8442
J . Phys. Chem. 1991, 95, 8442-8444
Evidence for the Encagement of Alkali Metal Ions through the Formation of Gas-Phage Clathrates: Cs+ In Water Clusters A. &lingert and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: August 5, 1991; In Final Form: September 13, 1991)
Reported herein is the first evidence for the encagement of an atomic ion in a cluster leading to the formation of a gas-phase clathrate, namely Cs+ contained within a complex comprised of 20 water molecules. Evidence is also presented for the encagement of Cs+ by other (distorted) clathrates involving 18,22, 24, 27, and 29 water molecules. All of these species were produced under thermal reaction conditions in a fast-flow reactor.
Introduction The encagement of ions by molecules is a well-known phenomenon in the condensed phase following the pioneering work by Cram and Lehn' on ions interacting with crown ethers and cryptands. In like manner, a wide variety of neutral atoms and molecules in aqueous phases is well established to be entrapped in clathrate structures of various sizes. According to Powell,2 clathrates are "structural combinations of two substances which remain associated, not through strong attractive forces but because strong mutual binding of the molecules of one sort makes possible the firm enclosure of the other". Very recently, considerable new interest into these clathrate hydrates has arisen since they have been identified as playing an important role in ocean sediments and in natural gas pipelines, and also having possible implications to the greenhouse effect.' In terms of smaller condensed units, clathrates are also suggested to influence the formation of aerosol particles in the atm~sphere.~ Investigations of these species have implications to other wide-ranging problems involving nucleation5 and atmospheric phenomena? and may even supply inferences about the entrapment of ions in condensed phases that may bear on Pauling's model' of the physical basis for anesthesiology where the formation of clathrates has been suggested to influence nerve conduction. In the case of the gas phase, there are only three known examples where structures of well-defined geometry can enclose ions, namely water molecules encaging NH4+, H30+, and OH-. The "solvation" of NH4+ by water clusters has been conjectured8 on the basis of magic numbers observed in the mass spectra of mixed water-ammonia coexpansions. More extensive interest and attention has been given to studies of the encagement of H30+by 20 water molecules, investigations which have extended over many years.9 However, the structure of this species was only recently revealedt0through titration experiments of the exposed hydrogen atoms which extend outward from the clathrate cage. The H30+ ion is now known to occupy a central position in the cage. An analogous anion species is inferred for clusters of OH-with water," which also shows a magic number for the 20-mer. Evidently, the attachment of other anions to water clusters leads to quite different structures.I2*I3 We report herein what we believe to be the first evidence for the encagement of an atomic ion in a cluster to form a gas-phase clathrate, in particular Cs+ contained within a complex composed of 20 water molecules. Similarly, evidence is also presented for cluster structures where Cs+ is encaged by other (distorted) clathrates involving 18, 22, 24, 27, and 29 water molecules. Experimental Section Unlike most experiments on gas-phase clusters which typically employ supersonic expansion to effect cluster formation, those reported herein were performed on a fast-flow reaction facility which produces clusters under thermalized reaction conditions. The experimental apparatus and its operation have been described
'Feodor Lynen Fellow, Alexander von Humboldt Foundation, Germany.
in detail elsewhere9JlJ4J5 and only those features salient to the new studies are discussed herein. The flow reactor is equipped with a high-pressure ion source comprising a thermionic filament exposed to the flow of a helium-water mixture at about 32 Torr (4200 Pa). The filament is covered with the usualI6 mixture of cesium nitrate and aluminum and silicon oxides, forming after heating a glasslike bead with a stoichiometry of about Cs20. Al203.2SiO2. This filament produces a copious quantity of cesium ions of a steady current, as is typical for ion sources produced in this manner. The gas mixture flows from the source through a 3 mm diameter orifice into the flow tube which is operated at 0.3 Torr (40Pa). The reactor and source were cooled to temperatures down to -120 'C and maintained a t preselected values for the duration of an experiment. As we have shown in previous s t u d i e ~ ? J l *the ~ ~ clusters J~ are produced by a sequential growth mechanism. A steady-state distribution is maintained which is sampled by the usual quadrupole mass filter located a t the exit of the flow tube. The individual mass spectra are obtained by 500 repetitive scans of 0.4 s each over the mass range of interest; the data are acquired in a multichannel scaler. The reproducibility of the results has been established by performing multiple experiments in which investigation was made of the influence of the relevant parameters, helium, water pressure, and temperature in the source and in the flow tube. Results Four mass spectra are displayed in Figure 1 which show the intensity distribution of clusters Cs+(H,O), as a function of cluster (1) Lehn, J. M. Angew. Chem., Inr. Ed. Engl. 1988, 27,89. Cram, D. J., Angew. Chem., Int. Ed. Engl. 1988, 27, 1009.
(2) Powell, H. M. J . Chem. SOC.1948, 61. (3) Appenzeller, T. Science 1991, 252, 1790. (4) Castleman, A. W., Jr. Enuiron. Sci. Technof. 1988, 22, 1265. (5) Castleman, A. W., Jr.; Keesee, R. G. Chem. Reu. 1986, 86, 589. (6) (a) Bjorn, G.; Arnold, F. Geophys. Res. Lerr. 1981,8, 1167. (b) Yang, X.; Castleman, A. W., Jr. Laboratory studies of Large Protonated Water Clusters Under the Conditions of Formation of Noctilucent Clouds (NLC) in the Summer Mesopause. J . Geophys. Res., in press. (7) Pauling. L. Science 1961, 134, 15. (8) Shinohara, H.; Nagashima, U.;Tanaka, H.; Nishi, N. J . Chem. Phys. 1985, 83, 4183. (9) Yang. X.; Castleman, A. W., Jr. J . Am. Chem. Soc. 1989, I l l , 6845 and references contained therein. (10) Wei, S.;Shi, 2.;Castleman, A. W., Jr. J . Chem. Phys. 1991, 94, 3268. (11) Yang, X.; Castleman, A. W., Jr. J . Phys. Chem. 1990, 94, 8500. (12) Zook, D. R.; Grimsrud, E. P. Inr. J . Mass Specrrom. Ion Processes 1991, 107, 293. (13) Perera, L.; Berkowitz, M. L. J . Chem. Phys. 1991, 95, 1954. (14) Yang. X.; Castleman, A. W., Jr. J . Chem. Phys. 1990, 93, 2405. (15) (a) Upschulte, B. L.; Shul, R. J.; Passarella, R.; Keesee. R. 0.; Castleman, A. W., Jr. Inr. J . Mass Spectrom. Ion Processes 1987, 75,27. (b) Castleman, A. W., Jr.; Sigsworth, S.;Leuchtner, R. E.; Weil, K.G.;Keesee, R. G. J . Chem. Phys. 1987,86, 3829. (c) Yang, X.; Castleman, A. W., Jr. J . Chem. Phys. 1991,95, 130. (d) Yang, X.; Castleman, A. W., Jr. J . Am. Chem. Soc. 1991,113, 6766. (16) Blewett, J. P.; Jones, E. J . Phys. Rev. 1936, 50, 464.
0022-3654191f 2095-8442$02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8443
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TABLE I: Comparison of Stable Structures in Aqueous Systems
1 cs+
. T =
-54'12
a)
Y+(H20), Where Cage Structures Are Assumeda clathrates in cond water clusters in gas phase water phase ref (22) (8) (20) (24) (this work) y+ =
-
H30+
NH4+
H30+
20
20
20
25 -
2
cs+ 18
n
20 22 24
OThe underlined numbers indicate the cases where we assume the enclosure of an additional water molecule. [ ] indicates n weak feature.
T =-117OC
200
300
400
500
d)
760
600
800
mass units
Figure 1. Ion abundance in arbitrary units as a function of mass for the clustering of water molecules to a cesium cation at four different temperatures.
size at four different temperatures. The temperatures were measured in the middle of the flow tube, halfway downstream. Except for employing the procedure of accumulating 500 single scans, the data are presented without any manipulation. The first spectrum, measured at -54 OC, reflects the comparatively low binding energy between the cesium cation and water molecules in the gas phase. This energy has been determined to be 57 kJ/mol (1 3.7 kcal/mol) for the first and 52 kJ/mol (12.5 kcal/mol) for the second water m~lecule.'~Accordingly, when operating the ion source and the flow tube at temperatures above -70 OC, we observe mainly the bare cesium ion with smaller amounts of Cs+(H20) and C S + ( H ~ O(Figure )~ la). The second spectrum (Figure 1b), obtained at -108 OC, displays a very smooth, exponential.type decrease in intensity up to the cluster C S + ( H ~ O ) with ' ~ , larger clusters just beginning to emerge. The largest cluster to be seen in this spectrum is CS+(H~O),~, with an appreciable intensity drop after that size. At slightly lower temperatures, as visible in Figure 1, c and d, the distribution is further shifted to larger cluster sizes; in the present study, the largest observed cluster was C S + ( H ~ O )The ~ ~ . intensity distribution of the clusters shows some fluctuations for the small clusters Cs+(H20), up to n = 18. But those are neither prominent for single cluster sizes nor identical under varied experimental conditions. By contrast, distinct and reproducible intensity anomalies ("magic numbers") are apparent for several of the clusters with sizes n > 17. These signals with enhanced intensities, followed by an intensity drop toward the (n + 1) cluster, are observed for n = 18,20, 22, 24, 27, and 29. Some results indicate a slightly enhanced signal for n = 31, too. Thereafter, the larger clusters again display a continuous featureless intensity distribution. The largest drop in intensity appears from n = 20 to 21, and is a little less pronounced in the cases of n = 27/28 and n = 18/19. ~
~
~
~
_
_
_
(17) Dzidic, I.; Kebarle, P. J. Phys. Chem. 1970, 74, 1466. Keesee, R. G.; Castleman, A. W., Jr. J. Phys. Chem. ReJ Data 1986, IS, 1011.
Although some features of the spectrum in Figure Id, obtained at a slightly lower temperature and with somewhat higher resolution, are different from those shown in Figure IC, the prominence of the magic numbers is exactly the same. Minor changes in temperature or water partial pressure (usually 0.2% of helium) may shift the overall shape of the cluster distribution considerably, but do not influence the enhanced intensities and the intensity drops that follow, which we have described. The absolute intensity of the C S + ( H ~ O )signal ' ~ compared with C S + ( H ~ Ois) a~ function of the p and T conditions, but it has to be pointed out that in all cases the intensity drop from n = 20/21 is significantly more pronounced than for n = 18/19. The bare cesium cation, Cs+, usually yields the highest signal. This is not surprising due to the short lifetime of the initial activated nucleation complex [Cs+H20]* which is a much weaker "heat bath" than the larger clusters. Clustering experiments on atomic ions typically reveal a "bottleneck" in the formation of the first association complex.
Discussion Magic numbers indicate clusters of enhanced stabilities.'* H+(H20)21,better written as H30+(H20)20,is a long known "magic number" in the size distribution of pure water cluster cations produced in supersonic expansion^,'^ as well as in clusters formed under thermalized conditions.20 The reason for the enhanced stability of this cluster has been found to be the formation of a pentagonal dodecahedron of 20 water molecules with the hydronium ion cradled in its centeralo The referred to titration experiments recently conducted in our laboratory led to a determination of the number of exposed hydrogens extending from the surface and, through topological considerations, established the uniqueness of the clathrate cage structure. The structure proposed* for mixed waterammonia clusters comprised of a cage of 20-water clusters with NH4+ at its center is consistent with this finding. The OH- anion would also be expected to give rise to a similar, though less stable structure, in view of the less favorable interaction of an anion with the partial negative charge on the oxygens occupying the interior of the clathrate cage. This expectation is consistent with the observation of a weak magic number for this system.'' The ionic radius of the cesium cation (167 pm) is quite different from the hydronium ion size (0-Hdistance = 100 pm; ref 21). However, we thought that a replacement of the latter by a cesium ion, if possible, could also yield considerable enhancement of the stabilization of the (H20)20-cluster,because the radius of the dodecahedron is likely larger than 350 pm (cavity radius 5250 pm) and should easily accommodate the Cs+ ion. Indeed, there are numerous examples2* for stabilization of the (H20),, cage (18) Wei, S.; Shi, Z.; Castleman, A.
8604.
W., Jr. J. Chem. Phys.
1991, 94,
(19) Searcy, J. Q.;Fenn, J. B. J. Chem. Phys. 1974,61,5282. Hermann, V.;Kay, B. D.; Castleman, A. W., Jr. J. Chem. Phys. 1982, 72, 185. (20) (a) Yang, X.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1989,111, 6845. (b) Yang, X.; Zhang, X.; Castleman, A. W., Jr. Kinetics and Mechanism Studies of Large Protonated Water Clusters, H*(H20)N,N=1-60, at Thermal Energy. Int. J. Mass Spectrom. Ion Processes, in press. (21) Newton, M. D. J . Chem. Phys. 1977,67, 5535. (22) Davidson, D.W. In Water, A Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1973; Vol. 2, Chapter 3.
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The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 0
b Figure 2. Proposed structure for Cs+(H20)20. This structure comprises 1 2 pentagons. White spheres, hydrogen; black spheres, oxygen; gray central sphere, Cs'; dashed lines, hydrogen bonds.
in the condensed phase, even by rare-gas atoms. While the cesium ion is not known to form clathrates in condensed phases, but instead displays a weakly bound hydration shell with about 8 water molecules,23the isoelectronic Xe is known to form clathrate hydrates (see ref 22). These considerations prompted the present investigations. Our observation of C S + ( H ~ Oas) ~the ~ most stable cluster is in perfect agreement with that prediction. We propose, therefore, the structure shown in Figure 2 for this cluster: 10 hydrogen atoms are arranged such that they extend perpendicularly outward from the surface of the dodecahedron. In view of the fact that all positions on the lattice should be able to be occupied by the 10 extending hydrogen atoms, and tunneling with rearrangement is likely, we assume that there is a fast fluctuation bet ween those per mutations. A remarkable feature of the magic numbers is the pattern of their spacing. One set contains even numbers at n = 18, 20, 22, and 24. Separated by two cluster sizes of small intensity, a pair of odd magic numbers follows at n = 27 and 29. Clusters produced in expansion experiments, as well as under thermal conditions, show enhancements for n = 21, 24, 26, and 28. Nishi reports magic numbers for NH,+(H20), with n = 20 and 27 and weakly at n = 22.* In the condensed phase, clathrate hydrates consist of cages containing 20, 24, 26, and 28 water molecules.22 In a recent paper, Michl reports enhanced bonding energies for the reactions leading to the clusters (H,O),H+ with n = 21, 24, 26, and 28.24 On the basis of more recent titration experiments of the protonated water clusters,I0 we have been able to make further assignments of the number of hydrogen atoms extending outward from various water clusters. In some cases these findings have enabled us to determine their structures and the positions of the protonated species which occupy them. These experiments have provided evidence for the possibility of two encaged water molecules (plus proton) in the case of the larger cages, so that most magic numbers can be explained by a small set of water cages with various contents. Table I summarizes the various experimental findings. For the underlined numbers, the encagement of a second particle (water molecule) is suggested. If we assume that the larger structures of 26 and 28 water molecules can encage a water molecule in addition to the cesium ion, the observed magic numbers at n = 24, 27, and 29 would correspond to the structures shown in Figure 3a-c. The additional magic numbers at n = 18 and n = 22 have rarely been seen in other experiments; we suggest structures as in Figure 4, a and b. We realize that the combination of square and hexagonal sites, which is unavoidable in order to form those cages, will weaken the hydrogen bonds considerably and hence the overall stability of the clathrates of these particular sizes. Explanation of magic numbers in this size range by other structures, for example in terms of a second closed hydration shell, is not very likely. However, the first solvation shell has been (23) Szlsz, Gy. I.; Heinzinger, K. Z. Nururforsch. 1983, 38u, 214. (24) Magnera, T. F.; David, D. E.; Michl, J. Chem. fhys. LRtt. 1991, 182, 363.
Figure 3. Proposed structures for (a) C S + ( H ~ O(12 ) ~ ~pentagons and 2 hexagons); (b) C S + ( H ~ O )(1~ ,2 pentagons and 3 hexagons); and (c) Cs+(H20)29(1 2 pentagons and 4 hexagons). The two larger structures contain the H20-Cs+ion. Q
Figure 4. Proposed structures for (a) C S + ( H ~ O (2 ) , ~squares, 8 pentagons and 1 hexagon); and (b) CS+(H,O)~~ (1 square, 8 pentagons, and 2 hexagons).
calculated to contain about 8 water molecules.23 In accordance with this, in many experiments we observe higher signals for Cs+ with 8 or 9 water molecules attached to it, compared with the general trend of an otherwise decreasing signal. Finally, it should be remarked that, in the case of water clusters with a neutral alkali(sodium) atom, no magic numbers have been observed by Hertel et al. in a study of the photoionization potentials of those Conclusion
Cesium-water cluster cations have been observed to form pronounced magic numbers under thermal conditions. By analogy to the H30+ system, there is strong evidence that these numbers are due to the formation of alkali metal ion gas-phase clathrates, a new type of species which to our knowledge has not been observed before. Based on the present findings and other recent considerations of water clusters comprised of cations and anions, a more complete picture of ion-solvent interaction and clathrate formation is slowly emerging.
Acknowledgment. Financial support by the U.S. Department of Energy, Grant No. DE-FG02-88ER60648, is gratefully acknowledged. We thank Ms. Xin Zhang for assistance in performing the measurement, and thank her and Dr. Philippe Bopp for their useful discussions. (25) Hertel, I. V.; Hiiglin, C.; Nitsch, C.; Schulz, C. P. fhys. Reo. Left. 1991, 67, 1767.
