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Reduction of Acetonitrile by Hydrated Magnesium Cations Mg+(H2O)n (n≈20–60) in the Gas Phase


  • Tim-Wai Lam,

    1. Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong (P. R. China), Fax: (+852) 3442-0522
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  • Dr. Christian van der Linde,

    1. Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel (Germany), Fax: (+49) 431-880-2830
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  • Amou Akhgarnusch,

    1. Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel (Germany), Fax: (+49) 431-880-2830
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  • Dr. Qiang Hao,

    1. Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong (P. R. China), Fax: (+852) 3442-0522
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  • Prof. Dr. Martin K. Beyer,

    Corresponding author
    1. Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel (Germany), Fax: (+49) 431-880-2830
    • Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel (Germany), Fax: (+49) 431-880-2830
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  • Prof. Dr. Chi-Kit Siu

    Corresponding author
    1. Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong (P. R. China), Fax: (+852) 3442-0522
    • Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong (P. R. China), Fax: (+852) 3442-0522
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Ion–molecule reactions of Mg+(H2O)n (n≈20–60) with CH3CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH3CN initiates the formation of MgOH+(H2O)n−1 together with CH3CHN. or CH3CNH., which is similar to the reaction of hydrated electrons (H2O)n with CH3CN. In subsequent reaction steps, three more CH3CN molecules are taken up by the clusters, to form MgOH+(CH3CN)3 after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH3CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(C[BOND]N)-like orbital of the bent CH3CN.−, which undergoes spontaneous proton transfer to form CH3CNH. or CH3CHN., with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH3CNH. or CH3CHN. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH3CN molecules taken up by the resulting MgOH+(H2O)n clusters coordinate directly to the first solvation shell of the MgOH+ core, as revealed by DFT calculations.


Singly charged hydrated metal ions in the gas phase have intriguing properties, which have been studied in detail over the last two decades with a fruitful combination of experiment and theory.1 Exchange experiments with D2O2 indicate that hydrated alkali-metal ions3 as well as most transition-metal ions M+(H2O)n, M=Cr, Fe, Co, Ni, Cu, and Zn,4 consist of a singly charged metal center embedded in a hydrogen-bonded network of intact H2O molecules, whereas room-temperature black-body infrared radiative dissociation (BIRD)5 activates an insertion reaction in Mn+(H2O)n, which for n≈8–20 are converted into HMnOH+(H2O)n−1.4 In other systems, water activation results in elimination of atomic or molecular hydrogen and oxidation of the metal to the metal hydroxide.614

Ion–molecule reactions provide insight into the subtle chemistry of these species. Reactions with a strong acid such as HCl induce the elimination of atomic or molecular hydrogen,10, 12, 15 and weak acids such as HCOOH also have a promoting effect.16 Precipitation reactions also work on the single-molecule level in water clusters.17 Coordination chemistry and charge transfer of hydrated transition-metal ions have been investigated in reactions with O2, CO2, and N2O18 as well as with NO.19

Mg+(H2O)n are certainly among the best-studied singly charged hydrated metal ions in the gas phase.6, 7, 9, 10, 13, 14, 20, 21 These species are only observed for n<6 or n>14, whereas exclusively MgOH+(H2O)n−1 are present in the mass spectra for 6≤n≤14.7 Quantum chemical calculations corroborate the interpretation of the experiments7, 9 that for n>14, Mg+(H2O)n consist of a doubly charged magnesium ion and a hydrated electron, with the spin density of the system distributed in a site remote from the metal center.13, 21, 22 In a combined experimental and theoretical study, we were able to show that Mg+(H2O)n also exhibit the chemistry of the hydrated electron23, 24 in reactions with O2 and CO2, albeit with significantly reduced reaction efficiencies.22 In the uptake of O2 or CO2, the hydrated electron is scavenged by the reactant, and formation of MgOH+ does not occur.

Like O2 and CO2, acetonitrile exhibits a specific reactivity toward the hydrated electron. Collisions of (H2O)n with CH3CN result in the formation of OH(H2O)m,25 which are detected by mass spectrometry. Thermochemical arguments require that CH3CHN. or CH3CNH. is formed as the neutral product. The former radical has been observed in ESR studies,26 the latter was generated by neutralization of protonated acetonitrile.27 Several experimental studies exist on the solvation and reactions of metal ions with acetonitrile in the gas phase. The interaction of acetonitrile with Nb+ has been studied by IR multiphoton dissociation spectroscopy,28 in which reductive nitrile coupling was observed for Nb+(CH3CN)5. Solvation of the doubly charged transition-metal ions Co2+ and Ni2+ with a mixture of acetonitrile and water molecules was studied by photodissociation.29 Collision-induced dissociation of doubly charged metal ions solvated with acetonitrile M2+(CH3CN)n occurs through solvent loss, electron transfer, proton transfer, or heterolytic cleavage of the C[BOND]C bond with formation of CH3+.29, 30

To learn more about the chemistry of the hydrated electron in Mg+(H2O)n, we investigated the interaction of Mg+(H2O)n (n≈20–60) with acetonitrile by Fourier-transform ion-cyclotron resonance (FTICR) mass spectrometry and quantum chemical calculations with density functional theory (DFT).

Experimental and Computational Details

The experiments were performed on a modified Bruker/Spectrospin CMS47X FTICR mass spectrometer with a 4.7 T superconducting magnet.31 Mg+(H2O)n ions were generated in a laser vaporization source and transferred to the ICR cell as described before.911, 22 The vaporization laser and frequency doubling crystal were heated by 20 laser shots to avoid changes in the initial cluster distribution, followed by 20 laser shots at 10 Hz and 5 mJ pulse energy to generate the ions. The reaction delay was measured relative to the end of the fill cycle, at nominal t=0 s, so the clusters resided up to 2 seconds in the ICR cell. For this reason, the reaction products were observed at nominal 0 second. Acetonitrile (spectroscopic grade, ≥99.9 %) was introduced into the ultrahigh-vacuum region through a needle valve at a constant pressure of 8.8×10−9 mbar, calibrated with the empirical method of Bartmess et al.32 using a geometry factor of 3.7.33 The reaction was monitored by measuring mass spectra at different reaction delays. Collision rates were calculated with the average dipole orientation (ADO)34 and hard-sphere average dipole orientation (HSA)35 models for a cluster size of n=30, by using literature values for the dipole moment, polarizability,36 and viscosity37 of gaseous acetonitrile.

DFT calculations were performed by using the Gaussian 09 quantum chemical package38 at the M06/6-31++G(d,p) level of theory, which predicts Mg+–ligand interaction with a quality comparable to that of high-level ab initio methods.22 The transition structures were located with one imaginary frequency calculated by the harmonic frequency analyses. Natural population analyses were used for the spin-density calculations. For bulk property calculations, the self-consistent reaction field (SCRF) method with the polarizable-continuum model (PCM) was employed.

Results and Discussion

Ion–molecule reactions

At the end of the trapping period at nominally 0 second reaction delay, Mg+(H2O)n and MgOH+(H2O)m are present in almost equal mass spectroscopic intensities (Figure 1 a) as well as minor intensities of MgOH+(CH3CN)(H2O)m. The inset in Figure 1 a illustrates that data interpretation is made complicated by the isotope pattern of magnesium, with isobaric peaks at 24Mg+(H2O)n and 25MgOH+(H2O)n−1. After a delay of 4 seconds (Figure 1 b), Mg+(H2O)n have almost disappeared, and MgOH+(CH3CN)(H2O)m are the dominant species in the mass spectrum. The dominant species at 10 seconds (Figure 1 c) are MgOH+(CH3CN)2(H2O)m, and MgOH+(CH3CN)3(H2O)m are also present in high intensities. The only product left after 60 seconds is MgOH+(CH3CN)3. The reaction with acetonitrile is accompanied by loss of water molecules owing to BIRD. For n≤21, black-body radiation activates formation of atomic hydrogen and MgOH+(H2O)m from Mg+(H2O)n as a parallel reaction.9, 10 Therefore, the cluster size distribution was optimized for large clusters to minimize contributions from this side reaction.

Figure 1.

Mass spectra of the reaction of Mg+(H2O)n with acetonitrile after different reaction delays at a pressure of 8.8×10−9 mbar. Formation of Mg(OH)+(H2O)m is followed by ligand exchange of up to three acetonitrile molecules.

For a quantitative kinetic analysis, the intensity of 24Mg+(H2O)n was corrected for the contribution of 25MgOH+(H2O)n−1, based on the intensity of 24MgOH+(H2O)n−1 and the natural isotope abundances of 24Mg and 25Mg, whereas the minor contribution of 24Mg+(H2O)n−1(HDO) was neglected. The correction is not exact, since the mass difference between 24Mg+(H2O)n and 25MgOH+(H2O)n−1 amounts to Δm=0.00703 u. However, pressure and lifetime broadening as a result of BIRD make it impossible to resolve the two peaks on our 4.7 T instrument. The kinetics, fitted with a genetic algorithm, nicely exhibits pseudo-first-order behavior for the first 5 seconds of the reaction, as illustrated in Figure 2. In the first reaction step, a hydrogen atom is transferred to acetonitrile, which leads to the formation of hydrated magnesium hydroxide and CH3CHN. or CH3CNH., written as [CH3CN,H] [Eq. (1)]:

equation image(\rm (1)
Figure 2.

Kinetic fit of the reaction of Mg+(H2O)n with acetonitrile at a pressure of 8.8×10−9 mbar for the first 5 seconds. The kinetics shows four consecutive reactions with pseudo-first-order behavior.

In subsequent collisions, three molecules of acetonitrile are taken up by the clusters through ligand exchange [Eq. (2)]:

equation image(\rm (2)

Table 1 summarizes the absolute rate constants derived from the pseudo-first-order rates and the acetonitrile reactant pressure. In collisions with acetonitrile, Mg+(H2O)n are efficiently converted into MgOH+(H2O)m. As shown previously for HCl, O2, and CO2,10, 22, 23, 39 this reflects the reactivity of the hydrated electron (H2O)n, which in collisions with acetonitrile reacts to form OH(H2O)m.25 In principle, uptake of CH3CN could also trigger the formation and elimination of a hydrogen atom, but the required selective evaporation of CH3CN to rationalize the exclusive formation of MgOH+(H2O)m is, in view of the high efficiency of reaction (2), highly improbable. In addition, the zero-point-corrected binding energy of a hydrogen atom to CH3CN was calculated as 80 kJ mol−1 at the G3 level of theory,40 103 kJ mol−1 at the B3LYP/6-311++G(d,p) level of theory,41 and 98 kJ mol−1 at the M06/6-31++G(d,p) level of theory, with formation of CH3CHN. as the energetically most favorable adduct and the global minimum on the [CH3CN,H] potential energy surface.

Table 1. Absolute rate constants kabs [cm3 s−1] of the reaction of Mg+(H2O)n with CH3CN. Efficiencies ΦADO and ΦHSA for collision rates of kADO=2.4×10−9 and kHSA=2.8×10−9 cm3 s−1 calculated from the ADO and HSA models for n=30, respectively.



kabs [cm−3 s−1]

ΦADO [%]

ΦHSA [%]





















Interpretation of the ligand-exchange reaction [Eq. (2)] seems straightforward. Acetonitrile molecules are added until, together with the hydroxide ion, a coordination number of four is reached at the Mg2+ center. Spectroscopic studies on Mg2+ in bulk CD3CN, however, report a primary coordination number of six,42 and a preferential coordination of Mg2+ with H2O in mixtures of water with deuterated acetonitrile, studied for mole fractions of up to 20 % H2O in CD3CN.42 But even at the highest water content, the signature of CD3CN coordinated to Mg2+ is strong, thus showing that water does not replace all coordinated acetonitrile molecules in the bulk.42 If one takes into account that the hydroxide ligand, the internal low temperature of the cluster, and the gas-phase environment have a subtle influence on the coordination chemistry, uptake of three acetonitrile molecules with direct coordination to the Mg2+ center is plausible and not in conflict with the bulk spectroscopic result. It is, however, probable that in large clusters, Mg2+ is hexacoordinated, with three acetonitrile molecules, one hydroxide ion, and two water molecules.

Magnesium hydroxide formation [Eq. (1)] proceeds, depending on the model, with 64–75 % collision rate. The rate constant for reaction (1) is 24 and 115 times higher than the low values observed for uptake of O2 and CO2, respectively.22 This is reasonable, because acetonitrile forms hydrogen bonds, in contrast to O2 and CO2, and owing to its large dipole moment it interacts more strongly with the positively charged metal center. The residence time of CH3CN in the cluster, before a chemical reaction occurs, is therefore significantly longer than for O2 or CO2. If the neutral reactant arrives at the cluster surface in a position remote from the reactive site, that is, the hydrated electron, O2 and CO2 evaporate quickly, whereas CH3CN integrates into the hydrogen-bonded network or even coordinates directly to Mg2+. The collision complex has sufficient time to perform the extensive rearrangements, including proton transfer, which are required to form the hydroxide ion and to eliminate the neutral product [CH3CN,H]. Uptake of additional acetonitrile molecules [Eq. (2)] becomes less efficient with increasing number of occupied coordination sites at the Mg2+ center.

DFT calculations

Structures and energies of CH3CN and CH3CN.−

An electron attachment to small gas-phase clusters of CH3CN can form dipole-bound anions with the excess electron being weakly solvated by the methyl groups of the linear CH3CN molecules.43, 44 With solvation, the excess electron locates favorably at an orbital with a significant C[BOND]N π* character to form a valence-bound anion, which has a bent geometry with a CCN angle of around 130°.45 Energies of CH3CN and CH3CN.− evaluated at the M06/6-31++G(d,p) level of theory are summarized in Table 2. For CH3CN, bending the linear CCN bond increases the relative energy to 22 kJ mol−1 at 155° and 89 kJ mol−1 at 130°. These energies are comparable with those for the molecule being solvated in aqueous solution described by the SCRF method with the PCM. The 6-31++G(d,p) basis set is not adequate to describe the linear dipole-bound state of CH3CN.−, which is incorrectly predicted to be higher-lying than CH3CN.4345 Single-point energy calculations performed with a larger basis set, composed of aug-cc-pVDZ for all atoms and three additional diffuse s and p orbitals for the methyl carbon atom,44 significantly reduce the relative energy of the dipole-bound CH3CN.− to −14 kJ mol−1. The basis set effect becomes insignificant for solvated CH3CN.− because the bent valence-bound geometry is much more favorable with a relative energy of −116 and −121 kJ mol−1 as predicted with the 6-31++G(d,p) and the larger basis sets, respectively.

Table 2. Relative energies of CH3CN and CH3CN.− with different CCN angles in the gas phase or aqueous solution as described by the polarizable-continuum model (PCM). All geometric parameters (except the constrained angle) are optimized at the M06/6-31++G(d,p) level. Relative energies are obtained at this level. The energies shown in parentheses are single-point energy calculations for the M06/6-31++G(d,p) optimized geometries using aug-cc-pVDZ for all atoms and three additional diffuse functions for the methyl carbon.




angle CCN [°]









relative energy [kJ mol−1]

gas phase

0 (0)

22 (22)

89 (87)


66 (−14)

80 (7)

105 (50)


solvated (PCM)

0 (0)

24 (23)

95 (94)


−65 (−72)

−92 (−98)

−116 (−121)

Reactions of hexacoordinated Mg+(H2O)6 (6+0) with CH3CN

Additions of CH3CN to small six-water model clusters Mg+(H2O)6 having the complete hexacoordinated first solvation shell are shown in Figure 3 a. The C3 structure (6+0) is −9 kJ mol−1 lower-lying than the previously reported C2 structure,14 evaluated at the M06/6-31++G(d,p) level. Natural population analysis (NPA) shows that 0.95 of the total spin density is distributed among Mg (0.35) and three water molecules (0.20 each). The CH3CN addition complex (6+0)-1 with the CH3 group pointing to this solvated electron, an analogue of the dipole-bound structure, is 16 kJ mol−1 higher than the reactant. Adding CH3CN to a position remote from the solvated electron to form the lowest-energy complex (6+0)-2 is exothermic by 66 kJ mol−1. The small Mg+(H2O)6 is already fluxional enough to allow facile movements of the solvated electron to various positions, for instance, to produce complexes (6+0)-3 and (6+0)-4. The binding energies of CH3CN in these solvation structures are around 63–66 kJ mol−1. The relative energies of all these complexes are only slightly changed after the larger basis set with very diffuse functions for the methyl carbon atom44 is applied, which suggests that the unpaired electron is stabilized by the water cluster rather than the large dipole moment of CH3CN.

Figure 3.

Reaction energy of Mg+(H2O)6 with CH3CN calculated at the M06/6-31++G(d,p) level of theory. Hexa- and pentacoordinated Mg structures ((6+0) and (5+1), respectively) were considered. Energies in parentheses are calculated with the very large basis set described by Takayanagi.44 Spin density isosurfaces are shown in yellow.

In (6+0)-2, CH3CN is linear. The bond angle tends to decrease if the solvated electron migrates closer to CH3CN, as in (6+0)-3. Further decreasing the bond angle to 157° results in a transition structure (6+0)-3 TS5 with a relative energy of −45 kJ mol−1. This bending energy of around 18–21 kJ mol−1 is comparable with the value predicted for bending a neutral CH3CN molecule to a similar angle of 155° (22–24 kJ mol−1, Table 2). This indicates that the solvated electron in (6+0) does not influence the bending of CH3CN; instead, it is the bending motion of CH3CN that provides a π*(C[BOND]N)-like orbital to accommodate some electron density, which is depicted with the spin-density distribution plot for (6+0)-3 TS5 in Figure 3 a. NPA shows that in (6+0)-3 TS5 there is 0.36 of the total spin density localized in CH3CN (0.27 at the cyano carbon and 0.05 at the nitrogen). Intrinsic reaction coordinate calculations for (6+0)-3 TS5 result in the reactant (6+0)-3 or a spontaneous proton transfer from a water molecule to the nitrogen, to yield the product (6+0)-5E, which is a solvation complex between MgOH+(H2O)5 ((6+0)-H) and the E configuration of CH3CNH. with the spin density mainly located at the cyano carbon (0.81). The relative energy of this E-isomer product is −98 kJ mol−1, which is 10 kJ mol−1 below the less stable Z isomer (6+0)-5Z.

Another transition structure (6+0)-4 TS6 with a relative energy of −38 kJ mol−1 is also located. In this structure, the CH3CN is also bent (159°) and gives the π*(C[BOND]N)-like orbital to accommodate 0.29 of the total spin density in CH3CN (0.10 on the cyano carbon and 0.15 on the nitrogen). The relative energy of the product (6+0)-6 is −119 kJ mol−1 and the spin density is almost exclusively located at the nitrogen atom (0.97). The energies of the two transition structures leading, respectively, to CH3CNH. and CH3CHN. are comparable, which suggests that formation of these two products is possible with the former being slightly more favorable kinetically. The binding energies of E/Z-CH3CNH. and CH3CHN. to the corresponding cluster are similar, around 56–58 kJ mol−1, which are comparable with the energy required for water evaporation.

Reactions of pentacoordinated Mg+(H2O)6 (5+1) with CH3CN

Figure 3 b shows the solvation structures for the pentacoordinated Mg+(H2O)6 (5+1), which is 20 kJ mol−1 higher-lying than (6+0). The binding energy of CH3CN onto (5+1) giving (5+1)-1 is 64 kJ mol−1 (−44 kJ mol−1 relative to (6+0)), which is comparable with that for (6+0). No transition structure associated with the formation of CH3CN.− can be located. This is probably because in (5+1)-1 the spin density is still mainly located at the Mg center with a value of 0.82, which is significantly larger than that, for instance, in (6+0)-3 of 0.29. The spin density of Mg remains constant (0.80–0.82) during a CCN angle scan calculation for (5+1)-1 from linear down to around 140°, at which still only 0.03 spin density is transferred to CH3CN. For comparison with the hexacoordinated clusters, the presumable reaction products (5+1)-2E (−98 kJ mol−1), (5+1)-2Z (−87 kJ mol−1), and (5+1)-3 (−131 kJ mol−1) are also located. The binding energies of E/Z-CH3CNH. and CH3CHN. in the corresponding complexes are 62–72 kJ mol−1.

Another, more favorable, addition is with the CH3CN directly attached to the vacant coordination site of (5+1) in the first solvation shell to yield (5+1)-4 with the spin density located at the cyano carbon (0.38) and the nitrogen (0.45). Product (5+1)-4 is metastable against proton transfer via the transition structure (5+1)-4 TS5 with the same relative energy of −100 kJ mol−1, to form (5+1)-5 (−145 kJ mol−1), the lowest-energy structure on the potential energy surface for Mg+(H2O)6+CH3CN as shown in Figure 3. Similar Mg[BOND]N binding structures for E/Z-CH3CNH. are also located as (5+1)-6(Z/E). These first-shell Mg[BOND]N binding energies are around 81–95 kJ mol−1.

The relative energetics for product formation are quite similar for the (6+0) and (5+1) pathways, whereas the release of the neutral product requires considerably more energy from the (5+1)-5 and (5+1)-6(Z/E) intermediates than from their hexacoordinated counterparts.

Reactions of hexa- and pentacoordinated Mg+(H2O)16 with CH3CN

The reactions of larger clusters Mg+(H2O)1621, 22 with CH3CN, as illustrated in Figure 4, are similar to those of the smaller-cluster analogues. Several surface-solvated structures, similar to the one shown as pathway a in Figure 4, are located with relative energies ranging from −40 to −48 kJ mol−1. All products of pathways b–d feature a [CH3CN,H] radical and a hydroxide ion in the first solvation shell. This suggests that concerted proton transfer2, 46 to form these structures from CH3CN and Mg+(H2O)16 faces very small to vanishing barriers. For the hexacoordinated structures, the reaction energies for the formation of MgOH+(H2O)15(L) with L=E-CH3CNH. or CH3CHN. (pathway b in Figure 4) are −114 or −145 kJ mol−1, respectively, which are of similar magnitudes to that for the formation of the solvent-separated ion pair of Mg+(H2O)16(CO2) (−129 kJ mol−1), as evaluated at the same M06/6-31++G(d,p) level of theory.22

Figure 4.

Reaction energy of Mg+(H2O)16 with CH3CN calculated at the M06/6-31++G(d,p) level of theory. a) Mg+(H2O)16(CH3CN) (hexacoordinated) with CH3CN solvated on the surface; b) MgOH+(H2O)15(L) (hexacoordinated); c) MgOH+(H2O)15(L) (pentacoordinated); d) MgOH+(H2O)15(L) (hexacoordinated including L in the first solvation shell). Spin density isosurfaces are shown in yellow.

The reaction energies for the pentacoordinated structures (pathway c in Figure 4) are less exothermic, with a relative energy for L=E-CH3CNH. and CH3CHN. of −79 and −96 kJ mol−1, respectively. The hydrated electron in Mg+(H2O)16 has been completely scavenged by CH3CNH. or CH3CHN. to give MgOH+(H2O)15, for which the hexacoordinated geometry becomes thermodynamically more favorable, which would explain the reduced exothermicity for the pentacoordinated structures. Geometry optimizations after L is removed relax to geometries that have relative energies of −75 kJ mol−1 (L=E-CH3CNH.) and −98 kJ mol−1 (L=CH3CHN.) for the hexacoordinated MgOH+(H2O)15 and −51 kJ mol−1 (L=E-CH3CNH.) and −73 kJ mol−1 (L=CH3CHN.) for the pentacoordinated MgOH+(H2O)15. These numbers correspond to binding energies of L of 39–47 kJ mol−1 for the hexacoordinated clusters and 23–28 kJ mol−1 for the pentacoordinated clusters.

Binding of E-CH3CNH. and CH3CHN. to the vacant coordination site (pathway d in Figure 4) gives products with relative energies of −105 and −128 kJ mol−1, respectively. The Mg⋅⋅⋅N distances of these E-CH3CNH. and CH3CHN. products are 2.30 and 2.25 Å, respectively, which are longer than the Mg⋅⋅⋅O distances of around 2.02–2.12 Å.

The present DFT geometry optimizations are certainly not able to describe the fluxional properties of the Mg+(H2O)16 clusters. Nevertheless, they give an estimated picture for the reaction thermodynamics, which suggests that once CH3CNH. or CH3CHN. is formed, the resulting Mg2+ prefers to be hexacoordinated. Migration of CH3CNH. or CH3CHN. to the first solvation shell requires a change from the hexacoordinated structures to the pentacoordinated intermediates, which are higher-lying by around 35–49 kJ mol−1, so that losing the neutral ligands that are initially formed at the cluster surface would be favorable. The reaction predicted theoretically by using the present medium-sized Mg+(H2O)16 model is aligned with the experimental observations for the larger clusters.

Solvation structures of MgOH+(CH3CN)p(H2O)5−p (p=1–5)

To gain insight into the further uptake of CH3CN by MgOH+(H2O)n through reaction (2), solvent exchange between water at the first solvation shell with acetonitrile was examined by DFT calculations using MgOH+(CH3CN)p(H2O)5−p (p=1–5) as the model systems [Eq. (3)]:

equation image(\rm (3)

Each model cluster consists of the MgOH+ ion core and five solvent molecules. Hexacoordinated structures for the Mg center were constructed by putting all five solvent molecules together with the hydroxide ion to the first solvation shell of Mg2+ (6+0)-H. Moving one solvent molecule from the first solvation shell to the second results in pentacoordinated structures (5+1)-H.

Reaction energies of the solvent exchange, reaction (3) for p=1–5, and the lowest-lying structures are shown in Figures 5 and 6, respectively. The solvent-exchange energy increases with the number of acetonitrile molecules added to the first solvation shell of Mg. Regardless of the coordination structures the reaction is exothermic for exchanging up to three acetonitrile molecules (p=1–3), which agrees with the experimental results that only clusters with no more than three acetonitrile molecules were observed. The reaction energies in bulk solution were also calculated by using the SCRF method with the PCM (Figure 5). The reaction energy in bulk solution shows less dependence on the number of acetonitrile molecules, especially for p=1–3 for which the reaction is almost thermoneutral.

Figure 5.

Reaction energies of the solvent exchange [Eq. (3) for p=1–5] at the M06/6-31++G(d,p) level of theory. Results for the gas phase (filled symbols) are compared with calculations in bulk water (open symbols), the latter described by the SCRF method with the PCM.

Figure 6.

The lowest-lying structures MgOH+(CH3CN)p(H2O)5−p for the solvent exchange [Eq. (3) for p=1–5] calculated at the M06/6-31++G(d,p) level of theory.

The theoretical results suggest that in the large clusters MgOH+(H2O)n, a solvated acetonitrile molecule can coordinate to the MgOH+ core by freely exchanging one water molecule from the first solvation shell. After CH3CN molecules are coordinated directly to the Mg center, the MgOH+(CH3CN)p core is presumably exposed to the cluster surface because of the hydrophobic character of the methyl group. Exchanging the water molecules of the first solvation shell of the ionic core with more than three acetonitrile molecules is thermodynamically unfavorable.


The ion–molecule reaction of CH3CN with Mg+(H2O)n (n≈20–60) proceeds by a mechanism that is similar to that with a hydrated electron (H2O)n, as revealed by FTICR mass spectrometric experiments and DFT quantum chemical calculations. Collisions of CH3CN with Mg+(H2O)n initiate the formation of MgOH+(H2O)n−1 by concomitant loss of CH3CHN. or CH3CNH.. The hydrated electron in Mg+(H2O)n can partially occupy an orbital with π*(C[BOND]N) character in the transition structure, in which CH3CN is bent with a CCN angle of around 157–159°. This leads to the formation of CH3CN.−, which undergoes spontaneous proton transfer to form CH3CNH. or CH3CHN., with the former being kinetically more favorable whereas the latter is energetically preferred. The reaction with clusters having a hexacoordinated Mg center is energetically more favorable than that for the pentacoordinated case. The resulting CH3CNH. or CH3CHN. is preferentially solvated on the cluster surface rather than in the first solvation shell of the Mg center. Up to three additional CH3CN molecules are taken up by the resulting MgOH+(H2O)n, replacing water molecules in the first solvation shell of the MgOH+ core. Compared with the reactions of O2 and CO2, CH3CN reacts much more efficiently with Mg+(H2O)n.


Financial support from the Deutsche Forschungsgemeinschaft (grant no. BE2505/4-2), DAAD, and PPP Hong Kong (project-ID 50750748) is gratefully acknowledged (M.K.B.). C.-K.S. is grateful for financial support from the Research Grants Council, Hong Kong Special Administrative Region: the General Research Fund, grant number CityU 102911, and the Germany/Hong Kong Joint Research Scheme (grant no. G_HK006/10).