CISD/cc-pVTZ//CISD/6-31G* calculations have been performed in order to compute the difference between the bond dissociation enthalpies (BDEs) of a C-H bond at C-3 of the 1-propyl radical and a primary C-H bond in propane. The implications of the computational finding that these two BDEs are almost the same are discussed with reference to both the amount of through-bond interaction between the radical centers at the (0,0) geometry of propane-1,3-diyl (1) and thermochemical estimates of DeltaH(f)(298) of 1. To compare DeltaH(f)(715) of 1 with AH(f)(715) of the transition structure for cis-trans isomerization of cyclopropane, the difference between the heat capacities of I and cyclopropane, integrated between 298 and 715 K, has also been calculated. This difference is computed to increase the calculated heat of formation of the diradical, relative to that of the transition structure, by ca. 1 kcal/mol. The results of our calculations, when combined with experimental values for the heats of formation of cyclopropane and propyl radical at 298 K and with the literature value for the enthalpy of activation for cis-trans isomerization of cyclopropane at 715 K, place the enthalpy of diradical I ca. 2 kcal/mol below that of the activated molecules which undergo this reaction. This result, which is at odds with our finding that the (0,0) geometry of 1 is the transition structure for conrotatory ring opening and ring closure of cyclopropane, is discussed in terms of contributions to the activation enthalpy from higher energy pathways for this reaction.