The electronic spectrum of benzaldehyde has been studied by using multiconfigurational second-order perturbation theory through the multistate extension (MS-CASPT2). The n#PI#~* II A II state, placed vertically at 3.71 e V, is assigned to the lowest band. The Ii A I -21 A I and 1.1 A I -31 A I transitions, of #PI##PI#~*nature, located at 4.33 and 4.89 e V, are responsible for the second- and third-energy bands, respectively. The most intense .feature involves .the 41A' and 51A' #PI##PI#~*excited states, calculated to be 5.98 and 6.23 eV above the ground state. In addition, excited states corresponding to the low-l.ying Rydberg series are related to the available experimental. data. Geometry optimizations for the ground state and low-Iying excited states of both n#PI#~* and #PI##PI#~*characters have been carried out at the CASSCF level. The relative ordering of the lowest n#PI#~* and #PI##PI#~*triplet states varies depending on the geometry employed. Furthermore, the SI(n#PI#~*) and T2( #PI##PI#~*) potential hypersurfaces are found to intersect upon relaxation in SI, providing a possible explanation for the efficient singlet-triplet intersystem crossing occurring in benzaldehyde.
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机译：通过多态扩展（MS-CASPT2），使用多构型二阶摄动理论研究了苯甲醛的电子光谱。垂直放置在3.71 e V处的n＃PI＃〜* II A II状态分配给最低频段。 ＃PI ## PI＃〜* nature的Ii A I -21 A I和1.1 A I -31 A I跃迁分别位于第二和第三能带，分别位于4.33和4.89 eV。最强烈的特征涉及41A'和51A'＃PI ## PI＃〜*激发态，计算出的激发态比基态高5.98和6.23 eV。此外，与低lyying Rydberg级数相对应的激发态与可用的实验有关。数据。已经在CASSCF级别对n＃PI＃〜*和＃PI ## PI＃〜*两个字符的基态和低Iying激发态进行了几何优化。最低的n＃PI＃〜*和＃PI ## PI＃〜*三重态的相对顺序取决于所采用的几何形状。此外，发现SI（n＃PI＃〜*）和T2（＃PI ## PI＃〜*）势超表面在SI松弛时相交，从而为苯甲醛中有效的单重态-三重态系统间交叉提供了可能的解释。 。
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