Toshiyuki Takayanagi

Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br(4)HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

Theoretical Study on the Mechanism of Low-Energy Dissociative Electron Attachment for Uracil†

The lowest electronically adiabatic potential energy surface of the uracil anion has been theoretically investigated with density-functional theory methods in order to understand the mechanism of the N-H bond dissociation induced by low-energy electron attachment. We found that the BH&HLYP level can reasonably describe both the dipole-bound and valence anionic states in a balanced way. With this density-functional theory level, we have constructed two-dimensional potential energy surfaces as a function of appropriate internal coordinates and discuss the importance of electronic coupling between the dipole-bound and valence anion states in dissociative electron attachment of uracil. The transition state geometry for the electronic isomerization between the dipole-bound anion and the pi* valence anion was successfully optimized and the barrier height for this isomerization was found to be relatively low. It was found that the out-of-plane motion of H at the C6 position plays the most important role in this isomerization process. Reduced-dimensionality quantum wave packet calculations taking two active internal coordinates into account have also been performed to interpret the resonance structures observed in cross sections for the N-H dissociation channel at a qualitative level.

Quantum proton transfer in hydrated sulfuric acid clusters: a perspective from semiempirical path integral simulations.

We have carried out path-integral molecular dynamics simulations for hydrated sulfuric acid clusters to understand acid-dissociation and hydrogen-bonded structural rearrangement processes in these clusters from a quantum mechanical viewpoint. The simulations were performed using the PM6 semiempirical electronic structure level whose parameters were modified on the basis of the specific reaction parameters strategy so that relative energies of optimized structures, as well as water binding energies reproduce ab initio and density-functional theory calculations. We have found that the acid dissociation processes, first and second deprotonation, effectively occur in a hydrated cluster with a specific cluster size. The mechanisms of the proton-transfer processes were analyzed in detail and it was found that the distance between O in sulfuric acid and O in the proton-accepting water is playing an important role. We also found that the water coordination number of the poton-accepting water is important in the proton-transfer processes.

QUASICLASSICAL TRAJECTORY STUDIES OF N(2D) + H2 REACTION ON A FITTED AB INITIO POTENTIAL-ENERGY SURFACE

The dynamics of the N(2D)+ H2 reaction has been investigated by quasiclassical trajectory calculations on a new potential-energy surface which was constructed on the basis of ab initio results. The calculated vibrational distribution for the NH product agreed well with that measured by Dodd et al.(J. A. Dodd, S. J. Lipson, D. J. Flanagan and W. A. M. Blumberg, J. Chem. Phys., 1991, 94, 4301). The thermal rate constants and isotope effects calculated on this surface were in moderate agreement with those recently determined. At a low collision energy, the N atom mainly approaches the H2 molecule collinearly and an NH radical is produced via the deep well corresponding to a stable NH2. At a high collision energy, a perpendicular approach is found to be important in addition to the collinear approach.

The photodissociation dynamics of dichloroethenes at 214 and 220 nm

The nascent rotational distributions of HCl (v=0, 1, and 2) generated in the photodissociation of three isomers of dichloroethenes (DCE) at 214 and 220 nm were measured under molecular beam conditions. HCl molecules were probed by a (2+1) resonantly enhanced multiphoton ionization technique combined with time‐of‐flight mass spectrometry. The rotational distributions of vibrationally excited HCl (v=1 and 2) molecules were Boltzmann‐type, while those of HCl (v=0) could not be represented by a Boltzmann distribution and consisted of two components. These results suggest that there are more than two processes in the photodissociation of DCE. Cl(2P3/2) and Cl*(2P1/2) could also be detected when DCE were photodissociated. The branching ratios of Cl*(2P1/2) to Cl(2P3/2) obtained in the present work were much larger than those obtained at 193 nm.

Kinetic measurements for the reactions of ozone with crotonaldehyde and its methyl derivatives and calculations of transition-state theoryElectronic supplementary information (ESI) available: The stationary-point geometries optimized at B3LYP/6-31G(d,p) for the reactions of ozone with nine unsaturated carbonyls. See http://www.rsc.org/suppdata/cp/b4/b402496f/

The rate coefficients for the reactions of O3 with six unsaturated carbonyls have been measured with the relative-rate method in the presence of a sufficient radical scavenger. The experiments were conducted using a 6-m3 reaction chamber combined with a long-path FTIR system. The rate coefficients (measured in 10−18 cm3 molecule−1 s−1) were 1.58 ± 0.23 for crotonaldehyde, 1.59 ± 0.22 for trans-2-pentenal, 1.82 ± 0.26 for 3-methyl-2-butenal, 5.34 ± 0.73 for trans-2-methyl-2-butenal, 29.5 ± 4.1 for 3-pentene-2-one and 8.3 ± 1.1 for mesityl oxide. Conventional transition-state theory (CTST) calculations based on ab initio molecular orbital and density functional methods were performed to evaluate the rate constants for nine unsaturated carbonyls including six compounds examined in the present experiment as well as acrolein, methacrolein, and methyl vinyl ketone. A log–log plot of the rate coefficients measured in the present and previous works vs. the calculated results of the rate constants, showed a linear relationship.

Theoretical Study on the Excess Electron Binding Mechanism in the [CH3NO2·(H2O)n]− (n = 1−6) Anion Clusters

The excess electron binding mechanism of the anionic nitromethane-water clusters was theoretically investigated using the potential energy surfaces calculated by high-level electronic structure theories. The mechanism was first studied for the dipole-bound and valence-bound anionic states of the CH(3)NO(2)(-) monomer with the ab initio multireference configuration interaction method to reveal the electron transformation process between these anionic states in detail. As a result, it was found that both the NO(2) tilting angle and NO distances play an essential role in this electron transformation. Following this result, various water solvation structures of the valence-bound CH(3)NO(2)(-) anion were optimized with up to six water solvents using the second-order Møller-Plesset (MP2) method. The calculated results predicted that the vertical detachment energy of the valence-bound CH(3)NO(2)(-) anion increases gradually with the hydration number, as is consistent with recent experimental observations. We also investigated metastable complexes composed of CH(3)NO(2) and (H(2)O)(6)(-) by using the MP2 and long-range corrected density functional theory calculations. Two types of dipole-bound forms were obtained for the [CH(3)NO(2).(H(2)O)(6)] anion complex. In one form the excess electron is internally suspended between the two moieties while in the other form two dipolar moieties are cooperatively arranged to reinforce the electron-dipole interaction.

Dynamics of dipole- and valence bound anions in iodide-adenine binary complexes: A time-resolved photoelectron imaging and quantum mechanical investigation.

Dipole bound (DB) and valence bound (VB) anions of binary iodide-adenine complexes have been studied using one-color and time-resolved photoelectron imaging at excitation energies near the vertical detachment energy. The experiments are complemented by quantum chemical calculations. One-color spectra show evidence for two adenine tautomers, the canonical, biologically relevant A9 tautomer and the A3 tautomer. In the UV-pump/IR-probe time-resolved experiments, transient adenine anions can be formed by electron transfer from the iodide. These experiments show signals from both DB and VB states of adenine anions formed on femto- and picosecond time scales, respectively. Analysis of the spectra and comparison with calculations suggest that while both the A9 and A3 tautomers contribute to the DB signal, only the DB state of the A3 tautomer undergoes a transition to the VB anion. The VB anion of A9 is higher in energy than both the DB anion and the neutral, and the VB anion is therefore not accessible through the DB state. Experimental evidence of the metastable A9 VB anion is instead observed as a shape resonance in the one-color photoelectron spectra, as a result of UV absorption by A9 and subsequent electron transfer from iodide into the empty π-orbital. In contrast, the iodide-A3 complex constitutes an excellent example of how DB states can act as doorway state for VB anion formation when the VB state is energetically available.

Ab initio study of small acetonitrile cluster anions

Ab initio electronic structure calculations have been performed for (CH(3)CN)(2) (-) and (CH(3)CN)(3) (-) cluster anions using a diffuse basis set. We found both the dipole-bound structures and internal structures, where in the former structure an excess electron is mainly distributed on the surface of the cluster while an excess electron is internally trapped in the latter configuration. The optimized structures found for cluster anions were compared to those for neutral clusters. Potential-energy surfaces were also plotted as a function of appropriate internal coordinates in order to understand the interconversions of the optimized structures of clusters. The relative stabilities of the optimized confirmers have been discussed on the basis of the characteristics of these potential surfaces, relative energies, and electron vertical detachment energies.

Reaction dynamics for the N(2D)+ H2 reaction

The dynamics of the N(2D)+ H2→ NH + H reaction has been studied by both accurate three-dimensional quantum scattering calculations and quasiclassical trajectory calculations. The potential-energy surface which has recently been developed on the basis of ab initio molecular-orbital calculations is employed. A hyperspherical coordinate system is used for the quantum scattering calculations. The quantum calculations are carried out only for the total angular momentum J= 0. The J-shifting approximation is used to compute reaction cross-sections and thermal rate constants. Both the quantum and quasiclassical calculations are found to reproduce the experimental vibrational distribution of the product NH at 300 K. For the thermal rate constants, an excellent agreement is obtained between the quantum results and the experimental results, although the quantum rate constants are restricted to the N(2D)+ H2(vi= 0,ji= 0) reaction. On the other hand, it is shown that the quasiclassical trajectory calculations give larger rate constants than the experimental ones.