J. Phys. Chem. 1992, 96, 507-510
How Sensitive Are Cluster Compositions to Energetics? A Joint Beam Expansion/Thermochemical Study of Water-Methanol-Trimethylamine Clusters M. Samy El-Shall,* George M. Daly, Junling Gao? Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006
Michael Meot-Ner (Mautner), and L. Wayne Sieck Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: October 8, 1991; In Final Form: December 6, 1991)
We report the results of joint beam expansion and thermochemical studies of clusters containing ionic hydrogen bonds between water (W), trimethylamine (T), and methanol (M). The cluster distributions H+W,T, show magic numbers for (n, m) = (1, 3), (2,4), and (3, 5), where the solvent shells are completed by blocking groups around a H30+core ion. Larger clusters with n > 3 show deviation from the n + 2 rule, suggesting structural changes. In H+M,T, clusters, the H+M,T2 sequence is always more predominant than the H+M,,T series independent of expansion or ionization conditions. In the three component clusters, the predominant series is H+W,MmT,+2. The cluster distributions from the beam expansion are quite sensitive to the clusters' binding energies as measured by high-pressure mass spectrometry. Even though ionization involves large excess energies, the observed cluster distributions show sensitivity to thermochemical differences as small as 1-3 kcal/mol.
Introduction Several studies have been reported involving mixed clusters composed of a protic component (HzO, NH3, C H 3 0 H ) and a methyl-blocked aprotic component that has a higher proton affinity.l-s The first investigation was by Kebarle et al. on (CH3)20/H20and (CH3)20/CH30H,1followed by equilibrium and a b initio studies by Deakyne et a1.: and collision-induced dissociation studies by Lifshitz et al., on CH3CN/H20,2v3and (CH3)2CO/H20.4 Recently, Castleman and co-workers investigated (CH3)2CO/NH3,5!CHJ3N/NH3,6 and (CH3)3N/H20,7 and discussed the factors influencing the location of the proton as a function of constituent proton affinity. The general observation is that a hydrogen-bonded network is formed with an H30+ or NH4+ core ion, with additional H20 or NH3 molecules in the center and the blocked aprotic component at the periphery. The proton is associated with the species of the lower proton affinity. The blocking of the outer hydrogen-bonding positions results in strong shell effects, disfavoring the addition of further blocked components after all the hydrogen bonding sites are occupied. In clusters containing water and blocking molecules B, shell filling occurs at the compositions B,+2(H20),H+. We recently reported joint beam expansion and thermochemical studies of blocked hydrogen bonding in binary clusters of methanol and acetonitrile.8 In this combination, the hydrogen-bonded network incorporating CH30H allows only the formation of a chain, rather than a two- or three-dimensional network. Although the binary components have similar proton affinities (APA = 6 kcal/mol) and various mixed clusters are therefore close in energy, the cluster distribution showed an unexpected degree of sensitivity to subtle differences in thermochemical stability. This sensitivity to small differences (1-2 kcal/mol) was observed even when the first stages of the ionization and protonation processes are very exothermic. It is of interest whether the proton is still located on the protic component, and if similar shell effects occur, even when the components have very large proton affinity differences. For the present study we selected two binary systems, H20-(CH3)3N and CH30H-(CH3)3N, where the proton affinities between the components differ by 66.2 and 51 kcal/mol, respectively. In this Letter, we report the results from parallel measurements by beam expansion and high-pressure mass spectrometry (HPMS)on these clusters. The results show very strong correlation between the magic numbers observed in beam expansion and the stability of *To whom correspondence should be addressed. Permanent address: Guangzhou TelecommunicationBureau, P.R. China.
these ions measured under equilibrium conditions. We also report the first observation of magic numbers in ternary clusters composed of water, methanol, and trimethylamine.
Experimental Section Trimethylaminewater binary clusters were generated by pulsed adiabatic expansion in a supersonic cluster beam apparatus, ionized by electron impact (El) and mass analyzed by a quadrupole mass Equilibrium measurements were carried out using the NIST pulsed high-pressure mass spectrometer.lOJ1 The enthalpies (AHo) and entropies (ASo) of cluster addition and exchange reactions were derived from van't Hoff plots. The standard deviation in these plots is usually less than 0.5 kcal/mol. Results Figure 1 displays a typical 70-eV mass spectrum observed from a cluster beam generated by the expansion of (CH3)3Ncontaining traces of H 2 0 (about 0.01%) in H e carrier gas at a stagnation pressure of 2.5 bar and T = 295 K. The use of low concentrations of water allows the generation of (CHJ3N clusters containing few (1-4) water molecules. Upon E1 ionization most of the dopant molecules that are weakly bound to the cluster should be very labile to 'boiloff" since they would most likely be located near the terminal sites. Alternatively, water molecules located inside (CH3)3N clusters would be stable toward evaporation. The spectrum in Figure 1 shows mixed clusters of the type H+W,Tm with m 1 n. The maximum ion intensity within the n = 1 and 2 series occurs for H+WT3 and H+W2T4clusters, respectively. These features are observed over the entire range of vapor com(1) Hiraoka, K.; Grimsrud, E. P.; Kebarle, P. J. Am. Chem. SOC.1974, 96, 5359. (2) Deakyne, C. A.; Meot-Ner (Mautner), M.; Campbell, C. L.; Hughes, M. G.; Murphy, S. P. J. Chem. Phys. 1986,84, 4958. (3) Lifshitz, C.; Louage, F. J . Phys. Chem. 1989, 93, 5633. (4) Iraqi, M.; Lifshitz, C. Int. J. Mass. Spectrom. Ion Processes 1989,88, 4.
(5) Tzeng, W. B.; Wei, S.; Neyer, D. W.; Keesee, R. G.; Castleman, Jr., A. W. J. Am. Chem. SOC.1990, 112,4097. (6) Wei, S.; Tzeng, W. B.; Castleman, Jr., A. W. J. Chem. Phys. 1991, 95, 585. (7) Wei, S.; Shi, Z.; Castleman, Jr., A. W. J. Chem. Phys. 1991, 94, 3268. (8) El-Shall, M. S.; Olafsdottir, S. R.; Meot-Ner (Mautner), M.; Sieck, L. W. Chem. Phys. Lett. 1991, 185, 193. (9) El-Shall, M. S.; Marks, C. J . Phys. Chem. 1991, 95, 4932. (10) Meot-Ner (Mautner), M.; Sieck,L. W. J. Am. Chem. Soc. 1991, 113, 4448. (1 1) El-Shall, M. S.; Meot-Ner (Mautner), M. J. Phys. Chem. 1987, 91, 1088.
0022-365419212096-507$03.00/0 0 1992 American Chemical Society
508 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
tn an bn
an = H+W~T, bn = H+W2Tn
dn = H+M2Tn en = H+M,T"
Figure 1. Raw 70-eV E1 mass spectrum of a cluster beam generated from the expansion of a vapor mixture (100 mbar of (CH3)3Ncontaining 0.01% water) in 2.5 bar of He. Cluster notations: T and W correspond
to trimethylamine and water, respectively.
Figure 3. Raw 70-eV E1 mass spectrum observed from the expansion of 1:l:l H20-CH30H-(CH3)3Nvapor mixture in 2 bar of He. Cluster notations: T, M, and W correspond to trimethylamine, methanol. and
water, respectively. 20
+ Y ->
Figure 4. Van't Hoff plots for the clustering reactions as indicated. Cluster notations: T and M correspond to trimethylamine and methanol,
respectively. Numbr of W (n)
Numkr of 1 (m)
Cluster Number (n)
Figure 2. (a) Ion intensity distribution observed from the expansion of 1:l H20-(CH3)3N vapor mixture in 2 bar of He. (b) Ion intensity distribution for H+M,T and H+M,T2 series observed from the expansion of 1:l CH3OH-(CH&N vapor mixture in 2 bar of He.
positions (1-9976 water with respect to (CH,),N) and at different stagnation pressures (1-4 bar) and electron impact energies (1 6-70
eV). Maximum ion intensities in clusters containing more than two water molecules were observed for those generated from the expansion of 1:l vapor mixture of water and (CH3)3N in He. These distributions are shown in Figure 2a. It is evident from these plots that the H+W,T, clusters with n = 3-6 invariably exhibit significant drops in ion intensities past m = 5. The distributions observed for m e t h a n ~ l / ( C H ~ ) clusters ~N (displayed in Figure 2b) show that the H+M,T2 sequence is predominant over the H+M,+,T series. Again, this behavior is observed not only for 1:1 mixture but over a wide range of vapor composition (30-8076 T with respect to M), stagnation pressures, and electron impact energy, indicating that the H+M,T2 ions have a particularly stable structure independent of the distribution of the neutral clusters produced. Ternary clusters composed of water, methanol, and (CH3),N were formed by expanding a 1:1:1 vapor mixture diluted in H e carrier gas. Figure 3 displays a representative portion of a typical 70-eV E1 mass spectrum. The prominent peaks in the spectrum are due to cluster ions of the general formula H+W,M,T,,+2. In other vapor mixtures with lower concentrations of (CH3)3N,only the H+WM,T3 series is observed. We note that these sequences represent combinations of the maximum ion intensities observed in ~ a t e r / ( C H ~ )and ~ Nm e t h a n ~ l / ( C H ~ )binary ~ N clusters. In order to define the origin of the magic numbers observed in the beam experiments, we used pulsed high-pressure mass spectrometry to derive the enthalpies and the entropies of the
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 509
TABLE I: -ASoValues As Obtained from van't Hoff Plots for the Clustering R~ctiotm"
-M0,cal/(mol K) 24.4 21.0 27.2 24.3 25.2 23.3 23.8 27.0
TWH' 3 TMH' 5 T2H' T2H' T2WH' 3 T2MH' 5 T3H+ TMH' Y TM2H' TM2H' TM3H'
"Cluster notations: T, M, and W correspond to trimethylamine, methanol, and water, respectively. clustering and exchange equilibria. Figure 4 shows some of the van't Hoff plots and Table I lists the entropies for some clustering reactions. Figure 5 summarizes the results for the enthalpies of clusters containing up to four molecules. Analogous data for higher clusters will be reported elsewhere.l2
Discussion The magic numbers observed in Figure 1-3 are consistent with shell filling structures around a core ion such as H 3 0 + or CH30H2+. For example, in the H+W,T, clusters, the magic numbers (1, 3), (2, 4), and (3, 5 ) can be attributed to blocked hydrogen-bonded structures with H 3 0 + as the core ion. The structure of the simplest ion in this series (1, 3) is shown below.
In this symmetric structure, the positive charge is equally distributed among the three hydrogen atoms, thus forming three equivalent ionic hydrogen bonds to trimethylamine molecules. Recently, this magic number has been observed by Castleman and co-workers in the MPI of ~ a t e r / ( C H ~ )binary ~ N clusters.' Our results indicate that the cluster ions H+W,TH2 with n = 1-3 are stable under the conditions of E1 ionization (17-90 eV) and therefore provide further evidence for the proposed structure with H 3 0 + as the core ion. Similarily, the enhanced ion intensity of the clusters H+M,T2 and H+W,M,T,+2 can be explained by analogous blocked structures as shown below.
n+ / -\
These configurations allow the formation of higher clusters through the incorporation of hydrogen-bonded methanol or water chains to the protic sites of the core ion. The incorporation of methanol or water will generate the H+WM,T3 or H+W,MT,+z series, respectively, which are the predominant ions observed in the mass spectrum of the ternary clusters (Figure 3). The equilibrium data given in Figure 5 indicate that within the (CH3),N containing clusters, the H'T2 dimer exhibits the strongest bonding in this network, with a binding energy typical of symmetric NH+N dimers. Because of the stability of the dimer, and the high proton affinity of (CH3)3N,the insertion of HzO to form H+WT2is exothermic only by 8.9 kcal/mol, which is much less, for example, than in the analogous insertion to form H+W(CH3CN)2, where AZP = -15.9 kcal/mo12. The energetics confirm that in the reverse direction, the energy required to lose (12) El-Shall, M.S.;Daly, G. M.; Mmt-Ner (Mautner), M.; Sicck, L. W. Manuscript in preparation.
W Figure 5. -AHo for clustering and exchange equilibria. Units are kcal/mol. Cluster notations: T, M, and W correspond to trimethylamine, methanol and water, respectively. Results are from present work unless noted otherwise; underlined values are calculated from cycles; a, ref 10; b, ref 15; c, average literature data; d, ref 16; e, ref 6; f, from AGO(225) = -2.5 kcal/mol; g, ref 17.
(CH3)3Nand HzO molecules from larger clusters in the beam, to form the H+Tzdimer is small. Therefore, the dimer constitutes a large peak in the cluster distributions in the beam studies, as shown in Figure 1. The stabilizing effect of a strong hydrogen bond network is reflected in the energetics involved when exchanging (CH3)3N for HzO molecules in the three-membered and four-membered clusters. In the thresmembered clusters, T2WH+> TWzH+,and in the four-membered clusters, T3WH+ > T2W2H+> TW3H+. The exothermicities of each T/W substitution in these series are 5 and 3 kcal/mol, respectively. It is interesting that these small differences are reflected in the cluster distributions. It is also of interest that the exothermicity of the T/W exchange in the mixed clusters is so small, despite the difference of 66.2 kcal/mol between the proton affinities of the components. This is due to the large stabilization of the proton, and the charge dissipation, by the first (CH3)3N molecule, which diminishes the effects of the further ligands. This situation is observed in Figure 5 which shows that the exchange reactions H+W, H+W,,T are exothermic by 47, 38, and 30 kcal/mol for n = 2, 3, and 4, respectively. Once the hydrogen-bond networks are blocked, further (CH&N for H 2 0 exchange becomes unfavorable. In the threemembered clusters, the exchange from H+WT2is endothermic by 2 kcal/mol, and in the four-membered clusters, the exchange from H+WT3 to H+T4is endothermic by 6.5 kcal/mol. Correspondingly, the blocked clusters are small as shown in Figures 1-3. Once again, this means that larger H+WT, clusters in the beam with m > 3 tend to lose (CH3)3N molecules to form stable species such as H+WT3 which are more resistant to further evaporation.
J . Phys. Chem. 1992, 96, 510-513
The thermochemistry also shows that the enthalpy change for methanol addition to H+T2to form H+MT2is more exothermic than for the analogous association of water to generate H’WT2. However, this trend is reversed in considering the addition to H+T3 to form H+MT3 or H+WT3. This condition is also reflected in the ternary cluster ion distribution resulting from the expansion of a 1:l:l mixture (Figure 3), which indicates that the ion intensities for H+MT2 and H+WT3 are greater than those for H+WT2 and H+MT3, respectively. The energetic effects associated with cluster composition are consistent with (CH3)3Nmolecules binding to a H30+core ion. The cluster distributions and magic numbers observed in the beam expansion are also consistent with clusters incorporating H 2 0 in the center and (CH3)3Non the periphery. Therefore, the present results provide evidence for the thermodynamic origin of magic numbers in this system. The distributions are therefore affected by small differences, 1-5 kcallmol, in the relative stabilities of the clusters, despite the substantial energy deposited during electron impact ionization. This probably reflects the results of the last steps in consecutive evaporation, where the first steps remove most of the energy. The last step that results in the observed cluster distributions occurs from clusters with small internal energies, and small differences in the required endothermicity for the various competitive channels may be significant. The correlation between cluster distributions and energies can be used to calculate the rate constants for consecutive cluster evaporation and to test different evaporation m0de1s.l~ (13) Klots, C. E. J. Phys. Chem. 1988, 92, 5864.
The distributions also reveal that the (n + 2) rule breaks down for large clusters. In these cases, the H20core may start to form cyclic structures, making less hydrogens available for bonding to (CH3),N. This effect is observed in the distributions given in Figure 2a. For clusters containing five or more water molecules, sharp decreases in ion intensities occur when the cluster contains more than five (CHJSN molecules, suggesting the formation of a cyclic water pentamer with an extra labile proton. The formation of cyclic structures in large ions was suggested by Newton and Ehrenson,I4and Deakyne calculated that many isomeric structures, presumably also cyclic ones, can be very close in energy to the most stable structure in large clusters2 More recently, Castleman and co-workers have made similar observations on cluster distributions following MP17 and have suggested the formation of cyclic structures for H+W20and H+W2,.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Thomas F. and Kate Miller Jeffress Memorial Trust for the partial support of this research. L.W.S. was supported by the Division of Chemical Science, Offrce of Basic Energy Science, US.Department of Energy. (14) Newton, M. D.; Ehrenson, S. J . Am. Chem. SOC.1971, 93, 4971. Newton, M. D. J . Chem. Phys. 1977,67, 5535. (15) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref Data 1984, 13, 685. (16) El-Shall, M. S.; Marks, C.; Sieck, L. W.; Meot-Ner (Mautner), M. J . Phys. Chem., in press. (17) Meot-Ner (Mautner), M. J . Am. Chem. Sac. 1989, 106, 1265.
Scanning Tunneling Microscopy of Cgoand CT0on Ordered Au( 111) and Au( 110): Molecular Structure and Electron Transmission Yun Zhang, Xiaoping Cao, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: October 1 1 , 1991; In Final Form: November 25, 1991)
Scanning tunneling microscopy (STM) images are reported for Csoand C70 layers on ordered Au( 111) and Au( 110) surfaces in air and in aqueous 0.1 M HC104. Hexagonal closepacked layers were obtained for both Cso and C70on Au( 111); however, the latter features distortions consistent with the presence of groups of ‘standing-up” and “lying-down” C70 orientations. While the fullerene layers are less ordered on Au( 1lo), the molecules are more rigidly held and can yield STM images with resolved intramolecular carbon rings. The likely modes of tipsurface electron tunneling via the adsorbed fullerenes, involving ‘superexchange” coupling, are discussed briefly in light of the observed STM imaging properties.
Recent reports of a straightforward procedure for synthesizing macroscopic quantities of C60(buckminsterfullerene) along with C701’2have triggered an intense broad-based effort aimed at elucidating the physical and chemical properties of these remarkable new allotropes of carbon. Substantial interest has been generated from an early stage in the scanning tunneling microscopy (STM) of Ca layers on metal and semiconductor surfaces, in both (1) (a) Kratschmer, W.; Fostiropoulous, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (b) Kratschmer, W.; Lamb, L. D.; Fostiropoulous, K.; Huffman, D. R. Nature 1990, 347, 354. (2) (a) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, N. E.; Flanagan, S.; Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, 2.;Billups, W. E.; Cuifolini, M. A,; Hauge, R. H.; Margrave, J. L.; Wilson, L. F.; Curl, R. F.; Smalley, R. E. J . Phys. Chem. 1990, 94, 8634. (b) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kratschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990,94,8630. (c) Parker, D. H.; Wurz, P.; Chatterjee, K.; Lykke, K. R.; Hunt, J. E.; Pellin, M. J.; Hemminger, J. C.; Gruen, D. M.; Stock, L. M. J. Am. Chem. SOC.1991, 113, 7499.
air and ultrahigh-vacuum (uhv) environments.” Related studies utilizing atomic force microscopy (AFM) have also been rep ~ r t e d . ~Besides ,~ seeking information on intermolecular packing (3) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Voies, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990,348,621. (4) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kritschmer, W.; Huffman, D. R. Nature 1990, 348, 623. ( 5 ) (a) Li, Y. 2.;Patrin, J. C.; Chandler, M.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991,252,547. (b) Li, Y. 2.;Chander, M.; Patrin, J. C.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991, 253, 429. (6) (a) Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Sarid, D.; Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D. J . Vac.Sci. Technol., in press. (b) Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Sarid, D.; Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D. Mater. Res. Sac. Symp. Proc. 1991, 206, 721. (7) Snyder, E. J.; Anderson, M. S.; Tong, W. M.; Williams, R. S.; Anz, S. J.; Alvarez, M. M.; Rubin, Y.; Diederich, F. N.; Whetten, R. L. Science 1991, 253, 171. (8) Sand, D.; Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Lichtenberger, D.L.; Nebesney, K. W.; Ray, C. D. Appl. Phys. Lett., in press.
0 1992 American Chemical Society