Suehiro Iwata

Photodissociation study on Mg+(H2O)n, n=1–5: Electronic structure and photoinduced intracluster reaction

Photodissociation spectra of Mg+(H2O)n (n=1–5) cluster ions were examined in the wavelength region from 720 to 250 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as 17u2009000 cm−1 with respect to the 2P–2S resonance line of the free Mg+ ion and were explained by the shift of this transition as a result of hydration. The spectra also exhibit clear evolution of solvation shell with the first shell closing at n=3, being consistent with the theoretical prediction. The mass spectra of the fragment ions show the existence of two dissociation processes: the evaporation of water molecules and the photoinduced intracluster reaction to produce the hydrated MgOH+ ion, MgOH+(H2O)m. The branching fraction between the two processes depends strongly on the solvent number n and also on the photolysis wavelength. The energetics and the dynamics of the dissociation processes were discussed in conjunction with the results of ab initio calculations.

The geometric and electronic structures of Arn+ (n=3–27)

The most stable structures of Arn+, n=3–27, are determined with the analytical gradient method for the diatomics‐in‐molecules Hamiltonian. The oscillator strength distribution is evaluated. The charge is found to be localized on the central three atoms, which form the trimeric ion core. The first solvation shell evolves around the ion core and is completed at n=25. The calculation shows that the photoabsorption band is in the visible region, which originates from the 2Σu+→ 2Σg+ transition of the Ar3+ ion core, and is red‐shifted with the increase of the cluster size, reproducing the experimental results. The red‐shift is explained in terms of the solvated ion core model, in which the excited state of the ion core interacts strongly with the surrounding solvent atoms.

An MCSCF study of the low‐lying states of trans‐butadiene

Ab initio MCSCF gradient method is applied to explore the potential surfaces of the low‐lying excited states of 1, 3‐trans‐butadiene. The determination of the equilibrium geometries and the force constants shows that there are several local minima for the 1u20093Bu (T1), 2u20091Ag (S1), and 1u20091Bu (S2) states. Since each excited state has a different electronic character, its potential energy surface is complicated with respect to the rotation of C–C double bond and C–C stretching motions. The second 1Ag state is found to have four minima with low symmetry (Ci, C2, and C1). A planar local minimum with C2h symmetry is found on the 1u20091Bu surface. The comparable planar minimum was found for the 1u20091Bu state of trans‐hexatriene. This is the first ab initio confirmation on the experimental analysis for the planar structure of the state. The present full π space MCSCF calculation gives not only reasonable transition energies but also detailed structures for low‐lying states.

Photodissociation of Ar+3 cluster ion

Abstract The time-of-flight spectra of Ar + produced in the photodissociation of Ar + 3 were observed in the wavelength range of 460–590 nm. Analysis revealed that (1) the kinetic energy distribution of Ar + fragments is bimodal: the components having a released kinetic energy of ≈ 0.5–1.0 eV (fast component) and almost zero kinetic energy (slow component), (2) the fragment angular distribution displays an anisotropy characteristic of direct dissociation involving a parallel-type transition, and (3) the formation of the fast component is the dominant channel with a branching fraction more than 80%. A dissociation mechanism involving a linear symmetric precursor is proposed on the basis of ab initio calculations for the potential energy surfaces of Ar + 3 .

Promotion of the proton transfer reaction by the intermolecular stretching mode: Application of the two‐dimensional finite element method to the nuclear Schrödinger equation

A model Hamiltonian is proposed to analyze the recently observed promotion of the proton transfer reaction by the intermolecular stretching vibrational mode. The Schrodinger equation was solved numerically with the two‐dimensional finite element method. The contour maps of the wave functions clearly indicate that when the intermolecular stretching mode is excited, the proton transfers from one site to the other over the ridge, taking the detour path. Only when the intermolecular mode is in the lowest states, does the proton transfer under the barrier with the tunneling mechanism. It is demonstrated that the full two‐dimensional analysis is necessary in the analysis of the large amplitude mode coupling and that the finite element method is a powerful tool to solve the Schrodinger equation for the nuclear motion in molecules.

A theoretical study of the photodissociation of acetylene in its lowest excited singlet state

Abstract The potential energy surface of acetylene in its lowest excited single state (S 1 ) is studied by using an ab initio MCSCF method to explore the mechanism of the photodissociation process. Predissociation leading to C 2 H and H suggested by the LIF spectrum is shown to occur on the single potential surface of the S 1 state, which is directly correlated with the excited state of C 2 H( 2 Π). The dissociation of the Cue5f8H bond from trans-bent acetylene in the S 1 state is found to proceed via a cis-bent transition state. The calculated energy barrier from trans-bent to cis-bent acetylene is lower than that of the Cue5f8H cleavage. The present results suggest that dissociation follows trans—cis isomerization.

Theoretical study of hydrated Be2+ ions

Abstract Hydrated Be 2+ ions [Be(H 2 O) n ] 2+ , n = 1−4 and 6, were examined theoretically. The structure of the hydrated ions was determined and the hydration energy estimated with and without electron correlation. The bond between the Be 2+ ion and the oxygen of water is very strong and has the nature of a dative bond. The non-additivity of the binding energy is so profound that without taking it into account the structure and dynamics of Be 2+ ions cannot be explained. The hydration number in water is found to be 4. The fifth and sixth water molecules prefer forming the second coordination shell to the Be 2+ ion. The result is in agreement with X-ray analysis of the aqueous solution, but not with a recent molecular dynamics simulation. In addition, the harmonic vibrational frequencies for the complexes are evaluated and compared with some experiments.

Second-order jahn-teller effect of cyclobutadiene in low-lying states. An MCSCF study

Abstract The molecular structures and vibrational frequencies of cyclobutadiene in the ground and low-lying excited states were examined with the ab initio multi-configuration self-consistent-field (MCSCF) method. The harmonic vibrational frequencies at a square structure in both the ground state 1B1g and the excited state 1A1s are analyzed in terms of the second-order Jahn-Teller effect. The energy barrier between two rectangular isomers in the ground state is estimated to be 3.2 kcal/mol with the zero-point vibration correction. The lowest excited singlet state 1A1g is found to have one imaginary frequency toward the rhomboidal distortion from the square structure. The frequency shift about the ring deformation due to the vibronic coupling is compared with the case of the low-lying states of benzene. The geometrical distortion from square to rectangle or rhomboid is also discussed with that of cyclobutadiene radical cation, which shows a typical deformation by the normal Jahn-Teller theorem.

Theoretical studies of the internal rotation of the methyl group in o-, m-, and p-fluorotoluenes and their cations

The potential energy curves of the methyl rotation of o-, m-, and p-fluorotoluenes and their cations are evaluated with the ab initio molecular orbital calculation and compared with the experimental values. The correlation between the barrier height and the difference of the electron distribution in the side and the other side of benzene ring is found.

Photodissociation dynamics of Ar+3

The nonadiabatic trajectory calculations were performed for the photodissociation process of Ar3+. Two methods—hemiquantal dynamics and Tully’s surface‐hopping method—were applied and the results were compared. The calculated velocities of the photofragments had slow and fast bimodal distributions, as were experimentally observed. The ratio of the slow Ar+ fragment to the fast one decreased with the excitation wavelength, also in good agreement with the experimental results. It was shown that the slow component of Ar+ was produced only through the nonadiabatic transition during the photoissociation process, and that the nonadiabatic transition was dependent on the excitation energy. In addition, the vibrational motion, especially the bending motion, was shown to play an important role in the nonadiabatic process.