The gas phase reactions between metal ions (alkaline earth metal cations, lanthanide cations, and transition metal cations) and the molecular clusters (CH
3OH)
n were studied in a pick-up source. It was found that the association complexes M
+(CH
3OH)
n (M = metal atom) undergo intracluster reactions when they reach certain sizes. The reactions include dehydrogenation induced by alkaline earth metal cations and lanthanide cations, and dehydration induced by Fe
+. The existence of critical sizes for the reactions indicates the importance of the formation of the solvation shell in reducing the activation energy.
+ 3 n 3+ 3 n-1...[
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The gas phase reactions between metal ions (alkaline earth metal cations, lanthanide cations, and transition metal cations) and the molecular clusters (CH
3OH)
n were studied in a pick-up source. It was found that the association complexes M
+(CH
3OH)
n (M = metal atom) undergo intracluster reactions when they reach certain sizes. The reactions include dehydrogenation induced by alkaline earth metal cations and lanthanide cations, and dehydration induced by Fe
+. The existence of critical sizes for the reactions indicates the importance of the formation of the solvation shell in reducing the activation energy.
For the alkaline earth metal cations, the first product switching from M
+(CH
3OH)
n to MOCH
3+(CH
3OH)
n-1 occurs at n [is approximately equal to] 5. Ab initio calculation shows that the H-elimination products stabilize much more quickly than the reactants with increasing cluster size. This is due to the stronger electrostatic interactions between the metal cation and the methanol solvents since the former has a strongly polarized core MgOCH
3+, in which Mg possesses its favorite oxidation state of +2. In addition, MOCH
3+(CH
3OH)
n-1 shift back to Mg
+(CH
3OH)
n at n [is approximately equal to] 15. We believe that these large clusters can be described by a two-cage structure [M
2+(CH
3OH)
n][e
-(CH
3OH)
m], which is related to the electron solvations in solutions. The valence electrons of the metal ions are delocalized among methanol ligands and form a solvated electron cage.
The fourteen lanthanide cations (Ln
+) exhibit different reactivities towards methanol. La
+, Ce
+, Pr
+, Nd
+, Gd
+, Tb
+, Ho
+, Er
+ and Lu
+ can react with methanol molecules and give dehydrogenation products such as LnCH
2O
+ and LnCH
3O
+. For those relatively unreactive lanthanide ions such as Sm
+, Eu
+, Dy
+, Tm
+ and Yb
+, only when they are solvated by a sufficient number of methanol molecules does dehydrogenation reactions start to occur and give Ln(OCH
3)
2+(CH
3OH)
n-2 (except Eu
+). Eu
+ is quite special in that it tends to change to Eu
2+ with half-filled 4f orbitals (4f
7) and the reaction product is EuOCH
3+(CH
3OH)
n-1. The correlation between the reactivities and the electronic structures is consistent with an insertion mechanism, in which Ln
+ needs two reactive valence electrons to form two covalent bonds with the oxygen and hydrogen atoms.
The dehydration reaction of Fe
+(methanol)
n takes places for n ≥ 12. It is believed that the reaction starts to occur after the solvation shell is completed, in which the two neighboring methanol molecules may be arranged in a favorable configuration to make the reaction occur. The main purpose of the Fe
+ cation is to work as a charge center just as found for the reactions between alkali metal cations and methanol clusters.
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