Effect of Pressure on the e⁻ + Pyrimidine ⇌ Pyrimidine⁻ Equilibrium in Nonpolar Solvents

The thermodynamics of the equilibrium reaction e - + pyrimidine ⇌ pyrimidine - was studied in tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP) as solvents. In hexane, only the forward attachment reaction could be observed. The attachment rate constants ( k a ) increase with temperature in all solvents. The equilibrium shifts to the right with increasing pressure because of the large, negative reaction volumes (−200 to −330 cm 3 /mol). The volume changes are consistent with electrostriction by the pyrimidine anion when a glass shell of solvent molecules is included. The free energy of reaction at 298 K is −0.43 eV in tetramethylsilane at 1 bar and −0.70 eV in 2,2,4-trimethylpentane at 150 bar. The solution free energies indicate the gas-phase (adiabatic) electron affinity of pyrimidine is −0.24 eV.