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Quantum light is increasingly recognized as a promising resource for developing optical measurement techniques. Particular attention has been paid to enhancing the precision of the measurements beyond classical techniques by using nonclassical correlations between quantum entangled photons. Recent advances in the quantum optics technology have made it possible to manipulate spectral and temporal properties of entangled photons, and photon correlations can facilitate the extraction of matter information with relatively simple optical systems compared to conventional schemes. In these respects, the applications of entangled photons to time-resolved spectroscopy can open new avenues for unambiguously extracting information on dynamical processes in complex molecular and materials systems. Here, we propose time-resolved spectroscopy in which specific signal contributions are selectively enhanced by harnessing nonclassical correlations of entangled photons. The entanglement time characterizes the mutual delay between an entangled twin and determines the spectral distribution of photon correlations. The entanglement time plays a dual role as the knob for controlling the accessible time region of dynamical processes and the degrees of spectral selectivity. In this sense, the role of the entanglement time is substantially equivalent to the temporal width of the classical laser pulse. The results demonstrate that the application of quantum entangled photons to time-resolved spectroscopy leads to monitoring dynamical processes in complex molecular and materials systems by selectively extracting desired signal contributions from congested spectra. We anticipate that more elaborately engineered photon states would broaden the availability of quantum light spectroscopy. [ABSTRACT FROM AUTHOR]

We propose a new hybrid approach combining quantum chemistry and statistical mechanics of liquids for calculating the nuclear magnetic resonance (NMR) chemical shifts of solvated molecules. Based on the reference interaction site model self-consistent field with constrained spatial electron density distribution (RISM–SCF–cSED) method, the electronic structure of molecules in solution is obtained, and the expression for the nuclear magnetic shielding tensor is derived as the second-order derivative of the Helmholtz energy of the solution system. We implemented a method for calculating chemical shifts and applied it to an adenine molecule in water, where hydrogen bonding plays a crucial role in electronic and solvation structures. We also performed the calculations of 17O chemical shifts, which showed remarkable solvent dependence. While converged results could not be sometimes obtained using the conventional method, in the present framework with RISM–SCF–cSED, an adequate representation of electron density is guaranteed, making it possible to obtain an NMR shielding constant stably. This introduction of cSED is key to extending the method's applicability to obtain the chemical shift of various chemical species. The present demonstration illustrates our approach's superiority in terms of numerical robustness and accuracy. [ABSTRACT FROM AUTHOR]

We present a methodology for analyzing chemical bonds embedded in the electronic wave function of molecules, especially in terms of spin correlations or so-called "local spin." In this paper, based on biorthogonal second quantization, the spin correlation functions of molecules are naturally introduced, which enables us to extract local singlet and local triplet elements from the wave function. We also clarify the relationship between these spin correlations and traditional chemical concepts, i.e., resonance structures. Several chemical reactions, including the intramolecular radical cyclization and the formation of preoxetane, are demonstrated to verify the analysis method numerically. [ABSTRACT FROM AUTHOR]

A series of ferrocene(Fc)‐bridged pentacene(Pc)‐dimers [Fc−Ph(2,n)−(Pc)2: n=number of phenylene spacers] were synthesized to examine the tortional motion effect of Fc‐terminated phenylene linkers on strongly coupled quintet multiexciton (5TT) formation through intramolecular singlet fission (ISF). Fc−Ph(2,4)−(Pc)2 has a relatively small electronic coupling and large conformational flexibility according to spectroscopic and theoretical analyses. Fc−Ph(2,4)−(Pc)2 exhibits a high‐yield 5TT together with quantitative singlet TT (1TT) generation through ISF. This demonstrates a much more efficient ISF than those of other less flexible Pc dimers. The activation entropy in 1TT spin conversion of Fc−Ph(2,4)−(Pc)2 is larger than those of the other systems due to the larger conformational flexibility associated with the torsional motion of the linkers. The torsional motion of linkers in 1TT is attributable to weakened metal‐ligand bonding in the Fc due to hybridization of the hole level of Pc to Fc in 1TT unpaired orbitals. [ABSTRACT FROM AUTHOR]

The ground state and excited state electronic properties of chlorophyll (Chl) a and Chl b in diethyl ether, acetone, and ethanol solutions are investigated using quantum mechanical and molecular mechanical calculations with density functional theory (DFT) and time-dependent DFT (TDDFT). Although the DFT/TDDFT methods are widely used, the electronic structures of molecules, especially large molecules, calculated with these methods are known to be strongly dependent on the functionals and the parameters used in the functionals. Here, we optimize the range-separated parameter, μ, of the CAM-B3LYP functional of Chl a and Chl b to reproduce the experimental excitation energy differences of these Chl molecules in solution. The optimal values of μ for Chl a and Chl b are smaller than the default value of μ and that for bacteriochlorophyll a, indicating the change in the electronic distribution, i.e., an increase in electron delocalization, within the molecule. We find that the electronic distribution of Chl b with an extra formyl group is different from that of Chl a. We also find that the polarity of the solution and hydrogen bond cause the decrease in the excitation energies and the increase in the widths of excitation energy distributions of Chl a and Chl b. The present results are expected to be useful for understanding the electronic properties of each pigment molecule in a local heterogeneous environment, which will play an important role in the excitation energy transfer in light-harvesting complex II. [ABSTRACT FROM AUTHOR]

A new N−F reagent, N,N′‐difluoro‐2,2′‐bipyridinium bis(tetrafluoroborate), for iodoarene‐catalyzed fluorination reactions as a terminal oxidant is reported. This reaction is effective for the fluorination of simple alkenes, aromatic alkenes and 1,3‐dicarbonyl compounds, giving 1,2‐difluorinated alkanes, geminal difluorinated compounds, 2‐fluorinated‐1,3‐dicarbonyl compounds in good to high yields, respectively. This is a useful fluorination reaction that not only improves the issues of the fluorination reaction using conventional mCPBA as a terminal oxidant but also constructs a simpler catalytic cycle. The present N−F reagent can be recovered as 2,2'‐bipyridine. [ABSTRACT FROM AUTHOR]

This article proposes an ab initio quantum chemical method to evaluate the effective electronic coupling that determines the rate of superexchange electron transfer in donor–bridge–acceptor (D–B–A) systems. The method utilizes the fragment charge difference to define electronic diabatic states and to apply an electrostatic potential in a form of a uniform potential difference that mimics solvation effects on the relative energies of the electronic states. The two-state generalized Mulliken–Hush method is used to obtain the effective electronic coupling as the nondiagonal element of the effective Hamiltonian that is derived based on the Green's function approach and the quasi-degenerate perturbation theory. A theoretical basis is provided for the dependence of the calculated effective electronic coupling on the applied potential and for how to find the optimal potential to give the desired effective electronic coupling that coincides with the result of the minimum energy splitting method. The method is applied to typical D–B–A molecules and gives the effective electronic couplings in reasonable agreement with the experimental estimates. [ABSTRACT FROM AUTHOR]

A new method for calculating nuclear magnetic shielding in solutions is developed based on the reference interaction site model self-consistent field (RISM-SCF) with spatial electron density distribution (SEDD). In RISM-SCF-SEDD, the electrostatic interaction between the solute and the solvent is described by considering the spread of electron to obtain more realistic electronic structure in solutions. It is thus expected to allow us to predict more quantitative chemical shifts of a wide variety of chemical species in solutions. In this study, the method is applied to a water molecule in water and is validated by examining the dependence of the solvent temperature and density on chemical shifts. The dependence of solvent species is also investigated, and more accurate results are obtained for polar solvents compared to the previous RISM-SCF study. Another application example of this method is the 15N chemical shifts of two azines in water, which is difficult to predict with the polarizable continuum model (PCM). Our results are in good agreement with the previous quantum mechanical/molecular mechanics study and experimental results. It is also shown that our method gives more realistic results for methanol and acetone than the PCM. [ABSTRACT FROM AUTHOR]

The structural stability of a cytochrome c domain-swapped dimer compared with that of the monomer was investigated by molecular dynamics (MD) simulations and by three-dimensional reference interaction site model (3D-RISM) theory. The structural fluctuation and structural energy of cytochrome c were treated by MD simulations, and the solvation thermodynamics was treated by 3D-RISM theory. The domain-swapped dimer state is slightly less stable than the monomer state, which is consistent with experimental observations; the total free energy difference is calculated as 25 kcal mol-1. The conformational change and translational/rotational entropy change contribute to the destabilization of the dimer, whereas the hydration and vibrational entropy contribute to the stabilization. Further analyses on the residues located at the hinge loop for swapping were conducted, and the results reveal details at the molecular level of the structural and interaction changes upon dimerization. [ABSTRACT FROM AUTHOR]

Nitration of benzene is a representative aromatic substitution reaction related to the σ-complex (arenium ion or "Wheland" intermediate) concept. This reaction is typically carried out in a mixed acid solution to generate nitronium ions, and how solvent molecules play roles in the reaction has been of great interest. Here we will shed new light on the reaction, namely the electronic structure and the microscopic insights of the solvation, which have been rarely discussed so far. We studied this process using the reference interaction site model-self consistent field with constrained spatial electron density distribution (RISM-SCF-cSED) method, considering sulfuric acid or water molecules as a solvent. In this method, the electronic structure of the solute and the solvation structure are self-consistently determined based on quantum chemistry and statistical mechanics of molecular liquids. The solvation free energy surfaces in solution and solvation structures were verified. In the bond formation process of benzene and nitronium ions, the solvation structure by sulfuric acid molecules drastically changes and the solvation effect on the free energy is quite large. We revealed largely contributing resonance structures in the π-electron system of the σ-complex in gas and solution phases by analysing the valence electronic structures. [ABSTRACT FROM AUTHOR]

Electronic structures of [Fe2S2(SCH3)4](super 2-, 3-) in DMSO solution are calculated using reference interaction site model complete active space self-consistent field (RISM-CASSCF)/multireference second-order Moller-Plesset perturbation theory (MRMP) method. The effect of solvent environment and its fluctuation is crucial in describing the redox processes of Fe2S2 clusters.