Direct ab initio molecular dynamics study of the two photodissociation channels of formic acid

ギ酸の二つの解離反応についてのdirect ab initio molecular dynamics計算

黒崎 譲; 横山 啓一  ; 寺西 慶哲

Kurosaki, Yuzuru; Yokoyama, Keiichi; Teranishi, Yoshiaki

248及び193nmの光によるギ酸の二つの解離反応、HCOOHHO+CO(1)及びHCOOHCO+H(2)、について、RMP2(full)/cc-pVDZレベルでのdirect ab initio molecular dynamics法を用いて約1200本の古典トラジェクトリを計算した。その結果、反応2における相対並進モードへのエネルギー分配の割合は、反応1におけるそれよりもはるかに大きいことが明らかとなった。このことは反応1と2の遷移状態の構造の違いによるところが大きいと考えられる。また、生成物であるCOとHはともに振動,回転状態が励起されていることが予測された。本計算から見積もられたCOの振動状態分布は、248nm光による光解離実験の値とよく一致した。

A total of 1200 trajectories have been integrated for the two dissociation channels of formic acid, HCOOH HO + CO (1) and HCOOH CO + H (2), which occur with 248 and 193 nm photons, using the direct ab initio molecular dynamics method at the RMP2(full)/cc-pVDZ level of theory. It was found that the percentage of the energy distributed to a relative translational mode in reaction 2 is much larger than that in reaction 1. This is mainly due to the difference in the geometry of transition state (TS); the HO geometry in the TS of reaction 1 was predicted to significantly deviate from the equilibrium one, whereas the CO and H geometries in the TS of reaction 2 were found to be more similar to their equilibrium ones. It was also found that the product diatomic molecules, CO and H, are both vibrationally and rotationally excited. The calculated relative population of the vibrationally excited CO for the 248 nm photodissociation was consistent with experiment.