Chemistry and charge transfer phenomena in water cluster cations

and Ion Processes

E L S E V I E R international Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 723-734

Chemistry and charge transfer phenomena in water cluster cations

Christian Berg, U w e Achatz, Martin Beyer, Stefan Joos, Gerhard Albert, Thomas Schindler, Gereon Niedner-Schat teburg*, Vladimir E. Bondybey

lnstitut J~r Physikalische und Theoretische Chemie, Technische Universitiit Miinchen, 85747 Garching, Germany

Received 2 January 1997; accepted 6 July 1997

Abstract

The present paper discusses three case studies of water cluster cation stability and reactivity by F F - I C R technique: (1) HCI reactions with protonated water clusters that reveal mechanisms of ionic solvation and evaporative recombination, (2) the black body radiation induced fragmentation of hydrated magnesium, Mg+(HEO)n, that leads to mono-hydroxide formation and evaporation of a hydrogen atom once that a critical size of n = 17 is reached, and (3) the fragmentation of hydrated aluminum cations, Al+(H20)n, which leads to the formation of aluminum di-hydroxide and evaporation of a hydrogen molecule at a critical size of about 22 water molecules. Solvation shell effects and possible clathrate-like structures are discussed together with the liquid- versus solid-like character of the clusters. Future investigations are proposed. © 1997 Elsevier Science B.V.

Keywords: Molecular clusters; Metal cation solvation; Charge transfer; IT-ICR spectrometry

1. Introduction

The solvent can have a profound effect upon the properties and chemistry of a dissolved sub- stance, for instance acids, bases, and salts. Ionic dissociation of gaseous hydrogen chloride into H ÷ and C1- ions requires a considerable amount of energy, almost 1400kJmo1-1 (De(HC1) + IP(H) - Ea(C1)). On the other hand, upon dissol- ving HC1 in water, the same ionization reaction proceeds spontaneously, with development of a considerable quantity of heat, 74.85 kJ mo1-1. Similarly, 737.7 and 1450.6 kJ mc,1-1 are required to remove the first and the second electron from a gaseous Mg atom [1]. In spite of the high 2188.3kJmo1-1 energy required for the ionization, salts of magnesium and strong

* Corresponding author.

acids appear in solution as highly stable Mg 2÷ ions. These effects are due to high solvation ener- gies of ions in water, and to the ability of polar solvents in general to stabilize charged species.

As long as the solvent is abundant, strong acids and bases are completely ionized, basically independent of the specific concentration. When, however, the fraction of the solvent is reduced below some critical ratio, the solvation may be inadequate to stabilize the ions present, and interesting effects like charge transfer processes may take place. In small finite systems, such as water clusters containing ions, the amount of available solvent is limited. Removal of a single solvent molecule may destabilize the ionic equilibrium, inducing charge transfer processes and intracluster reactions, and therefore provide new insights into the ion solvation.

Water clusters, both anionic and cationic, were

0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S0168-1176 (97 )00 133 -X

724 C. Berg et aL/lnternational Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 723-734

of course previously studied by a number of investigators and a variety of techniques [2-12]. Also proton bound clusters involving nonaqueous solvents, such as ethanol [ 13], aceto- nitrile [14], and acetic acid [15] were the subject of the recent elegant studies of the group of C. Lifshitz.

We have recently shown that a Fourier transform-ion cyclotron resonance mass spectrometer provides a particularly valuable tool for studying ionic water clusters and ion solvation. Small and medium size clusters of water or 'nanodroplets' are easily generated and stored in an ICR mass spectrometer [ 16-21 ]. Our work has also demonstrated that the black body radiation from the room temperature walls of the vacuum chamber provides a gentle way of removing stepwise the solvent molecules from the cluster [16]. We summarize the studies of the reactivity and fragmentation of several types of size selected water clusters here, and discuss them in the light of possible structural and mechanistic conclusions.

2. Experimental setup

The present study was performed in a modified Fourier transform-ion cyclotron resonance (FT-ICR) [22] mass spectrometer, Spectrospin CMS 47X [23-25]. Either of two optional cluster ion sources is used to generate cold clusters in high abundance: An external C.W.-corona discharge ion source [26,27] with a 200-~m dia- meter orifice is used to generate the protonated water clusters. The sample gas ( ~ 200 mbar, Ar/H20, 200:1) ionized in the discharge (300 V-4 kV, 50/zA-4 mA), expands through the nozzle into a vacuum and cools adiabatically, resulting in clustering of the water molecules. Alternatively hydrated metal cations are generated in a pulsed laser vaporization source [25,28-30] where target materials (solid A1 or Mg) are vaporized by the frequency doubled out- put of a Nd:YAG laser (532 nm, 10-20 mJ per pulse of 5 ns at 25 Hz). This metal plasma is

subsequently cooled and clustered by expansion with a short pulse of helium carrier gas seeded with water. The cluster ions were accelerated downstream from a 400-/zm skimmer, transferred into the high-field region of the superconducting magnet, decelerated, and stored inside the ICR cell at a pressure of 4 x 10 -l° mbar.

In a typical experiment the ions are first injected into the ICR cell until a sufficient number is accumulated. When working with the corona discharge source a mechanical shutter, which blocks the (unwanted) molecular beam that emerges from the cw source orifice, is acti- vated. Thereby the ultrahigh vacuum inside the ICR cell is maintained at 4 x 10 -I° mbar even while operating the discharge source. When using the pulsed laser vaporization source the additional neutral gas load to the ICR-cell region is negligible. Stored cluster ions thus undergo collisions with background gas approximately every 100s while the experiments discussed here are performed on a significantly shorter time scale. In particular any quantitative results are extracted from those experiments that were performed within 0.1 to 4 s.

For mass selection of a given cluster size, all unwanted ions are ejected from the ICR-cell by a series of resonant RF pulses. Bimolecular reactions are enabled by addition of neutral reactant gas into the ICR-cell region until a pressure of typically 1 x 10 -8 mbar is reached. For the investigation of unimolecular reactions the base pressure is kept at the ultimate base pressure of 4 × 10 -j° mbar. To obtain intensity time profiles of unimolecular and bimolecular reactions, consecutive mass spectra were recorded after stepwise increasing the reaction delay.

It is known that corona discharges similar to our source can produce cold radicals [31]. An independent experiment with our source under similar conditions with N2 gas resulted in a strong N~ fluorescence, with a T ~ 60 K rota- tional level distribution. As has been discussed in a preceding publication [20], the actual temperature of the water clusters arriving in the

C. Berg et al./International Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 723-734 725

ICR cell some 100/zs after their formation is governed by a balance between radiative heating and evaporative cooling, so that the temperature of their formation in the source is essentially irrelevant. The final temperature depends on the cluster size, and ranges from around 130 to 140 K for clusters with n ~ 60-70, to the terminal n = 4 ions, which do not further fragment at room temperature.

3. Results and discussion

3.1. H+(H20)n + HCl--reactions

An interesting system involves clusters produced by the reaction of HC1 with water clusters and their subsequent fragmentation [16]. When

only small H+(H20), clusters with n <-- 11 are present, only fragmentation upon collisions with HC1 is observed. When on the other hand larger clusters are left to react with HC1, one observes that they dissolve gaseous HC1 (Fig. 1) with simultaneous loss of water ligands, and products of the type H+(H20),(HCI)m are formed If one allows the reactions to proceed for some time, an interesting pattern shown in Fig. 1 emerges, and several distinct regions are observed. Clusters with n -- 11 do not react with HC1. The clusters in the region between about n = 11 and 14 contain mostly one HC1 molecule and clusters with n > 15 dissolve two HCI molecules.

When the large H+(H20),(HC1)m clusters fragment, either collisionally, or by absorbing background infrared radiation, an interesting

100

80

60

40

20

0

H"(H=O)n

n=10 11

/ / / /

- H +,

/

/

i i i i [

200 250

,2/ nltl 0 J.l .

i i

15 ....f"

.;" t" /,o"

/

"120)ml

i

~ i i j n=20

\ 14"..

\ C l - 17

/ , ............... 15 15 / 4

300 350 m / z

A /

/

6

d Fig. 1. Intensity distribution of HC1 + H+(H20). reaction products. It takes ten water molecules to solvate a single molecule of HCI and it takes 13 water molecules to solvate a second HC1 molecule [16].

726 C. Berg et al./lnternational Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 723-734

behavior is observed. The clusters sequentially evaporate water molecules, but when the boundary between the individual regions on Fig. 1 is reached, a molecule of HCI is lost instead. This can be easily understood if one assumes that a sufficiently large cluster dissolves the HCI molecule with ionization, so that a cluster con- tains formally two H ÷ cations and a C1- anion, stabilized by the water ligands. As the solvent molecules gradually evaporate, the ionized system is progressively destabilized. When the critical number of ligands is reached, the system becomes 'bistable', and a recombination of C1- with the H + with formation of covalent HC1 may take place. The HC1 molecule is bound to the cluster less strongly than H20, and evaporates preferentially.

It is interesting to speculate on the structure of the clusters near the critical size of n ~ 11. When one observes the fragmentation of a distribution of large pure water clusters by the room temperature black body radiation, one obtains eventually the H+(H20)4 cluster as a final product, which is apparently stable at room temperature [20]. The structure of this cluster can undoubtedly be described as a hydrated hydroxonium cation,

(H30*)2(H20)8CI- ~1~ H*(H20)I 0 + HCI

Cl- :

. . . . . 4 - * . - - J

. 3 o - l ...... "

............ ...........

Fig. 2. Possible structure of (H30+)2(H20)sC1 - and mechanism of HCI evaporation. At least ten water molecules are needed to screen the two hydroxonium cations present and form a semi-cavity to host the chloride anion. When heating up this structure HC1 evaporates

which recombines from Haq and Cl~q via proton transfer.

with an oxygen of a water molecule bound to each of the protons of H3 O+. It appears that clusters near the critical H+(H20) 10HCI size com- prise two such hydrated hydroxonium entities, separated by the C1- anion and stabilized by the remaining water solvent as shown schematically in Fig. 2. At least 14 water molecules are required to dissolve two HC1 molecules, that is to solvate and stabilize three H + ions and two C1- anions [16].

3.2. Hydrated magnesium cations

Very interesting, and recently extensively studied, are hydrated Mg + clusters [32-37]. Using the laser vaporization method, magnesium-containing clusters with up to n > 50 can be efficiently generated [30]. While very small Mg+(H20), species with n = 2-4 as well as clusters with n > 21 are readily formed, in the intermediate range between 5 < n < 18 hydroxide species Mg(OH)+(H20), are observed exclusively. An interesting behavior is obtained, when large clusters fragment. Until about n = 21 the clusters sequentially lose one molecule of water at a time. When this limit is reached, one observes besides the fragmentation channel also a parallel intracluster reaction, leading to the formation of hydroxide, and loss of hydrogen atom:

Mg + (H20)n + hv ~ Mg + (H20)n_ 1

+ H 2 0 ; n > 21 (also n --< 4)

Mg + (H20)n + hv "--' MgOH + (H20)n_ 2

+ H 2 0 + H ; n = 1 6 - 2 1

This behavior is shown for the size selected n = 19 cluster in Fig. 3, where besides the Mg+(H20)I8 fragment one observes a parallel reaction channel leading to the hydrated hydroxide Mg(OH)+(H20)17 and a loss of a hydrogen atom. As one proceeds to lower values of n, the

C. Berg et al./lnternational Journal of Mass Spectrometry and Ion Processes 167/168 (1997) 723-734 727

." / 24 ÷ V Mg (H20). 'Y [ • 24MgOH÷(H20)~

n=18/V r ~

19 18 . . .V V ..... i

m=17

18 .V,

• 7 "",

15 17 .1' ..... V..

' [ 1 V 14.,-'" .........

250 300 350 400 450 m/z

Fig. 3. Black body radiation induced fragmentation of Mg+(H20)19 clusters. Besides some plain water ligand loss there is also reactive fragmentation taking place which is hydrogen atom loss in the present case. The critical size of about 18 water molecules coincides with the observed limit in the initial cluster distribution from the ion source, where Mg+(H20), dominates for n -> 17 and MgOH÷(H20), for

n--<15.

branching ratio shifts in favor of the hydroxide, and by the time n = 17 is reached all the Mg+(H20), clusters are converted to the Mg(OH)+(H20)n hydroxide species. For all the cluster sizes investigated individual rate constants summarized in Fig. 4 were derived from fitting the obtained intensity time profiles assuming first order kinetics. As can be seen the

rates increase linearly with cluster size, which can be interpreted as the linear dependence of the energy absorption rate on the number of molecular absorbers [20].

The interpretation of these observations goes back to the experiments of Weyl in 1867 who observed that metallic sodium dissolves in liquid ammonia to form an intensely blue solution [38].

728 C. Berg et al./lnternational Journal o f Mass Spectrometry and Ion Processes 167/168 (1997) 723-734

"7(I )

2 "E m

t - O 0 ®

15

14

13

12 11 10

9

8

7 6

5

4

3

2

1

0

O-O Mg+(H20). -> Mg+(H20)..~ + H20 B-B Mg+(H20). -> MgOH+(H20)n.2 + H20 + H H MgOH+(H20)m -> MgOH*(H20)m.~ + H20

0 5 10 15 20 25 30 35 40 45 cluster size n

Fig. 4. Rate constants for the black body infrared radiative fragmentation of Mg+(H20),, n = 2 - 3 , 16-41, and MgOH+(H20),,, m = 4-19. In the range of n = 16-21 one observes for the Mgaq clusters besides the loss of single water molecules a competing intracluster reaction under formation of MgOHa+q and with loss of a single H atom.

It is now well known that the sodium readily ionizes, not only in ammonia, but in a number of other polar solvents, and the blue color is due to the solvated electron [39-42] . Photoionization experiments on hydrated alkali atoms, e.g. Na [43] and Cs [44], demonstrated that the ionization potential of these clusters saturates for n --> 4 at a limiting value which coincides with the detachment energy of an excess electron when hydrated in bulk water [38]. One may note that the ion Mg ÷ is isoelectronic with the neutral atom Na, and a similar process apparently occurs when Mg + is dissolved in water. If a sufficient amount of solvent is present, the rather unstable Mg + cation is further oxidized, leading to the formation of a Mg 2+ cation, and an electron, e- [35]. While the second ionization potential of an Mg atom is rather high, 15.03 eV [1], this is more than compensated for by the much higher hydration energy of the compact, closed shell Mg 2+ cation [45], and the solvation energy of the electron [46] (Fig. 5(a)).

When the large clusters containing the very stable MgZq cation and e~q are losing successively solvent molecules the amount of water is no longer sufficient to completely solvate both the Mg 2+ and the electron, and the system is destabilized. From the well known size dependence of excess electron solvation in clusters [47-49] and from the energy scheme of Fig. 5(a) one can deduce that for clusters of about 21 water molecules and below the separate solvation of Mg 2+ and e- becomes energetically less favorable than that of Mg ÷. Thus around the n = 21 limit a charge transfer and subsequent chemical reaction may take place. Mga2q and eaq recombine to form

+ a highly reactive Mgaq cation, which hydrolyzes one of the nearby water ligands. As a conse- quence a hydrogen atom is set free and a hydrated MgOH+q product is formed. While at first sight this liberation of highly reactive hydrogen atoms might appear surprising, it may be worth pointing out that the process is similar to the evolution of atomic hydrogen 'in statu nascendi'


A

Mg 2÷ + e- + H20(lq)

2.1P : 1451

~r AHhyd(e-) : -160

M g 2+ + e-aq

~Hhyd(Mg2+~.= -1760(+/-100)

Mg ÷ +iH20(Iq)

~ AHhyd(Mg +) = -340(+/-40) / / / / / v / / / / / / / / / / / /

AHr=-130kJ/mol 7~,Z~.7.~.Z~/.X'/////////~/~////~/~/,~/

Mg2+~l + e-aq

AI 3* + 2e- + H20(Iq) & ~r AHhyd(2e -) : -320

AI + + 2e-aq 3.1P : 2745

i ~-Ihyd(AI 3+) - -4350(+/-250)

AI 2÷ + e- + H20(lq) • ~ M1hyd(e- ) = -160

2.1P = 1817

~.Hhyd(Al 2÷) =.=-1880(+I-I 50)

AI ÷ +iH20(lq) AI 2÷ "+ - - • , , - , , U/~,~z/,z/.z/.z/.~

: ;~- ; ; . ; ; ; ; ; ; ,~,~,~; ; .~.~ aHr=÷141kJ/mol / AI3*aq + 2e-aq ,,,,+ % / "~' aq % . . . . . . . . . . . . . . . .,"

&Hr=+256kJ/mol

Fig. 5. Energy schemes for the solvation of (a) magnesium cations Mg + and (b) a luminum cations AI ÷ in water. Hydration enthalpies

are estimated from ~Hhy d = 699.2ZZ/(rio, + ro) with r0 = 0.85 ,/, the oxygen atom radius[43]. (All values in kJ/mol.)

when metals are dissolved in acid solutions. Alternative explanations, e.g. in terms of prior H-Mg-OHa+q formation or in terms of a per- sistent MgOH.H30~+q structure, have been discussed before [35]. These more complicated alternatives are not supported by any forcing experimental evidence, however.

3.3. Mg+(H20)n + HCl--reactions

Similar interesting processes can be observed in water clusters, in which the excess proton has been replaced by an ionized metal atom, and we have investigated in some detail reactions of clusters containing Mg + with hydrochloric acid, HC1. All larger clusters, regardless of size react with the formation of chloride species. One can understand the reaction so that the hydrated electron interacts with the HCI, resulting in solvated CI-, and the excess neutral hydrogen atom is set free:

Mg 2+ (H20) , ( e - ) + HC1

---, Mg2+ (HzO)n(e-)(H+ CI - )

---* Mg 2+ (H20),C1- + H

Interestingly small clusters (n --< 4) do not react with HC1, although such a reaction may well be exothermic. Larger clusters containing enough solvent molecules can dissolve additional hydrochloric acid forming MgCI+(HzO),(HC1)m products, and a similar pattern is observed as for the reactions with pure hydrated proton clusters. It is seen that at least 15 water molecules (n --> 15) are needed to dissolve one additional HCI, while n --> 20 is needed for m = 2. Apparently 20 water molecules are appropriate to stabilize the six ions: Mg 2+, 2H +, and 3C1-. When these now large ions are selected and allowed to fragment, one observes similar behavior as described in Section 3.1. The clusters lose progressively water molecules, but whenever the 'stability boundary' for a given m is reached, neutralization of two ions and formation of covalent HC1 takes place. This is less strongly bound than any other water molecule of the outer cluster layers. Thus it evaporates in the next step:

Mg 2+ (H +)2(CI-)3(H20)n + hi,

----, Mg 2+ (H + )2(C1- )3 (H20),_ I

+ H 2 0 ; n > 21

730 C. Berg et al.llnternational Journal of Mass Spectrometr3., and Ion Processes 167/168 (1997) 723-734

Mg 2 + (H + )2(C1- )3 (H20)n + hz,

---* Mg2+(H+)(C1-)2(H20)n +HC1; n ~ 21

Mg 2 + (H + )(C1- )2(H20)n + hu

---* Mg2+(H+)(C1-)2(H20)n_I +H20; n -> 16

Mg 2+ (H +)(C1-)2(H20)n +hv

---* Mg2+(C1-)(HzO)n +HC1; n ~ 15

Mg 2 + (C1-)(H20)n + hv

-"* Mg 2 + (C1 - ) ( H 2 0 ) n _ 1 + H20

The last chloride remains, and upon further fragmentation only water ligands are lost. The final product of the fragmentation is then the MgCI+(H20)4 cluster, which again is stable at room temperature.

3.4. Hydrated aluminum cations

Besides hydrated magnesium clusters we have also explored aqueous clusters containing aluminum [21]. These show in many respects similar behavior, but also some interesting differences. Similar to magnesium clusters, small AI+(H20), clusters by absorbing infrared radiation also simply fragment with the loss of water ligands, but while this is in magnesium only observed up to n ~- 5, in aluminum this extends to n < 11-12. If a sufficiently long time is allowed, one observes tetracoordinated A1 + with four H20 ligands which fragments very slowly at room temperature:

AI+(HzO)n +hi' --' AI+(HzO)n_I + H 2 0 n <-- 12

Again similar to magnesium, above the region of very small clusters which form as hydrated AI+(H20), species a region from about n = 7 to n = 21 is observed, where mainly hydroxides of

i l0 ,fl~.. II 2~ . '

I ' ' ' .": ' '". ' I ~, Ill I :* " ~ II S #

" ':,/"" "I" ... (b)] ! -i v .

~ 0 . ~ E _ L J ? , --

[ m=5 ~'I"IX . . . . AI+(H,O). 8 (c) l

I /I111N:J ............ ' 1

1 O0 200 300 400 500 600 700 800 rn/z

Fig. 6. Temporal evolution of AI+(H20), clusters when stored inside a room temperature ion trap: (a) initial distribution, t = 0 s, (b) t = 1 s, (c) t = 8 s, (d) t = 120 s. The initially dominating Al+q clusters form di-hydroxides while shrinking below a critical size of about 22 molecules [21].

the general formula AI(OH)~(H20), form. For still larger clusters the preferred structure reverts to the AI+(H20), species. Very interesting is the fragmentation of the aluminum-containing clusters, which can be seen in Fig. 6 and is summarized by the following scheme of equations:

AI + (H20)n + hu ---* A1 + ( H 2 0 ) n _ 1 + H20; n >-- 24

A1 + (H20)n +hu

---* AI(OH)~-(H20)n-2 +H2; 24 >- n ~ 13

AI(OH)~" (H20)n + h~,

--* AI(OH)~ (HzO)n_ 1 + H20

---' Al(OU)~" (H20)3


AI+(H20)n clusters with n --> 24 fragment exclusively by the loss of solvent molecules. A differ- ent behavior is observed when this n ~ 24 limit is reached, as can be seen in Fig. 6. Absorption of further photons does not necessarily result in sequential loss of water molecules, but an additional, parallel channel appears, where an intracluster reaction takes place, resulting in elimination of H2, molecular hydrogen. The aluminum appears to be oxidized to A13÷, and hydrated hydroxide cations, AI(OH)~(H20)n are produced. The formation of this cation is perhaps not surprising, if one considers that AI(OH)~(H20)4 is reported to be the most abundant singly charged aluminum cation in solutions [50].

It is interesting to note the difference between the chemistry observed for aluminum and magnesium. In the aluminum case, the Al+(H20)n clusters are closed shell species, with an even number of electrons. By eliminating the H2 molecule, AI(OH)+(H20)n_I clusters are produced, again closed shell species. In the case of Mg+(H20)~ hydrates, the starting clusters have an odd number of electrons. By losing a hydrogen atom from the open shell species, the closed shell MgOH+(H20)~_1 clusters are formed. The higher stability of the closed shell species in general may be one of the reasons why atomic hydrogen is eliminated in the magnesium case.

More difficult to explain than the fact that molecular hydrogen is being formed is why this reaction only takes place in the rather sharply defined range of clusters with about 13-24 water ligands. While the observations are fairly unambiguous, their interpretation is somewhat uncertain, and three explanations can be advanced:

3.4.1. Prior electron detachment At a first glance an interpretation similar to

that used for observations in magnesium, i.e. in the large clusters the aluminum is present as Al]q + 2eaq, or perhaps Al]q + eaq, might appear reasonable. The problem is that the energetics

which works well for magnesium appears uncertain for aluminum. The second and third ionization potentials of A1 are 18.823 and 28.44 eV, respectively [1]. The hydration ener- gies of A13+ and A12+ are not known, but they seem to fall somewhat short of the values which would be necessary to make electron detachment and formation of solvated electrons and multiply charged cations exoergic. Our best estimate of the energetics of aluminum cations in water is shown in Fig. 5(b). The numbers seem to suggest that the solvated doubly and triply ionized A1 ions are endoergic by about 140 and 260kJmol -l, respectively, but the possibility of one of these processes being exoergic lies within the estimated uncertainties. In drawing Fig. 5(b) it was not considered that the ions in the cluster will not be produced at an infinite distance, and would therefore be further stabilized by the screened coulombic interaction.

3.4.2. Solvation shell effects Such effects have been observed before in

numerous high pressure mass spectrometric experiments which were conducted in order to obtain successive hydration enthalpies for e.g. hydrated metal cations [51-53]. The high stability of Al+(H20)4 could indicate that n = 4 completes the first hydration shell. If the further solvation proceeded in a manner similar to hydrated proton clusters, the second shell will contain eight water molecules, hydrogen bonded to the eight protons of the four water molecules in the first solvation layer, probably arranged tetrahedrally around the AI +. Thus the first two hydration shells would require 12 water molecules. The experimental observation that the oxidation of the A1 + to hydroxide requires 13 or more water ligands might then be interpreted to suggest that molecules in the third solvation shell are needed for this reaction to take place. The upper limit of n = 24 could very specula- tively be interpreted by suggesting that if enough molecules are present in the third solvation shell,

732 C. Berg et al./International Journal of Mass Spectrometo' and Ion Processes 167/168 (1997) 723-734

it is stabilized so that the reaction can no longer proceed.

When the third solvation shell is complete or nearly complete, the entire ion is relatively stable. Gradual removal of water ligands then destabilizes the third solvation shell so that oxygen of one of the third coordination sphere water molecules comes near one of the hydrogen atoms of the first coordination sphere, inducing a proton transfer and the formation of a hydroxyl group on aluminum. A series of further proton transfers then results in the elimination of molecular hydrogen and the formation of the second aluminum hydroxyl group [21 ].

3.4.3. Prior hydroxide formation Previous ab initio calculations (restricted

Hartree-Fock/MP2) [54] for AI+(HzO)n, n --< 4 and our own calculations by density functional methods [55] for n -< 6 suggested the possibility of aluminum into the OH bond, and formation of hydroxide-hydrides H-A1-OH+(HzO)n_I . Our computations indicate that these isomers lie 100-160 kJ mol -l lower than the corresponding unreacted clusters of equal size. Activation barriers and the relative stabilities of larger clusters are not known, however. It is thus con- ceivable that while such a hydroxide-hydride is stable when fully solvated, when the hydration shell drops below 22-24 water molecules, an activation of a second water molecule, formation of dihydroxide, and elimination of molecular hydrogen takes place.

but the freezing points of microscopic droplets and clusters are undoubtedly considerably lower than that of the bulk liquid. While in ice crystals each water molecule is bound by rather strong hydrogen bonds to four other molecules such hydrogen bonded networks cannot fully develop in very small finite systems. Furthermore, in small clusters a large fraction of the molecules lies on the surface, and the average coordination in the cluster will be much lower than in the bulk. In any event, the complex rearrangements and reactions which take place in the clusters must require considerable changes of their geometry, and even if one might hesitate to call them liquid, their geometry must be fluxional, and the molecules must possess considerable mobility.

It should, on the other hand, be noted that the existence of 'magic' sizes with clearly increased stability, e.g. n = 21 [56] or n = 55 [20], would tend to suggest a well-defined structure and geometry for these species. It was proposed that this is due to the formation of a highly symmetric, closed hydrogen bonded network around the central H30 + [56] or metal cation [57]. Such a 'clathrate' structure would tend to egalize the water molecules in the first solvation shell of the metal ion, and would make breaking a single molecule off the cluster for activation more difficult. The formation of clathrate-like structures around hydrophobic atoms (e.g. rare gases) and molecules (e.g. N2, CH4) in bulk liquids has been previously noted in several studies [58,59].

3.5. Solid- versus liquid-like clusters

One question which should be considered is whether our 'nanosolutions' are actually liquid, or more like solid pieces of ice. We have previously considered the temperature of the water clusters in our system and concluded that their temperature will be size dependent. For the species with 50-60 molecules we have crudely estimated it to lie near 130-140 K [20]. This is of course well below the freezing point of water,

4. Summary and conclusions

We have shown that solvation dynamics and charge transfer processes in solutions can be conveniently studied by means of the FT-ICR mass spectrometry. The black body evaporation provides a gentle method of removing one solvent molecule at a time. Using this technique, we have shown how the number of solvent molecules controls the stability and evaporation


dynamics of the clusters. HCI dissolves exo- thermically with ionization when enough solvent molecules are present, but the ions neutralize and HC1 evaporates when their number drops below a certain required minimum. Similarly metal cations Ala+q and Mgaq dissolve readily when the water solvent is abundant, but react to produce hydroxides with evolution of hydrogen when enough solvent molecules are removed.

A common feature of the above-described processes is that the charge distribution within the cluster is controlled by the solvent. Removal of ligands leads to changes of stability of the species present in the cluster, and to intracluster charge transfer processes and chemical reactions. More control over the solvent vaporization could be obtained with an ICR cell with a variable wall temperature. By decreasing the wall temperature, the fragmentation rate could be decreased or completely suppressed. Conversely, by heating the walls, the stability of more strongly bound species, such as biological polymers, poly- peptides, and similar systems can be studied [60-63].

Finally, by replacing the 'black body' source with a tunable infrared laser, one could obtain infrared spectra of size selected clusters, and thus get more detailed information about their properties and structure.

Acknowledgements

Financial support by the Deutsche Forschungs- gemeinschaft and by the Fond der Chemischen Industrie is gratefully acknowledged.

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Chemistry and charge transfer phenomena in water cluster cations