Singlet ground state, acetylene

The isomers of C2H2 are 2Ol(s), acetylene and 2O2(s), vinylidene. Both have singlet ground states. A third isomer was reported to have appeared in a theoretical calculation, but, it turned out to have been a computational artifact [8]. Table 17.1 presents relative self consistent field (SCF) energies, zero point vibrational energies (ZPVE), and dipole moments of the isomers. 

The excited states of the enediyne moiety were approximated by a weakly interacting ethylene acetylene model where the various states are expressed as linear combinations of the configurations for one ethylene and two acetylenes. The authors examined the first six excited states of 80, three triplets and three singlets. AH methods examined predicted that the spectroscopically forbidden triplet states (DBj, DBj, and DAj, respectively) He between the singlet ground state (S ) and the first singlet excited state (I Bj). 

Figure 2.42. (Top) DFT optimized structure of gas phase ground-state singlet and excited triplet state acetylene. (Bottom) Optimized structures of acetylene on cluster models of Cu(100) (left) and Cu(110) (right). From Ref. [84].

The acetylene anion radical undergoes autodetachment of the electron, but the vinylidene anion can be generated easily [83]. Since the calculated isomerization barrier is -45 kcal/mol, the 2B2 ground-state vinylidene anion radical is predicted to be stable with respect to the 1,2-hydrogen shift [30, 84, 85]. As mentioned before, the vinylidene anion radical was used as the precursor for the generation of the singlet vinylidene in Lineberger s experimental studies. 

C2H2 fragments then only have to twist about the (72-axis as they recede to form two acetylene molecules, one in the closed shell ground-state and the other as an excited singlet. 

Figure 5.14. Geometry of (a) ground and (b) first excited singlet states of acetylene.

---