分子の電子・振動状態は, 分子が可視・紫外光や赤外光に対する応答を観測することで調べられてきた。この応答を理論的に予測するには, 注目する分子系の励起スペクトルと遷移モーメントを定量的に評価する必要がある。本研究では近未来を見越し, 申請者が開発してきた理論手法の発展・応用研究を行った。
分子系に弱い定常電場を印加し, そのエネルギー変化から, 遷移双極子モーメントを応答量として計算するモジュールを作製し, スピン軌道配置間相互作用プログラムに追加し, 従来の計算方法と計算精度を比較した。ヨウ素分子に応用した結果, 特に従来法では, 実験値からの大きな数値誤差が生じていたX-A,X-C状態間においても, 基底関数に依存しない数値的に安定な結果を得ることが出来た。数年前にTellinghuisenが発表したX-B状態間の遷移モーメント関数と我々の計算値の間の大きな差の原因を分析することで, 彼らが最小自乗フィットに用いたサンプル点が不十分であることを明らかにした。従来は非常に困難な電子状態と見なされてきた希土類錯体の配位子場分裂やf-f遷移に応用し, 高精度な結果を得ることに成功した。この計算手法をICN分子の光分解反応に応用し, 特に解離波束が複数の電子励起状態を経由するために, 生成物の準位分布に量子干渉が見られる機構が明らかになった。
分子内OH基やNH基の伸縮振動が示す基音および倍音の吸収強度とその置換基効果や水素結合の影響を調べるために, その双極子モーメントの核座標微分を解析的に計算する手法を開発した。π電子系の置換基効果として, 誘起効果よりも共鳴効果が吸収強度に顕著に影響することや, OH基やNH基を持つ分子が水素結合受容体と相互作用することによって, 基音吸収強度は著しく増強されるが倍音強度はむしろ減少する理由を, ローカルモード描像を使って明らかにした。星間分子として注目されているポリインの変角振動の吸収・発光強度についても同様の計算手法を用いて定量的な結果を得た。
Electronic and vibration states of a molecule have been investigated by observing the response to applied electric and magnetic fields onto the molecule. To theoretically predict the response, it is necessary to quantitatively evaluate the excitation spectra and the transition moments of the system. In this research, we have further developed and applied our theoretical method in anticipation of such a research direction in near future.
An electronic transition dipole moment (TDM) function of a molecule can be evaluated by applying a weak electric field onto the molecular system and by computing the energy response caused by the change of the electronic structure. We have developed such a computational method including spin-orbit interactions and applied it to several systems, then examined the accuracy by comparing with the conventional computational method. The first application to iodine molecule showed that the new method gave numerically very stable result even for the TDM functions between the X-A and X-C states, for which conventional methods yielded largely different values from the experimental ones. Our theoretical TDM between the X-B states was significantly different from the TDM obtained several years ago by Tellinghuisen with analyzing the experimental spectra data. Our analysis showed that the origin of the disagreement was due to the unevenly distributed sample points he used for the least squares fit. We have successfully applied the method to the ligand field splittings and the TDMs for the f-f transitions of several rare earth complexes, which had been considered very difficult. We also applied the new method to the photodissociation reaction of ICN molecule to clarify the detailed mechanism for an interference pattern observed in the quantum level populations of the CN photoproducts.
We have also developed an analytic derivative method of the molecular electric dipole moment with respect to a proton coordinate to compute the absorption intensities of the fundamental and overtone stretching vibrations of OH and NH groups in various molecules. Our interest is in their substituent and hydrogen bonding effects. With the local mode picture, we have clarified that, as a cause of substituent effects in □-electron systems, resonant effect is much more efficient than inductive effect. We also found that, with hydrogen bonding interaction between OH or NH groups and the acceptors, the fundamental absorption intensity is increased remarkably while the overtone intensity is usually suppressed. We obtained a highly accurate result using a similar technique for the photo absorption and emission due to the bending vibrations of polyynes that have attracted attention as interstellar molecules.