An Experimental Thermochemical and Theoretical Study of Triquinacene:  Definitive Disproof of Its Neutral Homoaromaticity

Abstract

The enthalpy of formation ( = 57.51 ± 0.70 kcal/mol) of triquinacene (1), newly determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that previously reported and corresponds to ab initio and density functional theory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be −29.4 ± 0.3 kcal/mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC measurements (28.4 ± 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6-311+G**+ZPE.

Thermochemical and X-ray Crystallographic Investigations of Some (CH)10 Hydrocarbons: Basketene, Nenitzescu’s Hydrocarbon, and Snoutene

“…[14] In order to understand better to what extent thermodynamics plays a role in these isomerizations, we have initiated a program to determine the heats of formation of several of these polycyclic (CH) 10 hydrocarbons by precise measurements of their heats of combustion. We have previously discussed our experimental findings for the energetics of diademane (10) and triquinacene (11), [15] and here we report new determinations of the enthalpies of formation [∆H 0 f (g)] of basketene (9), snoutene (14), and Nenitzescu's hydrocarbon 8.…”

“…The energies of combustion of the auxiliary compounds used have been published previously. [15] Basketene (9) Snoutene (14) m (substance)/g [b] 0.017506 0.031287 mЈ (cotton)/g [b] 0.000817 0.000735 mЈЈ (mylar)/g [b] 0 …”

“…1 Ǟ 7 569.7 45 [28] 3 Ǟ 2 243.2 19.5 [22] 5 Ǟ 7 518.2 34 [28] 6 Ǟ 5 258.2 21 [28,29] 9 Ǟ syn-12 383.2 29 [30] 9 Ǟ syn-12 384.1 28.6 Ϯ 0.1 this work syn-12 Ǟ 8 338.2 23.5 Ϯ 1.0 [31] 8 Ǟ 7 553.2 45.1 [32] 10 Ǟ 11 298.2 28.4 Ϯ 0.2 [15] 10 Ǟ 11 363.2 28.3 Ϯ 1.0 [9b] Comparison of all the values in Table 1 and 2 does not, however, reveal any general direct correlation between the values for strain energy release (∆SE) and the enthalpies of activation ∆H ‡ for the corresponding rearrangement. Thus, ∆H ‡ for the transformation 3 Ǟ 2 is almost twice as large as ∆SE, but for 9 Ǟ 8 and 10 Ǟ 11 it amounts to only 44 and 30% of the ∆SE, respectively.…”

“…Thermochemical results at T ϭ 298 15. K in kcal·mol Ϫ1[a] Calculated from the specific energies of combustion inTable 5.…”

A New Target for Highly Stereoselective Katsuki–Sharpless Epoxidation – One‐Pot Synthesis of C2‐Symmetric 2,2′‐Bioxiranes

“…Homodesmotic equations have been successfully used to estimate long-distance interactions. [12] For 7 the calculated homodesmotic reaction energy is only -1.80 kcal mol -1 at the MP2/6-31G* level Table 1. MP2/6-31G*-optimized bond parameters and comparison with the available X-ray data.…”