This study is a multidisciplinary contribution to the thermochemistry of 1,4-cubanedicarboxylic acid (pentacyclo-[4.2.0.0 2,5 .0 3,8 .0 4,7 ]octane-1,4-dicarboxylic acid). An isoperibolic static microbomb calorimeter was used to determine the enthalpy of formation in the condensed phase at T = 298.15 K as Δ f H m °(cr) = (-355.9 ( 11.7) kJ 3 mol -1 . The enthalpy of sublimation was obtained by combining the vaporization enthalpy evaluated by correlation-gas chromatography and the fusion enthalpy measured by differential scanning calorimetry and adjusted to T = 298.15 K, which afforded the value Δ sub H m (298.15 K) = (126.5 ( 9.0) kJ 3 mol -1 . Combination of these two enthalpies gave Δ f H m °(g, 298.15 K) = (-229.4 ( 14.8) kJ 3 mol -1 . Additionally the enthalpy of sublimation of 1-adamantanecarboxylic acid (tricyclo[3.3.1.1 3,7 ]decane-1-carboxylic acid) was determined as Δ sub H m (298.15 K) = (98.7 ( 4.5) kJ 3 mol -1 . By means of theoretical calculations using isodesmic reactions, the enthalpy of formation of 2,6-cuneanedicarboxylic acid (pentacyclo[3.3.0.0 2,4 .0 3,7 .0 6,8 ]octane-2,6-dicarboxylic acid) was calculated. Strain energies of cubane and cuneane dicarboxylic acids were also calculated.' EXPERIMENTAL SECTION Materials. Purified cubanedicarboxylic acid was kindly supplied to us by Dr. A. Bashir-Hashemi (ERC at AFRL/PRSP), to whom we are very grateful. The material for the combustion experiments was used as provided, and that used for the differential scanning calorimetry (DSC) experiments was additionally recrystallized from glacial acetic acid and dried. 17 1-Adamantanecarboxylic acid (0.99 mass fraction) and all the other alkanoic acids used in this study were purchased from Aldrich

The enthalpies of combustion and sublimation of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide, 2) were measured by a rotating-bomb combustion calorimeter and the Knudsen effusion technique, and the gas-phase enthalpy of formation was determined, DeltafH degrees m(g) = -98.0 +/- 1.9 kJ mol(-1). This value is not as large (negative) as could have been expected from comparison with thermochemical data available for the thiane/thiane oxide reference system. High-level ab initio molecular orbital calculations at the MP2(FULL)/6-31G(3df,2p) level were performed, and the optimized molecular and electronic structures of 2 afforded valuable information on (1) the relative conformational energies of 2-axial and 2-equatorial--the latter being 7.1 kJ mol(-1) more stable than 2-axial, (2) the possible involvement of nS --> sigma*(C-S(O)) hyperconjugation in 2-equatorial, (3) the lack of computational evidence for sigma(S-C) --> sigma*(S-O) stereoelectronic interaction in 2-equatorial, and (4) the relevance of a repulsive electrostatic interaction between sulfur atoms in 1,3-dithiane sulfoxide, which apparently counterbalances any nS --> sigma*(C-S(O)) stabilizing hyperconjugative interaction and accounts for the lower than expected enthalpy of formation for sulfoxide 2.

Dedicated to Professor AbstractWe analyze and compare the energetics of arenoic acids from studies of thermodynamic data such as the enthalpy of formation, ∆ f H°m. This quantity offers a powerful procedure for the understanding of the contrasting structural, conformational, and reactivity trends exhibited by compounds.