利用交叉分子束方法形成C6D6O, C6H6S與C6D6S加合物以及基態硫原子與苯反應的量子化學計算

Abstract

本實驗中，我們利用交叉分子束生成氧原子(或硫原子)與苯(或氘代苯)的加合物，並對基態硫原子與苯的反應進行量子化學計算。 在交叉分子束實驗中，氧原子(或硫原子)以適當氣體經高壓放電產生，其中基態的氧原子及硫原子為主要產物。氧原子束(或硫原子束)與苯(或氘代苯)分子束交會後，反應的產物會在距離分子束碰撞點10公分處被同步輻射的真空紫外光游離，並由質譜分析偵測。 本實驗觀測了與加合物相關的母體離子及碎片離子，實驗數據顯示這些離子皆在相同的飛行時間抵達偵測器，其角度分布亦相互吻合，藉此我們推測這些離子可能來自同一中性產物。而在質心角度附近觀測到母體離子及碎片離子的訊號則顯示中性產物的速度向量在質心方向，且游離前中性產物並沒有分解而造成速度變化。根據交叉分子束實驗結果，我們判定觀測到的母體及碎片離子來自於長生命期的C6D6O、C6H6S及C6D6S加合物。此外，光游離效率圖更顯示硫原子與氘代苯反應產生的C6D6S加合物擁有較多的游離碎片與低游離能，其內能相較穩定的Thiophenol高了許多。然而，即使加合物的高內能使其熱力學上不穩定，我們發現這些加合物仍擁有超過百微秒的長生命期。 為了瞭解分子束實驗中C6D6S加合物可能的反應異構物及生成途徑，我們進一步進行基態硫原子與苯反應的量子化學計算。反應過渡態與中間態的幾何結構以及反應路徑(intrinsic reaction paths, IRC)首先以密度泛函方法B3LYP/6-311G(d,p)計算，之後過渡態與中間態再以較高基組aug-cc-pVTZ做幾何結構優化。由計算結果我們推測反應可能進行的途徑如下：反應物原先在三重態位能面，經由intersystem crossing後(可能發生在類似C6H6S diradical的幾何結構)，在單重態位能面生成產物。單重態的benzene sulfide和thiophenol 可能為intersystem crossing後首先生成的中間態，之後可能再形成其他的異構物。相較其他中間態，單重態的thiophenol擁有最低的位能，因此也較為穩定。此外，我們更進一步用time-dependent M06方法計算intersystem crossing可能發生區域的基態與激發態能量，並針對與intersystem crossing相關的中間態及過渡態進行M06/aug-cc-pVTZ幾何結構優化。

In this work, we studied the formation of long-lived hot adducts in the O+C6D6, S+C6H6 and S+C6D6 reactions with crossed molecular beam method. Theoretical calculations related to the C6H6S adduct formation are also performed. In the crossed molecular beam experiments, O (or S) atoms were generated with discharge and ground state O(3P) and S(3P) atoms were found to be predominant in these two atomic beams. After O (or S) atomic beam intercepted with C6H6 (or C6D6) beam, the products were photo-ionized by vacuum UV light from synchrotron radiation 10 cm away from the crossing point; the photo-ions were then mass-filtered and detected. For the studied reactions, the detected parent and daughter ions arrived at the detector almost at the same time and exhibited the same angular distribution, indicating they all correspond to the same neutral species. Based on our experimental results, long-lived hot adducts of C6D6O, C6H6S and C6D6S are formed in the crossed beam condition. These products are detected at the angle of the center of mass, indicating zero recoil velocity. Photoionzation efficiency (PIE) curves of the C6D6S adduct and thiophenol sample were measured. In the PIE curves, lower ionization energy and more fragmentation are observed for the C6D6S adduct, consistent with the fact that this adduct has higher internal energy than the thiolphenol sample. Although the observed adducts have high internal energy and are thermodynamically unstable, their lifetimes may exceed the experimental time of flight which is in the order of hundreds of microseconds. To determine the possible isomers for the C6H6S adduct and their formation pathways, we conducted theoretical calculation on the S(3P) + C6H6 reaction. Geometry optimization of stationary points and intrinsic reaction paths (IRC) are carried out at B3LYP/6-311G(d,p) level of theory. These stationary points are then optimized again at B3LYP/aug-cc-pVTZ level. From the present data, we propose the most possible reaction paths are as follows. Reactants are originally on the triplet potential energy surface. Through intersystem crossing (possibly near the C6H6S diradical geometry), most isomers are then deposited on the singlet potential energy surface. Singlet benzene sulfide and thiophenol are supposed to be the first-formed intermediates, which may undergo further isomerizations. For all possible isomers, singlet thiophenol is found to be the most stable one. Furthermore, energies of ground and excited states near the intersystem crossing region are calculated by time-dependent density functional method, TD-M06 method. And for some stationary points which might concern singlet-triplet intersystem crossing are optimized by M06/aug-cc-pVTZ for better accuracy.