利用弗蘭克-康登模擬闡述芥子酸分子在氣相與溶液 相吸收光譜中的電振運動

芥子酸分子在有機合成中具有獨特光物理和化學特性的共軛面，其吸收光譜也從實驗角度被廣泛研究。然而解釋吸收光譜我們需要特別貫注釐清其電振運動。在當前的密度泛函與含時密度泛函的芥子酸電子結構計算中，我們發現其基態與第一激發態均存有順式與反式兩種結構；在(TD) M06-2X/6-311++G(d,p)的量化計算中，我們得到過渡態的基態分子（第一激發態）與順式和反式分子的能量差大概為 5.5 (11.7) kcal/mol。當芥子酸分子的計算環境由氣相轉換為 PCM 模型下的水溶液相時，基態下的過渡態能量壁壘明顯降低，但激發態的能量壁壘仍可達到 12.7kcal/mol。這反映出在氣相中順式和反式芥子酸分子的吸收光譜可以完全分離，但在水溶劑環境中會完全混合。為了定量理解並解釋芥子酸在氣相與溶液相中的實驗光譜，我們採用位移諧振子近似下的弗蘭克-康登模擬來分析芥子酸的電振運動。弗蘭克-康登估算反應出了在氣相中，反式芥子酸的吸收光譜主要由三個振動態貢獻，分別為振動態 v1（72.9 cm-1，其黃琨因子 S=0.83），振動態 v3（205.6 cm-1，S=0.06），以及振動態 v4 （255.6 cm-1，S=0.25），而順式芥子酸分子的光譜則幾乎只由一個振動態 v1（73.3 cm-1， S=1.38）貢獻。我們計算出的氣相中的順式與反式芥子酸分子的吸收光譜在譜峰的強度與相對位置上都與實驗資料相當吻合。然而，芥子酸分子在溶液相中的吸收光譜要通過弗蘭克-康登模擬出的順式與反式分子混合與疊加來解釋，並且在振子強度的計算中表明，順式與反式的芥子酸分子對吸收光譜的貢獻幾乎相等。通過水溶液中的弗蘭克-康登模擬，我們分析光譜的強度主要由上述的反式芥子酸的三種振動模式與順式芥子酸的一種振動模式共同貢獻。但是光譜的譜寬則反映出我們需要考慮頻率在 739 cm-1到 1300cm-1 之間的所有 25 個主要振動模式，在溶液中，溶劑分子與這些主要振動模式都有相互作用，所以這些振動模式都需要被考慮，從而重現溶液相光譜的譜寬。將這些主要振動模式都加入了光譜的模擬後，水溶劑中的模擬光譜也能很好地重現實驗吸收光譜。本研究提供了新的物理見解，以理解和解釋重要的芥子酸分子在氣相和溶液相中的實驗吸收光譜。

關鍵字

弗蘭克-康登；芥子酸；電振運動

並列摘要

Sinapic acid molecule has conjugate plane with unique photophysical and chemical properties in organic synthesis and its absorption spectra are extensively studied from experimental aspect. However, interpretation of its absorption spectra needs special attention to unravel origin of vibronic motions. The present density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculation for electronic structure of sinapic acid found there exist cis-minima and trans-minima on both ground and the first excited states; (TD) M06-2X/6-311++G(d,p) quantum chemistry calculation shows there is transition state about 5.5 (11.7) kcal/mol between ground-state (the first excited-state) cis- and trans- minimum. However, this transition state barrier diminishes on the ground state while it still has 12.7kcal/mol in water solvent estimated by polarizable continuum model (PCM). This reflects that the absorption spectra on cis-region and trans-region can be completely separated in gas phase, but will be completely mixed in water solvent environment. In order to quantitatively understand and interpret experimentally observed absorption spectra in gas and solution phases for sinapic acid, the Franck-Condon (FC) simulation with displaced oscillator approximation is employed to analyze vibronic motions. FC simulation reveals that absorption spectrum is dominantly produced by three vibrational modes v1 (72.9 cm-1 and Huang – Rhys factor S=0.83) , v3(205.6 cm-1 ,S=0.06), and v4 (255.6 cm-1 ,S=0.25) in trans-region, but by only one mode v1 (73.3 cm-1 ,S=1.38) in cis-region. Simulated absorption spectra in both trans- and cis- regions well reproduce experimental spectral profiles for both intensities as well as peak positions. However, absorption spectrum of sinapic acid in water solvent is interpreted by mixing cis-region with trans-region from FC simulation, and oscillator strength calculation shows that both regions contribute absorption spectrum almost equally. Analysis from FC simulation in water solvent reveals that spectral intensities are dominantly reproduced by above three modes in trans-region plus one mode in cis-region, however, spectral broadness requires from all 25 active vibrational modes from frequency 739 cm-1 to 1300 cm-1. Solvent molecules interacting with all active modes of vibration in sinapic acid molecule is essential to make spectrum broadening. Simulated spectrum in water solvent also well reproduces experimental absorption spectrum. The present study provides a new physical insight to understand and interpret experimental absorption spectra in gas and solution phases simultaneously for important sinapic acid molecule.

並列關鍵字

Franck–Condon ； sinapic acid ； vibronic motions

參考文獻

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