Various experiments have suggested the symmetry distortion of para-benzoquinone (PBQ) at the lowest triplet (n-π*) state. The vibronic coupling of electronic states known as the pseudo-Jahn-Teller effect could be the origin of the symmetry distortion. However, previous quantum chemical calculations could not provide reliable evidence for the symmetry distortion. Here we show the pseudo-Jahn-Teller distortion is an intrinsic property of PBQ due to the vibronic coupling between two low-lying n-π* states. Consequently, PBQ takes a C_2v geometry having inequivalent two carbonyl groups, but the molecule remains planar. This state involves an inequivalent n-π* electronic transition and yields a mixed-valence state.

各种实验表明对-苯醌（PBQ）在最低三重态（n-π *）状态下的对称畸变。电子状态的振动耦合被称为伪Jahn-Teller效应，可能是对称畸变的起源。然而，先前的量子化学计算不能为对称畸变提供可靠的证据。在这里，我们显示伪Jahn-Teller失真是PBQ的固有属性，这是由于两个低位n - π *状态之间的振动耦合引起的。因此，PBQ的C _2v几何结构具有两个不等价的羰基基团，但分子仍保持平面状。该状态涉及不等价的n-π *电子跃迁，并产生混合价态。