3554
J. Phys. Chem. 1988, 92, 3554-3560
located inside the supercages of Y zeolite. The number of platinum atoms in the clusters is almost the same after vapor deposition of Mo. Molybdenum is deposited epitaxially on the Pt clusters, possibly on facets of the encaged Pt clusters which are directed toward the supercage windows. The bimetallic clusters show fcc structure with a nearest-neighbor distance of 277 pm, although Mo is a bcc metal with nearest-neighbor Mo-Mo distance of 272 pm. The temperature of 573 K used in reduction of these clusters is insufficient for complete reduction of Mo below Mo2+. There is one oxygen atom associated with each Mo atom. The results in this paper show that the combined use of AWAXS and EXAFS gives more information than EXAFS alone on morphology and
lattice structure, since information on several coordination shells can be obtained.
Acknowledgment. We thank R. D. Lorentz, K. F. Ludwig, Jr., and W. K. Warburton for their help with the experimental apparatus. This work was supported by a continuing N S F grant, most recently N S F CBT 8219066, and was carried out in part at the Stanford Synchrotron Radiation Laboratory (supported by the U S . Department of Energy). G.B., on leave from IRC, was supported by an NSF fellowship under the US.-France Exchange Award Program. Registry No. Mo, 7439-98-7; Pt, 7440-06-4.
Infrared Laser Study of Methanol, Ethanol, and SF, Solvation in Water Clusters J. Crooks, A. J. Stace,* and B. J. Whitaker School of Molecular Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, U.K. (Received: August 26, 1987; In Final Form: January 20, 1988)
Mass spectrometric detection in association with infrared laser-induced photofragmentation has been applied to a detailed study of molecular solvation in the following neutral clusters: (CH30H), (CH3CH,0H), CH30H.(H20),, CH3CH20H-(H,0),, SF,.(D,O),, and SF6.(H2O),,. Infrared absorption profiles for these clusters were measured by monitoring the depletion of ion signal intensity in a mass spectrometer as a function of wavelength using line-tunable radiation from a C 0 2 laser. Data are presented for n in the range 2-25. The results suggest that hydrogen bonding in small methanol and methanol/water clusters is dominated by the formation of proton acceptor bonds on individual methanol molecules. In large pure methanol clusters, the measured absorption profiles display features that are consistent with formation of the chainlike hydrogen-bonded structures found in bulk liquid methanol. Features of the ethanol and ethanol/water cluster spectra are also attributed to hydrogen bonding. The recorded spectra from mixed SF6/water clusters suggest that the SF6 sits on or close to the surface of the cluster.
Introduction The ability to select individual ion clusters and to study their kinetic and thermodynamic properties',2 has led to a considerable improvement in our understanding of ion solvation at the microscopic level. For a large number of both negative and positive ionsI4 there now exists a quantitative picture of the development of the first solvation shell, and at a qualitative level it has been possible to monitor proton solvation as far as the third solvation shelLs Primarily because of the difficulties associated with detection, the study of solvation in neutral clusters has not seen similar progress. The techniques currently available for monitoring neutral clusters are bol~metry,~?' laser-induced f l u o r e s c e n ~ e ,electron ~*~ diffraction,loJ' and mass s p e c t r ~ m e t r y . ' ~ - 'Although ~ the first (1) Kebarle, P. In Ion-Molecule Reactions; Franklin, J. L., Ed.; Plenum: New York. 1972. (2) Kebarle, P. Annu. Rev. Phys. Chem. 1977, 28, 445. (3) Castleman, A. W., Jr. In Kinetics of Ion-Molecule Reactions; Auloos, P. W., Ed.; Plenum: New York, 1979. (4) Castleman, A. W., Jr. Adu. Colloid Interface Sci. 1979, IO, 73. (5) Stace, A. J. J . Am. Chem. SOC.1984, 106, 2306. (6) Gough, T. E.; Knight, D. G.; Scoles, G. Chem. Phys. Lett. 1983, 97, 155
(7) Gough, T. E.; Knight, D. G.; Rowntree, P. A,; Scoles, G. J . Phys. Chem. 1986, 90, 4026. (8) Even, U.; Amirav, A.; Leutwyler, S.; Ondrechen, M. J.; BerkovitchYellin, Z . ; Jortner, J. Faraday SOC.Discuss. 1982, 73, 153. (9) Levy, D. H.; Haynam, C. A.; Brumbaugh, D. V. Faraday SOC.Discuss. 1982, 73, 137. (10) Farges, J.; De Feraudy, M. E.; Raouit, B.; Torchet, G. J . Chem. Phys. 1983, 78, 5067. (1 1) Heenan, R. K.; Valente, E. J.; Bartell, L. S . J . Chem. Phys. 1983, 78, 243.
0022-3654/88/2092-3554.$01.50/0
three techniques are nondestructive, they only provide an average picture of the response from all the different sized species present in the cluster beam. For tHe purposes of generating mixed clusters, the absence of any form of mass selection can be a serious drawback in determining the composition of a cluster beam. While mass spectrometry has obvious advantages in this direction, the degree of fragmentation that results from either electron- or photon-impact ionization makes it difficult to correlate the signal intensity of any ion with the presence of a particular neutral precursor. 5 ~ 1 6 Despite the difficulties associated with identification, considerable progress has been made in the study of homogeneous clusters, such as (SF6),,'J2J7 and (N20)n18and, to a lesser degree, mixed clusters, such as SF6Arn6,19,20 and CH3F.ArW6In particular, structural information on clusters may be inferred from the ability of an absorbed photon to promote vibrational predissociation, with the resultant depletion in intensity being detected by using either (12) Geraedts, J.; Setiadi, S.; Stolte, S.; Reuss, J. Chem. Phys. Left. 1981, 78, 277. (13) Vernon, M. F.; Lisy, J. M.; Krajnovich, D. J.; Tramer, A,; Kwok, H.-S.; Shen, Y. R.;Lee, Y . T. Faraday SOC.Discuss. 1982, 73, 387. (14) Stace, A. J.; Bernard, D. M.; Crooks, J. J.; Reid, K. L. Mol. Phys. 1987, 60,671. (15) Stace, A. J.; Moore, C. Chem. Phys. Lett. 1983, 96, 80. (16) Buck, U.; Mayer, H. J . Chem. Phys. 1986, 84, 4854. (1 7) Geraedts, J.; Waayer, M.; Stolte, S.; Reuss, J. Faraday Soc. Discuss. 1982, 73, 375. (18) Miller, R. E.; Watts, R. 0.; Ding, A. Chem. Phys. 1984, 83, 155. (19) Philippoz, J.-M.; Zellweger, J.-N.; van den Bergh, H.; Monot, R. Surf. Sci. 1985, 156, 701. (20) Gough, T. E.; Mengel, M.; Rowntree, P. A,; Scoles,G. J . Chem. Phys. 1985, 83, 4958. McCombie, J.; Scoles, G., private communication.
0 1988 American Chemical Society
IR Laser Study of CH,OH, C 2 H 5 0 H ,and SF6 Solvation a bolometer6v7or a mass spectrometer.l2-I4 The line shapes and positions of the absorption features have been interpreted in terms of lifetimes and/or structures of the van der Waals molecules and clusters concerned.2' Our intention in this paper is to present the results of a study of solvation in neutral clusters. Infrared photodissociation, in conjunction with mass spectrometric detection, has been used to measure absorption features in the following cluster systems: (CH,OH),, (CH,CH,OH),, CH30H*(H20),, CH3CH20H. (H20),, SF6.(H20),, and SF6-(D20),. The alcohols and SF6 provide an interesting contrast due to their large differences in solubility when associated with bulk water. Whereas methanol and ethanol are miscible with water at all concentrations, SF6 is almost insoluble.22 SF6, however, is capable of forming clathrate hydrates.23 Each of these solutes has an infrared-active mode that falls within the range of a line-tunable C 0 2 laser: C H 3 0 H (v = 1033 cm-', gas phase, C-0 stretch); CH3CH20H(Y = 1032.6 cm-', gas phase, CH, wag); SF6 (v = 949 cm-l, gas phase S-F stretch). The effect of electron-impact ionization on neutral clusters is well d o c ~ m e n t e d , ' and ~ . ~ ~quantitative estimates of the degree of induced fragmentation are starting to appear in the literature.16 With regard to hydrogen-bonded clusters, similar investigations on their fragmentation indicate that it is less severe with only one or two molecules being lost as a result of i ~ n i z a t i o n . In ~ ~terms of the present experiments, this observation means that the relationship between a neutral cluster and the recorded ion signal depletion may not be as remote as one would expect if the neutral species were bound by weak van der Waals interactions rather than hydrogen bonds. As a result we have adopted the philosophy that measurements made on small ion clusters reflect the properties of small neutral clusters and that as the size of the ion cluster under study increases, so there is a corresponding increase in the narrow range of neutral clusters which contribute to the ion signal. It is certainly not possible for an ion cluster to be any larger than the neutral clusters that contribute to it. With regard to the use of a line-tunable laser radiation source, our intention is to search for gross features that may be indicative of the development of a solute-solvent relationship. In that respect, the 1-2-cm-' resolution provided by the C 0 2 laser does not present a problem, as long as the solvent-induced shifts are comparable to or greater than that magnitude. Experimental Section Neutral clusters were generated by the adiabatic expansion of either SF6or one of the alcohols, together with water and argon, through a 100-km pulsed nozzle operating at approximately 20 Hz. The cluster beam was collimated by a 0.5-cm-diameter skimmer situated 2 cm from the nozzle. In almost all the experiments the stagnation pressure behind the nozzle was 760 Torr. Approximately 20 cm beyond the skimmer the cluster beam was crossed with the infrared output from a line-tunable C 0 2 laser (Edinburgh Instruments Model PL 4). The laser beam was focused to a 1-mm-diameter spot by use of a 50-cm focal length zinc selenide lens and entered and left the vacuum system via two zinc selenide windows. A further 20 cm downstream from the laser excitation point, the cluster beam entered the ion source of a mass spectrometer. The mass spectrometer (AEI MS12) has been modified to facilitate the unimpeded entry of a cluster beam into the ion source. With an accelerating potential of 8 kV in the ion source, the MS12 is capable of detecting clusters with masses up to 1000 amu. Because the signal intensities from individual ion clusters were often very low, the mass spectrometer was operated with both the source and collector slits set to their maximum values. Under such conditions the instrument only has a resolution of 1000. (21) Janda, K. Chem. Reu. 1986, 86, 507. (22) Ashton, J. T.; Dawe, R. A,; Miller, K. W.; Smith, E. B.; Stickings, B. J. J . Chem. SOC.A 1968, 1793. (23) Cady, G. H. J . Phys. Chem. 1981, 85, 3225 (24) Buck, U., private communication
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3555 I
/
f+* 0
c
1022
1060 frequency(cm"1
Figure 1. Neutral cluster photodissociation spectra measured from the depletion of signal intensity for the ion clusters (CH30H),H+, for n = 2-25.
With the pulsed nozzle operating at a frequencyf, a reference pulse from the nozzle control unit was fed into a signal generator (Global 4001) which in turn pulsed the laser at a frequency f/2. Photodissociation was detected by correlating an f/2 reference signal with the ion signal from the mass spectrometer. A phase-sensitive detector (Brookdeal Model 9502SC) was used for this purpose. By utilizing the second harmonic control on the lock-in amplifier, it was possible to switch directly from the mass spectrum of a cluster to its depletion spectrum. In a typical experiment the mass spectrometer was tuned to a particular ion cluster and the depletion signal was then monitored as a function of laser wavelength. The laser fluence throughout was held constant at 60 mJ cm-2. Laser power dependence studies showed that, at this fluence, all the ion depletion signals were due to single photon absorption. None of the neutral clusters under consideration has a corresponding stable ion. The alcohol and mixed alcohol/water clusters produce protonated ion clusters upon ionization, Le., (ROH),H+ and ROH,+.(,H,O),. With SF, there is a further complication in that when ionized, the resultant stable ion is SFs+,which has the same nominal mass as (H20),H+. Hence, cluster ions in an SF6/H20expansion beyond 127 amu could be either (H20),8FS+ or (H20),,.+7H+. Because of this mass coincidence, all experiments on large SF6/water clusters were performed with D 2 0 . Initial use of D 2 0 resulted in the observation of mixed H 2 0 / H D O / D 2 0 ion clusters due to atom exchange on the walls of the inlet system and the nozzle. This problem was eliminated through a combinetion of baking and repeatedly washing the necessary components in D 2 0 . Results and Discussion Pure Clusters. Figure 1 presents a summary of the results for pure methanol clusters. The absorption profiles were measured by monitoring the intensities of the ions (CH30H),H+ as a function of laser frequency for n in the range 2-25. It is evident from the figure that, as the size of the cluster increases, there is a red shift in the absorption feature. The maximum moves from ~ 1 0 5 cm-' 0 for n = 2 to 1034-1040 cm-' for n = 25. Figure 2 combines the spectra for these two clusters in order to emphasize the extent of the shift. The photodissociation spectrum given by Hoffbauer et aLZ5for the methanol dimer is very similar to that we present for (CH30H),H+. Figure 3 compares the measured spectrum from ( C H 3 C H 2 0 H ) , H f with that from (25) Hoffbauer, M. A,; Giese, C. F.; Gentry, W. R. J . Phys. Chem. 1984, 88, 181.
3556 The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
(CH,OH),H
Crooks et al.
acceptor ( l o w freq.)
+
( C H,O H 1J-I
H,C-
/
H
0, '0-CH, donor(high freq 1
Figure 4. Pictorial summary of the conclusions from ref 27 and 28.
an oxygen atom's lone pair in a hydrogen bond (an acceptor bond) lowered the frequency of the C-0 stretch ( V = 1020 cm-I) for the molecule concerned. In contrast, the formation of a proton donor bond by a methanol molecule was observed to increase the frequency of the C-0 bond (v = 1040 cm-I). In pure liqiid I I I methanol the maximum in the absorption profile for the C - 0 bond ' 1616 1622 1628 10'34 K W O lo46 1052 lo58 "64 falls at 1036 cm-'. We interpret this latter value as a compromise frequency (cm-') between the two extremes in shift considered above. The reason is that the accepted picture for liquid m e t h a n 0 1 ~ ~involves 3~~ a Figure 2. Neutral cluster photodissociation spectra recorded from (CH30H)2H+and (CH30H)25H+.The error bars represent f i standard typical molecule being part of a chainlike structure and, therefore, deviation. forming one of each of the two types of hydrogen bond considered above. From our results it can be seen that (CH30H)2SH+has a (CH,CH,OHI,H+--- - ~ pronounced absorption feature in the 1036-cm-' region. However, the clusters will contain finite chains that could terminate in either of the ways shown in Figure 4, and therefore absorption features both above and below the liquid maximum at 1036 cm-' are to be expected. For small cluster sizes, the shift in the absorption maximum to higher frequencies is consistent with the observations on methanol polymers in CC14. As the of Kabisch and P01lmer~~ methanol concentration was reduced (and presumably the sizes of the polymers became smaller), they observed a shift in the absorption maximum for the C-0 stretch to 1044 cm-l. The shift we observe as the cluster size is reduced is slightly more pronounced and may be due to the fact that we can form small 1 polymers in complete isolation. In a cluster consisting of just three molecules, it is possible to form two proton donor bonds via the lone pairs on a single acceptor. If such bonding arrangements are dominant in a majority of the smaller clusters, then the I I I I lO'34 10'38 1042 lO46 lO!LO lo54 1062 lo66 presence of a high proportion of donor bonds could account for the absorption feature observed around 1050 cm-I. The fact that frequency (cm-' 1 individual molecules appear to undergo a shift in the arrangement Figure 3. Neutral cluster photodissociation spectra recorded from of hydrogen bonds as the cluster moves toward the bulk phase (CH3CH,0H),H+ and (CH3CH20H),5H+. could be due to steric effects. In a small cluster the proportion of molecules on the surface will be relatively high, and they will (CH3CH20H)15H+.These clusters also exhibit a red shift as a experience almost no steric interactions. In comparison, molecules function of increasing size, but its magnitude is only =1-2 cm-' at the center of a large cluster will be subjected to forces similar (the frequency separation between two adjacent C 0 2laser lines). to those experienced by molecules in the bulk liquid. Under such Although small, the shift is consistent across the range of cluster conditions the transition to a (folded) chain structure, similar to sizes studied. We do not present results for pure SF6clusters that adopted by the bulk liquid, could lead to a reduction in the because their spectra are already very well d o c ~ m e n t e d . ~ * ' ~ ~ ' ~ degree of steric interaction. There has been considerable discussion However, our measurements on, for example, the dimer are regarding the special stability of cyclic methanol tetramers in the identical with those presented previously. gas phase.29~31-3sCertainly there is no evidence in our results To interpret the methanol cluster results, we need to examine to indicate that a tetrameric cyclic structure (where all the C-0 some of the conclusions reached in previous spectroscopic studies stretches would be equivalent) is having a dominant influence on, of both the pure liquid and of methanol in various polar and for example, the absorption spectrum of (CH30H)5Hf. It could nonpolar solvents. In particular, we have to consider those factors be that better spectral resolution will be necessary to resolve this that influence the vibrational frequency of the C-0 stretching problem. modezGz8which is excited by the laser radiation. In the condensed In ethanol it has not been possible to establish if the same phase, a typical methanol molecule could be involved in the pattern of behavior is present. This is because the vibrational mode formation of two types of hydrogen bond. Either the hydroxyl excited by the laser is situated further from the hydrogen-bonding hydrogen atom could form a proton donor bond or the lone pair on the oxygen atom could form a proton acceptor bond. Both (29) Jorgensen, W. L. J. A m . Chem. SOC.1980, 102, 543. these types of bond are illustrated in Figure 4. From a Raman (30) Karpfen, A,; Schuster, P. Can. J . Chem. 1985, 63, 809. study of pure methanol and of methanol in organic solvent mix(31) Weltner, W., Jr.; Pitzer, K. S . J. A m . Chem. SOC.1951, 7 3 , 2606. tures, Kabisch and P ~ l l m e rconcluded ~ ~ . ~ ~ that the inclusion of (32) Miller, G. J . Chem. Eng. Data 1964, 9, 418. 1
'
1
(26) Kecki, Z. Spectrochim. Acta 1962, 18, 1155; Spectrochim. Acta 1962, 18, 1165. (27) Kabisch, G.; Pollmer, K. J . Mol. Strucf. 1982, 81, 35. (28) Kabisch, G.; Pollmer, K. Magy. Kem. Foly. 1985, 89, 145.
(33) Clague, A. D. H.; Govil, G.; Bernstein, H. J. Can. J . Chem. 1969, 47, 625. (34) Renner, T. A,; Kucera, G. H.; Blander, M . J. J. Chem. Phys. 1977, 66, 111. (35) Shuster, Ya. A,; Kozlova, V . A,; Granzhan, V . A. Zh. Fiz. Khim. 1978, 52. 199.
IR Laser Study of C H 3 0 H , C2HSOH,and SF6 Solvation
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3557
site than is the case for methanol (in methanol we excite the C - 0 stretch, whereas in ethanol it is the C H 3 wag). There is a very small red shift as the size of the cluster increases, and this could be due to two factors: (1) a much reduced but similar effect to that discussed above for methanol; (2) steric interference from the more bulky alkyl groups (compared with the CH, group of methanol) of the other molecules within the cluster. Solute-Solvent Clusters. In order to generate clusters consisting of a single solute molecule it was necessary to work with a considerable excess of water in the expansion. As a result there were always pure water ion clusters present in the mass spectra. For the alcohols, the above expansion conditions resulted in the observation of ion clusters of the form ROH2+.(H20),. From thermodynamic and kinetic experiment^,^^^^' it is well-known that, for small mixed ion clusters of the above type, the excess proton prefers to be solvated by the alcohol rather than the water molecules. Assuming that the excess proton comes from loss of O H rather than OR (see below), any additional alcohol molecules present in the neutral cluster would be retained by the resultant ion. We therefore propose that the ion clusters observed are formed by the ionization of neutral clusters that contain just a single alcohol molecule. If the excess proton came from an alcohol rather than a water molecule, then we should expect to see some response to laser excitation from the pure water ion clusters. This effect would arise from neutral ROH.(H20), losing RO upon ionization. However, no such response is observed. Further discussion on this aspect of the experiments will be given below. In the case of S F 6 / H 2 0or S F 6 / D 2 0 clusters the situation is complicated both by the range of ions observed and by the fact that they all showed a response to laser excitation of the neutral beam. Depending on the relative concentrations, the following ions could be observed: (D20),D+, (D20),-SFS+,SF,+(SF6).D20, SFS+(SF6),.D, and SFS+(sF6),. It was possible to obtain an absorption profile from each of these ions, and in all cases except (D20),D+ the profiles were characteristic of the SF6 dimer or higher polymers. In particular, the profile determined from (D20),-SF5+ showed two peaks split by =21 cm-' which is a promonent feature of the dimer s p e c t r ~ m . ~ Only J ~ ~ 'in~ the spectra obtained from monitoring the (D20),D+ and (H20),H+ ions were the dimer features absent. These spectra were, therefore, taken to be characteristic of neutral clusters containing just a single SF6 molecule. The fact that we can monitor a depletion signal from (D20),D+ implies that an SF6 molecule is lost from the ion [sF6-(D20),]+* ( m> n) immediately following electron-impact ionization (pure water clusters gave no response to photoexcitation over the entire C 0 2 laser range). It was not possible to determine whether photoexcitation can also displace an SF6 molecule from a neutral water cluster (possibly via predissociation). The presence of excess water meant that there was always a background (H,O),H+ or (D,O),D+ peak in the mass spectrum, against which the depletion signal had to be measured. Typically, the depletion signal amounted to = l % of the total ion signal. As stated above, the pure water ion clusters present in the alcohol/water experiments exhibited no response to laser excitation. This observation indicates that alcohol molecules are always retained by the water clusters following ionization. Such behavior provides further confirmation of the point discussed above concerning the mixed alcohol/water clusters containing just a single alcohol molecule. Figure 5 shows a plot of the absorption profiles measured from the ions CH30H2+.(H20), for n in the range 1-10. Figure 6 presents the two extremes of the data range, CH3OH2+-H2O and CH30H2+.(H20)lo.In comparison to the pure methanol results, two differences are evident: (1) the addition of water results in a single, narrow peak; (2) the red shift in small clusters, although slight, is more pronounced than that observed for the same size of pure methanol cluster. The direction of the shift is consistent across the range of different cluster sizes studied. The trend in red shift is identical with that observed by Kabisch and P ~ l l m e r ~ (36) Kebarle, P.;Haynes, R. N.; Collins, J. G.J . Am. Chem. SOC.1967, 89,5753. (37) Stace, A. J.; Shukla, A. K. J . Am. Chem. SOC.1982, 104, 5314.
/
1022
1060 frequency(cm-'1
Figure 5. Neutral cluster photodissociation spectra measured from the depletion of signal intensity for the ion clusters CH,OH,+.(H,O),, for n = 1-10,
[C H,O H . H,Ol H' c
( H,O)J
C
3
-
H+
e-0 -
KI
.-VI
C .-40 -
-
0,
Q.
0,
U
i I 1 I
1016
I
I
IO22 1028 1034 1040 1046 1652 1058
-4
frequencykm-'1 Figure 6. Neutral cluster photodissociation spectra recorded from
CH30H2+.H20and CH30H2+.(H20)lo. following the gradual dilution of methanol in bulk water. However, the range of our shift is smaller. The fact that we can only study clusters containing up to =lO water molecules may mean that the molecule is not completely solvated. It is interesting to note that the line widths (fwhm) of the absorption profiles given in Figure 5 are comparable to those measured by Kabisch and PollmerZ7 for methanol in a considerable excess of water. On the basis of their observed spectral shift, Kabisch and Pollmer2' concluded that single methanol molecules in bulk water predominantly form acceptor bonds between their lone pairs and the water hydrogen atoms. The fact that we observe just a single absorption feature, as opposed to the broad profile given by pure methanol clusters, is, we believe, consistent with the presence of just one type of methanol molecule. This, together with the fact that even a few water molecules are sufficient to give a pronounced red shift, leads us to believe that the methanol/water clusters contain methanol molecules that are only hydrogen bonded via their lone pairs, Le., only methanol proton acceptor bonds are present, and there are proton donor bonds in the clusters. Theoretical ~no ~ ~methanol ~ support for this conclusion can be found in a recent ab initio study of small methanol/water clusters.38 Curtiss and F r ~ r i p found ,~ (38) Curtiss, L. A,; Frurip, D. J. In?.J . Quantum Chem. 1981, 15, 189.
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
[CH,CH,OH H,OjHt
Crooks et al.
-
( 40),,D+
I
4
T
$4
4
t
1 T
4
4 CA, Tr
3 ld34 ld38
1642 1046 lo50 1054 1058 1062
lo66
f re q ue ncy ( c m-'1 Figure 7. Neutral cluster photodissociation spectra recorded from
CH,CH20H2+.H20and CH3CH20H2+-(H20)lo.
40'
' T 1
9b
966
Pi4 942 9%
4 8 966 I 974
902 1
frequency (c"'1 Figure 9. As for Figure 8, but from (D20)6D+and (D20)11D+
_3
I
i
;;
+
T
+
'I T
frequency (cm") Figure 8. Neutral cluster photodissociation spectra recorded for D 3 0 + and (D20)3D*,following the adiabatic expansion of a mixture of SF6 and D 2 0 . The error bars represent f l standard deviation.
that, for the trimer CH30H.(H20)2,a cyclic structure involving only the methanol lone pairs in hydrogen bonding is more stable than an open chain where the hydroxyl O H forms a proton donor bond. A similar, but much reduced, red spectral shift is observed in the ethanol/water clusters. Figure 7 shows a combined spectrum for CH3CH20H,+.H20and CH3CH20H2+.(H20)Io. Once again,
the direction of the shift is maintained across the range of different cluster sizes studied. With the addition of = l o water molecules the absorption maximum is red shifted by =1-2 cm-' from the maximum for (CH3CH20H)15Hf,which in turn is shifted by 1-2 cm-' from the maximum for dimer spectrum. The fact that water is more effective at generating a red shift than additional molecules of the pure liquid suggests that the shift observed in both systems is due to hydrogen bonding rather than steric interactions. The reduced magnitude of the shift is a reflection of the distance from the CH3 to the hydrogen-bonding site. In contrast to the situation for the methanol/water trimer, ab initio calculations by Alagona and Tan?' suggest that for CH3CH20H.(H20),the most stable configuration is an open chain structure with the hydroxyl O H participating in a donor bond. More recently, Alagona and Tani4' have used the potentials calculated in ref 39 to perform a Monte Carlo simulation of an aqueous solution of ethanol. They conclude that the CH2 group in ethanol is strongly perturbed by the solvation shell surrounding the polar OH group but that the perturbation does not extend as far as the methyl group. Such a conclusion confirms our proposal that the spectral shift originates from hydrogen-bonding interactions; however, the supporting ab initio c a l c ~ l a t i o n ssuggest ~~ that the geometry of the bonds is different from that considered to be responsible for the shift in methanol clusters. Unfortunately the observed shifts in both the pure ethanol and the ethanol/water cluster spectra are too small for us to make a clear distinction between effects due to either donor or acceptor hydrogen bonds. Methanol in an aqueous solution has also been the subject of recent Monte Carlo calcu(39) Alagona, G.; Tani, A. J . Chem. Phys. 1981, 7 4 , 3980. (40) Alagona, G.; Tani, A. Chem. Phys. Lert. 1982, 87, 337
IR Laser Study of C H 3 0 H , C z H 5 0 H , and SF6 Solvation
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3559
I!i
t
L
'
Vi0
916
Pi4 Vi2
940
958
966
974
982
.PI
H,016H
5
I'
frequency (cm-'1 Figure 11. Neutral cluster photodissociation spectra recorded for (H20)3H+and (H20)6H*,following the adiabatic expansion of a mixture of SF, and H 2 0 . The error bars represent k l standard deviation.
occupy surface sites. Those SF6 molecules that are solvated have an infrared absorption feature which is similar to that observed for the matrix-isolated m o l e c ~ l e .Philippoz ~ ~ ~ ~ et al.I9 have used a mass spectrometer to obtain infrared absorption profiles of SF6/argon clusters by monitoring depletion of the ion SF5+.Ar,. The results for n = 1-9 l9 give absorption profiles that are centered at 938 cm-' (the P(26) line of the C 0 2 laser) and very much narrower than those shown above for SF6/water clusters. When n is increased to 18,45the SF6/argon clusters only absorb on the single laser line given above. The infrared frequency of this narrow absorption feature coincides with one of the peaks in the spectrum In contrast to the situation for water, of matrix-isolated SF6.44,45 the Ar,' ions that appear in the SF6/argon mass spectra exhibit no response to laser excitation of the SF6.45Taken together, these observations suggest that the adiabatic expansion of an SF6/argon mixture generates clusters in which the SF6molecule is predominantly solvated by argon and that very few surface-bound molecules are observed. This latter feature appears to be particularly true in our experiments. Surface-bound SF6 molecules would have been expected to be displaced by either photoexcitation or electron-impact ionization and to give, therefore, a response from the laser similar to that observed for the water clusters. The production of argon clusters with a high proportion of surfacebound SF6 molecules appears to require the development of techniques other than simple adiabatic expansion.20 Preliminary results suggest that SF6/benZene clusters have absorption profiles (41) Jorgensen, W. L.; Madura, J. D. J . Am. Chem. SOC.1983,105, 1138. (42) Okazaki, S.; Nakanishi, K.; Touhara, H. J . Chem. Phys. 1983, 78, 454.
(43) Swanson, B. J.; Jones, L. H. J . Chem. Phys. 1980, 73, 986. (44) Boissel, P. Thesis, University of Orsay, 1985. (45) Stace, A. J.; Crooks, J. J.; Al-Mubarek, A., unpublished results.
3560
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that are intermediate between those of argon and water.45 Conclusion We have presented in this paper the first detailed study of molecular solvation in neutral clusters. Using mass spectrometric detection in association with laser-induced photofragmentation, it has been possible to determine infrared absorption profiles for the following cluster systems: (CH,OH),, (CH3CH20H),, CH30H.(H20),, CH3CH20H.(H20),, SF,*(D20),, and SF,. (H20),. In pure methanol clusters, the measured profiles display features that are consistent with the methanol moving toward bulk behavior as the cluster size increases. In small methanol clusters, the results suggest that hydrogen bonding between molecules is dominated by proton donor rather than proton acceptor bonds. Confirmation of this bonding arrangement is found in the spectra recorded from the mixed methanol/water clusters. To account for the observed peak width and red shift in these clusters, we
propose that individual methanol molecules predominantly form proton acceptor bonds with the water. The lack of a significant spectral shift from either pure ethanol clusters or mixed ethanol/water clusters prevents us from drawing any firm conclusions regarding their behavior. However, the evidence does suggest that any spectral shift present is due to hydrogen bonding rather than steric interactions. The recorded spectra from mixed SF,/water clusters suggest that the SF, sits on or close to the surface of the cluster.
Acknowledgment. We thank the SERC for the award of a research studentship to J.C., for an Advanced Research Fellowship to B.J.W. and for an equipment grant. We also thank A. AlMubarek for assistance with some of the experiments. Registry No. CH,OH, 67-56-1; CH,CH,OH, 64-17-5; SF,, 255162-4; HzO, 7782-39-0.
Anion-Exchange Behavior of Polypyrrole Membranes E. W. Tsai, T. Pajkossy,? K. Rajeshwar,* and J. R. Reynolds* Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019 (Received: August 20, 1987; In Final Form: January 1 1 , 1988)
Anion exchange in anodically synthesized free-standing membranes of polypyrrole tosylate was studied by a combination of spectrophotometry, ac impedance, potentiometry, and electrochemical techniques. The short-time diffusion profiles could be analyzed in terms of a semiinfinite planar diffusion model. The apparent diffusion coefficients extracted via application of this model span the 10-12-10-10cm2/s range. These values are compared with the results of recent measurements in other laboratories on other polymer systems, including polyacetylene. The diffusion coefficient for chloride exchange was also measured by the low-frequency ac impedance technique; the value thus obtained was in good agreement with that culled from spectrophotometric analyses. The diffusion was strictly charge-compensating as gleaned from chemical analyses of the exchanged polypyrrole membranes in chloride and phosphate bathing media. Polypyrrole morphology and bathing medium pH were used as two other variables to study the anion-exchangeprocess. Finally, the ac resistance of the polypyrrole membranes was monitored prior to and after anion exchange. Contrary to the findings of previous authors, only modest changes in the membrane conductivity were noted as a function of anion composition.
Introduction The transport of charge-compensating ions that accompanies electrochemical redox switching of polypyrrole between the conductive and insulating states has elicited much attention in recent years.’-I0 While a full picture on the coupled processes of ion transport and faradaic current flow is gradually emerging from these studies, little is as yet known about the spontaneous ionexchange behavior of polypyrrole membranes in free-standing form.” This paper presents data on the temporal aspects of the exchange process using polypyrrole membranes containing ptoluenesulfonate (tosylate) dopant anions as the starting material. The bathing medium composition, its pH, and the polymer morphology were used as variables to study the anion-exchange characteristics. In all the cases, the exchange kinetics could be conveniently probed by the optical absorption of the tosylate = 219 nm). The identity of the ingressing chromophore (A, anions was established in selected instances by appropriate chemical analyses of the polypyrrole membrane after exchange. A second objective of this study was to clarify the role that the dopant anion plays in the electronic conductivity of the parent membrane. Previous authors have presented evidence pointing toward a strong correlation between the nature of the anion and the polypyrrole c o n d ~ c t i v i t y . ~Ion ~ . ~exchange ~ offers the possibility of maintaining the degree of polymer oxidation at a
* To whom correspondence should be addressed. ‘Permanent address: Central Research Institute for Physics, Hungarian Academy of Sciences, Budapest, Hungary. 0022-3654/88/2092-3560$01.50/0
constant level while simultaneously varying the counterion or “dopant” anion. The morphology of the parent membrane is also (1) Diaz, A. F.; Castillo, J. I.; Logan, J. A,; Lee, W.-Y. J . Electroanal. Chem. 1981, 129 115. (2) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lerr. 1984, 53, 2461. (3) Feldberg, S. W. J . Am. Chem. Sot. 1984, 106, 4671, and references therein. (4) (a) Burgmayer, P.; Murray, R. W. J. Phys. Chem. 1984,88,2515. (b) Burgmayer, P.; Murray, R. W. J . Am. Chem. Sot. 1982, 104, 6139. (c) Feldman, B. J.; Burgmayer, P.; Murray, R. W. J. Am. Chem. SOC.1985, 107, 812. ( 5 ) Pickup, P. G.; Osteryoung, R. W. J . Electroanal. Chem. 1985, 195, 271. (6) (a) Shimidzu, T.; Ohtani, A,; Iycda, T.; Honda, K. J . Elecrroanal. Chem. 1987,224, 123. (b) Shimidzu, T.; Ohtani, A,; Iyoda, T.; Honda, K. J. Chem. SOC.,Chem. Commun. 1987, 321. (7) (a) Zhou, Q.-X.; Kolaskie, C. J.; Miller, L. L. J . Electroanal. Chem. 1987,223,283. (b) Miller, L. L.; Zinger, B.; Zhou, Q.-X. J. Am. Chem. SOC. 1987, 109, 2267. (8) Sundarean, N. S.;Basak, S.; Pomerantz, M.; Reynolds, J. R. J . Chem. Soc., Chem. Commun. 1987, 621. (9)Pickuu. P. G. J . Electroanal. Chem. 1987. 225. 273. (10) Tsai: E. W.; Jang, G.-W.; Rajeshwar, K. J . Chem. SOC.,Chem. Commun. 1987. 1776. -~ (1 1) Preliminary work in this direction has appeared recently: (a) Schlenoff, J. B. Ph.D. Dissertation, University of Massachusetts, Feb 1987. (b) Schenoff, J. B.; Chien, J. C. W. J . Am. Chem. SOC.1987, 109, 6269. (12) (a) Diaz, A. F. Chem. Scr. 1981, 17, 145. (b) Salmon, M.; Diaz, A. F., Logan, A. J.; Krounbi, M.; Bargon, J. Mol. Cryst. Liq. Crysr. 1982, 83, 265. (13) Warren, L. F.; Anderson, D. P. J. Electrochem. Sot. 1987, 134. 101.
0 1988 American Chemical Society
