Table 1.

Zero-point energy corrected water and hydrogen binding energies (in kcal/mol) from B3LYP/6-311++G** calculations for [X(H₂O)_n]⁺, X = Mg, Ca, MgOH, and CaOH. MP2/6-31G** water binding energies to [Mg(H₂O)_n]⁺ and hydrogen loss barrier energies from [50], and experimental CID results from [60], are included for comparison.

	CIDa	MP2/6-31G*		B3LYP/6-311++G**
X	Mg	Mg		Mg		MgOH	Ca		CaOH
	H2O	H2Ob	H	H2O	H	H2O	H2O	H	H2O
1	28.4 ± 3.0	36.8	71.9c	31.0	78.1	50.4	28.1	36.8	29.9
2	22.4 ± 1.6	30.4	49.0c	24.0	51.7	37.3	25.3	32.2	27.0
3	17.3 ± 2.1	26.2	33.9c	21.1	35.5	29.2	19.2	24.5	23.7
4	11.5 ± 2.1	19.1	31.5c (22.0d)	15.0	21.3	20.6	17.5	18.2	19.6
5	---	16.5	17.9c (14.2d)	8.7	9.4	16.6	16.1	14.7	17.3
6	---	14.8	12.4e (6.9d)	24.1	17.0	---	17.8	15.3	---

^aZero K experimental bond dissociation energies from [60] upon collision of $\mathrm{Mg}{\left({\mathrm{H}}_2\mathrm{O}\right)}_n^{+}$, n = 1 – 4, with xenon gas.

^bWe calculated MP2/6-31G** energies for Mg⁺ and H₂O. These values and those determined in [50] were used to determine zero-point energy corrected adiabatic water binding energies for these ions.

^cDissociation barrier for direct hydrogen loss from the lowest-energy [Mg(H₂O)_n]⁺ structure determined in [50].

^dDissociation barrier for hydrogen loss from [Mg(H₂O)_n]⁺ allowing structural isomerization barrier prior to hydrogen loss. For selected structures studied, the isomerization barriers were less than the hydrogen loss dissociation barriers [50].

^eThe dissociation barrier for direct hydrogen loss from the lowest energy [Mg(H₂O)₆]⁺ structure (4+2a) was not reported; the value for a very similar, albeit slightly higher energy, structure (4+2b) is reported instead [50]. Based on the similarities between the two structures, we would expect them to have similar dissociation barriers for direct hydrogen loss.