Theoretical Studies of the Energies and Geometries of Strained Hydrocarbons
Theoretical models are used to explore problems of interest to organic chemists in four projects. The first three apply existing methodologies to investigate a series of strained cyclic hydrocarbon molecules; the fourth develops methodology. Project I. Semiempirical calculations have been performed on a variety of hydrocarbons composed of fused cyclobutane rings using the MNDO, AM1 and MM2 methods. The systems studied include linearly concatenated cyclobutanes (ladderanes), cyclic structures (prismanes) and two 'star-shaped' C24H24 hydrocarbons, helvetane and israelane. The results reported include optimized geometries, heats of formation and vibrational frequencies. The performance of the theoretical models is evaluated and the relative stabilities of the structures are discussed. Project II. An ab initio molecular orbital study of the energetics of the successive hydrogenations of triquinacene provides no indication of homoaromatic stabilization of this system, a conclusion at variance with a recent thermochemical study. The absence of this effect persists even when energies are computed in the 6-31G*basis to second order in electron correlation, and the contributions of zero-point energies and thermal corrections to enthalpies are taken into account. Project III. Ab initio calculations on bicyclo (2.2.2) octa-2,5,7-triene (barrelene), bicyclo (2.2.2) octa-2,5-diene, bicyclo (2.2.2) oct-2-ene, and bicyclo (2.2.2) octane have been performed at the 3-21G, 6-31G* and 6-31G** SCF levels and at the 6-31G* RMP2 level and the contributions of zero-point energies and thermal corrections to enthalpies have been evaluated. The results for these compounds enable comparison with the experimental thermochemistry of the barrelene series and a reassessment of the extent of destabilization in barrelene. Project IV. The fourth project involves the development of a procedure to reduce the cost of ab initio geometry optimizations in large basis sets using the results of similar optimizations in smaller bases. In most cases, optimization in the larger basis can be accomplished with as few as 3 or 4 gradient calculations. The method has been applied to 13 strained hydrocarbon molecules and has been found to work in each case. Energies and geometric parameters agree with those obtained from complete optimizations. (Abstract shortened with permission of author.)
City University of New York - Graduate Center
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