J . Phys. Chem. 1989, 93, 5903-5906
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Semiequilibrium Dialysis Study of the Solubilization of Benzoate Anion by Aqueous Hexadecylpyridinium Chloride Faten Z. Mahmoud,t,l Sherril D. Christian,*,*,§Edwin E. Tucker,*,§Ahmed A. Taha,t'* and John F. Scamehorng-l Department of Chemistry, University College f o r Girls, Ain Shams University, Cairo, Egypt, and Department of Chemistry, Institute f o r Applied Surfactant Research, and School of Chemical Engineering, University of Oklahoma, Norman, Oklahoma 73019 (Received: January 17, 1989; In Final Form: March 22, 1989)
Semiequilibrium dialysis (SED)measurements have been used to investigate the solubilization and binding of benzoate anion by micelles of the cationic surfactant hexadecylpyridiniumchloride. The equilibrium results are correlated with an ion-binding model described previously, with an additional parameter introduced to account for the solubilization of benzoate at least partly within the micelles. The benzoate ions bind, as expected, as counterions to the positively charged micelles, and in addition, they appear to solubilize similarly to neutral phenols and aromatic carboxylic acids, with the polar group anchored in the head-group region of the micelles and the aromatic moiety extending into the hydrophobic micellar interior.
Introduction Recent reports from our laboratories have indicated the effectiveness of colloid-enhanced ultrafiltration methods in removing either multivalent ions or organic solutes from aqueous streams.'-" Micellar-enhanced ultrafiltration (MEUF) makes use of the fact that aqueous surfactant micelles, at concentrations greater than the critical micelle concentration (cmc), can bind or solubilize target solute species and prevent them from passing through ultrafiltration membranes having molecular weight cutoff values in the range 1000-50000. In several previous studies, it has been shown that a simple experimental method, semiequilibrium dialysis (SED), can be used to determine the extent of binding of ions and organic solutes to surfactant micelles and that equilibrium binding isotherms obtained by SED can be used to predict ultrafiltration res~Its.~*~J2,'3 We thought it would be of interest to investigate the binding of an organic anion (benzoate) by positively charged surfactant micelles, because the removal of a charged organic solute by oppositely charged micelles could potentially involve both counterion binding at the micellar surface and solubilization analogous to that obtained with polar, but neutral, solute molecules. Hexadecylpyridinium chloride (also called cetylpyridinium chloride and abbreviated as CPC) was chosen as the aqueous cationic surfactant, primarily because studies had been made previously of the solubilization of numerous organic solutes by CPC7*12-'7 and of the binding of a divalent counterion (GO:-) to the micellar surface.' It was hoped that the results of these studies would be useful in interpreting the binding and/or solubilization of benzoate and other organic ions by CPC micelles. The present paper reports semiequilibrium dialysis data at 25 "C, for sodium benzoate at molarities in the retentate varying from 0.013 to 0.067 M, at CPC concentrations varying from 0.04 to 0.21 M, and at NaCl concentrations varying from 0 to 0.1 1 M. Results are correlated with an ion-binding model introduced modified to include a parameter to account for the partitioning of benzoate anion between the electrical double-layer region outside the micelle and the micellar interior. The model provides a good correlation of all the data. Experimental Section Sodium benzoate, from Aldrich Chemical Co., and cetylpyridinium chloride, from Hexcel Corp., were used as received. Solutions were prepared with redistilled, deionized water; ordinary equilibrium dialysis cells (Fisher Scientific) were used with regenerated cellulose membranes (6000 molecular weight cutoff).
'Ain Shams University.
*Department of Chemistry, University of Oklahoma. Institute for Applied Surfactant Research, University of Oklahoma. School of Chemical Engineering, University of Oklahoma.
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Membranes were washed thoroughly in distilled water for at least 30 min prior to use. Semiequilibrium dialysis experiments were performed as described p r e v i o ~ s l y , ~ ~ 'allowing ~ - ' ~ ~ ' ~18-24 h at 25 "C for the attainment of equilibrium with respect to the solubilized benzoate anion. Initially, an aqueous solution containing known concentrations of sodium benzoate, CPC, and NaCl (if present) is placed on one side of the membrane (in the retentate compartment), and the other compartment (the permeate) is filled with either pure water or aqueous NaCl at the same concentration as in the retentate. At semiequilibrium, the electrolytes (sodium chloride and sodium benzoate) are assumed to be present at the same thermodynamic activities in the solutions on both sides of the membrane, although the thermodynamic activity of the surfactant will continue to be somewhat greater in the retentate than in the permeate. The concentrations of benzoate and CPC in the permeate solution were determined simultaneously from ultraviolet (1) Christian, S. D.; Bhat, S. N.; Tucker, E. E.; Scamehorn, J. F.; ElSayed, D. A. AIChE J . 1988, 34, 189. (2) Sasaki, K. J.; Burnett, S. L.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1989, 5, 363. (3) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 257. (4) Scamehorn, J. F.; Ellington, R. T.; Christian, S. D.; Penney, W.; Dunn, R. 0.;Bhat, S. N. AIChE Symp. Ser. 1986, 82, 48. ( 5 ) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1987, 22, 763. (6) Gibbs, L. L.; Scamehorn, J. F.; Christian, S. D. J . Membr. Sci. 1987, 30, 67. (7) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. In Use of Ordered Media in Chemical Separations; Hinze, W. L., Armstrong, D. W., Eds.; ACS Symposium Series 342; American Chemical Society: Washington, DC, 1987; p 184. (8) Scamehorn, J. F.; Christian, S.D.; Ellington, R. T. In SurfactantBased Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Dekker: New York, in press; Chapter 2. (9) Christian, S. D.; Scamehorn, J. F. In Surfactant-Eased Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Dekker: New York, in press; Chapter 1. (10) Dunn, R. O., Jr.; Scamehorn, J. F.; Christian, S. D. Colloids and SurJ 1989, 35, 46. (1 1) Bhat, S. N.; Smith, G. A.; Tucker, E. E.; Christian, S. D.; Scamehorn, J. F.; Smith, W. Ind. Eng. Chem. Res. 1987, 26, 1217. (12) Christian S. D.; Smith, G. A,; Tucker, E. E.; Scamehorn, J. F. Langmuir 1985, 1, 564. (13) Christian S. D.; Smith, G. A,; Tucker, E. E.; Scamehorn, J. F. J . Solution Chem. 1986, 15, 519. (14) Higazy, W. S.; Mahmoud, F. Z.; Taha, A. A.; Christian, S. D. J . Solution Chem. 1988, 17, 191. (15) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1987, 3, 598. (16) Bhat, S. N.; Smith, G. A.; Tucker, E. E.; Christian, S. D.; Scamehorn, J . F.; Smith, W. Ind. Eng. Chem. Res. 1987, 26, 1217. (17) Christian, S. D.; Tucker, E. E.; Smith, G. A,; Bushong, D. S. J . Colloid Interface Sci. 1986, 113, 439.
0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989
TABLE I: Solubilization Results for Sodium Benzoate in Hexadecvlovridinium Chloride Micelles [Na BzIrel,L7 M 0.021 75 0.021 73 0.03 I 92 0.019 79 0.019 83 0.066 56 0.035 99 0.036 03 0.020 73 0.038 25 0.038 06 0.020 69 0.031 32 0.02007 0.020 12 0.020 73 0.01 2 97 0.01 2 99 0.01 7 69 0.018 16 0.01774 0.064 68 0.065 68 0.030 93 0.022 52 0.022 37 0.022 46 0.01282 0.01283 0.012 86 0.026 00 0.026 04 0.026 05 0.01429 0.01432 0.046 09 0.022 36 0.002 29 0.022 61
Wac11 M 0
0 0 0 0 0
0
0 0 0 0
0 0 0 0
0 0 0 0.047 89 0.047 89 0.047 89 0.086 90 0.086 90 0.04277 0.070 20 0.070 20 0.070 20 0.040 I O 0.040 I O 0.040 I O 0.1 I O 1 0.1 I O 1 0.1 I O 1 0.060 85 0.060 85 0.1067 0.052 87 0.05287 0.528 87
[CPCI
M 0.2070 0.2070 0.1667 0.1035 0.1034 0.1988 0.1085 0.1085 0.0623 0.0980 0.0980 0.0623 0.0632 0.05 18 0.05 18 0.063 1 0.0678 0.0678 0.0392 0.0393 0.0393 0.0921 0.0922 0.0662 0.0837 0.0838 0.838 0.0473 0.0172 0.0473 0.0930 0.0930 0.0930 0.05 13 0.0513 0.1007 0.0499 0.0499 0.0499
[NaBzl,,, M observedd predicted‘
0.000 093 0.000 1 13 0.000 246 0.000 182 0.000 143 0.000679 0.000 554 0.000556 0.000 326 0.000684 0.000 7 16 0.000 325 0.000 857 0.000 386 0.000 388 0.000 326 0.000 14 1 0.000 117 0.001 705 0.001 238 0.001 663 0.005 870 0.004 888 0.003 785 0.000 770 0.000914 0.000832 0.000488 0.000 478 0.004 446 0.001 137 0.001 096 0.001 085 0.000 699 0.000 672 0.002 452 0.001 698 0.001 767 0.001 444
0.000095 0.000 096 0.000 242 0.000 152 0.000 153 0.000 948 0.000 5 I6 0.000518 0.000 304 0.000 688 0.000 680 0.000 303 0.000 839 0.000 366 0.000369 0.000 299 0.000 I O 1 0.000 101 0.001 226 0.001 284 0.001 228 0.006 501 0.006 757 0.001 542 0.000 835 0.000826 0.000831 0.000488 0.000489 0.00 489 0.001 245 0.001 248 0.001 249 0.000 694 0.000 696 0.002 886 0.001 406 0.001 398 0.001 438
“Final molarity of sodium benzoate in retentate. bMolarity of sodium chloride initially in retentate and permeate. ‘Final molarity of hexadecylpyridinium chloride in retentate. Measured final molarity of sodium benzoate in permeate. ‘Predicted final molarity of sodium benzoate in permeate, from model described in text.
spectral measurements at several wavelengths; concentrations of surfactant and benzoate in the retentate were inferred from initial concentrations and known amounts of these solutes transferred into the permeate compartment.
Results and Data Analysis Table 1 includes the experimental SED results for aqueous solutions containing known concentrations of both CPC and sodium benzoate. Values of sodium benzoate concentrations in the permeate, calculated from the ion-binding model (vide infra), are included in the last column of the table. Ion-Binding Model. Previously, we utilized simple counterion-binding theories to correlate results of ultrafiltration and dialysis experiments in which divalent ions are removed from an aqueous solution.’S2 A fraction of the ions are bound to the surface of positively or negatively charged colloids, comprising either ionic surfactant micelles or polyelectrolytes; effects of added 1 :1 electrolytes (normally NaC1) are also considered. Our model incorporates a two-state binding theory proposed by Oosawa’* to divide the counterions into a fraction closely associated with the surface of the colloid (as bound counterions) and a fraction existing more or less free in the ”bulk” aqueous phase. Equation 1 derived from Oosawa’s theory can be used to predict the degree of dissociation (0)of counterions from a spherical (18) Oosawa, F. Polyelectrolytes; Dekker: New York, 1971
Mahmoud et al. polyelectrolyte (here, the CPC micelle), for ions having an absolute value of charge equal to z (for chloride, z = 1 ) ; the parameter 4 represents the volume fraction of the solution in which the bound counterions are located. P is a dimensionless variable relating In ( I - p ) / p = In + / ( I - 4)
+ pzP(1 - 4’/3)
(1)
to the electrical potential of the polyion, and in principle, P can be estimated from the radius and charge density of the In previous studies of the binding of counterions to CPC micelles,’ however, we treated P as a variable parameter, which had an optimum value of 55.1 f 3.5; the same value of P was used in correlating the present results. $I is assumed to be equal to the product of the molar volume of the surfactant and the molarity of the surfactant in micellar form. In applying the polyelectrolyte ion-binding model to solutions of CPC and sodium benzoate, both chloride and benzoate counterions are considered to be bound at or near the surface of the polyelectrolyte. Thus, equations analogous to eq 1 may be written to account for the simultaneous binding of the counterions chloride and benzoate In ( 1 - p’)/p’ = In + ( I - 4)
+ (Pq + P/q?P(I
- +‘I3)
(3)
where the unprimed terms refer to chloride ions and the primed terms denote benzoate and where q and q’ are the fractions of the free ion charge carried by the two types of counterion (chloride and benzoate, respectively). Equations 2 and 3 predict that the two anions, which have the same charge, will have the same binding fractions (i.e., 0 = (3’). However, the binding of benzoate by surfactant micelles should arise from both nonspecific electrostatic effects, accounted for by eq 2 and 3, and the tendency of the aromatic moiety of benzoate ions to solubilize within the micelle. We assume that the concentration of benzoate solubilized inside the micelle is equal to a factor of K (an unknown partition ratio) times the concentration of benzoate occurring as bound counterions, predicted by eq 2 and 3. Thus, a reasonable description of the binding is that the benzoate ion will be distributed between (a) the bound state near the external surface of the charged micelles, randomly interspersed with the chloride counterions, and (b) a solubilized state in which the COO- ion is anchored in the positive head-group region of the pyridinium ions of CPC, with the phenyl group extending toward the hydrophobic micellar interior. The constant K is interpreted as a dimensionless partition coefficient for the benzoate anion, distributed between the two states. It is possible to correlate the equilibrium concentrations of benzoate and chloride ions in the permeate and retentate solutions by using eq 2 and 3, applying charge and mass balance equations, and making the assumption that the thermodynamic activities are the same for the electrolytes on the two sides of the dialysis At the relatively small ionic strengths of the present experiments, it has been assumed that the activities of sodium benzoate and sodium chloride in each compartment are equal to the ion concentration products [Na’] [C6H5COO-]and [Na’l[Cl-1, respectively, neglecting the effects of activity coefficients. In the retentate compartment, the ion concentrations appearing in these products are equated to the concentrations of free ions, i.e., the concentrations of these species not bound to micelles. In the permeate, very low concentrations of surfactant micelles are formed during the time period (less than 1 day) required for semiequilibrium to be established; therefore, the free ionic concentrations of chloride, benzoate, CP’, and Na+ ions are practically equal to the total concentrations of these species. A nonlinear least-squares method described previously20i2’has been used to infer the value of the single unknown, the partition (19) Dunn, R. 0. Ph.D. Dissertation, University of Oklahoma, 1987. (20) Christian, S. D.; Tucker, E. E. A m . Lob. (Fairfield, Conn.)1984, / 4 ( 8 ) , 36. (21) Christian, S. D.; Tucker, E. E. Am. Lab. (Foirfield, Conn.) 1984, / 4 ( 9 ) , 31.
Semiequilibrium Dialysis Study of Benzoate Anion coefficient K , by correlating the data in Table I with the model described in the preceding paragraphs. The value of P in eq 1-3 was assumed to be equal to 55, the value determined previously.’ By using a provisional value of K (the partition coefficient for C6H,COO- between the outer ion-bound and inner solubilized regions of the micelle) and the known total concentrations of CPC, sodium benzoate, and sodium chloride in the retentate compartment, eq 2 and 3 are solved for and p’ and the concentrations of chloride and benzoate ions bound to the micelle and existing “free” in the retentate solution are calculated. If the conditions of electrical neutrality are applied and the ion products for NaCI, CPC, and N ~ C ~ H S C OonOthe two sides of the membrane are set equal, it is possible to predict the equilibrium concentrations of these salts in the permeate and the retentate solutions. The nonlinear least-squares method ultimately produces the optimum value of the single fitting parameter, K , by determining the minimum value of the sum of squares of residuals (i.e., differences between benzoate concentrations, calculated and observed, in the permeate compartment) for all 39 experiments. The final column in Table I lists values of the calculated sodium benzoate concentrations in the permeate, corresponding to the parameter value K = 44.4 i 1.4. The results are correlated with a 17.7% root mean square relative deviation between the predicted and observed sodium benzoate concentrations. As the model used to predict the transfer of benzoate from the retentate to the permeate at semiequilibrium was developed, attempts were made to improve the goodness of fit by including a term that would reduce the effective value of P (the potential parameter of the Oosawa model) at increasing concentrations of NaCl in the solution. In our previous studies of the removal of divalent ions by micellar-enhanced ultrafiltration,’ inclusion of an equation P = Po/(1 + C T [ N ~ C I ] ’where ~ ~ ) , a is an adjustable parameter and Po is the value of the potential parameter in the absence of added NaCI, led to a considerable improvement in fitting results for the removal of Cr0:- by CPC. Also, in studying the removal of Cu2+by polyelectrolyte ultrafiltration, the effect of NaCl in decreasing the apparent value of the potential parameter Q (for presumed rod-shaped polyions) was calculated by a similar equation.2 However in modeling the present results, there was no significant diminution of the apparent value of P at added sodium chloride concentrations as great as 0.1 1 M. In fact, the value of a inferred by least-squares analysis did not differ significantly from zero, and the relative error was not reduced below the value of 17.7% obtained with the model described above. This somewhat surprising, but salutary, result implies that the effectiveness of a positive colloid in removing an anion having a charge of -1 will be better than would have been expected from analogous studies of the removal of divalent ions.
Discussion An interesting consequence of the present experiments and the model used to represent the solubilization of benzoate anions by the CPC micelles is that the relative degree of solubilization depends strongly on the concentration of surfactant and other electrolytes. Commonly, the solubilization equilibrium constant is defined by K,,, = [solubilizate in micelle]/( [micellar surfactant] [free solubilizate]) or by Ksol= X/[free solubilizate], where X is the mole fraction of the solubilized species in the micelle. Whichever definition is used, K,,, changes considerably as X and the total concentrations of surfactant, sodium benzoate, and sodium chloride are varied. Figure l shows theoretical plots of K,,, against X,for given total concentrations of the surfactant; the curves are calculated by using the value of the partition ratio from the model ( K = 44.4) and using Oosawa’s two-state theory to account for the influence of total ion concentrations on the binding of benzoate. Doubling the concentration of surfactant at any given value of X decreases K,,, by nearly a factor of 2 , and increasing X at a constant concentration of CPC drastically decreases K,,,. The effect of added NaCl (not shown in the figure) is also to diminish the extent of solubilization of benzoate. Previous investigations of the binding or solubilization of organic ions by surfactant micelle^^^-^^ have not led to accurate or systematic
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5905
0.05MCPC
1
\
i
0.20MCPC
0.0
0.1
‘
0.2
0.3
0.4
0.5
X
mole fraction benzoate in micelle
Figure 1. Theoretical dependence of K,,, = mole fraction of solubilized benzoate ion in micelle/[free benzoate in bulk solution] on mole fraction of benzoate in micelle, in aqueous solutions containing 0.05, 0.10, and 0.20 M hexadecylpyridinium chloride (CPC).
determinations of the extent of binding as a function of the intramicellar composition or concentration of added electrolyte; however, vapor pressure osmometry data for sodium phenylacetate in CPC micelles2*did indicate that the dependence of K,,, on X is similar to that shown in Figure 1. The ion-binding model used here (employing an optimized value of the dimensionless partition coefficient, K , in addition to the value of the potential parameter, P, determined previously) quite successfully accounts for all of the semiequilibrium dialysis results. The simple equations involved in the model are very convenient for predicting and correlating results for other systems in which ionic solutes are solubilized by oppositely charged colloids. Figure 1 clearly shows that the concept of a solubilization equilibrium constant for charged organic species is of limited utility, because the electrical nature of the interaction with the ionic micelle causes Ksolto vary rapidly with variations in concentrations of the surfactant, the solubilizate, and added electrolytes. This result is in contrast with most systems in which molecular solutes are solubilized by ionic or nonionic micelles; for such solutions, K,,, frequently varies relatively little with changes in concentration ~~J~ of surfactant, organic solute or added 1 :1 e l e ~ t r o l y t e . ~Fortuitously, in the present study, the partition coefficient K is apparently nearly constant for the benzoate anion throughout a range of concentrations of surfactant, sodium benzoate, and sodium chloride. However, one might anticipate that K would vary if the mole fraction of benzoate in the micelle were increased to values larger than 0.3 or 0.4. The semiequilibrium dialysis method is one of the most direct and generally applicable techniques for determining the extent of binding of either ions or neutral molecules to surfactant micelles.26 Semiequilibrium dialysis results obtained in the present investigation indicate that colloid-enhanced ultrafiltration methods should be quite effective in removing monovalent or multivalent organic anions or cations from aqueous streams. In both micellar-enhanced ultrafiltration (MEUF) and polyelectrolyte ultrafiltration (PEUF) separations, it has been shown that the concentrations of counterions in the permeate solution closely approximate those observed in analogous dialysis experiments.’s2 The possibility of using MEUF to remove organic ions from aqueous streams is thus an appealing one, because both the electrical forces responsible for counterion binding and the tendency of polar organic solutes to solubilize in the vicinity of the head-group region of ionic micelles will act to reduce the concentration of these species in the permeate stream. In the limit (22) Miyake, Y . ; Shigeto, M.; Teramoto, M. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 1515. (23) Ige, J.; Soriyan, 0. J . Chem. SOC.,Faraday Trans. 1 1986,82, 201 1. (24) Zoltewicz, J. A,; Munoz, S . J . Phys. Chem. 1986, 90,5820. (25) Bushong, D. S. Ph.D. Dissertation, University of Oklahoma, 1985. (26) Nguyen, C. M.; Christian, S. D.; Scamehorn, J. F. Tenside Dererg., in press
J . Phys. Chem. 1989, 93, 5906-5910
5906
as the mole fraction of benzoate in the micelle approaches zero, at low ionic strength, the ratio of the concentration of benzoate in the retentate to that in the permeate will approach tremendously large values.
Energy, Grant No. DE-FGOI-87FE61146, and the National Science Foundation, Grant C H E 8701887. F.Z.M. expresses her appreciation for the award of a Peace Fellowship from the Egyptian Ministry of Higher Education, supported by the U S . Agency for International Development.
Acknowledgment. Financial support for this work was provided by the Office of Basic Energy Sciences of the Department of
Registry No. CPC, 123-03-5; sodium benzoate, 532-32-1.
Observation of Novel Photochemistry in the Multiphoton Ionization of Mo(CO), van der Waals Clusters William R. Peifer and James F. Garvey* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 (Received: January 31, 1989)
van der Waals clusters of Mo(CO)~generated in the free-jet expansion of a pulsed beam of seeded helium are subjected to multiphoton ionization and the product ions analyzed by quadrupole mass spectrometry. Oxomolybdenum and dioxomolybdenum ions are observed to be produced with high efficiency. This behavior is in striking contrast to that of metal carbonyl monomers and covalently bound cluster carbonyls, which under complete ligand loss prior to ionization. The observed photochemistry is ascribed to reactions between a photoproduced molybdenum atom and the ligands of neighboring Mo(CO), "solvent" molecules within the van der Waals cluster.
Introduction The study of transition-metal clusters in the gas phase has experienced tremendous growth over the past decade.' The development of pulsed laser ablation/molecular beam sources2 has made possible the study of clusters of even the most refractory of transition metals. These clusters are of fundamental significance in our understanding of metal-metal and metal-ligand bonding interactions, and they serve as model systems for the study of catalysis and surface phenomena. Studies of the effects of cluster size and structure on reactivity provide us the necessary experimental database with which to test and refine our theories concerning the chemistry and physics of bulk metals. Reactivities of the atomic ions of the first-row transition metals in the activation of H-H bond^^-^ and C-H bonds6-* have been systematically studied as functions of translational and electronic energy. Reactivity in these cases can be rationalized in terms of qualitative molecular orbital arguments and spin conservation. By contrast, dimer ions of some first- and second-row transition metals exhibit reactivities (or lack thereof) toward saturated hydrocarbons which markedly differ from those of the atomic ions.*I3 Furthermore, chemisorptionl"l6 and dehydrogenati~n'~.'~ ( I ) Cole, J. L. In Metal Clusrers; Moskovits, M., Ed.; Wiley: New York, 1986: Chaoter 6. (2) Dick, T. G.; Duncan, M A,; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1981, 74, 65 1 1 . (3) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1985, 89, 5626. (4) Elkind, J. L.; Armentrout, P. B. J . Chem. Phys. 1986, 84, 4862. ( 5 ) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1987, 91, 2037. (6) Aristov, N.; Armentrout, P. B. J . Phys. Chem. 1987, 91, 6178 (7) Georgiadis, R.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 7067. (8) Sanders, L.; Hanton, S.; Weisshaar, J. C. J . Phys. Chem. 1987, 91, 5145. (9) Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC.1980, 102, 7129. (10) Jacobson, D. B.; Freiser, B. S . J . Am. Chem. SOC.1985, 107, 1581. (11) Hettich, R. L.; Freiser, B. S . J . Am. Chem. SOC.1985, 107, 6222. (12) Tews, E. C.; Freiser, B. S. J . A m . Chem. SOC.1987, 109, 4433. (13) Huang, Y.; Freiser, B. S. J . Am. Chem. SOC.1988, 110, 387. (14) Geusic, M. E.; Morse, M. D.; Smalley, R. E. J . Chem. Phys. 1985, 82 ~-. 590. ( I 5 ) Whetten. R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Phys. Reu. Letr.
.-
-.
1985. - - - - -54 , -1494 - .
(16) Richtsmeier, S.; Parks, E. K.; Liu, K.; Pobo, L. G.; Riley, S. J . J . Chem. Phys. 1985, 82, 3659. (17) St. Pierre, R. J.; El-Sayed, M. A. J . Phys. Chem. 1987, 91, 763. ( I 8) St. Pierre, R. J.; Chronister, E. L.; Song, L.; El-Sayed, M. A. J . Phys. Chem. 1987, 91, 4648.
0022-3654/89/2093-5906$01 .50/0
reactions of Nb, Co, and Fe clusters are observed to be sensitive to both the size of the cluster and the identity of the metal. The rationalization of these trends in reactivity in terms of metal-metal bonding awaits a more comprehensive theoretical understanding of the electronic structure of these larger clusters. One might ask how trends in the reactivity of small clusters are modulated in the transition from bare metal clusters to ligated clusters (Le., multinuclear cluster coordination compounds). Of particular importance are the large class of transition-metal carbonyl compounds. These compounds are inherently interesting because of the variety exhibited in the types of bonding between carbonyl ligands and metals.I9 In addition, many coordinatively unsaturated metal carbonyl species are thought to play an important role in catalysis.20 Coordinatively unsaturated metal carbonyls may be efficiently generated via photolysis of their saturated counterparts. While the condensed-phase photochemistry of metal carbonyls is dominated by the loss of a single ligand,21the gas-phase photochemistry of these compounds is characterized by sequential loss of several ligands and statistical22 partitioning of excess energy among the vibrational, rotational, and translational modes of the fragment^.^^-^* Photofragmentation is even more extensive in the case of multiphoton absorption. Multiphoton ionization (MPI) of mononuclear metal carbonyls is characterized by initial multiphoton dissociation (MPD) of all ligands, yielding the bare metal atom, followed by subsequent ionization of the neutral meta1.29-31 (19) Wade, K. In Transition Metal Clusters; Johnson, B. F. G., Ed.; Wiley: New York, 1980; Chapter 3. (20) Whetten, R. G.; Fu, K. J.; Grant, E. R. J A m . Chem. Soc. 1982,104, 4270. (21) Geoffroy, G. L.; Wrighton, M. S. Organomerallic Photochemistry; Academic: New York, 1979. (22) It has been recently proposed that energy disposal is rigorously described by statistical theory only in the limit of a steep exit potential surface. See: Holland, J . P.; Rosenfeld, R. N. J . Chem. Phys. 1988, 89, 7217. (23) Nathanson, G.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J . Chem. Phys. 1981, 74, 361. (24) Ouderkirk, A. J.; Weitz, E. J . Chem. Phys. 1983, 79, 1089. (25) Seder, T. A,; Ouderkirk, A. J.; Weitz, E. J . Chem. Phys. 1986, 85, 1977 ..
(26) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC.1985, 107, 2203. (27) Waller, 1. M.; Davis, H. F.; Hepburn, J. W. J . Phys. Chem. 1987, 91. 506. (28) Waller, I . M.; Hepburn, J . W. J . Chem. Phys. 1988, 88, 6658.
0 1989 American Chemical Society
