Theoretical Investigation of the C 2 H 2 B 2 Potential Energy Surface

Abstract

Fifteen unique energy minima and thirteen transition states on the C 2H2B2 potential surface have been located and optimized at the MP2 level of theory with the 6-311G(d,p) basis set. The planar four-membered ring isomer \({\text{HC}}{\text{ = }}{\text{ = }}{\text{CHB}}{\text{ = }}{\text{ = }}{\text{B}}\), 1, an analog of cyclobutadiene, is a transition state lying 37 kcal/mol above the nonplanar four-membered ring \({\text{HC}}{\text{ = }}{\text{ = }}{\text{CHB - B - }}\), 3. The planar \({\text{HC}}{\text{ = }}{\text{ = }}{\text{B - CH}}{\text{ = }}{\text{ = }}{\text{C}}\), 10, is the second most stable species found, lying 72.2 kcal/mol below 3. The nonplanar, butterfly-shaped \({\text{B}}{\text{ = }}{\text{ = }}{\text{CH - CH}}{\text{ = }}{\text{ = }}{\text{B}}\) ring, 4, is a local minimum 33.7 kcal/mol more stable than 3. A four-membered ring isomer with alternating boron–carbon locations, \({\text{HC}}{\text{ = }}{\text{ = }}{\text{B}}{\text{ - }}{\text{ - }}{\text{CH}}{\text{ = }}{\text{ = }}{\text{B}}\), 5, lies 67.0 kcal/mol below 3 and 33.3 kcal/mol below 4. The ring of 5 is planar with one hydrogen above and one below the plane (C2h symmetry). The borylene-substituted boracyclopropene, \({\text{HC}}{\text{ = }}{\text{ = }}{\text{B}}{\text{ - }}{\text{ - }}{\text{CH}}{\text{ = }}{\text{ = }}{\text{B}}\), 8, is a planar local minimum lying 36.0 kcal/mol above 5. The most stable C2H2B2 isomer found was the planar, four-membered ring system 22 (D2h symmetry) composed of two BCC three-membered rings fused across the C-C bond. Structure 22 lies 22.2 kcal/mole below 10, 105.4 kcal/mol below 3, 71.7 kcal/mol below 4, and 38.2 kcal/mol below 5. Isomer 22 is the structural analog of the trialene form of C4H2. The most stable linear isomer, HB\({\text{ = }}{\text{ = C}}{\text{ = }}{\text{ = }}{\text{C}}{\text{ = }}{\text{ = }}\)BH, 26, was surprisingly 50.5 kcal/mol less stable than 22. The stabilities of the two most stable cyclic isomers 10 and 22 may be explained by aromaticity.