J. Phys. Chem. 1992,96,4143-4145 character, thereby strengthening the C-0 bond and raising the frequency. Because the antibonding orbital is predominately on the carbon, we suspect that the structure of the complex is OC-Clz rather than C0-Cl2. However, isotopic substitution (I3Cl6O)can establish the C O orientation unambiguously. If we assume that the carbon is attached to the C12,then the C-Cl bond length would be 3.12 A which is approximately the sum of the van der Waals radii for these two atoms. (15) DeKock, R. L.; Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1971,10, 38.
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In summary, the CO-Clz complex has been studied using infrared spectroscopy and is found to be linear (&, = 4.78 A), with a 6.28-cm-' blue shift relative to uncomplexed CO. Like HF-ClZ, the structure can be rationalized as a donor-acceptor complex between a weak Lewis base and a halogen molecule.
Acknowledgment. S.W.B. thanks the Department of the Army and the U S . Army Ballistic Research Laboratory for support in the form of a Long Term Professional Training Fellowship. This research was supported by the US.Army Research Office Center for the Study of Fast Transient Processes and by the Department of Energy under grant no. DE-FG03-89ER4053 (R.A.B.).
Vibrational Spectroscopy of Ammoniated Sodium Ions: Na+(NH&, M = 6-12 Thomas J. Selegue and James M. Lisy* Department of Chemistry, University of Illinois, Urbana, Illinois 61801 (Received: January 17, 1992; In Final Form: March 26, 1992)
Vibrational spectra of Na+(NH3),,,, M = 6-12, in the region from 1020 to 1090 cm-I (9.6-pm band of the COzlaser) are reported. The spectra, associated with the u2 mode of NH,, reflect the influence of the Na+-NH, and NH3-NH3 interactions. From the spectral dependence on cluster size, it appears that six ammonia molecules form the first solvent shell about the Na+ ion. Ammonia molecules in a bulklike environment appear for ammoniated sodium ions with 10 or more solvent molecules.
Introduction There have been a number of recent gas-phase studies on ammoniated ammonium ions. The vibrational spectrum of the ammoniated ammonium ion was reported in the 2500-4000-~m-~ region.' Spectral features were assigned to the NH4+core and ammonia molecules in the first solvent shell. Four ammonia molecules were found to complete the first solvent shell. Protonated ammonia clusters were investigated mass spectrometrically using both electron impact ionization2 and multiphoton i~nization.~ The translational kinetic energy release, following unimolecular decomposition of NH4+(NH3)N,has been measured for N = 2-17.z*3 Measurements of decay fractions have been used to determine relative binding energies4 All of the dynamical studiesz4 have found unusual stability associated with NH4+(NH3)4,where the first solvent shell is filled. The solvation of other ions by ammonia has been mainly characterized by high-pressure mass spectrometry (HPMS).S-9 A significant decrease in the binding energy was observed for the attachment of a fifth ammonia for both Li' and Na', suggesting the existence of a solvation shell of four ammonia molecules for these two ions.6 This decrease was not observed for the larger K+ and Rb+ ions.5 We have recently investigated the vibrational spectra of C S + ( C H ~ O H )N~ ,= 4-25,1° and Na+(CH30H)p,P = 6-25,l' (1) 2182. (2) (3) 332. (4) 2506.
Price, J. M.; Crofton, M. W.; Lee, Y . T. J . Phys. Chem. 1991, 95, Lifshitz, C.; Louage, F. J. Phys. Chem. 1989, 93, 5633. Wei, S.;Tzeng, W. B.; Castleman Jr., A. W. J . Chem. Phys. 1990,92,
Wei: S.;Tzeng,W. B.; Castleman Jr., A. W. J. Chem. Phys. 1990,93, Wei, S.; Kilgore, D.; Tzeng, W. B.; Castleman Jr., A. W. J . Phys. Chem. 1991, 95, 8306. (5) Castleman Jr., A. W. Chem. Phys. Leu. 1978, 53, 560. (6) Castleman Jr., A. W.; Holland, P. W.; Lindsay, D. M.; Peterson, K. I. J . Am. Chem. SOC.1978, 100, 6039. (7) Holland, P. M.;Castleman Jr., A. W. J . Chem. Phys. 1982, 76, 4195. (8) Evans, D. H.; Keesee, R. G.; Castleman Jr., A. W. J . Chem. Phys. 1987,86, 2927. (9) Gleim, K. L.; Guo, B. C.; Kcesec, R. G.; Castleman Jr., A. W. J . Phys. Chem. 1989, 93, 6805. (10) Draves, J. A.; Luthey-Schulten, 2.;Liu, W.-L.; Lisy, J. M. J . Chem. Phys. 1990, 93, 4589.
in the 1020-1060-~m-~region. From the spectral dependence of the methanol C-O stretch on cluster size and from supporting Monte Carlo simulations, we observed that 10 and 6 methanols filled the first solvation shells of Cs+ and Na+, respectively. A similar sensitivity to bonding environment is also exhibited by the u2 vibrational mode of NH,, varying from 950 cm-' for the monomer12 to 980 and 1004 cm-l for (NH3)*,I3-.l71016 cm-l for (NH3)3,14-17 1044-1050 cm-l for larger aggregates,I6J7and 1115 cm-'when complexed to Na+.I8 Since the vz or "umbrella" mode of ammonia is affected by coordination with either the nitrogen lone pair or the hydrogens, sensitivity to the position of the ammonia in the cluster is expected. A large portion of this region in the infrared is covered by the 9.6-pm R- and P-branch transitions of the COzlaser, making Na+(NH3)Mideal candidates for an experimental study.
Experimental Section A detailed description of the experimental apparatus may be found in our previous publications.I0J1 Briefly, Na+ ions are generated from a thermionic emitter downstream from the nozzle of a continuous molecular beam source. The ions merge into the cluster beam and collide with and are solvated by the ammonia clusters. The solvated ions are stabilized by evaporative cooling and possess a significant amount of internal energy.l0J1 The cluster ions are guided by a series of aperture lenses into a quadrupole mass filter, where a single cluster ion size is selected. A CW C 0 2 laser with from 4 to 20 W of power (depending on the particular laser transition) propagates along the axis of the quadrupole. (1 1) Selegue, T. J.; Moe, N.; Draves, J. A.; Lisy, J. M. J . Chem. Phys., in press. (12) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; Natl. Stand. Ref. Data Ser.; GPO: Washington, DC, 1972; Vol. I. (13) Fraser, G. T.; Nelson Jr., D. D.; Charo, A.; Klemperer, W. J. Chem. Phys. 1985, 82, 2353. (14) Snels, M.; Fantoni, R.; Sanders, R.; Meerts, W. L. Chem. Phys. 1987, 115, 19. ( 1 5 ) Heijmen, B.; Bizzari, A.; Stoke, S.; Reuss, J. Chem. Phys. 1988, 126, 201. (16) Huisken, F.; Pertsch, T. Chem. Phys. 1988, 126, 213. (17) Siizer, S.;Andrews, L. J. Chem. Phys. 1987, 87, 5131. (18) Ault, B. S . J. Am. Chem. SOC.1978, 100, 5773.
0022-3654/92/2096-4143$03.00/0 0 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Letters
rii
E
Q
200e-18
C 3
.*
.-
l00e-I8
v1
-= oooe+o
3
2' 1065
1075
1085
1095
Frequency (cm-' ) Fijyre 1. Vibrational predissociation spectra of Na+(NH3),,,, M = 6-9, in the 9.6-rm R-branch region. Note the sharp onset of depletion occurring for M = 7 and the persistent peak at 1085 cm-' for M = 7, 8, and 9.
When in the proper frequency region, the laser can excite a vibrational transition in one of the solvent molecules. This vibrational energy rapidly flows into other vibrational degrees of freedom and is sufficient to induce the evaporation of an ammonia molecule while the cluster is still inside the quadrupole. The resulting daughter ion has the incorrect mass-to-charge ratio and is ejected from the quadrupole mass filter, thus reducing the cluster ion beam intensity. The depletion of the cluster ion beam signal is corrected for the laser fluence to yield a photodissociation cross section that is plotted versus laser frequency. Laser-cluster interaction times are from 20 to 30 ps with laser intensities of 30-1 50 W/cm2. We quickly discovered our standard ion source with a 100pm-diameter pinhole nozzle was only able to produce significant numbers of small cluster ions, Na+(NH3)Mwith M I10. By using a conical nozzle with a 180-pm-diameter throat and a 30° expanding cone of 1 cm in length, we were able to generate appreciable signal out to M = 50. The thermionic emitter was positioned about 8-10 mm downstream from the end of the cone. For these studies, we used a gas mixture of -70% NH3 in argon at a backing pressure of 250 Torr.
Results and Discussion Vibrational predissociation spectra were recorded for Na+(NH,),, M 1 6, in the 9.6-pm band of the C02 laser. No absorption was detected in this spectral region for M C 6. For 9 1 M 1 7, predissociation was observed only in the R-branch region from 1068 to 1090 cm-'. These spectra are shown in Figure 1. There is an abrupt onset in the photodissociation cross section upon increasing from M = 6 to M = 7 at 1085 cm-I. The band associated with this transition appears to be quite narrow. The magnitude of the cross section increases in this region as M increases up through M = 9. These absorptions, assigned to the v2 mode, are between liquid N H 3 or large neutral N H 3 clusters at about 1050 cm-l and the binary Na+-NH3 complex at 1115 cm-I observed in a low-temperature Ar matrix.'* This implies an environment for these ammonias that is intermediate to the strong electrostatic influence of the unshielded sodium ion and the weaker influence of hydrogen bonding. This would be consistent with ammonia molecules occupying a region outside of the first solvation shell of the Na+ ion but not yet completely free of the ion field. The lack of observable predissociation for M I6 coupled with the sharp onset of dissociation at M = 7 implies that the first six molecules are in close proximity forming a first solvation shell about the ion and shield the subsequent ammonias. The inference that six molecules comprise the first solvation shell of Na+ is supported by Monte Carlo simulations of Na+(NH3)M cluster ions19 and Na+ in liquid ammonia.20 It should be noted (19) Selegue, T. J.; Moe, N.; Lisy, J. M. To be published.
1025
1035
1045
1055
1065
1075
1085
1095
Frequency (cm-') Figure 2. Vibrational predissociation spectra of Na+(NHJM, M = 9-12.
M = 9, the peak dissociation cross section shifts from 1085 to -1058 cm-' with the growth of the second solvent shell. As M increases beyond
that lower occupation numbers of four and five ammonias for the first solvation shell have been reported, based on HPMS studies6 and Monte Carlo simulations,21 respectively. A smaller first solvation shell of four or five would be inconsistent with our spectroscopic observations. An abrupt change in the infrared spectrum would occur with the addition of a second shell ammonia at M = 5 or 6. However, we observe this change at M = 7. It is possible that if the first solvation shell is filled at M = 4 or 5, the M = 5 or 6 ammonias could absorb in the 25-cm-I region between the Na+-NH3 absorption at 1115 cm-I and the frequency limit of our C 0 2 laser at 1090 cm-I. We would then detect no photodissociation for M C 7 at frequencies less than 1090 cm-I. This would be inconsistent, however, with our studies of methanol solvated alkali-metal ions10J1in which the spectral shifts of molecules filling a second or third solvation shell were found to be only on the order of 2 cm-l. If four molecules filled the first shell and the next two absorb between 1115 and 1085 cm-I, this suggests a second solvation shell occupation number of two, which is unreasonably small. As M increases beyond nine, dissociation in the 9.6-pm P-branch region becomes evident. The dissociation cross section grows rapidly in the 1050-1060-cm-' region at the expense of the 1085-cm-' peak as shown in Figure 2. The gap in the spectra between 1059 and 1068 cm-I is at the origin of the OOol 02OO vibrational transition of the C02laser, where the low-J transitions have insufficient gain to lase. Absorptions in the 1044-1050- and 1050-60-~m-~regions have been identified with large neutral ammonia clusters and condensed-phase ammonia,22respectively. Thus, it appears that the onset of bulklike behavior occurs for Na+(NH3)Mclusters at M = 10. The sharp feature at 1085 cm-I does not disappear for M > 9 but seems to stabilize for M L 11. The feature at 1085 cm-l may be characteristic of an isolated ammonia coordinated to ammonias in the first solvent shell. As the number of ammonias in the second solvent shell increases, the molecules would tend to aggregate due to energetically favorable multiple-body interactions. These mutually interacting secondshell molecules would then tend to resemble bulklike ammonia and absorb in the 1050-1060-~m-~region. Until the second shell is completely filled, an isolated ammonia in this region would still absorb at 1085 cm-l. This explanation is consistent with the experimental observations. An alternative explanation for the change in the spectra as M increases beyond 9 involves an alteration in the cluster structure. The appearance of a peak at 1071 cm-' for M = 10 could be due to a restructuring of the cluster to a smaller (four or five molecule) first solvent shell. At M = 11 the restructuring is complete with
-
(20) Marchi, M.; Sprik, M.; Klein, M. L. Faroday Discuss. Chem. SOC. 1988, 85, 373. (21) Marchi, M.; Sprik, M.; Klein, M. L. J . Phys.: Condem. Matter 1990, 2, 5833. (22) Bromberg. A,; Kimel, S.; Ron, A. Chem. Phys. Lett. 1977,46,262
and references therein.
J. Phys. Chem. 1992,96,4145-4148
the disappearance of the 1071-cm-l peak and the growth of the bulklike band at about 1055 cm-I. However, this interpretation does not account for the continued presence of the 1085-cm-’ band and is not our first choice. It is also appropriate to note that in Na+(CH30H)N”and CS+(CH,OH)~’Ono evidence for restructuring of the first solvent shell has been observed. It is somewhat surprising that the onset of bulklike behavior in Na+(NH3)Moccurs at the low value of M = 10. In Na+( C H 3 0 H ) , similar behavior was not observed until N = 21.” However, in comparison to the ammoniated ammonium ion, this low threshold appears to be reasonable, since Lee and co-workers’ observed ammonias in a bulklike environment for N H 4 + ( N H 3 ) , M 1 8, differing only by two molecules from our observations. It is interesting to conjecture at this point why the onset of bulk solvent regions in the cluster ions is so different for Na+(NH3)M and Na+(CH,OH)N. When the first solvent shell of the ion is filled, the ion is somewhat screened from subsequent solvent molecules by those in the first shell. The ion now interacts with the second-shell molecules in two ways: first through residual electric field strength that is not “screened” by the first-shell molecules and second through interaction with the first-shell
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methanols that have been polarized by the ion. The molecular polarizability of NH, is about 30% less than that for CH30H. Hence, the ammonia molecules in the first shell of Na+ will be less strongly polarized by the ion than the methanol molecules and the interaction between first- and second-shell ammonia molecules will not be much greater than in the bulk. The influence of the ion will extend only a short distance into the solvent, and the interaction between ammonia molecules will become dominant, resulting in a net environment very similar to bulk ammonia. We are currently looking at larger cluster ions Na+(NH3)Mwith 50 1 M 1 l2I9 as well as other ammoniated alkali-metal ions23to better understand ion solvation by ammonia.
Acknowledgment. This work has been supported in part by the National Science Foundation (Grant CHE-9111930) and the University of Illinois Research Board. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. (23) Selegue, T. J.; Moe, N.; Draves, J. A.; Lisy, J. M. Work in progress.
Polymerization of Pyrrole over Pd and SnO, Supported on KL Zeolite Gustavo Larsen, Gary L. Hailer,* Department of Chemical Engineering, Yale University, P.O. Box 2159, Yale Station, New Haven, Connecticut 06520
and Manuel Marquez Chemistry Department, Yale University, P.O. Box 6666, Yale Station, New Haven, Connecticut 06520 (Received: January 28, 1992; In Final Form: March 24, 1992)
We have prepared polypyrrolezeolite systems by oxidative polymerization over small SnOz particles and oxygen-covered Pd clusters supported in KL zeolite. The local structure of the Pd and Sn02clusters was studied by means of extended X-ray absorption fine structure (EXAFS). Electron spin resonance (ESR) and Fourier transform infrared (FTIR)spectroscopies and solvent extraction techniques were used to characterize the polymeric species in the zeolite cavities. A monomer/oxidant ratio larger than stoichiometric appears to indicate that the oxidation reaction may proceed catalytically after air exposure rather than in a redox fashion.
Introduction The advent of conducting polymer/zeolite composite materials is relatively recent and has been pioneered by Bein et al.’-, and other research group^.^.^ It offers a fascinating opportunity for preparing highly oriented “wires” of molecular dimensions that may ultimately lead to simple electric circuits of the smallest conceivable size.6 Zeolite cations, generally Na+ or K+,can be ion-exchanged with aqueous solutions of metal ions that are desired to be incorporated into the zeolite lattice. The importance of the ion-exchange technique is that redox-active species can be easily loaded into the crystalline host allowing oxidative polymerization reactions to take place within cavities. We have chosen L zeolite which has a structure of parallel channels? with the goal of constraining the polymer chain growth process to one dimension, thereby minimizing the Occurrence of uncontrolled ‘coiling” throughout the whole zeolite lattice. To provide the zeolite host with the required redox-active species we Enzel, P.;Bein, T. Synth. Met. 1989, 29, E163. Enzel, P.;Bein, T. J. Phys. Chem. 1989, 93, 6270. Enzel, P.;Bein, T. J. Chem. Soc., Chem. Commun. 1989, 1326. Chao, T. H.; Erf, H. A. J. Caral. 1986, 100, 492. Dutta, P.K.;Puri, M. J. Cafal. 1988, I l l , 453. Molecular Elecfronic Devices I& Carter, F. L., Ed.; Marcel Dekker: New York, 1987. (7) Newsam, P. J. J. Phys. Chem. 1989, 93, 7689. (1) (2) (3) (4) (5) (6)
proceeded in a different (and somewhat more complex) way. Very small Pd8 and SnO, clusters are deposited by impregnation techniques and will be shown to act, in all probability, as oxygen activators rather than oxidants per se. There are at least two motivations for choosing such peculiar zeolite environments. First of all, we have reason to believe that, when performed within zeolite cavities, these five-member heterocycles polymerizations may be viewed as rather facile reactions, i.e., they may occur fairly easily without requiring very restrictive polymerization conditions. For example, Caspar et al.9 have recently reported the oligomerization of thiophene in zeolite Na-8 without intentionally introducing oxidant cations. Although in that paper the nature of the redox process appeared unclear, monomer units were not reported to remain after the zeoliteloading step, suggesting that the polymerization might not be solely due to a stoichiometric reaction of the monomer with some zeolite trace impurity. Second, one can speculate that the presence of metal and metal oxide clusters (or any other species) randomly distributed in the L-zeolite channels may play some role in the macroscopic properties of the final material, e.g., conductivity, since they might well introduce doping-like effects. The latter (8) Larsen, G.; Haller, G. L. To be published: Proc. 1OfhInr. Congr. on Cafal.,Budapest, July 1992. (9) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J . Am. Chem. SOC. 1991, 113, 600.
0022-3654,I92 12096-4145S03.00,IO 0 1992 American Chemical Society -I
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