The Journal of
Physical Chemistry
~
0 Copyright, 1982, by the American Chemical Society
VOLUME 86, NUMBER 19
SEPTEMBER 16,1982
LETTERS Evidence of Stable Structures In Hydrogen-Bonded Ion Clusters A. J. Stace' and C. Moore Depeflmnt of Chmbby, The Unlversny. Southampton, Hampshire, SO9 5NH United K i m (Received: Mey 13. 1982; I n Final Form: Ju& 13, 1082)
A combined molecular beam-mass spectrometer apparatus has been used to monitor the relative intensities of ion clusters of the type X,(H20),H+ where X is acetone, diethyl ketone, dimethyl ether, and diethyl ether. We show that it is possible to correlate our observations with the formation of stable hydrogen-bonded ion clusters and that the fragmentation data on these species support the view that the combination p = n - 2 results in the formation of ion clusters with very characteristic properties.
Introduction Calculations' support the view that the ion's stability can be attributed to the formation of a pentagonal dodecaFrom the study of gaseous ion clusters it has become hedron structure with an H30+unit residing at the center.* recognized that stability in hydrogen-bonded species is In this Letter we report the results of a study of the achieved by the symmetric positioning of molecules around relative intensities of some mixed ion clusters of the type a central proton or protonated For small ion X,(H,O)fl+ where X is acetone, diethyl ketone, dimethyl clusters, such as (Hz0)4H+, (CH3COCH3)2H+, and ether, and diethyl ether. Our observations are rationalized (CH30CH3)3.H30+,there is accurate thermodynamic data in terms of stable hydrogen-bonded c o n f i a t i o n s and our to support this and for each of the above species conclusions supported by the fragmentation patterns of an appropriate symmetric hydrogen-bonded configuration the species studied. While it is acknowledged that qualcan be drawn. In the case of larger ion clusters the necitative studies of this type are no substitute for quantitative essary thermodynamic data are not yet available; however, thermodynamic information, the nonequilibrium condievidence based primarily on relative ion intensities does tions present in our experiment &ow us to generate much indicate the presence of stable structures. A prime examlarger ion clusters than can presently be accommodated ple is (H20)21H+. A number of different experimental in equilibrium experiments. It is therefore important to techniques have been used to form this ion and in each know if local intensity variations can be correlated with case it has appeared with an unusually high i n t e n ~ i t y . ~ ~ (1)E.P.Grimerud and P. Kebarle, J. Am. Chem. SOC.95,7939(1973). (2)K. Hiraoka, E.P. Grimsntd, and P. Kebarle, J Am. Chem. Soc. 96, 3359 (1974). (3)Y.K. Lau, P. P. S. Saluja, and P. Kebarle, J. Am. SOC.,102,7429 (1980). (4)J. Q.Searcy and J. B. Fenn, J. Chem. Phys. 61,5282(1974). 0022-3654/82/2086-3681$01.25/0
(6) S. Lin, Rev. Sci. Zmtmm., 44,516 (1973). (6)G. M.Lanwter, F. Honda, Y. Fukuda, and J. W. R a w , J. Am. Chem. SOC.,101,1951 (1979). (7)P.M.Holland and A. W. Castleman, Jr., J.Chem. Phys., 72,5984 (1980). (8)J. L. Kaaener and D. E. Hagen, J. Chem. Phys., 64,1860 (1976).
0 1982 American Chemical Society
Letters A
A
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2
n=2 7 6
3
I 1
2
3
4
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6
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;, 2
3
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,
,
4
5
6
B
B
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1
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ii"^ 1
2
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Flguro 7. Relathre Intensities of ion clusters of the type X,(H20)pH' expressed as a percentage of the most Intense peak (A) acetone: (B) diethyl ketone. In the indivkluai graphs the intensltles are plotted a8 a functlon of p , and the value of n Is given on the rlght-hand side of each graph.
stable ion shcturea, or whether thay are either an artefact of the experiment or some reflection of particularly stable neutral clusters.9J0
Experimental Section Details of the cluster formation process have been given elsewhere.l1J2 Briefly, neutral clusters are generated by the expansion of a mixture of either the ether or ketone with water through a 0.005-cm orifice with argon as a carrier gas. Following collimation the modulated cluster beam is ionized by electron impact and mass analyzed on a modified AEX MS 12 mass spectrometer. In addition to providing relative ion intensities, the mas spectrometer is also used to monitor the decomposition routes of the ions. If an ion has a lifetime in the range 10b-104 a, there is a high probability that it will decompose in the field-free region between the ion source and the magnet. Under such circumstances the product ion is not properly focused by the instrument and appears as a diffuse peak at a noninM e r position on the mass scale. Such peaks are normally referred to as metastable peaks. It can be shown that only the reaction path with the lowest critical energy will produce a metastable peak of significant intensity.12 The relative intensities of the particular ion clusters discussed in this paper appear to be almost independent of the conditions present in the expansion nozzle. Varying the carrier gas pressure and the water concentration does (9) D.Dreyfuns and H. Y. Wachman, J. Chem. PhYS., 76,2031 (1982). (10) 0.Echt, K. Sattler, and E. Racknegal, %ye. Rev. Lett., 47,1121 (1981). (11) A. J. Stace and A. K. Shukla, J. PhYS. Chem. 86, 885 (1982). (12) A. J. Stace and A. K.ShuLla, J . Am. Chem. SOC.,in press.
Relative lntensltles of ion cluster of the type X,(H,O)pH+ expressed as a percentage of the most intense peak: (A) dimethyl ether: (B) diethyl ether. (See caption to Flgure 1.) Flguro 2.
influence the overall distribution of species in the beam. However, ions of the type X,(H20)pH+ do not result directly from the ionization of a neutral cluster of that composition; one of the monomer units has to fragment in order to provide the proton. It is most likely, therefore, that this reaction, together with the stability of the resultant ion, dominates the process and is effective in reducing any relationship between the composition of the neutral cluster beam and the composition of the protonated ion clusters.
Results and Discussion Figure 1shows the measured relative intensities for ion clusters of the type X,(H20)#+ where X is either acetone or diethyl ketone. In each case the intensities are expressed as a percentage of the most intense peak. Figure 2 presents similar data for X as either dimethyl ether or diethyl ether. For both the examples in Figure 2 no ion clusters of the above type are observed when p < n - 2. For the small ion clusters this observation is consistent with the results of Kebarle et al.,'v2 Le., their experiments showed no evidence of (CH30CH3)4-H30+,and (CH30CH3)3H+was found to have a comparatively low heat of formation and would not be expected, therefore, to exhibit any significant stability. For all four sets of data the results diplay the same general trend up to n = 5, with the most intense peak being given by the combinationp = n - 2. It is possible to equate these combinations to p and n with a series of structures which could be adopted by the ion clusters and these are given in Figure 3. In each case the structure corresponds to a configuration where all the available hydrogen-bonding positions are filled. For some combinations more than one
The Journal of phvslcai Chemistry, Voi. 86, No. 19, 1982 3803
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Letters n.2, p=O X-
X
H -X
H 'H '0'
X
I
n=3,p:1
E,(H20)pH+
X
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H
E,(HzO)pH+ E,(H20),1H+
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Flguro 3. Possible hydrogen-bonded structures for X,,(H,O),H+ clusters wlth the combination p = n 2.
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ion
structure can be drawn but the overall conclusion remains the same. Obviously, some structures could not continue to grown indefinitly, and our results would suggest that by the time n = 6 they are beginning to show signs of weakness. There are a number of factors which will contribute to this, these are as follows: (1) As the number of active or labile hydrogen atoms increases, the stabilizing influence of the positive charge will diminish. (2) Large structures will be unable to gain stability from the formation of a compact three-dimensional unit, cf. (H20)21H+,6*7 Two-dimensional configurations will tend to be more fragile. (3) The extent of the steric hindrance between the alkyl groups on the ethers and ketones will increase as the structure grows and this will have a destabilizing effect. Apart from appearing as the most intense of the protonated ions, the clusters corresponding to the combination p = n -2 also exhibit a characteristic fragmentation pattern as determined from the observed metastable transitions. For the ketones there are metastable peaks corresponding to the following reactions: when p = K,(H20)pH+ K,(H20)p,1H+ HzO n - 2 (1)
-
K,(H20)pH+
-
+
K,-l(HzO)pH++ K
For both ethers the reactions are
when p # n - 2 (2)
+
E,-1(H20)pH+ E
+ H20
when p = n - 2 (3) when p # n - 2 (4)
As can be seen, not only do the stable configurations decompose in a very specific manner, but also there is a difference in behavior between the ethers and the ketones. For the combination p = n - 2 the above reaction sequence breaks down when n > 6, and this would support the discussion regarding a size limit on the stable structures. There are also deviations under those conditions when either n >> p or p >> n. The nonexistence of a metastable peak does not mean that a particular decomposition process is absent, but it is possible to show that only the reaction step with the lowest critical energy produces a peak of significant intensity.12 The difference in behavior between the ethers and the ketones could arise from the fact that they have very different dipole momenta. Those for the ethers are lower than that of water (dimethyl ether, p = 1.32 D; water, p = 1.85 D), whereas the dipole momenta of the ketones are much larger (acetone, p = 2.95 D). This in turn would indicate that it is the long range ion-permanent dipole interaction which dictates the decomposition process rather than the shorter ranged ion-induced dipole interaction. The available data on polarizabilities and proton affiiities would suggest that if short-range forces were important then the ethers and ketones would be expected to behave in a similar manner. One possible explanation for this is that the positive charge is sufficiently dispersed for it not to be effective in polarizing the alkyl groups on either the ethers or the ketonea. A similar argument could be applied to the other reactions given above, i.e, (2) and (4). However, the occurrence of these particular reactions must also be related to two further observations: (1)In the protonated ether-water clusters we find no ions with more ether molecules than there are hydrogen-bonding positions available. The fact that those species with excess water prefer to lose water molecules agrees with the results of Kebarle et aL2 (2) h the protonated ketonewater clusters the intensities of those species with excess water declines rapidly beyond the combination p = n - 2. The fact that we observe a consistent trend in the behavior of two different classes of aprotic solvent when clustered with protonated water, and that it is possible to rationalize this behavior in terms of the formation of a series of hydrogen-bonded structures, leads us to believe that it is possible to correlate ion intensity data with the formation of stable ion structures. Acknowledgment. We thank the Science and Engineering Research Council'for an equipment grant and for the award of an Advanced Research Fellowship to A.J.S. We are also grateful to Miss P. Bilboa for assistance with some of the experimental work.
