Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701-4004, USA.
J Phys Chem A. 2009 Nov 26;113(47):13318-26. doi: 10.1021/jp9041364.
The cavity ringdown absorption spectrum of 4-cyclopentene-1,3-dione was recorded near 487 nm in a room-temperature gas cell. The very weak band system (epsilon approximately 0.05 dm3 mol-1 cm-1) in this region is due to the T1(n,pi*) <-- S0 electronic transition. The origin-band maximum was observed at 20540.0 +/- 0.5 cm-1. We have assigned about 40 vibronically resolved bands in a region extending to +1100 cm-1 relative to the origin. Assignments were aided by quantum-chemical calculations of the T1 <-- S0 0-0 excitation energy as well as ground-state vibrational frequencies. From the CRD spectral assignments, we determined fundamental frequencies for several vibrational modes in the T1 excited state, including the lowest-energy ring-bending and -twisting modes, nu19' (b1) and nu14' (a2), respectively. Their fundamentals in the T1 state are 160.5 and 246 cm-1, compared to 99 and 239 cm-1, respectively, in the S0 ground state. The increases in these ring frequencies upon electronic excitation signify that the nominal n --> pi* chromophore is delocalized to include the conjugated ring atoms. The extent of this delocalization is different in the T1(n,pi*) vs S1(n,pi*) excited states. This conclusion is based on observed differences in T1 vs S1 ring fundamental frequencies.
在室温气体池中，记录了 4-环戊烯-1,3-二酮在近 487nm 处的腔衰荡吸收光谱。该区域非常弱的带系（epsilon 约为 0.05 dm3 mol-1 cm-1）归因于 T1(n,pi*) <-- S0 电子跃迁。观察到的起始带最大值位于 20540.0 +/- 0.5 cm-1。我们在相对于起始的 +1100 cm-1 范围内分配了大约 40 个振动态分辨的带。通过量子化学计算 T1 <-- S0 0-0 激发能以及基态振动频率，辅助了分配。从 CRD 光谱分配，我们确定了 T1 激发态中几个振动模式的基本频率，包括最低能量的环弯曲和扭转模式，nu19'（b1）和 nu14'（a2），分别为 160.5 和 246 cm-1，而在 S0 基态分别为 99 和 239 cm-1。这些环频率在电子激发时的增加表明，名义上的 n --> pi发色团被离域以包括共轭环原子。在 T1(n,pi)和 S1(n,pi*)激发态中，这种离域程度不同。这一结论基于 T1 和 S1 环基本频率的观察差异得出。
