8300
J . Phys. Chem. 1989, 93, 8300-8303
Solvation Shells of the Proton Surrounded by Acetonitrile, Ethanol, and Water Molecules J. M. Mestdagh,* A. Binet, and 0. Sublemontier Service de Physique des Atomes et des Surfaces, C.E.N. Saclay, 91 191 Gif-sur-Yvette Cedex, France (Received: January 3, 1989; In Final Form: May 10, 1989)
The filling of solvent shells around the proton was investigated by the method of cluster ionization and fragmentation. Hydrogen-bonded clusters of H20, CH3CN, C2H50H,and mixtures of these molecules were studied. Well-defined first shells corresponding to (CH3CN)2H+,(CH3CN),(H20)H+,and (H20)4H+were observed. Most significantly,only one shell was found in clusters of pure CH3CN, whereas a structure corresponding to (CH3CN),(H20),2H+ was observed up to n = 7 in multicomponent clusters of water and acetonitrile. These results illustrate how results derived from small clusters in high-pressure mass spectrometric experiments may be extended to large clusters.
Introduction
A series of both experimental and theoretical studies have drawn attention to the stability and structural properties of cluster ions where a proton is surrounded by molecules such as HzO, CH3CN, CH,OH, HCN, NH,, or mixtures of these One topic considered is whether well-defined solvent shells around the proton are filled. Quantitative criteria to answer this question have been developed by Meot-Ner and S ~ e l l e r .For ~ example, it was shown that the systems (HZO)4H+, (CH3CN),H+, and ( H2O)(CH,CN),H+ form well-defined closed These criteria are based on measurements of the thermochemical quantities AGO and AHo for the stepwise growth of protonated cluster ions in a pulsed high-pressure mass spectrometer. Due to both the necessarily limited pressure and relatively high temperature in the mass spectrometer, these experiments are restricted to the study of cluster ions having a maximum of seven solvent molecules attached to the proton. Ab initio calculations have been performed in close conjunction with these experimental studies. This provides information on the structure of solvated proton^.^^^ Sample systems for the calculations are (H20),H+ ( n = 1-4), (H20),(CH3CN),H+ ( n = 1-3, m = 1 , 3 ) and (H20),(NH3)H+ (n = 2-4). Here again the information is limited to fairly small clusters. The topic of solvent shell filling may be studied by another method, using ionization and fragmentation of clusters generated in supersonic expansions.6 The following point is now well established, but it gave rise to considerable controversy as reviewed in ref 6 : substantial rearrangement of the cluster molecules, as well as internal exothermic ion molecule reactions, occurs immediately after ionization of the neutral clusters. The cluster ions that are formed thus have a large excess energy and several neutral molecules are evaporated. Hydrogen-bonded clusters are representative of this behavior. Proton-transfer reactions and substantial evaporation occur upon ionization, and it was shown that the mass spectra reflect the stability of the protonated cluster ions. The observation of these can be used to provide information on solvation shell structures around the proton.6 For example, the existence of a close solvation shell at n = 5 in the system (NH3)"H+ has been shown by this m e t h ~ d . The ~ method has been refined even further. For example, observation of metastable peaks in mass spectra has allowed the monitoring of competitive decomposition (1) Meot-Ner (Mautner), M. J . A m . Chem. SOC.1978, 100, 4694. (2) Deakyne, C. A.; Meot-Ner, M.; Campbell, C. L.; Hughes, M. G.; Murphy, S. P. J . Chem. Phys. 1986, 84, 4958. (3) Meot-Ner (Mautner), M. J . Am. Chem. SOC.1986, 108, 6189. (4) Meot-Ner (Mautner), M.: Speller, C. V. J . Phys. Chem. 1986, 90, 66 16. (5) Deakyne, C. A. J . Phys. Chem. 1986, 90, 6625. (6) Castleman Jr., A. W.; Keese, R. G. Chem. Rev. 1986,86, 589. Castlemen Jr., A. W.; Maerk, T. D. Gaseous Ion Chemistry and Mass Spectrometry; Futrell, J . H., Wiley: New York, 987; Chapter 12. (7) Echt, 0.;Dao, P. D.; Morgan, S.; Castleman Jr., A. W. J. Chem. Phys. 1985, 82, 4076.
0022-3654/89/2093-8300$01.50/0
processes in (ROH),(H20),H+ ion clusters.8 Another example is the evidence of delayed reactions within ionized methanol clusters as shown in a reflecting field time-of-flight mass spectrometer (refle~tron).~ The method of ion cluster decay, even in its simplest form where direct mass spectra are observed, is complementary to that of cluster growth in high-pressure mass spectrometers. It can be used to study the filling of solvent shells beyond the first shell. All that is required is to adjust the supersonic expansion so that many large-size clusters are present in the beam. The present work aims at applying the method of cluster ion decay, so as to see how results derived from equilibrium data in high-pressure mass spectrometer measurements may be extended from small ion clusters to large ones. Sample systems for this investigation are hydrogen-bonded clusters of H 2 0 , C2H50H, CH,CN, and mixtures of these molecules. Studying mass spectra of beams containing heteroclusters of water and acetonitrile has a second interest. It allows discussion of the work of Nishi et a1.I0 that was published before the overriding importance of cluster ion stability for the interpretation of mass spectra was recognized. Description of the Apparatus
The present experiments were conducted using the perturber beam arm of a crossed molecular beam apparatus described elsewhere." Only the elements that are directly relevant to the present work are reviewed briefly. The beam is generated by the liquid expansion method introduced by Nishi et al.'O The beam source produces collinear mixing of a liquid jet (acetonitrile, ethanol, water, or mixtures of these liquids) with a flow of argon. The liquid jet comes out of a 2 m long fused silica capillary tubing of 0.25 mm internal diameter and 0.4 mm external diameter. The outlet of the capillary tubing is centered inside a heated cylindrical gas nozzle of 0.8 mm internal diameter and 2 mm total length. The outlet of the capillary tubing is 0.5 mm upstream from the outlet of the gas nozzle. This relative position of the two nozzles is adjusted to optimize the generation of stable beams containing clusters in large quantities. The axial part of the jet is extracted by a 0.5-mm skimmer and a 1.0collimator. With a proper choice of the source parameters, Le., argon nozzle temperature, liquid backing pressure, and argon gas stagnation pressure, the beam contains many large-size clusters. Downstream from the collimator, the beam enters vacuum mbar). The mass spectrometer used for chambers (p < 5 X the present studies is at the end of a time-of-flight arm, at about 2 m from the source nozzle. (8) Stace, A. J.; Shuda, A. K. J . A m . Chem. SOC.1982, 104, 5314. (9) Morgan, S.;Castleman Jr., A. W. J. Am. Chem. Soc. 1987,109,2867. (10) Nishi, N.; Yamamoto, K.; Shinohara, H.; Nagashima, U.; Okuyama, T. Chem. Phys. Lett. 1985, 122, 599. (1 1) Cuvellier, J.; Mastdagh, J. M.; Berlande, J.; de Pujo, P.; Binet, A. Reu. Phys. Appf. 1981, 16, 679. Mestdagh, J. M.; Berlande, J.; Cuvellier, J.; de Pujo, P.; Binet, A. J. Phys. B 1982, 15, 439.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 26, 1989 8301
Solvation Shells of the Proton TABLE I: Mass Swctrum of Acetonitrile Given in Ref 12
41 amu
CHBCH masses, amu
1'2
40 519
41 1000
peak intensity, au
38 115
39 192
12 55
14 113
26 34
28
13 33
32
I
OC!
'lA0
' ;1;
2;O 3dO Mars (a.m.1~1
2AO
3;O
Pi0
4;O
500
Figure 1. Mass spectrum observed for the expansion of pure acetonitrile. The temperature of the source nozzle was 270 O C .
The mass spectrometer is a commercial NERMAG R 10-10 quadrupole mass spectrometer working in the 10-1000 amu range. The mass selection follows a 40-eV electron impact ionization. The mass spectra are recorded with the molecular beam turned alternatively on and off so that the computer for data acquisition can subtract the mass spectrum of the residual background. The mass spectra presented below are background subtracted. They are not corrected for the transmission of the mass spectrometer, but this is not a limitation since only the general form of the spectra is considered below.
Results and Discussion 1 , Systems Containing Acetonitrile. 1 . I . The (CH3CM,,H+ System. Figure 1 shows the mass spectrum recorded over the range 0-500 amu of a beam generated as follows. Acetonitrile is expanded into argon, the temperature of the argon gas nozzle is 270 OC, the argon gas stagnation pressure is 1.7 bar, and the backing pressure of the liquid is 1 bar. The mass range 20-100 amu was studied at higher resolution so as to resolved the cluster peaks at masses 38-42. The corresponding spectrum is shown in Figure 2. Let us identify first the peaks observed in Figure 2. Table I gives the mass spectrum of acetonitrile molecules.I2 The main peak in the table is 41 (CH3CN+)which identifies the peak at 41 amu in Figure 2. Table I then indicates secondary peaks at mass 38 and 39. The peaks of Figure 2 at masses 38 and 39 are assigned to these fragments since their intensities relative to mass 41 are in agreement with the abundances given in Table I. Finally Table I indicates a fragment at mass 40. Both acetonitrile and argon thus contribute to the peak at mass 40 in Figure 2. As stated in the Introduction, peaks corresponding to protonated acetonitrile clusters (CH3CN),H+ are found in the mass spectra because the following series of intracluster processes occurs rapidly upon ionization: CH3CN
CH3CN' CH,CNH'.*
(CHsCN),
proton transfer
(CHaCN), CH3CNH'-
+
CH&N
evaporation
(CH3CN),.p
+ pCH3CN
(1)
The second point to observe in Figure 2 is the significant peaks at masses 42 (CH3CNH+)and 83 (CH3CN.-CH3CNH+), Le., at the masses of protonated acetonitrile and acetonitrile dimers. (12) Cornu, A.; Massot, R. Compilation of Mass Spectral Data, 2nd ed.; Heyden: London, 1975.
Figure 2. Same spectrum as in Figure 1, but for clarity the mass range is limited to 20-100 amu.
0
+
=
.I
i
r
2
0
I
1
I
I
.
b
-
8302 The Journal of Physical Chemistry, Vol. 93, No. 26, 1989
p# 'i , !;KH, CNI,
O.+jl
0'
Mestdagh et al.
50
100
150
200
250
300
Mass 1 a . i ul
350
400
450
IH, Oln-2 H'
0
500
Figure 4. Mass spectrum observed for the expansion of the liquid mixture: 99% CH3CN, 1% H20 by volume. The gas nozzle temperature was 230 OC.
2. The excess energy due to process 2 plus that due to the 40-eV electron impact ionization process is removed from the cluster ion by successive evaporations of CH3CN molecules. The evaporation process stops when an additional evaporation step would remove more energy than is available in the cluster ion. 3. As shown by the method of cluster growth in high-pressure mass spectrometric measurements, the proton H+ surrounded by CH3CN molecules has a well-defined first solvation shell which is filled with two molecules.',2 Considering points 1-3 above and following ref 6, one thus expects that forming (CH3CN),H+ cluster ions by a chain of evaporation processes starting from large parent ions would give a mass spectrum where the (CH3CN),H+ peak intensities versus n present steplike irregularities corresponding to filling of the various solvation shells. The step in Figure 3 between n = 2 and n = 3 is directly related to the first solvent shell of the CH3CN/H+ system that is filled with two molecules. The data given in ref 1 and 2 from high-pressure mass spectrometric measurements are limited to the first shell of acetonitrile around the proton. The present measurements give the complementary information that no further closed shell is visible beyond the first one. Indeed, no structure is observed in Figure 3 above n = 3. This result can be understood by considering that after the first solvent shell is filled with the structure2 C H 3-CN-H+...N C-C H 3 no sites are available for forming further hydrogen bonds. The two methyl groups of the above closed-shell ion cluster cannot act as good donors in hydrogen bonds. Additional molecules thus cannot be bonded by strong H bonds to the closed-shell ion. This results in no further solvent shell. 1.2. The (H20),(CH3CN),,H+ System. The source conditions of the previous section can be used to also generate beams of heteroclusters. The pure liquid acetonitrile is replaced by a mixture of 99% acetonitrile and 1% H 2 0by volume. Figure 4 shows the corresponding mass spectrum. The temperature of the gas nozzle was 2 3 0 OC. The peaks observed in Figure 4 are identified as heterocluster ions of the general formula (CH3CN),(H20),H+ corresponding to various values of n and p. For clarity, only labels corresponding to the four families p = 0, p = n-I, p = n-2, and p = n-3 are shown in the figure. Even with such a small amount of water, the spectrum differs corlsiderably from that of pure acetonitrile shown in Figure 1. Here in Figure 4 the series of main peaks corresponds to (CH3CN),(H20),-2H+ whereas in Figure 1 it was (CH,CN),H+
n = 2, 3, 4, ...
(3)
n = 1, 2, 3 , 4, ...
(4) ~
~~~
(1 3) Lias, S. G.; Ausloos, P. Ion-Molecule Reactions; American Chemical Society: Washington, DC, 1975. Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1984-1985.
2
4
6
to
8
n
Figure 5. Integrated peak intensities versus n for three group of peaks corresponding to the formulas (CH3CN),(H20)H+, (CH3CN),(H2O)3H+,and (CH3CN),(H20)5H+as obtained from Figure 4. The peak of the main sequence in each group has been normalized to 1.
This effect was observed in ref 10 and was discussed on the basis of a direct relationship between observed ions and neutral parents. The authors then went even further by suggesting the existence of water chains surrounded by acetonitrile molecules in the liquid mixture from which the beam was extracted. This picture may be correct, but we believe that it cannot be proved simply from the observations of spectra similar to that of Figure 4 even if, as in ref 10, they are complemented by other spectra where the acetonitrile is deuterated. We have already recalled that cluster ions generated by electron impact ionization undergo extensive rearrangement and fragmentation before mass selection and detection. Let us show that the spectrum of Figure 4 can be rationalized in a way similar to that in the previous section by considering the stability of ions that have the general formula: (CH3CN),(H20),H+
( n and p are integers)
(5)
Deakyne et al.2 provide extensive experimental and theoretical results on the stability and structure of the smallest of these ions (those corresponding t o p = 1-7, n = 1-3, with n p C 7). They showed the overstability of the closed-shell ions (CH3CN)2H+and (CH3CN),(H20)H+ and derived a rule for cluster ion stability for larger clusters: the most stable cluster ions (5) are those with p = n-2, Le., those that have the general formula (3). They were not able to check this rule for clusters containing more than three molecules of acetonitrile.2 The results of Figure 4 clearly show that the ions following the overstability rule of Deakyne et aL2 are those forming the main sequence of peaks in the mass spectrum. The important aspect of Figure 4 is that the stability rule can be applied to clusters having as many as seven acetonitrile and five water molecules attached to the proton. Figure 5 allows one to examine how valid the overstability rule is as the number of CH3CN molecules bound to a fvred number n of H20molecules is increased. Figure 5 shows the integrated peak intensities corresponding to the three groups of peaks (CH3CN),(H20)H+, (CH3CN),(H20),H+, and (CH3CN),(H20)5H+.Each group of peaks has been normalized with respect to the peak which belongs to the sequence of peak (3), i.e. (CH3CN)3(H20)H+, (CH3CN)5(H20)3H+,and (CH3CN),(H20),H+, respectively. The overstability rule is apparent in Figure 5 since the highest peak of each group is that belonging to the sequence (3), but it is more distinct within the group of small clusters (one water molecule attached) than within the group of large clusters containing five water molecules. This observation suggests that the overstability of cluster ions (3) preventing the removal of one acetonitrile molecule becomes lesser as the average size of the cluster increases. It is interesting to notice that among the ions corresponding to formula 3, the one with n = 3, Le., (CH3CN)3(H20)H+,can
+
Solvation Shells of the Proton
The Journal of Physical Chemistry, Vol. 93, No. 26, 1989 8303
1.2
I
0.E
1 hJj,3,1;t? f;y?,y;? lC,H,OHl,
0 2
5
;
1
0'
(H20)3H30+previously reported by Meot-Ner from high-pressure mass spectrometric measurements.' Otherwise, the spectrum has no significant irregularity. This observation indicates that if the water-ethanol system is shell structured around the proton, the structures are too weak to be observable by the present method. The situation encountered here is thus very different from that encountered above with the water-acetonitrile system. This is due to the fact that C2HSOHis both an acceptor and donor of hydrogen bonds whereas CH3CN is only acceptor. In the case of water and ethanol molecules solvating the proton, it is thus possible to construct networks of strong H-0-H hydrogen bonds that do not require defined numbers of molecules. To our knowledge, the water-ethanol system has not yet been studied by the method of cluster ion growth in high-pressure mass spectrometry. However, the water-methanol system which is similar to the present one has been studied. Interestingly, only a faint shell structure corresponding to (CH30H)(HzO)zH+was visible at the limit of the experimental uncertainties. This result is consistent with the present information that the water-ethanol system has no, or has an extremely weak, shell structure around the proton.
20
40
;
9
:
;
;
EO 100 120 Mass la m u l
60
140
10
;
160
;
100
,
IH20), H'
IC, H, OH) IH, 01, H'
(H,OI, 200
H*
Figure 6. Mass spectrum observed for the expansion of the liquid mixture: 5% C2H50H,95% H 2 0 by volume. The gas nozzle temperature was 280 OC.
truly be considered as a closed solvent shell. In contrast, those with r > 3 are better described by a filament structure as was suggested in ref 2. The structure proposed was CHjCN * * H-0-H*
**(
/
0-H * * ) * * 0 - H * * NCCH3 / /
H
H CHaCN'
CH3CN'
H
CHsCN'
It is based on a network of strong Ha-0-H hydrogen bonds. The CH3CN molecules are obligatory chain terminators in the filament since they are acceptors and not donors of H bonds. As a result, additional CH3CN molecules would be less efficiently bound to the cluster ion. This suggests that upon ionization the cluster loses CH3CN molecules rather than HzO and may explain why, as noticed at the beginning of the present section, so many water containing cluster ions are observed. Besides the filament structure, an alternative structure may be imagined when the ion cluster contains four water molecules. It is a branched structure where three water molecules fill an inner shell about H30+. In this case the proton would be solvated by almost pure water since the six acetonitrile molecules would be pushed to the periphery of the ion cluster. A possible overstability of this structure would result in a drop off similar to that of Figure 3 when integrated peak intensities are plotted as a function of the number of water molecules contained in the ion cluster. Although mass spectra in this mass region have rather low signal to noise ratio, no such drop off was observed. This suggests that the branched structure does not have a particular stability. The filament structure thus would be preferred over the branched structure. A similar conclusion was reached in ref 2 by extrapolating qualitatively a trend observed in small clusters: acetonitrile molecules next to the protonated center are stabilizing. 2. Systems Containing Water and Ethanol. Figure 6 shows the mass spectrum of a beam generated from a mixture of 5% ethanol into 95% water by volume. The temperature of the gas nozzle is 280 OC. The spectrum was taken up to mass 600, but for clarity, it is shown over the range 0-200 amu. The peaks observed in the mass spectrum correspond to the families (H,O),H+
n = 1-24 ( n = 1-1'0 in Figure 6 )
(CzHsOH)(HzO),H+
n = 1-22 ( n = 1-8 in Figure 6)
(CzHSOH)z(HzO),H+
n = 1-20 ( n = 1-5 in Figure 6)
n = 1-17 (n = 1-3 in Figure 6 ) (C2HSOH),(H20),H+ A small irregularity is seen in the family of peaks (H20),H+ for n = 4. It is associated with the closed-shell structure
ConcIusion The present work has derived information on solvent shells around the proton by observing mass spectra of supersonic beams containing hydrogen-bonded clusters of organic and water molecules. The method was applied to the solvent molecules HzO and CH3CN and the mixtures Hz0/CH3CN and HzO/CzHsOH. Well-defined first solvation shells corresponding to (CH3CN)2H+,(CH3CN)3(Hz0)H+,and (Hz0)4H+ have been found. In contrast, no such structures exist, or they are weak, when the proton is surrounded by pure CzHsOH or mixtures of HzO and CzHSOH. These observations are in agreement with available theoretical information based on ab initio calculations, and experimental data based-on high-pressure mass spectrometric measurements. The central part of the present work concerns the large cluster ions having more molecules attached to the proton than are needed to build the first closed solvation shell. Our observations reveal that in (CH3CN),H+ clusters no further shell exists with significant overstability around the first one. In contrast an overstable solvation structure of the proton is found beyond the first shell for solvation by mixtures of water and acetonitrile., It corresponds to the formula (CH3CN),(H20),2H+
(n is an integer)
and has probably a filament structure formed by strong H-0-H hydrogen bonds. The overstability of this structure has not previously been proved experimentally for n > 3. It was studied in the present work up to n = 7, and we observed that it is more pronounced at low n values. This result well illustrates to what extent results derived from small cluster studies can be extended to large clusters. The conclusion is that such an extension is possible to do, at least in the present case where ion clusters are strongly H bonded. It would certainly be useful to develop this conclusion further, for example by considering heteroclusters of water with dipolar aprotic solvents such as DMF ((CH,),NCHO) and DMSO ((CH,),SO). These molecules indeed can be compared to the CH3CN that is studied in the present work, but have much larger dipolar moments the effect of which could thus be examined. Acknowledgment. Thanks are due to C. Roland0 (Ecole Normale SupEriewe) who lent the mass spectrometer used in the present work. We are grateful to J. Berlande and J. P. Visticot (SPAS-CEN Saclay) and to H. Mestdagh (ENS) for numerous discussions. Registry No. MeCN, 75-05-8; EtOH, 64-17-5; H+, 12408-02-5.
