Yasuhiro Ikabata

Unveiling a New Aspect of Simple Arylboronic Esters: Long-Lived Room-Temperature Phosphorescence from Heavy-Atom-Free Molecules

Arylboronic esters can be used as versatile reagents in organic synthesis, as represented by Suzuki-Miyaura cross-coupling. Here we report a serendipitous finding that simple arylboronic esters are phosphorescent in the solid state at room temperature with a lifetime on the order of several seconds. The phosphorescence properties of arylboronic esters are remarkable in light of the general notion that phosphorescent organic molecules require heavy atoms and/or carbonyl groups for the efficient generation of a triplet excited state. Theoretical calculations on phenylboronic acid pinacol ester indicated that this molecule undergoes an out-of-plane distortion at the (pinacol)B-Cipso moiety in the T1 excited state, which is responsible for its phosphorescence. A compound survey with 19 arylboron compounds suggested that the phosphorescence properties might be determined by solid-state molecular packing rather than by the patterns and numbers of boron substituents on the aryl units. The present finding may update the general notion of phosphorescent organic molecules.

Interpretation of intermolecular geometric isotope effect in hydrogen bonds: nuclear orbital plus molecular orbital study.

The intermolecular geometric isotope effect (GIE) in hydrogen bond A-X···B (X = H and D) is investigated theoretically using the nuclear orbital plus molecular orbital (NOMO) theory. To interpret the GIE in terms of physically meaningful energy components such as electrostatic and exchange-repulsion interactions, the reduced variational space self-consistent-field method is extended to the NOMO scheme. The intermolecular GIE is analyzed as a two-stage process: the intramolecular bond shrinkage and the intermolecular bond elongation. According to the isotopic shifts of energy components described by the NOMO/MP2 method, the intermolecular GIE is approximately interpreted as a process reducing the exchange-repulsion interaction after the decrease of electrostatic interaction.

Effect of Hartree-Fock exact exchange on intramolecular magnetic coupling constants of organic diradicals

The intramolecular magnetic coupling constant (J) of diradical systems linked with five- or six-membered aromatic rings was calculated to obtain the scaling factor (experimental J/calculated J ratio) for various density functional theory (DFT) functionals. Scaling factors of group A (PBE, TPSSh, B3LYP, B97-1, X3LYP, PBE0, and BH&HLYP) and B (M06-L, M06, M06-2X, and M06-HF) were shown to decrease as the amount of Hartree-Fock exact exchange (HFx) increases, in other words, overestimation of calculated J becomes more severe as the HFx increases. We further investigated the effect of HFx fraction of DFT functional on J value, spin contamination, and spin density distributions by comparing the B3LYP analogues containing different amount of HFx. It was revealed that spin contamination and spin densities at each atom increases as the HFx increases. Above all, newly developed BLYP-5 functional, which has 5% of HFx, was found to have the scaling factor of 1.029, indicating that calculated J values are very close to that of experimental values without scaling. BLYP-5 has potential to be utilized for accurate evaluation of intramolecular magnetic coupling constant (J) of diradicals linked by five- or six-membered aromatic ring couplers.

Extension of local response dispersion method to excited-state calculation based on time-dependent density functional theory

We report the extension of the local response dispersion (LRD) method to the excited-state calculation based on time-dependent density functional theory. The difference density matrix, which is usually used for excited-state response properties, enables a state-specific dispersion correction. The numerical assessment proves that interaction energies of exciton-localized molecular complexes and their shifts from the ground state are accurately reproduced by the LRD method. Furthermore, we find that the dispersion correction is important in reproducing binding energies of aromatic excimers, despite the existence of other attractive forces such as exciton delocalization and charge-transfer interaction.

Semi-local machine-learned kinetic energy density functional with third-order gradients of electron density

A semi-local kinetic energy density functional (KEDF) was constructed based on machine learning (ML). The present scheme adopts electron densities and their gradients up to third-order as the explanatory variables for ML and the Kohn-Sham (KS) kinetic energy density as the response variable in atoms and molecules. Numerical assessments of the present scheme were performed in atomic and molecular systems, including first- and second-period elements. The results of 37 conventional KEDFs with explicit formulae were also compared with those of the ML KEDF with an implicit formula. The inclusion of the higher order gradients reduces the deviation of the total kinetic energies from the KS calculations in a stepwise manner. Furthermore, our scheme with the third-order gradient resulted in the closest kinetic energies to the KS calculations out of the presented functionals.

The divide-and-conquer second-order proton propagator method based on nuclear orbital plus molecular orbital theory for the efficient computation of proton binding energies

An efficient computational method to evaluate the binding energies of many protons in large systems was developed. Proton binding energy is calculated as a corrected nuclear orbital energy using the second-order proton propagator method, which is based on nuclear orbital plus molecular orbital theory. In the present scheme, the divide-and-conquer technique was applied to utilize local molecular orbitals. This use relies on the locality of electronic relaxation after deprotonation and the electron-nucleus correlation. Numerical assessment showed reduction in computational cost without the loss of accuracy. An initial application to model a protein resulted in reasonable binding energies that were in accordance with the electrostatic environment and solvent effects.

Local response dispersion method in periodic systems: Implementation and assessment

We report the implementation of the local response dispersion (LRD) method in an electronic structure program package aimed at periodic systems and an assessment combined with the Perdew–Burke–Ernzerhof (PBE) functional and its revised version (revPBE). The real‐space numerical integration was implemented and performed exploiting the electron distribution given by the plane‐wave basis set. The dispersion‐corrected density functionals revPBE+LRD was found to be suitable for reproducing energetics, structures, and electron distributions in simple substances, molecular crystals, and physical adsorptions.

Unveiling Controlling Factors of the S0/S1 Minimum Energy Conical Intersection: A Theoretical Study

The minimum energy conical intersection (MECI) geometries play an important role in photophysics and photochemistry. Although a number of MECI geometries can be identified using quantum chemical methods, their chemical interpretation remains unclear. In this study, a systematic analysis was performed on the MECIs between the singlet (S0) and lowest singlet excited (S1) states of organic molecules. The frozen orbital analysis (FZOA), which approximates the excited states with minimal main configurations, was adopted to analyze the excitation energy components at the S0/S1 MECI geometries as well as the S0 and S1 equilibrium geometries. At the S0/S1 MECI geometries, the HOMO-LUMO gaps decreased as expected but did not disappear. The remaining gaps were balanced with the HOMO-LUMO Coulomb integrals. Furthermore, we discovered that the HOMO-LUMO exchange integrals became approximately zero. On the basis of this fact, a systematic interpretation of the S0/S1 MECI geometries has been described.

RAQET: Large-Scale Two-Component Relativistic Quantum Chemistry Program Package

The Relativistic And Quantum Electronic Theory (RAQET) program is a new software package, which is designed for large‐scale two‐component relativistic quantum chemical (QC) calculations. The package includes several efficient schemes and algorithms for calculations involving large molecules which contain heavy elements in accurate relativistic formalisms. These calculations can be carried out in terms of the two‐component relativistic Hamiltonian, wavefunction theory, density functional theory, core potential scheme, and evaluation of electron repulsion integrals. Furthermore, several techniques, which have frequently been used in non‐relativistic QC calculations, have been customized for relativistic calculations. This article introduces the brief theories and capabilities of RAQET with several calculation examples.

Quantum chemical approach for positron annihilation spectra of atoms and molecules beyond plane-wave approximation

We report theoretical calculations of positron-electron annihilation spectra of noble gas atoms and small molecules using the nuclear orbital plus molecular orbital method. Instead of a nuclear wavefunction, the positronic wavefunction is obtained as the solution of the coupled Hartree-Fock or Kohn-Sham equation for a positron and the electrons. The molecular field is included in the positronic Fock operator, which allows an appropriate treatment of the positron-molecule repulsion. The present treatment succeeds in reproducing the Doppler shift, i.e., full width at half maximum (FWHM) of experimentally measured annihilation (γ-ray) spectra for molecules with a mean absolute error less than 10%. The numerical results indicate that the interpretation of the FWHM in terms of a specific molecular orbital is not appropriate.