Theoretical verification of nonthermal microwave effects on intramolecular reactions.

There have been a growing number of articles that report dramatic improvements in the experimental performance of chemical reactions by microwave irradiation compared to that under conventional heating conditions. We theoretically examined whether nonthermal microwave effects on intramolecular reactions exist or not, in particular, on Newman-Kwart rearrangements and intramolecular Diels-Alder reactions. The reaction rates of the former calculated by the transition state theory, which consider only the thermal effects of microwaves, agree quantitatively with experimental data, and thus, the increases in reaction rates can be ascribed to dielectric heating of the solvent by microwaves. In contrast, for the latter, the temperature dependence of reaction rates can be explained qualitatively by thermal effects but the possibility of nonthermal effects still remains regardless of whether competitive processes are present or not. The effective intramolecular potential energy surface in the presence of a microwave field suggests that nonthermal effects arising from potential distortion are vanishingly small in intramolecular reactions. It is useful in the elucidation of the reaction mechanisms of microwave synthesis to apply the present theoretical approach with reference to the experiments where thermal and nonthermal effects are separated by screening microwave fields.

Experimental and theoretical investigations of isomerization reactions of ionized acetone and its dimer.

Ionization dynamics of acetone and its dimer in supersonic jets is investigated by a combination of experimental and theoretical techniques, both of which have recently been developed. In experiments, the neutral and the cationic structures are explored by infrared predissociation spectroscopy with the vacuum-ultraviolet photoionization detection schemes. Reaction paths following the one-photon ionization of the acetone monomer and its dimer have been studied by the joint use of several theoretical methods including the ab initio molecular dynamics, the global reaction route mapping, the intrinsic reaction coordinate, and the artificial force induced reaction calculations. Upon one-photon ionization, the dimer isomerizes to the H-bonded form, in which the enol cation of acetone is bound to the neutral molecule, while this enolization is energetically forbidden in the acetone monomer. The enolization of the dimer cation occurs through a two-step proton-transfer from the methyl group of the ionized moiety, and is catalyzed by the neutral moiety within the dimer cation.

Quantum dynamics of light-driven chiral molecular motors.

The results of theoretical studies on quantum dynamics of light-driven molecular motors with internal rotation are presented. Characteristic features of chiral motors driven by a non-helical, linearly polarized electric field of light are explained on the basis of symmetry argument. The rotational potential of the chiral motor is characterized by a ratchet form. The asymmetric potential determines the directional motion: the rotational direction is toward the gentle slope of the asymmetric potential. This direction is called the intuitive direction. To confirm the unidirectional rotational motion, results of quantum dynamical calculations of randomly-oriented molecular motors are presented. A theoretical design of the smallest light-driven molecular machine is presented. The smallest chiral molecular machine has an optically driven engine and a running propeller on its body. The mechanisms of transmission of driving forces from the engine to the propeller are elucidated by using a quantum dynamical treatment. The results provide a principle for control of optically-driven molecular bevel gears. Temperature effects are discussed using the density operator formalism. An effective method for ultrafast control of rotational motions in any desired direction is presented with the help of a quantum control theory. In this method, visible or UV light pulses are applied to drive the motor via an electronic excited state. A method for driving a large molecular motor consisting of an aromatic hydrocarbon is presented. The molecular motor is operated by interactions between the induced dipole of the molecular motor and the electric field of light pulses.

Quantum optimal control of electron ring currents in chiral aromatic molecules.

We report the results of optimal control simulations of pi-electron rotation (ring current) in a six-membered chiral aromatic molecule, 2,5-dichloro[n](3,6)pyrazinophane (DCP), attached at a surface and excited by a linearly polarized UV laser. DCP has a pair of optically allowed, quasidegenerate pi-electronic excited states. The laser pulse to generate an approximate angular momentum eigenstate consisting of the quasidegenerate states was designed using the global optimal control theory. For both counterclockwise and clockwise pi-electron rotations, the calculated objective functional and target yield as a function of the angle of the photon polarization vector show two maxima and two minima. The origin of the two minima is coherent excitation to only one of the quasidegenerate states. The two maxima arise from creation of a superposition of the quasidegenerate states. The optimal control pulse at the maxima is a two-color laser field resonant with the quasidegenerate states. The electric field of the optimal control pulse consists of two parts: a slowly oscillating part with phase phi(env) and a rapidly oscillating one. The phase phi(env) is a crucial parameter for determination of the rotation direction of pi electrons at the end of control. The results of the optimal control simulations suggest that pi-electron rotation can be controlled by applying a two-color laser field with adjusted phases.

Theoretical design of an aromatic hydrocarbon rotor driven by a circularly polarized electric field.

An aromatic hydrocarbon rotor without functional groups is theoretically designed. Such a molecular rotor is free from long-range electrostatic interactions. Induced dipole interactions are the rotor-driving forces under a nonresonant excitation condition. As an example, a molecular rotor with a condensed aromatic ring, a pentacene moiety mounted on a phenyl-acetylene axle that is driven by a circularly polarized electric field is considered. Results of simulations of the quantum dynamics of a rotor that take into account short-range rotor-bath interactions are presented by numerically solving the density matrix equations of the rotational motions.

Mechanisms in adaptive feedback control: photoisomerization in a liquid.

The underlying mechanism for Adaptive Feedback Control in the experimental photoisomerization of 3,3'-diethyl-2,2'-thiacyanine iodide (NK88) in methanol is exposed theoretically. With given laboratory limitations on laser output, the complicated electric fields are shown to achieve their targets in qualitatively simple ways. Further, control over the cis population without laser limitations reveals an incoherent pump-dump scenario as the optimal isomerization strategy. In neither case are there substantial contributions from quantum multiple-path interference or from nuclear wave packet coherence. Environmentally induced decoherence is shown to justify the use of a simplified theoretical model.

Quantum control of a chiral molecular motor driven by laser pulses.

"Molecular motors or machines" are one of the hot subjects in chemistry because they play an important role in molecular devices. We have theoretically demonstrated that unidirectional rotations of a chiral molecular motor can be driven by using tailored linearly polarized laser pulses. The findings obtained here serve as a theoretical basis for control of functions such as gearing or acceleration of molecular motors.