Observation of X 1 a , Vinylidene by Photoelectron Spectroscopy of the C 2 Hf Ion

The vinylidene radical ( H2C=Cr) is the simplest unsaturated carbenc and is postulated to be a reactive intermediate in a number of reactions [ 1 ,?I, such as in the combination of two CH2 radicals to produce acetylene_ It is particularly interesting in that it rearranges by a 1.7 hydrogen shift. an important process in the chemistry of txrbenes [3]_ Extensive calculations have been done on the electronic structure of S 1 A, vinylidene [4-l?] _ which predict. among other things. a barrier fo rearrangement of 4-S kcal/niole [6. S]. or even as low as 0.9 kcal/mole [ 1 1 ] _ Dyn.unical calculations predict 141 a lifetime against tunneling of O-1-10 ps. in agreenlent with indirect experiments (2. l3]_ There have been no direct observations of sinL?let vinylidene, and. indeed. the espected short life3 time 1~~s led to speculation that it may not be spectroscopic.dly observable_ Calculations predict the lowest excited electronic state, jBz. to lie 30-50 kcal/mole above the singlet [S. S. 14-17) _ The triplet is predicted to have a much higher barrier fo rearrangement. and might live sufficiently long to be esperinlentally observed_ In fact. Laufer [l&-20] has observed a transient absorption in both photolyzed acetylene and in ketene, which