Calculation of anharmonic effects in the unimolecular dissociation of M2+ (H2O)(2) (M = Be, Mg, and Ca)

Abstract

The anharmonic and harmonic rate constants of the unimolecular dissociation of M2+(H2O)(2) (M = Be, Mg, and Ca) were calculated using the Rice-Ramsperger-Kassel-Marcus theory. The anharmonic effects of the reactions were investigated. The results show that the energy barrier of the dissociation of Be2+(H2O)(2) is 68.47 kcal/mol, and the anharmonic (T-4000K = 4.28x10(8) s(-1)) and harmonic (T-4000K = 4.22x10(8) s(-1)) rate constants were close in value in both the canonical and microcanonical systems. The energy barriers of the two steps for the dissociation, Mg2+(H2O)(2) MgOH++H3O+, were 37.41 and 11.39 kcal/mol, and those for the dissociation, Ca2+(H2O)(2) CaOH++H3O+, were 21.15 and 26.42 kcal/mol. The anharmonic effect of the two reactions is significant and cannot be neglected in both the canonical and microcanonical systems. The comparison also shows that the rate constants of the dissociation of Ca2+(H2O)(2) have the maximum values, while those of Be2+(H2O)(2) have the minimum values in the three reactions; however, the anharmonic effect also shows the similar trend in the comparison.