The energy profile of the Bergman equilibrium o-diethinyl-parameters for the hydrogen abstraction of 2 and 1,4-didehybenzene (1) F? 1,4-didehydronaphthalene (2) has been drobenzene (5) have been derived from the rate of the naphestablished from the temperature and NO dependence of the thalene and benzene formation, when 1 and cis-3-hexenetrapping rate of the diradicals 2 which leads to a heat of for-1,5-diyne (4) are heated in the presence of methanol, toluene, mation for 2 of AH: = 152.9 ? 1.4 kcal . mol-I. Activation and 1,4-cyclohexadiene.

From the NO-and 0,-dependence of the trapping rate of 1,6-cyclodecadiyne (3) in the gas phase and in supercritical C 0 2 in the temperature range 160-290°C, the enthalpy profile for the equilibrium 3 F? 4 has been established. For the diradical4, a heat of formation of 116.2 kcal . mol-I is obtained, which agrees well with the expectation value derived by the abstraction of two hydrogen atoms from hexahydronaphthalene (5).Gleiter and Ritter"] demonstrated recently that 1,6-cyclodecadiynes represent ideal model compounds with which to study the interaction of nonconjugated diynes. By means of trapping experiments using hydrogen donors of different reactivity (1,4-~yclohexadiene, 9,1O-dihydroanthracene, and cyclooctene), they obtained convincing evidence for an intermediate 1,4-diradical 2, when N,N'-diisopropyl-1,6-diazadeca-3,S-diyne (1) was heated to 100°C.

Radical Recombination Enthalpy. -Thermochemistry of the Ringopening of CyclophanesBy thermolysis of the cyclophanes 1, 5, 8, 11, 14, 18, and 21 in the presence of NO or 0, the enthalpy profiles for the ring opening were determined from the temperature, NO, or Obwohl 1 bei Temperaturen >200 "C im Gleichgewicht mit p-Xylylen (3) vorliegt, beobachtet man in der Gasphase bis zu Temperaturen von 300 "C praktisch keinen Massenverlust (knb < lop7 s~' ) .In Gegenwart von NO kommt es jedoch zu einer Abnahme des Substrats. Die Geschwindigkeit der Reaktion, die erster Ordnung in bezug auf das pCyclophan sowie NO ist, zeigt eine Abhangigkeit von der Abfanger-Konzentration, wie fur ein vorgelagertes Gleichgewicht zu fordern ist (s. Abb. 1): Der Anstieg der Abnahmegeschwindigkeit wird mit steigender NO-Konzentration immer langsamer und strebt asymptotisch einem Grenzwert zu, der die Situation beschreibt, bei der das Intermediat quantitativ abgefangen wird.Eine Identifizierung des abgefangenen Intermediats ergibt sich unmittelbar aus der kinetischen Auswertung der Abfangexperimente. Aus den bei acht Temperaturen (168-238 "C) und jeweils 5-8 NO-Drucken ermittelten Geschwindigkeitskonstanten (s. Tab

From the 0,-dependence of the trapping rate of 1,l-difluoro-2,3-diphenylcyclopropane in supercritical C 0 2 in the temperature range 110-180°C and the rates of its geometrical isomerization and racemization of the trans-isomer, the energy profile for the geometrical isomerization is derived. Assuming that 7 is a common intermediate, the temperature de-pendence of the ratio of the rates of isomerizationhacemization leads to a lowering of 1.2 kcal . mol-' for the activation energy for the disrotatory cyclization of the intermediate diradical compared to the conrotatory mode. The energy barriers associated with cyclization of the diradical amount to 2.1 and 0.9 kcal . mol-', respectively.

Resonance Energy of Diradicals. - 1,8-NaphthoquinodimethaneFrom the racemization of 19c and the temperature and NO dependence of the trapping rate of 20b between 218 and 288°C the energy profile for the equilibrium 19b F? 20b is derived. The singlet-triplet splitting of the diradical is 2.5 kcal . mol-I with the triplet being the ground state. By comparison of the experimental reaction enthalpy with the analogous value for 25, 26 or a hypothetical model reaction with non-interacting radicals it is shown that the stabilizing interaction of the radicals in the singlet state of 20b is negligible making the stabilization of the triplet equivalent to the singlet-triplet splitting. By using literature data it is shown by an analogous analysis that the interaction of the radicals in the bisallyl diradical 28 and in the triplet state of the trimethylenmethane derivative 29 is also negligible whereas in the singlet state of 29 the interaction is strongly destabilizing. Mit der Resonanzenergie korrigieren wir den