Single Molecule Imaging of Oxygenation of Cobalt Octaethylporphyrin at the Solution/Solid Interface: Thermodynamics & Kinetics from Microscopy
We have demonstrated for the first time that (1) the pressure and temperature dependent chemical reaction at the solid solution interface can be studied on a molecular level and that (2) a variable temperature STM can be used to determine thermodynamic data for these processes. In particular, we used the STM to study the reversible binding of O2 with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. We have shown that the solid support (in this case HOPG) can act in a manner similar to an electron-donating ligand bound to the fifth coordination site on the cobalt ion of CoOEP, thereby greatly increasing the compound’s affinity for oxygen. Over the temperature range of 10ºC to 40ºC we found that DHP = -87±10 kJ/mole and DSP = ‑339±30 J/K-mol. The free energy, enthalpy, and entropy change associated with the O2 binding process are found to be qualitatively correct but larger than previously observed for purely solution phase reactions.
Comparison of thermodynamic values for different Co-Porphyrins.
| Compound | Physical State | P1/2(25C) Torr | ∆Sº J/K-mol | ∆Hº kJ/mole |
|---|---|---|---|---|
| CoMb(sperm whale) | 0.1 M phosphate pH 7 | 57 | -232.4 | -40 |
| CoMb(horse) | 0.1 M phosphate pH 7 | 57 | -197.3 | -33 |
| CoMb(sperm whale) | 0.1 M phosphate pH 7 | 50 | -207.7 | -34 |
| CoTpivPP(N-Melm) | toluene | 140 | -213.2 ± 5.4 | -38 ± 1 |
| CoTpivPP (1.2-diMelm) | toluene | 900 | -206.1 ± 7.1 | -40 ± 2 |
| CoT(p-OCH3)PP(N-Melm) | toluene | 15,500 | -146.8 | -36 |
| CoPPIXDME(N-Melm) | toluene | 17,800 | -201.1 | -45 |
| CoPPIXDME(N-Melm) | toluene | 11,000 | -200.6 | -38 |
| CoOEP on HOPG | phenyloctane | 3,200 | -297 ± 10 | -68 ± 10 |