| Abstract
| - The potential energy surface for the intramolecular reaction of singlet state RR‘EERR‘ (E = C, Si, Ge, Sn,and Pb) has been explored using density functional theory. All the stationary points, including the unsymmetricalreactant (R‘R2E−ER‘), the transition state, the symmetric product (R‘REERR‘), and the monomer (R‘RE)were completely optimized at the B3LYP/LANL2DZdp level of theory. Our theoretical findings suggest thefollowing: (1) Both double-bonded RR‘CCRR‘ and RR‘SiSiRR‘ species are true minima on their potentialenergy surfaces and should be the only compounds existing at all temperatures. (2) The germanium systemwill occur either in the dimeric R2R‘Ge−GeR‘ and RR‘GeGeRR‘ structures or the monomeric RR‘Ge structure,depending on the temperature. (3) If the size of the substituent (R) is small, then the unsymmetrical single-bonded R2R‘Sn−SnR‘ molecule can exist at low temperatures. At room temperature, the unsymmetricalR2R‘Sn−SnR‘ species can exist in equilibrium with its RR‘Sn monomer. (4) The unsymmetrical R3Pb−PbRcompound may be kinetically stable at low temperatures. On the other hand, it is predicted that both theunsymmetrical R3Pb−PbR and the symmetric R2PbPbR2 species will spontaneously dissociate into R2Pbmonomers at room temperature. Our theoretical results are in good agreement with available experimentalobservations (J. Am. Chem. Soc.2003, 125, 7520), and the results obtained allow a number of predictions tobe made.
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