Sessions Anna G, McDonnell Michael P, Christianson Drew A, Drucker Stephen
Department of Chemistry , University of Wisconsin-Eau Claire , Eau Claire , Wisconsin 54701 , United States.
J Phys Chem A. 2019 Jul 25;123(29):6269-6280. doi: 10.1021/acs.jpca.9b04238. Epub 2019 Jul 12.
The 4-pyran-4-one (4PN) molecule serves as a model for investigating structural changes following π* ← n electronic excitation. We have recorded the cavity ringdown (CRD) absorption spectrum of 4PN vapor at room temperature, over the wavelength region from 350 to 370 nm. This spectral region includes the T(n,π*) ← S band system as well as the low-energy portion of the S(n,π*) ← S system. Aided by predictions from ab initio (equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles, EOM-EE-CCSD) and density functional theory (time-dependent density functional theory with PBE0 functional, TDPBE0) calculations, we have made vibronic assignments for about 30 features in the CRD spectrum, mostly T(n,π*) ← S transitions. We have used these results to correct certain vibronic assignments appearing in the previous literature for both T(n,π*) ← S and S(n,π*) ← S band systems. We conclude that the lowest-energy carbonyl wagging fundamentals (ν, in-plane and ν, out-of-plane) undergo significant frequency drops (28 and 50%, respectively) upon T(n,π*) ← S excitation and similar drops (29 and 39%, respectively) for S(n,π*) ← S excitation. We find that vibrational modes involving the conjugated ring atoms undergo relatively small frequency changes upon π* ← n excitation, for both T and S states. We have used the present spectroscopic results and vibronic assignments to test the accuracy of computed excited-state frequencies for 4PN. This benchmarking process shows that the economical time-dependent density functional theory method is impressively accurate for certain (but not all) vibrational modes. The highly correlated EOM-EE-CCSD ab initio method is capable of making accurate frequency predictions, but the results, unexpectedly, depend sensitively on basis set family. This anomaly is traceable to a computed conical intersection between the T(n,π*) and T(π,π*) surfaces near the T(n,π*) potential minimum. Relatively small errors in the location of the conical intersection lead to enhanced mixing of the two electronic states and incorrect T(n,π*) vibrational frequencies when certain triple-ζ quality basis sets are used.
4-吡喃-4-酮（4PN）分子是用于研究π*←n电子激发后结构变化的模型。我们记录了室温下4PN蒸气在350至370nm波长范围内的腔衰荡（CRD）吸收光谱。该光谱区域包括T(n,π*)←S能带系统以及S(n,π*)←S系统的低能量部分。借助从头算（在耦合簇单双激发水平纳入动态相关的运动方程激发能，EOM-EE-CCSD）和密度泛函理论（采用PBE0泛函的含时密度泛函理论，TDPBE0）计算的预测结果，我们对CRD光谱中的约30个特征进行了振转归属，大多是T(n,π*)←S跃迁。我们利用这些结果对先前文献中T(n,π*)←S和S(n,π*)←S能带系统的某些振转归属进行了修正。我们得出结论，最低能量的羰基摇摆基频（ν，面内和ν，面外）在T(n,π*)←S激发时经历显著的频率下降（分别为28%和50%），在S(n,π*)←S激发时也有类似下降（分别为29%和39%）。我们发现，对于T态和S态，涉及共轭环原子的振动模式在π*←n激发时频率变化相对较小。我们利用当前的光谱结果和振转归属来检验4PN计算激发态频率的准确性。这个基准测试过程表明，经济的含时密度泛函理论方法对于某些（但不是所有）振动模式具有令人印象深刻的准确性。高度相关的EOM-EE-CCSD从头算方法能够进行准确的频率预测，但结果出乎意料地对基组族敏感。这种异常可追溯到在T(n,π*)势阱最小值附近计算得到的T(n,π*)和T(π,π*)表面之间的锥形交叉。当使用某些三重ζ质量基组时，锥形交叉位置的相对小误差会导致两个电子态的混合增强以及T(n,π*)振动频率错误。
