The Hydrogen Bond Energies in ClHCl− and Cl−(HCl)n

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Abstract
The equilibrium constants for the gas phase reactions: Cl−(HCl)n = Cl−(HCl)n−1 + HCl, (n, n−1) were measured at different temperatures with a pulsed electron beam high pressure mass spectrometer. This allowed determination of ΔGn,n−10, ΔHn,n−10, and ΔSn,n−10 for reactions with n = 1 to n = 4. The enthalpy change for the reaction: (ClHCl)− = Cl− + HCl was ΔH1.00 = 23.7 kcal/mol. This value is much higher than the literature value of 14.2 kcal/mol based on Born cycles. The stabilities of the Cl−(HCl)n clusters are compared with those of OH−(H2O)n and Cl−(H2O)n measured earlier. It is found that the (ClHCl)− is nearly as stable as the (HOHOH)− species but that the stabilities of the higher Cl−(HCl)n clusters decreases much more rapidly than that of OH−(H2O)n. The initial strong interaction in (ClHCl) is assumed to be due to the high polarizability of Cl. For large n this effect becomes unimportant. Cl−HOH is much more weakly bound than (ClHCl)−, however, at high n the Cl−(H2O)n interactions become more favorable.

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