Molecular Orbital Studies of the Structures and Reactions of Singly Charged Magnesium Ion with Water Clusters, Mg+(H2O)n

Abstract

With ab initio molecular orbital calculations, the structures of the cation clusters Mg+(H20)" and their hydrogen-eliminated products (Mg0H)+(H20)"-i are optimized. In Mg+(H20)", the hydration number of the most stable isomer is 3. In (Mg0H)+(H20)"-i, all water molecules are directly bonded to Mg+ for n < 6. The hydration energy of (MgOH)+ is larger than that of Mg+ because of the strongly polarized (MgOH)+ molecular ion; Mg is oxidized halfway to Mg(II). The internal energy change of the hydrogen elimination of Mg+(H20)" is positive for n = 1-5, but becomes negative for n = 6, which is in good agreement with the product switching in the TOF spectrum reported in the preceding paper by Sanekata et al. The effects of isotope substitution and equilibrium constants of the hydrogen (deuterium) elimination reaction observed in their experiment can be explained qualitatively.

Reactions of Alkaline Earth Metal Ions with Methanol Clusters

“…This is similar to the situation in Mg + (H 2 O) 2 , where the two water molecules are staggered. 5 Despite the large steric hindrance expected for structure 2b, the two structures were found to be very close in energy. The structure 2b is found to be only 0.21 kJ/mol less stable than structure 2a.…”

“…The Mg + (CH 3 OH) n cluster ions dominate the products for small cluster ions until n ) 5. At n ) 5, a significant intensity dip in the mass spectrum in Figure 3 marks a sharp switching of the reaction product from Mg + (CH 3 OH) 5 to MgOCH 3 + (CH 3 OH) 4 through the following reaction:…”

“…For n ) 4, only a single large peak corresponding to Mg + (CH 3 -OH) 4 dominates along with two small peaks due to the isotopes of the Mg. At n ) 5, however, two relatively large peaks appeared, corresponding to MgOCH 3 + (CH 3 OH) 4 and Mg + (CH 3 OH) 5 , respectively. Here the intensity of the former is only slightly larger than that of the latter.…”

“…For CD 3 OH, the situation is very similar to that of CH 3 OH. The first product switching occurs at n ) 5, where the intensity of MgOCD 3 +(CD 3 OH) 4 becomes larger than that of Mg + (CD 3 OH) 5 . This similarity is not surprising since the dehydrogenation reaction involves only the O-H bond cleavage, on which the replacement of CH 3 with CD 3 has little effect.…”

“…Similar product distributions were observed for Ca + -H 2 O, though the exact switching points differ slightly. 3 To understand the reactions of M + (H 2 O) n (M ) Mg and Ca), Iwata and co-workers performed ab initio molecular orbital calculations 5,6 on these cluster ions and their corresponding dehydrogenation products, MOH + (H 2 O) n-1 , with n as large as 8 (for Ca + ). The two species were found to have very different structures.…”