EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE CLUSTERING OF H3O+ WITH CO2 AND N2O

The thermodynamics of the clustering reactions H3O+ + CO2 reversible H3O+CO2 and H3O+ + N2O reversible H3O+N2O were investigated. The enthalpies and entropies of reaction were determined to be -13.4 +/- 0.4 kcal/mol and -19.8 +/- 1.0 cal/(mol K) for H3O+CO2 and -15.6 +/- 0.4 kcal/mol and -24.1 +/- 0.8 cal/(mol K) for H3O+N2O. The values for H3O+CO2 compare favorably with the previous results of both Meot-Ner and Hiraoka. The results also compare well with theoretical calculations presented here using ab initio methods. The best calculated bond enthalpies are weaker by 0.1 kcal/mol for H3O+CO2 and 1.6 kcal/mol for H3O+N2O than the experimentally determined values. Three modes of interaction were considered between H3O+ and the terminal atoms of CO2 and N2O, namely, the single, double, and triple hydrogen-bonded species. It is noteworthy that AM1 and PM3 methods order the stability of the three species incorrectly with respect to ab initio methods. The lowest energy ab initio geometries feature a single hydrogen bridge nearly collinear with a pair of nonbonding electrons on the N2O or CO2 neutral species. Clustering at both the terminal oxygen and nitrogen of N2O was studied and found to be sensitive to the level of electron correlation. At the Hartree-Fock, MP3, MP4SDQ, and QCISD(T) levels, the oxygen end is favored. At the MP2 and MP4SDTQ levels, the nitrogen end is favored. Since it has been noted that the full MP4 level overestimates the effect of triple excitations, calculations at the QCISD(T)/6-31+G(2dp) level (estimated by an additivity approximation) are presented and show that clustering at the oxygen end of N2O is preferred over the nitrogen by 0.9 kcal/mol.