苯酚為酪胺酸的一個發色團,因此它對光的穩定性就相當重要,由實驗與理論計算得知,在氣相中的分子吸收紫外光後,被激發到ππ*電子激發態,然而ππ*電子激發態與πσ*耦合,最後會經由排斥態從O-H斷鍵,分解出氫原子。為了更進一步研究苯環上不同支鏈對此反應路徑的影響,本實驗選用鄰、間、對-羥基苯甲酸與鄰、間、對-羥基苯乙酮,並利用多重質量離子影像技術來探討個別的光分解機制,對於羥基苯甲酸,間、對羥基苯甲酸主要路徑為氫原子解離,而鄰羥基苯甲酸主要路徑則是H2O的解離,另一方面,鄰、間、對羥基苯乙酮主要光分解路徑都有COCH3的解離,但是只有鄰-羥基苯乙酮觀測不到氫的解離。因此在含有分子內氫鍵的分子,氫原子解離的路徑很明顯被抑制。
Phenol is a chromophore of amino acid tyrosine. The photostability of amino acids is very important to life. According to the experimental results and theoretical calculations, phenol absorb an UV photon are excited to ππ* electronic excited state. The ππ* state couples with a πσ* repulsive state. Then the H atom of O-H bond eliminates from the repulsive state. The photodissociation produce the radicals which cause further reaction. That is opposite to the photostability of amino acids. To go a step further to study the effect of the different side chains on H elimination channel, 2-,3-,4-hydroxybenzoic acid and 2-,3-,4-hydroxyacetophenone are chosen in this experiment. We study the photodissociation of each compound by using multimass ion imaging technique. For hydroxybenzoic acid, H elimination is the major channel for 3-,4-hydroxybenzoic acid and H2O elimination is the major channel for 2-hydroxybenzoic acid. On the other hand, COCH3 elimination is the major channel for 2-, 3-, 4-hydroxyacetophenone. H elimination is observed for 3-, 4-hydroxyacetophenone but it is not observed for 2-hydroxyacetophenone. In comparison with photodissociation channels and molecular structures, H elimination from the repulsive excited state is quenched for the molecules with intramolecular hydrogen bond.