超快光游離誘發酚-氨錯合物陽離子內之 質子轉移動態學研究

Ultrafast Photoionization Induced Proton Transfer Dynamic in Phenol-Ammonia Cation Complex

蔡宗廷(Tsai, Tsung Ting)

指導教授 ： 鄭博元

國立清華大學/理學院/化學系/碩士(2015年)

https://doi.org/10.6843/NTHU.2015.00079

摘要

在本論文中，本團隊利用飛秒雷射激發-探測光游離-光裂解技術(Femtosecond Pump-Probe Photoionization-Photofragmentation technique)結合質譜偵測技術，研究[PhOH-NH3]+陽離子錯合物的質子轉移反應及相關動力學。吾人以REMPI技術(1+1 , λ_pump = 261.3 ~ 273.3 nm)游離化PhOH-NH3，再藉由導入探測雷射(λ_probe = 392 ~ 410 nm)得到[PhOH-NH3]+離子碎裂強度隨時間變化的瞬時訊號。我們發現在Δt = 6 ps的離子損耗率光譜不僅與Mikami團隊報告過的[PhOH-NH3]+離子阱光裂解光譜圖相似，與苯氧自由基(PhO．)電子從基態(X ̃)躍遷至π-π*激發態(C ̃)的吸收光譜也有相像之處，這意謂著我們在Δt = 6 ps所觀察[PhOH-NH3]+損耗光譜的形狀就是質子轉移的結構。另外，吾人亦觀測[PhOH-H2O]+與[PhOH-Ar]+在不同波長的瞬時訊號，但發現[PhOH-H2O]+與[PhOH-Ar]+錯合物內的結構弛緩過程皆未發生質子轉移。我們將利用兩組時間常數適解在不同波長的瞬時訊號，分別是τ_1≈ 70 fs與τ_2≈ 1 ps。理論計算的結果顯示出[PhOH-NH3]FC+弛緩至最穩定的質子轉移結構約有0.3 eV的能量差，且集中在熱反應中心OPh⋯H⋯N。依此吾人推論反應過程為PhOH-NH3藉由REMPI游離至[PhOH-NH3]FC+能態後，進行一“起始波包運動(≈ 70 fs) ”，過程中，質子會在OPh與N之間快速移動，接著熱反應中心的弛緩能量在H+移動期間透過分子內振動能量重新分佈(IVR)進行“質子轉移弛緩(≈ 1 ps)”，最後產生穩定的質子轉移結構[PhO−NH4]+。本論文研究的結果顯示:錯合物中路易式鹼分子的質子親和力與錯合物陽離子弛緩至最穩定結構的弛緩能量，是影響質子轉移過程的關鍵。

關鍵字

質子轉移 ； 陽離子動態學 ； 錯合物 ； 飛秒雷射 ； 酚 ； 氨

並列摘要

We report studies of ultrafast proton transfer (PT) reaction of PhOH-NH3 cation complex by using femtosecond pump-probe photoionization-photofragmentation spectroscopy (fs-PIPF). Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond [1+1] resonance-enhanced multiphoton ionization via the S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. The spectrum of the relative [PhOH-NH3]+ ion depletion yields measured at ~6 ps with λ_probe = 392 ~ 410 nm was found to be similar to the photofragmentation spectrum of [PhOH-NH3]+ reported by Mikami et al. and it resembles the X ̃→C ̃ band of the phenoxy radical. Thus, it suggests that the observed temporal evolutions of the photofragmentation spectra are consistent with an intracomplex PT reaction. Besides, we also compare fs-PIPF [PhOH-NH3]+ ion transients at a series of pump and probe wavelengths with [PhOH-H2O]+ and [PhOH-Ar]+ transients. The very different temporal evolutions of [PhOH-H2O]+ and [PhOH-Ar]+ transients from those of [PhOH-NH3]+ further confirm the above assignments. The experiments revealed that PT in [PhOH-NH3]+ cation proceeds in two distinct steps: an initial wave-packet motion (∼70 fs) followed by a slower relaxation (∼1 ps) that stabilizes the system into the equilibrium PT structure. Our results support the mechanism of intracomplex PT in [PhOH-NH3]+ cation complex and suggest that the proton affinity of the base is an important factor for the proton-transfer dynamics.

並列關鍵字

proton transfer ； complex ； cation dynamics ； femtosecond laser ； phenol ； ammonia

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