The thermochemistry of cubane 50 years after its synthesis: a high-level theoretical study of cubane and its derivatives.

The gas-phase enthalpy of formation of cubane (603.4 ± 4 kJ mol(-1)) was calculated using an explicitly correlated composite method (W1-F12). The result obtained for cubane, together with the experimental value for the enthalpy of sublimation, 54.8 ± 2.0 kJ mol(-1), led to 548.6 ± 4.5 kJ mol(-1) for the solid-phase enthalpy of formation. This value is only 6.8 kJ mol(-1) higher than the 50-year-old original calorimetric result. The carbon-hydrogen bond dissociation enthalpy (C-H BDE) of cubane (438.4 ± 4 kJ mol(-1)), together with properties relevant for its experimental determination using gas-phase ion thermochemistry, namely the cubane gas-phase acidity (1704.6 ± 4 kJ mol(-1)), cubyl radical electron affinity (45.8 ± 4 kJ mol(-1)), cubane ionization energy (1435.1 ± 4 kJ mol(-1)), cubyl radical cation proton affinity (918.8 ± 4 kJ mol(-1)), cubane cation appearance energy (1099.6 ± 4 kJ mol(-1)), and cubyl ionization energy (661.2 ± 4 kJ mol(-1)), were also determined. These values were compared with those calculated for unstrained hydrocarbons (viz., methane, ethane, and isobutane). The strain energy of cubane (667.2 kJ mol(-1)) and cubyl radical (689.4 kJ mol(-1)) were independently estimated via quasihomodesmotic reactions. These values were related via a simple model to the C-H BDE in cubane. Taking into account the accuracy of the computational method, the comparison with high-precision experimental results, and the data consistency afforded by the relevant thermodynamic cycles, we claim an uncertainty better than ±4 kJ mol(-1) for the new enthalpy of formation values presented.

Energetics of nonbonded ortho interactions in alkylbenzenes.

The gas-phase enthalpies of formation for a set of ortho-substituted alkylbenzenes were obtained from CCSD(T*)-F12 and W1-F12 calculations. Most values are in keeping with available experimental data. The gas-phase enthalpies of formation of 1-ethyl-2-propylbenzene, 1-ethyl-2-isopropylbenzene, 1,2-diisopropylbenzene, 1,2,4-triethylbenzene, and 1,2,4,5-tetraethylbenzene, for which no experimental data are available, were determined as -46.0, -46.8, -68.7, -76.9, and -116.8 kJ mol(-1), respectively (estimated error bar ±4 kJ mol(-1)). The whole set of experimental and theoretical values are in good agreement with the estimates obtained using the extended Laidler bond additivity (ELBA) method. This agreement supports the approximation used in ELBA that interactions between ortho alkyl groups (other than tertiary alkyl groups) have roughly the same magnitude as a methyl-methyl interaction.

Energetics and structure of hydroxynicotinic acids. Crystal structures of 2-, 4-, 6-hydroxynicotinic and 5-chloro-6-hydroxynicotinic acids.

The relationship between energetics and structure in 2-, 4-, 5-, and 6-hydroxynicotinic and 5-chloro-6-hydroxynicotinic acids (2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA, respectively) was investigated in the solid and gaseous phases by means of a variety of experimental and computational chemistry techniques. The molecular and crystal structures of the 2HNA, 4HNA, 6HNA, and 5Cl6HNA solid forms used in this study were determined by single crystal X-ray diffraction at 293 +/- 2 K. The 2HNA, 4HNA, and 5Cl6HNA samples were monoclinic (space groups: P2(1)/n for 2HNA and P2(1)/c for 4HNA and 5Cl6HNA), and that of 6HNA was found to be triclinic (space group: P1). The 2HNA sample investigated corresponds to a new polymorphic form of this compound. The 2HNA, 4HNA, 6HNA, and 5Cl6HNA molecules crystallize as oxo tautomers exhibiting N-H and Cring=O bonds. This is also supported by the observation of bands typical of N-H and Cring=O stretching frequencies in the corresponding FT-IR spectra. The absence of these bands in the spectrum of 5HNA indicates that a hydroxy tautomer with an unprotonated N heteroatom and a Cring-OH bond is likely to be present in this case. Results of theoretical calculations carried out at the G3MP2 and CBS-QB3 levels of theory suggest that in the ideal gas phase, at 298.15 K, 2HNA favors the oxo form, 4HNA prefers the hydroxy form, and no strong dominance of one of the two tautomers exists in the case of 6HNA and 5Cl6HNA. The standard molar enthalpies of formation of 2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA in the crystalline state, at 298.15 K, Delta(f)H(m)(o)(cr), were determined by micro combustion calorimetry. The corresponding enthalpies of sublimation, Delta(sub)H(m)(o), were also derived from vapor pressure versus temperature measurements by the Knudsen effusion method. The obtained Delta(f)H(m)(o)(cr) and Delta(sub)H(m)(o) values led to the enthalpies of formation of 2HNA, 4HNA, 5HNA, 6HNA, and 5Cl6HNA in the gaseous phase. These were discussed together with the corresponding predictions by the B3LYP/cc-pVTZ, B3LYP/aug-cc-pVTZ, G3MP2, and CBS-QB3 methods on the basis of isodesmic or atomization reactions. The experimental "stability" order (more stable meaning a more negative Delta(f)H(m)(o)(g) value) found was 5Cl6HNA > 2HNA > 6HNA > 4HNA > 5HNA, and it was accurately captured by the CBS-QB3 and G3MP2 methods, which give 5Cl6HNA > 2HNA approximately 6HNA > 4HNA > 5HNA, irrespective of the use of isodesmic or atomization reactions. In contrast, only when well-balanced isodesmic reactions were considered did the DFT results agree with the experimental ones. The picture that emerged from the structural and energetic studies carried out in this work was also discussed in light of that typical of hydroxypyridines, which are generally regarded as the archetype systems for the study of the hydroxy <--> oxo tautomerization in N-heterocyclic compounds.

Energetics of tert-butoxyl addition reaction to norbornadiene: a method for estimating the pi-bond strength of a carbon-carbon double bond.

The energetics of tert-butoxyl radical addition reaction to norbornadiene was investigated by time-resolved photoacoustic calorimetry (TR-PAC). The result, together with the C-O bond dissociation enthalpy (BDE) in the addition product, allowed us to calculate the pi-bond dissociation enthalpy in norbornadiene. Quantum chemistry (QC) methods were also used to obtain several enthalpies of reaction of the addition of oxygen-centered radicals to alkenes. The pi-bond dissociation enthalpies in these molecules were calculated by a procedure similar to that used in the case of norbornadiene and were compared with the pi-BDE values obtained by the method proposed by Benson. These two different approaches yield similar values for the pi-BDEs in alkenes, indicating that the addition method proposed in the present study is a valid way to derive that quantity. The influence of strain in the pi-BDEs of cyclic alkenes was investigated and allowed us to justify the difference between the pi-BDE in norbornene and norbornadiene. Finally, the thermochemistry of the addition and abstraction reactions involving these two molecules and tert-butoxyl radical was analyzed.

Energetics of the thermal dimerization of acenaphthylene to heptacyclene.

The energetics of the thermal dimerization of acenaphthylene to give Z- or E-heptacyclene was investigated. The standard molar enthalpy of the formation of monoclinic Z- and E-heptacyclene isomers at 298.15 K was determined as Delta(f)H(m)o (E-C24H16, cr) = 269.3 +/- 5.6 kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, cr) = 317.7 +/- 5.6 kJ x mol(-1), respectively, by microcombustion calorimetry. The corresponding enthalpies of sublimation, Delta(sub)H(m)o (E-C24H16) = (149.0 +/- 3.1) kJ x mol(-1) and Delta(sub)H(m)o (Z-C24H16) = (128.5 +/- 2.3) kJ x mol(-1) were also obtained by Knudsen effusion and Calvet-drop microcalorimetry methods, leading to Delta(f)H(m)o (E-C24H16, g) = (418.3 +/- 6.4) kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, g) = (446.2 +/- 6.1) kJ x mol(-1), respectively. These results, in conjunction with the reported enthalpies of formation of solid and gaseous acenaphthylene, and the entropies of acenaphthylene and both hepatcyclene isomers obtained by the B3LYP/6-31G(d,p) method led to the conclusion that at 298.15 K the thermal dimerization of acenaphthylene is considerably exothermic and exergonic in the solid and gaseous states (although more favorable when the E isomer is the product), suggesting that the nonobservation of the reaction under these conditions is of kinetic nature. A full determination of the molecular and crystal structure of the E dimer by X-ray diffraction is reported for the first time. Finally, molecular dynamics computer simulations on acenaphthylene and the heptacyclene solids were carried out and the results discussed in light of the corresponding structural and Delta(sub)H(m)o data experimentally obtained.