The way to read this diagram is it's as if it were my brain and I am facing this way.
查看这张示意图的方法是,设想这是我的大脑,面朝这个方向
So pi bonds have electron density both above and below the bond axis, but they actually have a nodal plane at this z, this bond axis here.
键在键轴之上,和之下都有电子密度,但它们在z方向有节面,这是键轴的地方。
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