Controlling Vibrational Energy Flow in Liquid Alkylbenzenes
Controlling Vibrational Energy Flow in Liquid Alkylbenzenes
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DOI：
10.1021/jp406528u
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发表时间：
2013-09-19
影响因子：
3.3
通讯作者：
Dlott, Dana D.
中科院分区：
文献类型：
--
作者：
Pein, Brandt C.;Sun, Yuxiao;Dlott, Dana D.
用超快红外（IR）拉曼光谱研究了φ-S烷基苯的振动能，其中φ = C6 H5，取代基S为CH 3-（甲苯），（CH 3）（2）CH-（异丙苯，IPB），（CH 3）（3）C-（叔丁基苯，TBB）.使用先前描述的方法，（1）将简正模式分类为苯基（0）、取代基（5）或全局（G）。IR脉冲被调谐以找到使初始CH-伸缩激发在0或S上的定位最大化的条件。反斯托克斯拉曼光谱测量拉曼活性S、0和G模式的瞬态能量含量，以确定苯基与取代基的速率（φ. -> S）或取代基到苯基（S -> phi）的转移，此时能量转移主要是分子内的。由于苯基CH-Y伸缩为90-130，因此感兴趣的是S -> phi过程，其中分子结构和局部耦合i比能量差更重要。-> S过程效率很小，并且与所有三种取代基大致相等。增加取代基的大小可以提高S -> 4（I）转移效率。这与基于较大取代基的较大态密度所预测的相反，并且它提供了一条控制向前到向后振动能量转移比的途径。S -> phi传递效率被理解为是由局部非谐耦合的增加引起的。当振动时，较重的取代基更有效地将能量转移到苯基。
Ultrafast infrared (IR) Raman spectroscopy was used to study vibrational energy in phi-S alkylbenzenes, where phi = C6H5 and substituents S were CH3- (toluene), (CH3)(2)CH- (isopropylbenzene, IPB), or (CH3)(3)C- (tbutylbenzene, TBB). Using methods described previously,(1) the normal modes were classified as phenyl (0), substituent (5), or global (G). IR pulses were tuned to find conditions that maximized the localization of initial CH-stretch excitations on 0 or S. Anti-Stokes Raman spectroscopy measured transient energy content of Raman-active S, 0, and G modes, to determine the rates of phenyl to substituent (phi. -> S) or substituent to phenyl (S -> phi) transfer during the first few picoseconds, when energy transfer was mainly intramolecular. Since phenyl CH-Y stretches were 90-130 interest were S -> phi processes where molecular structure and local couplings i were more important than energy differences. The -> S process efficiencies were small and about equal with all three substituents. The S -> 4 ( I) transfer efficiencies could be increased by increasing substituent size. This was opposite to what would be predicted on the basis of the larger density of states of larger substituents, and it provides a path toward controlling forward-to-backward vibrational energy transfer ratios. The S -> phi transfer efficiency is understood as resulting from an increase in the local anharmonic couplings. A heavier substituent, when vibrating, transfers energy more effectively to the phenyl group.