, and E(a) = 12.7 kcal/mol and E(exo) = 24.8 kcal/mol in the third course, indicating that fluoride anion accelerates the transmetalation via the second and third reaction courses. The acceleration of transmetalation by fluoride anion is clearly interpreted in terms of the formation of a very strong Si-F bond and the stabilization of the transition state by the hypervalent Si center, which is induced by the fluoride anion. Our computational results show that hydroxide anion accelerates the transmetalation in a manner similar to that observed with fluoride anion. From these results, we predict that the electronegative anion accelerates this transmetalation because the electronegative group forms a strong covalent bond with the silyl group and facilitates the formation of the hypervalent Si center in the transition state.

Ru-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods. In the presence of water molecules, the reaction proceeds as follows: (1) Carbon dioxide forms the adduct cis-Ru(H) 2 (PMe 3 ) 3 (H 2 O)(CO 2 ), in which the C and O atoms of CO 2 interact with the H (hydride) ligand and the H atom of H 2 O, respectively. (2) Nucleophilic attack of the H ligand to CO 2 takes place easily to afford a Ru-(η 1 -formate) intermediate, Ru(H)(PMe 3 ) 3 (η 1 -OCOH)(H 2 O), with a much smaller activation barrier than that of the CO 2 insertion into the Ru-H bond, which is the ratedetermining step in the absence of water molecules. (3) The rate-determining step is the coordination of a dihydrogen molecule with the Ru-(η 2 -formate) complex, Ru(H)(PMe 3 ) 3 (η 2 -O 2 CH)(H 2 O), the activation barrier of which is smaller than that of the CO 2 insertion into the Ru-H bond. (4) The metathesis of the Ru-(η 1 -fomate) moiety with the dihydrogen molecule easily occurs in Ru(H)(PMe 3 ) 3 (η 1 -OCOH)(H 2 )-(H 2 O) to afford formic acid with a moderate activation barrier. On the basis of these results, it should be concluded that the early half of the reaction mechanism changes by the presence of water molecules, which is the reason for the acceleration by water molecules. One of the most important results is that the aqua ligand accelerates the nucleophilic attack of the H ligand to CO 2 because the hydrogen-bonding interaction between the aqua ligand and carbon dioxide decreases the activation barrier and increases the exothermicity. Theoretical calculations clearly show that similar acceleration is induced by amines and alcohols.

The explicitly correlated approach is one of the most important breakthroughs in ab initio electronic structure theory, providing arguably the most compact, accurate, and efficient ansatz for describing the correlated motion of electrons. Since Hylleraas first used an explicitly correlated wave function for the He atom in 1929, numerous attempts have been made to tackle the significant challenges involved in constructing practical explicitly correlated methods that are applicable to larger systems. These include identifying suitable mathematical forms of a correlated wave function and an efficient evaluation of many-electron integrals. R12 theory, which employs the resolution of the identity approximation, emerged in 1985, followed by the introduction of novel correlation factors and wave function ansätze, leading to the establishment of F12 theory in the 2000s. Rapid progress in recent years has significantly extended the application range of explicitly correlated theory, offering the potential of an accurate wave-function treatment of complex systems such as photosystems and semiconductors. This perspective surveys explicitly correlated electronic structure theory, with an emphasis on recent stochastic and deterministic approaches that hold significant promise for applications to large and complex systems including solids. Published by AIP Publishing. [http://dx

The isosinglet combination of the chiral-odd twist-3 distribution function e u (x) + e d (x) of the nucleon has outstanding properties that its first moment is proportional to the well-known πN sigma-term and that it contains a δ-function singularity at x = 0. These two features are inseparably connected in that the above sum rule would be violated, if there is no such a singularity in e u (x) + e d (x). Very recently, we found that the physical origin of this δ-function singularity can be traced back to the long-range quark-quark correlation of scalar type, which signals the spontaneous chiral symmetry breaking of the QCD vacuum. The main purpose of the present paper is to give complete theoretical predictions for the chiral-odd twist-3 distribution function e a (x) of each flavor a on the basis of the chiral quark soliton model, without recourse to the derivative expansion type approximation. These theoretical predictions are then compared with the empirical information extracted from the CLAS data of the semi-inclusive DIS processes by assuming the Collins mechanism only. A good agreement with the CLAS data is indicative of a sizable violation of the πN sigma-term sum rule, or equivalently, the existence of a δ-function singularity in e u (x) + e d (x).