In there compounds, the cyclopentadiene ring has only four hydrogens.在这些化合物里, 环戊二烯环只有4个氢.The cyclopentadiene is prepared by the thermal cracking of dicyclopentadiene.以热裂解双环戊二烯的方法制备环戊二烯.The chemistry of cyclopentadiene or...
我有了头痛的先兆。I had the beginnings of a headache.乌云是下雨的明显先兆.Black clouds are an unmistakable sign of coming rain.那被认为是一种不幸的先兆.It is said to be an omen of misfortune....