Estimation of Ozone-Cluster Using MOPAC Program

Akira MURAIa*, Tsuyoshi NAKAJIMAb and Norio TAHARAb

aTechnical Research Center, Kansai Electric Power Company
11-20 Nakoji 3-Chome, Amagasaki, 661-0974, Japan
bFaculty of Engineering, Shinshu University
4-17-1 Wakasato, Nagano, 380-8553, Japan

(Received: January 7, 2002; Accepted for publication: August 29, 2002; Published on Web: November 15, 2002)

Ozone, made from air or oxygen by electric discharge, is used in a process called ozonation to purify water (by sterilization, bleaching, and deodorization) in a filtration plant. During an experimental study on an ozone storage process using silica gel, we found that ozone desorption from silica gel proceeds under the same temperature and pressure as ozone adsorption. The extra energy is not required for the ozone desorption except for elution by the oxygen gas stream.
The ozone gas desorbed from silica gel seemed to be different from normal ozone generated by an ozone generator. First, it smelled a little different from the normal one and remained on clothes for a longer time. Second, it seemed to have a higher specific gravity than the normal one, so that it was necessary to set an ozone leak detector lower to the floor to recover its original sensitivity. We thought that the desorbed ozone contained some kinds of ozone cluster.
By semi-empirical calculations using MOPAC(PM3), we estimated the energy of association between two ozone molecules as approximately 1 kcal/mol, suggesting the formation of ozone clusters. The value was confirmed by ab initio calculations (MP4 and QCISD(T) using 6-311+G(d) basis set) of the same ozone dimer. This confirmation supported the results obtained by MOPAC(PM3) calculations and thus suggested the existence of the ozone cluster, and indicated the usefulness of the MOPAC(PM3) calculations for qualitative studies on systems associated weakly by van der Waals interaction.
To get experimental evidence for the ozone cluster, we compared the specific gravity of the desorbed ozone from silica gel with that of Freon gas, HFC-125 (MW=120) and showed that it was heavier than the Freon gas. This fact suggests that the desorbed ozone gas contains some ozone cluster (O9 of mass 144). We analyzed the desorbed ozone by GC/MS and found elution peaks of m/e=96 and 144, corresponding to the dimer and trimer of ozone, respectively. These GC/MS data gave further evidence for the existence of ozone clusters. We named the clusters "Kansai Electric Power Composed Ozone-clusters (KEPCO)."

Keywords: Ozone, Ozone-cluster, Ozone storage, Bonding energy, MOPAC

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