REEXAMINATION OF THE STRUCTURES OF C2LI2 AND OF C4LI4

Geometry optimization and frequency analysis at MP2(full)/6-31+G* indicates the linear (1) and doubly bridged D2h (3) C2Li2 forms to be the only minima on the dilithioacetylene potential energy surface. The transition structure (2) connecting these isomere has Cs symmetry, i.e., the two CLi arrangements are not identical. At MP4stdq/6-311+G* + ZPE, the energy of 1 is 10.7 kcal/mol higher than 3, and rearrangement (via 2) would only require 0.3 kcal/mol. Hence, only 3 is likely to be observed experimentally. No C2v minima, neither doubly bridged nor of vinylidene type, could be located. In contrast, H2C=C is a singlet minimum and the doubly bridged form of C2H2 (an even higher energy transition structure) prefers C2v symmetry (7) over D2h strongly. The tetrahedral face-centered form of C4Li4 (4) was indicated to be a local minimum (despite claims to the contrary in the literature) at MP2/6-31G*. Nevertheless, 4 is unlikely to be a viable candidate for experimental observations as an isolated entity, as other forms are far more stable. © 1990 American Chemical Society.