胞嘧啶微溶合團簇之超快激發態動態學研究

本論文研究protic solvent—水、甲醇與aprotic solvent—四氫呋喃與胞嘧啶形成之為溶合團簇之激發態衰減動態學。我們利用飛秒雷射激發–探測多光子游離技術結合飛行時間質譜儀進行實驗，以激發波長為266 nm；探測波長為800 nm之pump-probe REMPI(1+3')游離氣相胞嘧啶與溶劑分子所形成的團簇，觀察離子訊號大小隨激發–探測脈衝時間差變化。胞嘧啶在氣相中有三種較穩定的互變異構物—keto、enol、imino結構，在胞嘧啶單體的氣相動力學實驗中可以看見兩種生命期，分別為keto與enol構型的貢獻。但當胞嘧啶(Cy)在分子束在與protic solvent分子影響下，瞬時光譜中只會看見keto結構的貢獻。與前人的實驗相比，發現Cy-(H2O)n=2-6的生命期都約為0.2 ps，和在液態水相的胞嘧啶動力學實驗有相似的時間尺度。Cy-(MeOH)1與Cy-(H2O)1瞬時光譜形狀相似，生命期都約為0.5 ps；而Cy-(MeOH)2和Cy-(MeOH)3的激發態衰減生命期分別為0.48 ps和0.33 ps 較Cy-(H2O)n>2長，推測可能是因為甲醇分子的極性與氫鍵都較水分子弱所造成。當胞嘧啶與aprotic solvent分子作用時，瞬時光譜形狀與胞嘧啶單體的瞬時光譜較相似。Cy-(THF)1的瞬時光譜包含兩個生命期，一個為keto構型的貢獻，生命期為0.48 ps，；另一為enol結構的貢獻，生命期為10 ps。由上述的生命期可知，四氫呋喃對enol結構的胞嘧啶影響較大，生命期長了兩倍以上。

關鍵字

胞嘧啶；團簇；超快動態學；激發態；溶劑；氣態

並列摘要

Gas-phase ultrafast excited-state dynamics of cytosine(Cy)-(H2O)n=4-6, Cy-(MeOH)n=1-3, and Cy-(THF)1 micro-solvated clusters were investigated in molecular beams using femtosecond pump-probe photoionization spectroscopy at about 266 nm excitation to identify the effect of protic and aprotic solvents to the cytosine intrinsic dynamics. The results suggest that the presence of protic solvent molecules allows an extensive relaxation of Cy-(H2O)n=1-6, and Cy-(MeOH)n=1-3 to the lowest-energy tautomer isomers, but aprotic solvent molecules do not exhibit such behaviors. Excited-state lifetimes of Cy-(H2O)n=1-6 and Cy-(MeOH)n=1-3 measured here are 0.2-0.5 ps. On the other hand, there are two components in the Cy-(THF)1 transient: one with an excited-state lifetime of 0.48 ps, and the other ~ 10 ps. The shorter component is ascribed to the keto form of Cy-THF, whereas the longer component is attributed to the enol form of Cy-THF. Comparisons of the Cy-(THF)1 and cytosine monomer transients suggest that THF has a stronger effect on enol-cytosine than keto-cytosine.

並列關鍵字

cytosine ； clusters ； ultrafast dynamics ； excited state ； soivent ； gas phase

參考文獻

21. 顏弘建, 胞嘧啶及其水合團簇之氣相超快激發態動態學研究. 2007.

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胞嘧啶微溶合團簇之超快激發態動態學研究

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