Thermochemical data, ΔHn−1,n and ΔSn−1,n (n=1–7), of clustering reactions, H+3(Ar)n−1+Ar=H+3(Ar)n and D+3(Ar)n−1+Ar=D+3(Ar)n, were measured with a pulsed electron‐beam high‐pressure mass spectrometer. The shell formation with n=3 and 6 was observed for both H+3(Ar)n and D+3(Ar)n clusters. The binding energies of D+3(Ar)n are found to be about 0.2 kcal/mol greater than those of H+3(Ar)n with n=1–3. With n≥4, the binding energies for both clusters become about the same. The Ar ligands in the cluster D+3(Ar)n are found to have slightly more restricted freedoms of motion than those in H+3(Ar)n, probably due to the smaller size of the core ion D+3 than H+3. The binding energy of H+3‐‐‐Ne was also measured. The obtained binding energy (∼0.4 kcal/mol) is more than one order of magnitude smaller than that of H+3‐‐‐Ar (6.69 kcal/mol). This is mainly due to the much smaller polarizability of Ne than Ar. A careful remeasurement of thermochemical data for the clustering reaction H+3+H2=H+3(H2) was also made. The obtained −ΔH°(7.01 kcal/mol) and −ΔS°(18.1 cal/mol K) values are in excellent agreement with our previous measurement.

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