Stability and bonding nature of positronic lithium molecular dianion.

We studied the stability of a system consisting of a positron (e+) and two lithium anions, [Li-; e+; Li-], using first-principles quantum Monte Carlo calculations combined with the multi-component molecular orbital method. While diatomic lithium molecular dianions Li22- are unstable, we found that its positronic complex can form a bound state with respect to the lowest energy decay into the dissociation channel Li2- and a positronium (Ps). The [Li-; e+; Li-] system has the minimum energy at the internuclear distance of ∼3 Å, which is close to the equilibrium internuclear distance of Li2-. At the minimum energy structure both an excess electron and a positron are delocalized as orbiting around the Li2- molecular anion core. A dominant feature of such a positron bonding structure is described as the Ps fraction bound to Li2-, unlike the covalent positron bonding scheme for the electronically isovalent [H-; e+; H-] complex.

Theoretical investigations of positron affinities and their structure-dependent properties of carbon dioxide clusters (CO2)n (n = 1-5).

Although positron binding to van der Waals intermolecularly bonded clusters of non-polar carbon dioxide (CO2) molecules was experimentally suggested, the positron binding feature has been poorly understood. We investigated positron affinities (PAs) by means of multi-component configuration interaction calculations for various structures of (CO2)n (n = 1-5) obtained by the single-component artificial force induced reaction (SC-AFIR) method. Our calculations showed that CO2 monomers do not bind a positron, whereas positron affinities for clusters tend to increase with an increase in the cluster size. Our regression analyses for determining PAs with electrostatic and structural properties of conformations revealed a significant conformer effect due to which structural characteristics such as flatness may have a strong influence on PA for loosely bound positronic complexes of (CO2)n.

Decomposition analysis on the excitation behaviors of thiazolothiazole (TTz)-based dyes via the time-dependent dielectric density functional theory approach.

Thiazolothiazole (TTz)-based materials have been attracting much attention because of their widespread applications. In this paper, we discuss the excited electronic behaviors of asymmetric TTz dyes in solvents based on the time-dependent dielectric density functional theory method. Based on dipole moment and charge distribution (population) analyses, we discuss large intramolecular electron transfers, which are triggered by photon excitations, toward the acceptor part of dyes. In addition, we explore the contributions of geometrical changes and solvent reorientations (reorganizations) to the solvatofluorochromic phenomena based on a decomposition technique. The decomposition analysis shows that the solvent reorientation effect mainly contributes to changes in the fluorescent spectra in response to solvents.

A comprehensive theoretical study of positron binding and annihilation properties of hydrogen bonded binary molecular clusters.

We studied the positron binding and annihilation of hydrogen bonded binary molecular clusters containing small inorganic molecules such as water, hydrogen fluoride, ammonia, hydrogen sulfide, hydrogen chloride, and phosphine, using first-principles calculation. While unimolecular systems of these species mostly exhibit no or very small positron binding energies (positron affinities), we found that all of their hydrogen bonding clusters have greater positive positron affinities. The permanent dipole moment enhanced by the formation of the intermolecular hydrogen bond acts as a dominant parameter to bind a positron for a given proton donor, whereas it is insufficient for reproducing the dependence of the positron affinity on substitutions of the proton donor. By multiple regression analyses with inherent properties of the clusters, we found a reasonable model with additional effective parameters represented by, particularly, the number of hydrogen atoms free from the hydrogen bond. By density analyses for the single-particle and electron-positron collision probabilities, we revealed that these effective parameters are associated with the electronic structure changes induced by the hydrogen bond and positron binding, which have important roles to enhance the electron-positron contact densities due to the proton-screening effect.

Positron Binding and Annihilation Properties of Amino Acid Systems.

Despite the fact that the positron annihilation has been used in biomedical applications, the detailed mechanism of the positron annihilation on biological molecules remains poorly understood so far. In this work, we investigated the positron binding and positron annihilation properties for both global minimum and hydrogen-bonded structures of 20 amino acid molecules using the multicomponent molecular orbital method. By regression analysis, we confirmed that positron affinity can increase with an increase of the permanent dipole moment of the parent amino acids as reported in previous studies, while the annihilation rate linearly increases with respect to the square root of positron affinity. By the one-particle property analyses for probabilities of electron-positron contacts, we found that delocalization characteristics of both electrons and positrons play key roles to enhance the positron annihilation rate arising from both the valence electrons in σ- and π-type molecular orbitals from 2p atomic orbitals but not from the highest occupied molecular orbital electrons, particularly for comparatively weakly bound positronic amino acid systems.

Theoretical investigation of the enhancement of positron affinity by the vibration and dimerization of non-polar carbon disulfide.

The positronic bound state for the non-polar carbon disulfide (CS2) has been experimentally identified, although previous theoretical investigations, which were dedicated to studying the positronic CS2 monomer, could not reasonably reproduce the experimentally measured positron affinity. In the present study, we performed analysis of the vibrational averaged positron affinity for the positronic CS2 dimer, [C2S4; e+], using the Hartree-Fock and configuration interaction levels of the multi-component molecular orbital method combined with the self-consistent field level of the vibrational variational Monte Carlo method. We demonstrated that the equilibrium structure of the non-polar C2S4 can have the positronic bound state with a positron affinity of about 46.18 meV in the configuration interaction level, while this is 0 meV in the Hartree-Fock level. Furthermore, by taking into account the vibrational effect, we succeeded in reproducing the resonant positron kinetic energies lying close to the experimental value, where the vibrational averaged positron affinity becomes greater with an increased dipole moment and dipole polarizability. We also showed possible mechanisms to effectively enhance the resonant positron capture for [C2S4; e+], associated with both the infrared active and infrared inactive vibrational modes.

First-principles quantum Monte Carlo studies for prediction of double minima for positronic hydrogen molecular dianion.

We studied the positron (e+) interaction with the hydrogen molecular dianion H2 2- to form the positronic bound state of [H-; e+; H-] using the first-principles quantum Monte Carlo method combined with the multi-component molecular orbital one. H2 2- itself is unstable, but it was shown that such an unbound H2 2- may become stable by intermediating a positron and forming the positronic covalent bond of the [H-; e+; H-] system [J. Charry et al., Angew. Chem., Int. Ed. 57, 8859-8864 (2018)]. We newly found that [H-; e+; H-] has double minima containing another positronic bound state of [H2; Ps-]-like configuration with the positronium negative ion Ps- at the bond distance approximately equal to the equilibrium H2 molecule. Our multi-component variational Monte Carlo calculation and the multi-component configuration interaction one resulted in the positronic covalent bonded structure being the global minimum, whereas a more sophisticated multi-component diffusion Monte Carlo calculation clearly showed that the [H2; Ps-]-like structure at the short bond distance is energetically more stable than the positronic covalent bonded one. The relaxation due to interparticle correlation effects pertinent to Ps- (or Ps) formation is crucial for the formation of the Ps-A2-like structure for binding a positron to the non-polar negatively charged dihydrogen.

Hydration Effect on Positron Binding Ability of Proline: Positron Attachment Induces Proton-Transfer To Form Zwitterionic Structure.

The positron binding abilities of proline and its hydrated clusters were theoretically studied using multicomponent molecular orbital (MC_MO) calculations. Low-lying equilibrium structures of the neutral proline·(H2O) n ( n = 0-2) clusters were systematically explored with the aid of global reaction route mapping code. The positron binding energies were then calculated for the equilibrium structures. We found that the zwitterionic forms generally displayed high positron binding energies owing to their highly polar nature. We also found that nonzwitterionic forms can exhibit high positron binding energies if the addition of water significantly increases the total electric dipole moment. These results demonstrate that the most stable positron-bound proline·(H2O) n ( n = 1 and 2) structures are more stable than the most stable neutral structures, which implies that the attachment of a positron to a neutral hydrated proline cluster can induce structural changes if the positron-electron annihilation lifetime is sufficiently long relative to the time scale of nuclear motion. We also examined the influence of positron binding on the energy profiles of the proton transfer reactions from the nonzwitterionic form to the zwitterionic form.

Reduction of OH vibrational frequencies in amino acids by positron attachment.

Positron affinities have been calculated for five amino acid molecules (asparagine, cysteine, glycine, proline, and serine) with the intramolecular COOH···NH2  hydrogen-bonded motif as a function of the OH distance using two computational methods, namely multicomponent molecular orbital theory and pseudopotential model. Since the elongation of the carboxylic OH bond leads to the formation of highly polarized zwitterionic amino acid with COO- ···NH3+  structure, the calculated positron affinity significantly increases with the elongation of the OH distance. This indicates that the OH bond strength is significantly weakened by positron attachment. We discuss the reduction of OH vibrational frequencies using effective one-dimensional potential energy curves for neutral and positron-attached amino acid molecules. © 2018 Wiley Periodicals, Inc.

Ferromagnetic spin coupling in the chromium dimer cation: measurements by photodissociation spectroscopy combined with coupled-cluster calculations.

The magnetic coupling of the chromium dimer cation, Cr2 (+), has been an outstanding problem for decades. An optical absorption spectrum of Cr2 (+) has been obtained by photodissociation spectroscopy in the photon-energy range from 2.0 to 5.0 eV. Besides, calculations have been performed by the equation-of-motion coupled-cluster singles and doubles method for vertical excitation of the species. Their coincidence supports our assignment that the ground electronic state exhibits a ferromagnetic spin coupling, which is contrary to those of neutral and negatively charged dimers, Cr2 and Cr2 (-), in their lowest spin states.

Binding of a positron to nucleic base molecules and their pairs.

Hole attack: A theoretical one-electron oxidation of nucleic base molecules and their pairs by positron is proposed, based on the calculations for positron-attached neutral forms of species, adenine (A), thymine (T), guanine (G), cytosine (C), and their Watson-Crick base pairs (A-T and G-C). The results reveal that binding of a positron to neutral isolated nucleic base molecules is base-selective.

Positron-attachment to nonpolar or small dipole CXY (X, Y = O, S, and Se) molecules: vibrational enhancement of positron affinities with configuration interaction level of multi-component molecular orbital approach.

To theoretically demonstrate the binding of a positron to a nonpolar or small dipole molecule, we have calculated the vibrational averaged positron affinity (PA) values along the harmonic asymmetric stretching vibrational coordinate with the configuration interaction level of multi-component molecular orbital method for CXY (X, Y = O, S, and Se) molecules. For CSe2 and CSSe molecules, a positron can even be attached at the equilibrium structures, due to the effect of the induced dipole moment with large polarizability values. For a CS2 molecule, the positive PA value is obtained at the lowest vibrational excited state in our scheme. Although there is no direct experimental evidence for the positron-binding to CO2, COS, and COSe molecules, we have predicted positron-binding for these molecules at higher vibrational excited states.

Ab initio quantum Monte Carlo study of the binding of a positron to alkali-metal hydrides.

Quantum Monte Carlo methods are used to investigate the binding of a positron to the alkali-metal hydrides, XH (X = Na and K). We obtain positron affinities for the NaH and KH molecules of 1.422(10) eV and 2.051(39) eV, respectively. These are considerably larger than the previous results of 1.035 eV and 1.273 eV obtained from multireference single- and double-excitation configuration interaction calculations. Together with our previous results for [LiH;e(+)] [Y. Kita et al., J. Chem. Phys. 131, 134310 (2009)], our study confirms the strong correlation between the positron affinity and dipole moment of alkali-metal hydrides.

Bound states of the positron with nitrile species with a configuration interaction multi-component molecular orbital approach.

Characteristic features of the positron binding structure of some nitrile (-CN functional group) species such as acetonitrile, cyanoacetylene, acrylonitrile, and propionitrile are discussed with the configuration interaction scheme of multi-component molecular orbital calculations. This method can take the electron-positron correlation contribution into account through single electronic-single positronic excitation configurations. Our PA value of acetonitrile with the electronic 6-31++G(2df,2pd) and positronic [15s15p3d2f1g] basis set is calculated as 4.96 mhartree, which agrees to within 25% with the recent experimental value of 6.6 mhartree by Danielson et al. [Phys. Rev. Lett., 2010, 104, 233201]. Our PA values of acrylonitrile and propionitrile (5.70 and 6.04 mhartree) are the largest among these species, which is consistent with the relatively large dipole moments of the latter two systems.

Formation of Schiff-base for photoreaction mechanism of red shift of GFP spectra.

We have proposed the formation of Schiff-base between R96 and chromophore (CRO) to elucidate the reaction mechanism for the irreversible red shift of green fluorescent protein (GFP) spectra under the absence of oxygen. The difference between absorption energies of reactant and product for our GFP models with CIS(D)/6-31G* level is 0.21eV, which is in reasonable agreement with the corresponding experimental value of 0.25eV. We have suggested the irreversible photoreaction mechanism, where the CRO excited from ground (S(0)) state to first excited singlet (S(1)) state immediately turns to the first excited triplet (T(1)) state, and the nucleophilic addition reaction occurs on the T(1) state.

Ab initio quantum Monte Carlo study of the positronic hydrogen cyanide molecule.

Quantum Monte Carlo methods are used to investigate the binding of a positron to the hydrogen cyanide (HCN) and lithium hydride (LiH) molecules. Our value of the adiabatic positron affinity (PA) of LiH of 1.010(3) eV is very close to the best theoretical value of 1.005 eV, obtained from variational calculations using explicitly correlated Gaussian basis sets [K. Strasburger, J. Chem. Phys. 114, 00615 (2001)]. We have obtained a reliable estimate of 0.0378(48) eV for the PA of the HCN molecule, which is almost 20 times larger than that obtained at the Hartree-Fock level, and strongly supports the binding of a positron in the electrostatic field of the HCN molecule. Our results show the importance of correlation effects for describing weakly bound positronic molecular complexes.

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals.

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.