Stilbenes enthalpy difference

The enthalpy difference of cis- compared with frans-stilbenes (Table 2) has been determined for a few cases [55, 59-64], For stilbene an energy difference of 10-13kJ mol" 1 has been reported, the result of a reexamination [63] is 19kJ mol"1. This value has to be added to the barrier for the thermal cis - trans isomerization in order to obtain the energy of the twisted ground state. 

We are being somewhat disingenuous in that we are taking the difference here of the enthalpy of formation of the naturally liquid (Z)-isomer and of the liquid (/ )-isomer obtained by summing the value for the solid and the ca 27 kJmol-1 enthalpy of fusion (at 398 K) from Domalski. We know of no measurement of the enthalpy of fusion for (Z)-stilbene at any temperature from which to derive an enthalpy of formation for the solid. 

The second ozonide is that from fraw -stilbene, i.e. the 3,5-diphenyl trioxolane derivative. The ozonation experiment was done on both the cis and trans olefins resulting in reaction enthalpies of —471.5 and —428.9 kJmoU, respectively. The difference between these two values is ca 43 kJ moU, which is close to the difference between the two olefins enthalpies of formation (one as liquid and the other as solid), 46 2 kJ moU. This result vindicates the original authors suggestion of using the ozonation enthalpies as a method of determining the difference between enthalpies of formation of a Z/ pair of olefins. 

We acknowledge that there is some steric repulsion between the various groups on the olefinic carbons that lessens both resonance stabilization and intermolecular hydrogen bonding. The former is documented by the ca 16 kJ mol -1 difference in the enthalpies of formation of gaseous (E)- and (Z)-stilbene. 

An alternative model, the hot-band mechanism, found support in studies of the temperature dependence of NVET rates in stilbene sensitization.114,115 In this model, thermal population of vibrational modes of the ground-state acceptor is assumed to provide access to geometries near to the relaxed excited state, as depicted in Figure 2.20. It is supported by the fact that energy differences between triplet donors are reflected almost entirely in the activation entropy rather than in the activation enthalpy, as would be expected from Balzani s treatment. 

T = 184kJmol 1 for trans- and cis-stilbene has been obtained by photoacoustic calorimetry [443]. Later, Caldwell and co-workers [444] reported ET = 195 and 178kj mol 1 for trans- and cis-stilbene, respectively. The latter value refers to the relaxed triplet relative to the c state and therefore corresponds to 19 kJ mol 1 difference in enthalpy between the two isomers [444],... 

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