The Journal of
Physical Chemistry
0 Copyright, 1989, by the American Chemical Society
VOLUME 93, NUMBER 15 JULY 27, 1989
LETTERS Magic Numbers in Kinetic Energy Releases for Unimolecular Decompositions of (NH,), H+ Ion Clusters Chava Lifshitz* and Frank Louage Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: March 28, 1989)
The average kinetic energy ( T ) released during unimolecular decomposition of protonated ammonia ion clusters, (NH3)*H+, has been measured as a function of cluster size for n = 2-8. There is a pronounced discontinuity in the generally rising function of ( T ) with n, reflecting the filling of the first solvation shell at n = 5 .
Introduction Cluster ions have aroused great interest in recent Mass spectra of clusters very often demonstrate pronounced abundances for selected ions of special stability. These are the so-called magic numbers in ionized cluster distributions. It has been demonstrated that cluster ions produced by ionization of neutral clusters are metastable toward dissociation. This has been demonstrated for protonated ammonia cluster ions, (NH3)"H+,which are formed by multiphoton ionization of N H 3 cluster^.^^^ As many as six NH3 molecules have been observed to dissociate ("evaporate") from the protonated nonamer in the time scale of microseconds to tens of microsecond^.^ These observations have enabled rates of unimolecular dissociation to be derived and studies to be made of the origin of certain magic numbers in the ammonia system, most notably the protonated ent tamer.*,^ The special stability of the protonated pentamer is due to the completion of the inner solvation shell around the NH4+ ion and has been noticed also ( 1 ) Mark, T. D. Inr. J . Mass Spectrom. Ion Processes 1987, 79, 1. (2) Castleman, A. W., Jr.; Keesee, R. G. Chem. Reu. 1986, 86, 589. (3) Shinohara, H.; Nishi, N. Chem. Phys. Lett. 1982, 87, 561. (4) Echt, 0.;Dao, P. D.; Morgan, S.; Castleman, A. W., Jr. J . Chem. Phys. 1985, 82, 4076.
0022-36S4/89/2093-S633$01 .SO/O
for hydrated ammonia cluster ions.5 Pulsed high-pressure mass spectrometry has demonstrated the filling of the solvent shell for the protonated ammonia pentamer by a discontinuous drop in the attachment energy for the sixth NH3 m ~ l e c u l e . Furthermore, ~~~ distinct solvent shells have been distinguished for (NH3),,H+by ab initio calculations.s Several mechanisms have been proposed for the dissociation of metastable cluster ions:9 electronic predissociation, tunneling through a centrifugal barrier, and vibrational predissociation. Vibrational predissociation is attractive since it can be treated by statistical theories such as RRKM/QET.'O Whether vibrational predissociation is dominating on all time scales is not known; also, (5) Shinohara, H.; Nishi, N.; Washida, N. Chem. Phys. Lett. 1988, 153, 417. (6) Payzant, J. D.; Cunningham, A. J.; Kebarle, P. Can. J . Chem. 1973, 51, 3242. (7) Meot-Ner (Mautner), M.; Speller, C. V. J . Phys. Chem. 1986, 90, 6616. ( 8 ) Deakyne, C. A. J . Phys. Chem. 1986, 90, 6625. (9) Mark, T. D. In Electronic and Atomic Collisions; Gilbody, H. B.,
Newell, W. R., Read, F. H., Smith, A. C. H. Eds.; Elsevier Science Publishers: Amsterdam, 1988; p 705. (10) For a recent review on the current status of the theory, see: Lifshitz, C. Adu. Mass Spectrom., in press.
0 1989 American Chemical Society
5634 The Journal of Physical Chemistry, Vol. 93, No. IS, 1989
Letters
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Figure 1. Mass spectrum of protonated ammonia clusters; high-pressure ion source conditions: -105 O C , 0.25 Torr of ammonia, ionizing electron energy 100 eV.
the contribution of kinetic energy release to energy disposal and the dependence of the extent and rate of dissociation on the initial size of the cluster are poorly understood." An increase in kinetic energy release with increasing cluster size has k e n found in several different some of which have been modeled by statistical theories. The kinetic energy release distributions for the metastable fragmentation of nascent ion/molecule clusters such as NH3.NH4+were modeled by using statistical phase space theory. In this Letter we report on kinetic energy release distributions (KERDs) and average kinetic energy releases (KERs) as a function of cluster size for the protonated ammonia cluster reaction series (NH3),H+ (NH3),,..lH+ N H 3 where n = 2-8. It will be demonstrated that the solvent shell structure becomes apparent in the KERs. The statistical nature of the reactions will be discussed.
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Experimental Section Measurements were performed on a high-resolution doublefocusing mass spectrometer of reversed geometry-the VG ZAB-2F.'9*20 Ions are formed by electron impact in a temperature- and pressure-variable source.20~21The ionizing electron energy was 100 eV. A unique aspect of this source is that the electron beam is coaxial with the ion exit slit. The electron entrance aperture is a hole 0.05 cm in diameter, and the ion exit slit dimensions are 0.01 X 1.5 cm. An extraction voltage of 10 V was applied to the ion exit plate. Ions move through the source at constant velocity due to the presence of a constant drift potential gradient. The ion source was cooled to 168 K in order to promote clustering. The pressure employed in most experiments was 0.25 Torr, measured with a capacitance manometer. Somewhat lower pressures were employed to maximize the dimer and trimer abundances. The metastable fragmentations were studied by mass-analyzed ion kinetic energy spectrometry The ion accelerating voltage was 7 kV. Most of the experiments were carried out at an energy resolution E / A E 1 7000. The background pressure in the second field-free region (2-FFR), where the ( 1 I ) Castleman, A. W., Jr.; Keesee, R. G. Science 1988, 241, 36. ( 1 2 ) Stace, A . J.; Shukla, A . K. In!. J . Mass Specfrom.Ion Phys. 1980, 36, 119 .
(13) Stace, A. J.; Moore, C. Chem. Phys. Lett. 1983, 96, 80. (14) Stace, A . J.; Shukla, A. K. Chem. Phys. Left. 1982, 85, 157. ( I 5 ) Illies, A. J.; Jarrold, M. F.; Bowers, M. T. Int. J . Mass Spectrom. Ion Phys. 1983, 47, 93. (16) Stace, A. J. J . Chem. Phys. 1986, 85, 5774. ( 1 7 ) Iraqi, M.; Lifshitz, C. Int. J . Mass Spectrom. Ion Processes 1989,88, A._. 4
(18) Illies, A. J.; Jarrold, M. F.; Bass, L. M.; Bowers, M. T. J . Am. Chem. SOC.1983, 105, 5 7 7 5 . (19) Morgan, R. P.; Beynon, J. H.; Bateman, R. H.; Green, B. N. In!. J . Mass Spectrom. Ion Phys. 1978, 28, 171. (20) Kirchner, N. J.; Bowers, M. T. J . Phys. Chem. 1987, 91, 2573. ( 2 1 ) van Koppen, P. A. M.; Kemper, P. R.; Illies, A. J.; Bowers, M. T. Inf. J . Mass Spectrom. Ion Processes 1983, 54, 2 6 3 . ( 2 2 ) Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Ions; Elsevier: Amsterdam, 1973.
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Figure 2. Metastable ion peak shapes: (a) (NH&Ht NH4++ NH3; (b) (NH3)sH* (NH3)4H*+ NH3. Ion intensity is plotted versus the electrostatic energy analyzer (ESA) voltage. The main beam passed a t an ESA voltage of 7 1 8 . 4 V ( 6 8 4 2 - e V ion energy).
metastable reactions of interest occur, was 13.8 X Torr as indicated on an ion gauge mounted at the entrance to one of the diffusion pumps. The metastable products are mass and energy analyzed by an electrostatic analyzer (ESA) and detected by using single ion counting. Ion counting is achieved by a combination of an electron multiplier, amplifier/discriminator, and multichannel analyzer. The electron multiplier is mounted off-axis, the positive ion beam being converted to electrons by collision with a negatively charged conversion dynode.23 Multiplier current is converted to a TTL compatible 20-11s pulse by means of an SSR instrument Model 1120 amplifier/discriminator, and the data are accumulated by an EG&G Ortec MCA Model 5604. Pulses are counted for the duration of the channel dwell time, 0.5-8 ms per channel. Up to 1500 repetitive scans were collected for low-intensity metastable ions in one single experiment. The metastable time window for the 2-FFR region is (enter) 4.38 X lod s and (exit) 9.66 X IO" s for (NH3)2H+and (enter) 8.65 X s and (exit) 19.05 X s for (NH3)*H+. Results and Discussion A typical cluster ion mass spectrum is represented in Figure 1. The pentamer ion (NH3)5H+is the most intense cluster ion observed. All of the nascent cluster ions formed in the highpressure ion source, (NH3),H+, n = 2-8, were observed to undergo dissociation in the 2-FFR, "evaporating" one ammonia molecule each. The tetramer also demonstrated the loss of a second ammonia molecule and the pentamer up to three ammonia molecules. (23) Holmes, J. L.; Szulejko, J. E. Org. Muss Spectrollt. 1983, 18, 273.
l
~
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5635
Letters
(NHJ"H+-
(NHJ"-, H++ NH,
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i
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'
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Figure 4. Average kinetic energy releases ( T ) in meV for (NH3),H+ (NH,),,H+ + NH,,as a function of cluster size n. The data including error bars represent results deduced from several independent experiments.
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KINETIC E N E R G Y , e V
Figure 3. Kinetic energy release distributions (KERDs) for the reactions (NH3)"H+ (NH3)-,H+ + NH, (n = 2-8) taking place in the second field-free region of the Z A B - 2 F (a) n = 2, 3 , 4 , 5 ; (b) n = 5 , 6 , 7 , 8. The data for n = 5 are reproduced in both panels for comparison.
Metastable peak shapes were measured for the reactions involving the loss of a single ammonia molecule. That for the dimer was the least intense metastable peak and demonstrated a weak broad underlying peak due to collision-induced dissociation (CID). A small contribution from CID was also observed for the trimer. Typical results for the dimer and pentamer are represented in Figure 2. KERDs were obtained from the first derivatives of the metastable ion peak and are represented in Figure 3. Average KERs (( T ) )were obtained from the metastable peak shapes of several repetitive experiments, and their dependence on cluster size, n, is represented in Figure 4. The value for the dimer reaction, ( T ) = I . 1 meV, is in excellent agreement, within experimental errors, with the previous values obtained by Bowers and c ~ - w o r k e r s . I ~ ~ ~ ~ The average KER is observed to increase with cluster size, as has previously been observed for other system^.'^-'^ The unique feature of the present results, which to the best of our knowledge has not been observed before, is the marked discontinuity in ( T ) versus n for n = 6. A decrease in ( T ) is observed at this point, where the second solvation shell begins and a second rise of ( T ) with n occurs for n I 7 . These data demonstrate quite clearly the statistical nature of the dissociations. As has been noted earlier,I4 the nature of the experiment is such that metastable peaks sampled arise from the decomposition of ions, not with fixed values of excess energy, but with lifetimes which lie within a specific range. The rise of (7')with n reflects the higher internal energy, E , necessary to bring about dissociation in the microsecond time (24) Holmes, J. L.; Osborne, A. D. Int. J . Mass Spectrom. Ion Phys. 1977, 23, 189. (25) Lifshitz, C.; Tzidony, E. Int. J. Mass Spectrom. Ion Phys. 1981, 39, 181.
(26) Jarrold, M. F.; Wagner-Redeker, W.; Illies, A. J.; Kirchner, N. J.; Bowers, M. T. Int. J . Mass Spectrom. Ion Processes 1984, 58, 63.
frame, in the larger sized clusters. This is easily understood for the (C02),+ system,I4 since the binding energy is independent of the cluster size.27 For the same critical energy of dissociation Eo,there is a larger density of vibrational states in the reactant ion as n increases and therefore a reduced microcanonical rate coefficient k ( E o ) . For proton bound clusters, such as (CH30H),H+,17the binding energy decreases with increasing cluster size.28,29For ( T ) to increase with increasing n, the degrees of freedom effect on reducing k(Eo) has to overcome the effect of rising k(Eo) due to lowering of the critical energy, Eo. A monotonically rising function of ( T )with n reflects a continuously smooth drop in binding energy. However, for (NH3),H+ there is a discontinuity in the binding energy curve versus n, with a sharp drop between n = 5 and 66*7due to the completion of the first solvation shell. This is reflected in the discontinuity in the rise of ( T ) with n, between n = 5 and 6 (Figure 4). The increasing number of degrees of freedom is unable to compensate for the rise of k(Eo)due to the much lower critical energy for boiling off the sixth ammonia molecule. As a result, a lower excess energy is required to bring about dissociation of the hexamer in the 2-FFR than is required for the pentamer. The very low KERs observed in this study are characteristic for ion-dipole dissociations. We are planning theoretical modeling of these data. This is not an easy task due to the special nature of ion-permanent dipole interactions i n v o l ~ e d . The ~ ~ ~major ~~ problem in treating ion-dipole dissociations t h e o r e t i ~ a l l yhas ~~~~~ been the angular dependence of the interaction potential, which dictates employment of numerical procedures. These procedures have to be employed in conjunction with RRKM/QET or adiabatic channel theory.32 It is quite clear however that while rigorous modeling may be difficult, cluster ions are in principle better candidates for statistical modeling than ordinary molecule^.^^-^^ (27) Engelking, P. C. J. Chem. Phys. 1987, 87, 936. (28) Grimsrud, E. P.; Kebarle, P. J . Am. Chem. SOC.1973, 95, 7939. (29) Meot-Ner (Mautner), M. J . Am. Chem. SOC.1986, 208, 6189. (30) Shao, J.-D.; Baer, T.; Morrow, J. C.;Fraser-Monteiro, M. L. J . Chem. Phys. 1987,87, 5242. (31) Bass, L. M.; Bowers, M. T. J . Chem. Phys. 1987, 86, 2611. (32) Ruttink, P. J. A. J . Phys. Chem. 1987, 91, 703. (33) Engelking, P. C. Chem. Phys. Lett. 1987, 139, 6. (34) Klots, C. E. J . Chem. Phys. 1985,83, 5854. (35) Brechignac, C.; Cahuzac, Ph.; Leynier, J.; Weiner, J. J . Chem. Phys. 1989, 90, 1492.
