PREDICTIVE FORCE-FIELD CALCULATIONS FOR THE EQUILIBRIUM DIMERIZATION OF ISOPRENE

The Diels-Alder dimerization of isoprene is an important reaction; dip entene (or racemic limonene), one of the products formed in this react ion, has major applications in the manufacture of polymers and adhesiv es.' Dipentene also has various uses in the food and pharmaceutical in dustries.(7) In the present work, the QCFF force field program(2) was used to calculate gas-phase thermodynamic properties of the monomer (i soprene) and the dimers 1-methyl-5-(1-methylethenyl)cyclohexene (dipre ne) and 1-methyl-4-(1-methylethenyl)cyclohexene (dipentene) for the te mperature range 298.15-1000 K. These QCFF-calculated thermodynamic val ues were compared, when possible, with corresponding values obtained e xperimentally or from other force field programs, and the agreement wa s found to be satisfactory. The QCFF values were further used to deriv e gas-phase equilibrium properties-Delta H degrees, Delta S degrees, D elta G degrees, and K-p-for the isoprene dimerization reaction at vari ous temperatures. These computational results suggest that, based upon thermodynamics, diprene and dipentene are about equally favored dimer ization products of isoprene and that reported experimental data refle ct kinetic control of dipentene formation at lower temperatures. The e quilibrium thermodynamics data for the isoprene dimerization reaction presented in this paper are felt to be sufficiently reliable that they can be utilized in the absence of any additional experimental data; t he present work thus illustrates the power of predictive thermodynamic computational techniques in extending the thermochemical database.