Energetics of the Thermal Dimerization of Acenaphthylene to Heptacyclene Rui C. Santos, ² Carlos E. S. Bernardes, ‡ Hermı ´nio P. Diogo, ² M. Fa ´ tima M. Piedade, ²,‡ Jose ´ . N. Canongia Lopes, ² and Manuel E. Minas da Piedade* ,‡ Centro de Quı ´mica Estrutural, Complexo Interdisciplinar, Instituto Superior Te ´ cnico, 1049-001 Lisboa, Portugal, and Departamento de Quı ´mica e Bioquı ´mica, Faculdade de Cie ˆ ncias, UniVersidade de Lisboa, 1649-016 Lisboa, Portugal ReceiVed: October 31, 2005; In Final Form: December 14, 2005 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 Δ f H m o (E-C 24 H 16 , cr) ) 269.3 ( 5.6 kJ‚mol -1 and Δ f H m o (Z-C 24 H 16 , cr) ) 317.7 ( 5.6 kJ‚mol -1 , respectively, by microcombustion calorimetry. The corresponding enthalpies of sublimation, Δ sub H m o (E-C 24 H 16 ) ) (149.0 ( 3.1) kJ‚mol -1 and Δ sub H m o (Z-C 24 H 16 ) ) (128.5 ( 2.3) kJ‚mol -1 were also obtained by Knudsen effusion and Calvet-drop microcalorimetry methods, leading to Δ f H m o (E-C 24 H 16 , g) ) (418.3 ( 6.4) kJ‚mol -1 and Δ f H m o (Z-C 24 H 16 , g) ) (446.2 ( 6.1) kJ‚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 Δ sub H m o data experimentally obtained. Introduction The dimerization of acenaphthylene in solution upon exposure to sunlight (eq 1) was observed and reported for the first time over 90 years ago by Dziewonski and co-workers. 1,2 These authors were able to show that the reaction leads to two isomeric cyclobutane dimers and that the relative yields of the products strongly depend on the solvent and on the acenaphthylene concentration. 2 Due to the presence of the seven rings, the compounds were dubbed heptacyclenes. The dimer with the higher fusion temperature (T f ) 579-580 K) was named R-heptacyclene 2 and was later shown to correspond to the E form in eq 1 (cyclobuta[1,2-a:3,4-a′]diacenaphthylene, 6b,6c-12b,12c-tetrahydro-(6bR,6c,12b,12cR), CAS Registry Number, 14620-98-5) from a partial determination of the molecular structure by X-ray diffraction. 3 The compound with the lower fusion temperature (T f ) 505-507 K) was designated by -heptacyclene (cyclobuta[1,2-a:3,4-a′]diacenaphthylene,6b,- 6c,12b,12c-tetrahydro-(6bR,6cR,12bR,12cR), CAS Registry Num- ber, 15065-28-8). The assignment of the Z structure to -hep- tacyclene was first based on the results of ozonolysis studies 4 and was subsequently supported by an X-ray diffraction determination of the molecular structure. 5 A variety of studies have been undertaken to elucidate the mechanism of the photodimerization of acenaphthylene and of the reverse photodissociation of the dimers in solution. 6-14 Practically unexplored are, however, the thermal aspects of the acenaphthylene/hepatcyclene system, although Dziewonski and Paschalski 2 had already noted that acenaphthylene can be regenerated by decomposition of the E dimer upon heating to its fusion temperature in an open vessel. If the reaction is carried out in a closed tube, then only products resulting from the subsequent polymerization of acenaphthylene are observed. 2 In this work, the energetics of the interconversion between acenaphthylene and the E or Z dimers in the solid and gaseous states was investigated, by combining the results of combustion calorimetry, Calvet-drop microcalorimetry, and Knudsen effu- sion measurements, with data from density functional theory and molecular simulation calculations. In addition, a full determination of the molecular and crystal structure of the E dimer by X-ray diffraction is reported for the first time. Methods General. Elemental analyses were carried out on a Fisons Instruments EA1108 apparatus. The IR spectra were recorded in a Jasco 430 spectrophotometer calibrated with polystyrene film, using KBr plates. The 1 H NMR spectra were obtained at ambient temperature on a Varian 300 MHz spectrometer. X-ray powder diffractometry (XRD) was carried out over the range 5° e 2θ e 35°, on a Rigaku diffractometer employing Cu KR * To whom correspondence should be addressed. E-mail: memp@fc.ul.pt. ² Instituto Superior Te ´cnico. ‡ Universidade de Lisboa. 2299 J. Phys. Chem. A 2006, 110, 2299-2307 10.1021/jp056275o CCC: $33.50 © 2006 American Chemical Society Published on Web 01/20/2006