J. Phys. Chem. 1992,96, 9106-9111
9106
References and Notes
(12) Nicolis, G.; Prigogine, I. SelfOrganizarion in Nonequilibrium Sys-
(1) Belouaov, B. P. In Oscillations and Traveling Waves in Chemical Sysrems; Field, R. J., Burger, M., Eds.; Wiley: New York, 1985. (2) Zhnbotinsky, A. M. In Oscillations and Trawling Waves in Chemical Sysrems; Field, R. J., Burger, M., Us.; Wiley: New York, 1985. (3) Hudson. J. L.: Mankin. J. C. J . Chem. Phvs. 1981. 74.6171-6177. (4) Gybrgyi; L.; Field, R. J.;Noszticzi~~, Z.; M ~ r m i W.'D.; c ~ Swinney, H. J . Phys. Chem. 1992,96, 1228-1233. (5) R o w J.-C.; Simoyi, R. H.; Swinney, H. L. Physica 1983, A?, 257-266. (6) Gybrgyi, L.; Field, R. J. Nature 1992, 355, 808-810. (7) Schneidcr. F. W.: MIinstrr. A. F. J . Phvs. Chem.1991.95.2130-2138. (8) Field, R. J.; Noyes, R. M. Faraday Symp. Chem. Soc.' lfi4,9,21-27. (9) Ruoff, P. Chem. Phys. Lett. 1982,90, 76-80. (10) Ruoff, P.; Varga, M.; KbrBs, E. Acc. Chem. Res. 1988,21,326-332. (11) Troy, W. C. In Theorefical Chemistry; Eyring, H., Henderson, D., Eds.; Academic: New York, 1978; Vol. 4.
rems; Wiley: New York, 1977; Chapters 13.5 and 15.4.
(13) Ruoff, P.; Noycs, R. M. J . Chem. Phys. 1986,84, 1413-1423. (14) Field, R. J. J . Chem. Phys. 1975, 63, 2289-2296. (15) Ruoff, P. Chem. Phys. Lrrr. 1983, 96, 374-378. (16) Ruoff, P.; Noycs, R. M. J . Phys. Chem. 1989,93, 7394-7398. (17) Ruoff, P. Chem. Phys. Lrrr. 1982, 92, 239-244. (18) Field, R. J.; Noycs, R. M. J . Chem. Phys. 1974, 60, 1877-1884. (19) Field, R. J.; Fbrsterling, H.-D. J. Phys. Chrm. 1986,W, 5400-5407. (20) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University h: Cambridge, 1989; Chapter 7. (21) Ruoff, P.; Vcstvik, J. J . Phys. Chem. 1989, 93, 7798-7801. (22) Gybrgyi, L.; Field, R. J. J. Chem. Phys. 1989, 91, 6131-6141. (23) Blittersdorf, R.; Mlnstcr, A. F.; Schneider, F. W. J . Phys. Chem. 1992, 96, 5893-5897.
ARTICLES Charge Transfer in the Photodissociation of Metal Ion-Benzene Complexes K.F. Willey, C.S.Yeh, D.L.Robbius, and M.A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602 (Received: June 8, 1992; In Final Form: August 19, 1992) Photodiiation dynamics, spectroscopy, and binding energetics are investigated for a variety of gas-phase metal ion-benzene complexes. These complexes are produced and cooled by pulsed laser vaporization in a s d e d supersonic expansion. They are mass-selected and studied with laser photodissociation in a reflectron time-of-flight mass spectrometer. A prominent photoprocess for many of these complexes at low energy is "dissociative charge transfer", which produces the benzene cation photofragment. The relative importance of this channel depends on the energy of excitation and on the density of metal ion electronic states in the same energy region as the charge-transfer electronic state. The measurement of the appearance threshold for the charge-transfer channel establishes an upper limit for the metal ion-benzene dissociation energy. charge-transfer dissociation processes in a variety of metal ionIntroduction benzene complexes (e.g., Fe+-bz, Mg+-bz, Ag+-bz, Bi+-bz). The binding of metal atoms or metal ions to organic ligands These studies reveal previously unavailable energetic information such as benzene has been a topic of interest for many years.'+ for the bonding in these complexes. The data obtained here are The resulting complexes have bonding interactions representative compared to those from other techniques, when available, and to of those in many inorganic or organometallic systems, and they the predictions of theory. are also important as models for adsorption on bulk metal surfaces. Metal *-complexes with benzene or other unsaturated molecules Experimental Section have been studied since the early work of Mulliken.s However, Metal-bonzene ion-molecule complexes are produced by laser the detailed structures of these complexes and their binding envaporization at 532 nm (NdYAGsbcond harmonic) in a pulsed ergetics remain to be characterized. nozzle cluster source using naethodsdacribed pre~iously.'~-'~ The Recent technical developments stimulated by molecular beam complexes are mass-selected one at a time from the distribution cluster research have improved both the experimental and theoproduced by the source in a reflectron time-of-flight mass specretical capabilities for studying gas-phase complexes containing tr~meter.~'Laser excitation of selected ions takes place in the metals.622 In particular, metal-molecule complexes have been reflection field at the turning point in the ion trajectory. Phoproduced by pulsed laser vaporization sources for study with ' ~ J ~ ~ ~ products and parent ion depletion are measured with molecular beam laser and mass spectroscopy t e c I ~ n i q u e s . ~ ~ . ~ ~todissociation a second stage of time-of-flight mass analysis. The operation of These complexes have been studied with laser photodissociathe reflectron instrument for photodissociation experiments has tion9J0J3JsJ7-19 and collision-induced dissociation16in low-pressure been described previ~usly.~' The wavelength dependence of environments and with equilibrium methods in high-pressure mass photodhciation processes kimestigatcd with a NdYAG pumped spectrometry." In recent work in our laboratory, we have shown tunable dye laser (SpectraPhysics PDL2). Frapent ion intensity that photodissociation of metal ion complexes often leads to a is recorded as a function of the laser wavelength to obtain phopreviously unrecognized process known as "dissociative charge tofragment excitation spectra. transfer".I7 In this process, the complex dissociates in an excited electronic state in which the metal and its molecular ligand Results and Discussion separate with the charge remaining on the species with the higher Mssocition Ch8naels and Brrncbing Ratios. Figure 1 shows ionization potential. The observation of this process and its energy an example of metal-benzene complexes produced with our laser dependence can lead to the determination of the complex binding vaporization source. The Fe+-benzene cluster ion distribution energy. In the present report, we describe our observations of shown here is measured by pulsing the mass spectrometer acTo whom correspondence should be addressed. celeration plates to extract the cluster ions from the molecular 0022-3654/92/2096-9 106$03.00/0 0 1992 American Chemical Society
Photodissociation of Metal Ion-Benzene Complexes
The Journal of Physical Chemisfry, Vol. 96, No. 23, 1992 9107
Fe+-( benzene)x
d L . - - j L :
d----L-
0
4 0
Figure 1. Mass distribution of iron-benzene ion-molecule complexes produced by laser vaporization in a d e d beam expansion. Clutcr ions are mass-selected from this kind of distribution prior to studics of photodissociation.
beam. These complexes are ionized in the laser vaporization prooess, and they are cooled in a supersonic expansion after their formation. We therefore believe that these ions are internally cold. As shown in the figure, we observe primarily ions of the form [M-(benzene)# under these conditions. There is little, if any, evidence for fragmentation. Ion masses between the main cluster peaks can be assigned to oxides or iron cluster (dimer, etc.) complexes. The clean mass spectra observed and the photofragmentation patterns discussed below indicate that these are weakly bound metal complexes. As shown here, the mass peaks corrtsponding to mone and dibenzene adducts appear with much greater intensity than those for the large aggregates. This is generally true in our studies of a variety of metakbmme systems, and it may be indicative of the preferred formation of sandwich structures which are familiar for condensed-phasemetal-benzene complexes. In the present paper we focus on the monobenzene complexes; multibenzene complexes like these are discussed in other reports from our laboratory. In essentially every system studied here, the metal atom in the complex has a much lower ionization potential than that of bmzene (9.24 eV). For the metals studied here the IP's are as follows: Ag, 7.576; Fe, 7.870; Bi, 7.289; Mg, 7.646; Co, 7.86; Cu, 7.726. Therefore, in the ground electronic state of these complexes, the charge is expected to be localized on the metal atom, is., M+-(benzene)* These systems are therefore properly regarded as metal ion-benzene complexes. Ab initio calculations on these systems verify this simple picture, finding that the bonding interactions are largely electrostatic in naturesz2 In other work in our laboratory, detailed spectroscopy of different metal complexes produced under these same conditions also supports the picture of weak electrostatic bonding.Ig F i i 2 shows the photodissociation mass spectra obtained for the complexes Fe+-benzene and Co+-benzene. The laser wavelengths d are 315 and 608 nm, rtspcctively. These spectra are accumulated by a difference method in which the parent M+benzene parent ion intensity without the photodissociation laser is subtracted from the intensity measured with laser excitation. The parent ion signal loss from dissociation is therefore plotted as a depletion signal (negative polarity peaks), and the fragment ions appear as new signal (positive polarity). These dissociation spectra are also consistent with the picture of weakly bound ion-molecule complexes. The only charged photofragment from these systems is the atomic metal cation, M+. Production of this fragment occw8 by simple cleavage of the metal ion electrostatic bond with the benzme molecule. The benzene molecule does not absorb at visible or near-ultraviolet wavelengths, while the iron or cobalt atomic ions do absorb in this region. Even allowing for shifts in the spectra of the species upon complexation, the lowest T* transition on benzene (260 nm in the isolated energy T system) is not likely to be shifted to these energies. The chromophore in these systems, therefore, is most likely the metal ion. Previous experiments on metal ion-benzene complexes involving
-
100
50
150
4 6 Number of Molecules ( x ) 2
co+
0
50
Co+-benzene
100
150
Mass (amu) Figure 2. Photodissociation mass spectra of cobalt-benzene and ironbenzene complexes. The cobalt data is obtained at 608 nm, while the iron data is obtained at 317 nm. As shown, both of these complexes dissociate to the atomic metal ion at these wavelengths.
the fmt-row transition metals have also o k e d a near-continuous absorption in the visible and near-UV wavelength region derived from the large number of transition-metal-ion states at low energy." The electronic structure of the silver atomic cation, Ag+,is very different from that of open-shell transition-metal ions. Ag+ is a closed-shell atom with no low-energy atomic transitions. Cu+ and Au+ have electronic structures similar to that of silver, but the d9s' configurations for these atoms are at lower energy than in AB+. Ion-molecule complexes containing Ag+, therefore, can have no metal ion chromophore for absorption at low energy. As shown in Figure 3, however, the Ag+-benzene complex does absorb and dissociate at wavelengths in the near-ultraviolet region of the spectrum (303 nm here). Unlike the iron and cobalt complexes described above, the fragment from dissociation of Ag+-benzene is the benzene cation. This channel is easy to recognize because the parent ion mass peak has the doublet pattern resulting from the two i s o t m of silver (107,109), while the fragment mass peak is a singlet. Because of the limited mass resolution in the parent ion selection, both silver isotope species are selected and irradiated with the laser. It should therefore be possible to see either a doublet lo7*'OSAg+ fragment, a singlet C,&+ fragment, or branching into both. If a single isotopic complex were selected initially, only correspondingisotopic fragments would be papsible. The c&+fragment is the only channel observed for this complex in this wavelength region under any conditions. Formation of the benzene cation fragment is not the lowest energy dissociation pathway for this complex because, as discussed above, the silver atom has a much lower ionization potential than benzene. The dissociation observed, therefore, must have occurred out of an electronically excited state correlating to the C6H6++ Ag asymptote. This kind of state, in which the usual charge distribution is reversed, is known as a charge-transfer electronic transition. Charge-transfer transitions are often found in organometallic and transition-metal chemistry, and they usually have extremely large oscillator strengths. Charge transfer electronic transitions are well-known in condensed-phase metal complexes, and there are
Willey et al.
9108 The Journal of Physical Chemistry, Vol. 96, No, 23, 1992
TABLE I: AIP Vduea, Dhociitioo Energies, and Dimdative Charge-Transfer Energies Predicted for a Variety of Mebl-&azcee Ion-Molecule Complexes metal
PIP
DO (kcal/mol)"
Mg
1.59 2.42 2.50 2.41 1.81 1.37 1.38 1.61 1.51 2.36 1.61
30.4 62.8 51.1 31.4 35.1 51.1 62.6 59.3 50.1 52.1 36.5
Ti
V Cr Mn Fe
Co 0
50
100
200
150
Ni Cu Nb Ag
charge-transfer charge-transfer energy wavelength (ev) (nm) 2.91 5.14 4.72 4.10 3.33 3.59 4.10 4.18 3.69 4.62 3.25
426 24 1 263 303 313 346 303 291 336 268 382
*Dissociation energies are calculated by Bauschlicher and co-worktential. The electrostatic interaction is expected to be significant in this state, because the near-point charge on Ag+ is actin on the benzene molecule, which has a high polarizability (25.1 Consistent with this, Bauschlicher has calculated a dissociation energy for this elcctrostaticbond of 36.5 kcal/mol. In the excited state, the charge is on benzene, where it can be delocalized over the molecular *-bonding network. The electrostatic interaction, therefore, has to a first approximation a delocalized charge polarizing a silver atom. This interaction should be significantly weaker than that in the ground state, resulting in a shallow potential well shifted to longer bond distance. The general features of these potential curves should apply for many metal ion-benzene complexes. Because of the differences in bonding between the ground and excited states, photoexcitation in these systems is likely to the repulsive wall of the upper state. If the photon energy exceeds the combined ground-state dissociation energy and the ionization potential difference between the metal atom and benzene (AIP), dissociation on the excited surface is energetically possible. Observation of 10046 yield for the benzene cation fragment in the Ag+-benme system requires that dimciation procaeds exclusively out of this excited state. This is not surprising for dissociation of the silver-benzene complex, because there are no other electronic states at low energy. A similar situation might be expected for a complex of Cu+ with benzene. Although the copper ion has more low-energy configurations than silver, its electronic state density is more similar to that of silver than it is to an open-shell transition-metal ion. As shown in Figure 3, the c o p b e n z e n e complex also dinsociates via the charge-transfer route. (Data shown are at 355 nm.) As in the case of the silver complex, this dissociation process is the only one observed. If the photoexcitation energy is great enough, all metal complexes of the form demibed here should have an excited electronic state corresponding to charge transfer. As indicated in Figure 4, the excitation energy must exceed the combined complex dissociation energy (Do) and the ionization potential difference (AIP) between the metal atom and benzene. Charge-transfer transitions, therefore, may occur in very different energy regions depending on the exact values of Do and AP. Using the calculated binding energy for the Ag+-bz complex (36.5 kcal/mol, 1.58 eV) and the AIP value of 1.67 eV, the threshold for charge-transfer dissociation should be located at 3.25 eV or about 381.5 nm. The observation of this process at 303 nm (see Figure 3) is therefore not surprising. Similar considerations predict the threshold for this process in Cu+-benzene at 336 nm, slightly higher in energy than our observation at 355 nm. For the open-shell transitionmetal species, however, significantly higher dissociation energies of 51.1 (Fe+-bz) and 62.6 kcal/mol (Co+-bz) are predicted. Charge-transfer dissociation in these systems should require excitation at 346 and 303 nm, respectively. Table I contains PIP values, calculated dissociation energies, and the corresponding predictions for the threshold energies for charge-transfer disso-
k).
4 0
50
150
100
w
200
Mass (amu)
Figure 3. Photodissociation mass spectra for copper-benzene and sib ver-benzene ion complexes. The copper data is obtained at 355 nm, and the silver data is obtained at 302.5 nm. Both complexes undergo dissociative charge transfer at these wavelengths, yielding the benzene cation as the only photofragment.
benzene
R (M-benzene)
+
Figure 4. Generalized level diagram for metal-benzene ion complexes showing the relative positions expected for the ground state and the charge-transfer excited state. As shown, the asymptote between these two statm is the ionization potential difference between the metal atom and benzene. As indicated, vertical excitation is often likely to athe excited-state potential surface above its dissociation limit.
previous examples of charge transfer in gas-phase ion-molecule complexes not containing However, chargetransfer transitions in metal ion-molecule complex systems like those described here were not observed before the first reports from our research group." The mechanism of this photochemical process can be understood by referring to Figure 4, where a schematic energy level diagram appropriate for Ag+-benzene is presented. The potential curves indicated are those in the metal-benzene stretching coordinate, which is also a convenient coordinate to consider both charge transfer and dissociation of the complex. In the ground state, the charge is on the silver atom, which has the lower ionization po-
Photodissociation of Metal Ion-Benzene Complexes
Mg+-benzene
lMg+
I I
The 3ournal of Physical Chemistry, Vol. 96, No. 23, 1992 9109 Ag+benrene Photodissociation
I
0
100
50
Fe+
150
Fe+- benzene
800
700
500
600
400
309
200
Wnm)
Figure 6. Photodissociation spectrum for the complex Ag+-benzene obtained by detecting the benzene cation channel as a function of the excitation laser wavelength. The threshold determined from this spectrum is a t 418 nm. As shown, there is no vibrational structure in this spectrum, consistent with excitation into a repulsive region of the upper state potential surface.
4 0
I
50
I
bz+
i
100
150
Bi+- benzene
of metal complexes which exhibit an intermediate efficiency for dissociative charge transfer. The efficiency for this channel in a single complex should also vary with the excitation energy region chosen for study. Figure 5 shows that these predictions are in fact valid. The complexes Mg+-benzene and Bi+-benzene both dissociate via partial charge transfer and simple cleavage of the electrostatic bond at the selected wavelengths shown (278 and 355 nm). The same is true for the Fe+-benzene complex when it is studied at a shorter wavelength (280 nm) than that used in Figure 2, so that dissociative charge transfer is energetically possible. In the bismuth complex, the charge-transfer channel is most prominent, but it less so in the magnesium and iron complexes. These branching ratios presumably result from a competition for absorption between the charge-transfer resonance and other electronic states of the complexes derived from metal ion excited states. For example, the Mg+ 2S 2Presonance line occurs at about 280 nm, and complex transitions derived from this asymptote may also occur in the region studied here. 'I"hW nnd Dissoc&lion l h e r g k We have described above how a knowledge of the complex dissociation energy and the metal-benzene AIP value can be used to predict the energy required for dissociative charge transfer. However, the dissociation energies required are in general not known. The data presented in Table I use theoretical values for the dissociation energies. Instead of predicting charge-transfer thresholds from calculated dissociation energies, it is possible to measure the thresholds directly and therefore determine the dissociation energies experimentally. At the threshold energy where charge transfer is first observed, the laser energy must be greater than or equal to the combined values of Do and AIP. Therefore, subtraction of the AIP value from the threshold energy measured gives an upper bound on the dissociation energy: Do'' Ih ~ m - AIP
-
4
0
100
200 Mass (amu)
300
Figure 5. Photodissociation mass spectra of systems exhibiting intermediate amounts of charge transfer. Spectra of Mg+-benzene (278 nm), Fe+-benzene (280 nm), and Bi+-benzene (355 nm) have different signal-to-noise levels determined by the absolute cross sections for dissociation, the laser power available at these wavelengths, and the amount of parent ion which could be produced.
ciation for a group of selected metal-benzene complexes. It is not surprising, therefore, that the cobalt and iron systems above do not exhibit charge transfer at the wavelengths (608 and 3 15 nm) used in Figure 2. The discussion above suggests that all metal complexes will have excited states corresponding to charge transfer. These electronic transitionsare expected in general to have large oscillator strengths. The observation of dissociative charge transfer, however, depends on the efficiency of exciting the charge-transfer state above its dissociation limit. Two factors are expected to be important in determining the efficiency of this process: the Franck-Condon factors for the vertical transition and the density of other competing electronic states in the same energy region. Without a detailed knowledge of all the excited states in the complex, one cannot make quantitative predictions, but qualitative estimates of efficiencies should be possible based on the relative number of metal ion atomic states at low energy. In addition to the extremes of behavior illustrated above, there should be a variety
An upper limit is obtained becaue the exact threshold may not be accessible in the vertical electronic transition. We have used tunable dye laser radiation to scan the photodissociation spectrum for the determination of charge-transfer thresholds. To do this, we record the intensity of the benzene cation fragment as a function of the dye laser energy. Figure 6 shows an example of this kind of experiment in the spectrum obtained for the complex Ag+-benzene. It is a structureless continuum with a threshold at 418 nm in the visible blue region of the spectrum. From the energetic value of this threshold we derive an upper limit on the Ag+-benzene complex dissociation energy of 30.1 kcal/mol. Similar experiments performed for the complexes Mg+-benzene and Fe+-benzene yield thresholds at 450
9110 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Willey et al.
of ions produced in a Fourier transform (FT) mass spectrometer. In their experiment, dissociation is believed to occur on the ground-state surface following internal conversion from a higher state where absorption occurred. The products detected in that experiment, Fe+ (+benzene), have a threshold at lower energy (- 520 nm) than the charge-transfer threshold measured here. The upper limit on Doderived from their study (55 f 5 kcal/mol) is in reasonably good agreement with, but slightly lower than, our number (62.1). The ions produced in the Freiser experiment are at a temperature of greater than or equal to room temperature, and therefore a slight red shift in their threshold might be expected. Several research groups have investigated the Ag+-benzene system using different techniques. McMahon and -workers have “In each case, the energy cited represents an upper bound on the used equilibrium concentration measurements in a high-temexact dissociation energy. Numbers obtained by other experimental perature/high-pressure mass spectrometer system to obtain the groups used photodissociation, equilibrium high-pressure mass specvalue Do5 35 kcal/m01.~’ Armentrout and co-workers obtained trometry, and collision-induced dissociation. Bauschlicher and coworkers.22 e Freiser and co-worker~.’~ “McMahon and co-w~rkers.~~ Do= 37 kcal/mol in a guided ion collisional dissociation experArmentrout and co-workers.I6 iment. Both of these values are somewhat higher than our photodissociation value of 30.1 kcal/mol. Freiser and co-workers and 305 nm and dissociation energies of 26.9 and 62.1 kcal/mol, obtain a value higher than any of these via photodissociation in respectively. These data are presented in Table 11. In both of the FT mass spectrometer environment (55 kcal/mol).lk Freiser these other spectra (not shown) the benzene cation has a wavedetects the charge-transfer channel, but at a higher energy onset length dependence without any resolved structure, consistent with than that which we measure. The three data points other than an electronic transition onto the repulsive wall of the upper that of Freiser are in general agreement with each other and with electronic state. We have not yet scanned the dissociation the value predicted by theory. However, all of these methods are thresholds for other metal-benzene complexes. However, we have subject to errors which are difficult to quantify. Both equilibrium observed the charge-transfer channel at the selected fixed and ion beam collision experiments may have thresholds brcadened wavelength of 355 nm for the bismuth and copper complexes. or shifted by the thermal energy content in the ions. This effect Upper limits derived from these data may be reduced after further would cause measured thresholds to be lower than expected. experiments, but these data are still the only information available Thresholds measured by ion beam collisions may have a kinetic on these complexes to date. Preliminary upper limits on the shift to higher apparent energy because of the finite time scale dissociation energies from these measurements are 35.5 (Bi) and required for dissociation to occur and because collisional energy 45.7 kcal/mol (Cu). transfer may not be complete. Kinetic shifts are also possible in Table I1 provides a comparison between the dissociation energies photodissociation experiments, but energy deposition is more derived here, the results of other laboratories, and the predictions accurately known. In principle, supersonically cooled ions which of theory. The comparison with theory is most complete. Aldissociate on a repulsive upper surface should provide the best-case though only a few complexes have been studied, some trends in scenario for the determination of a rigorous upper limit for the binding strengths to the different metals are already apparent. dissociation energy. However, even this experiment may produce The iron ion binds much more strongly than either silver or nonthermal ions depending on the exact operating conditions. magnesium. As an atom with a partially filled d shell, iron may Without spectroscopic verification, it is impossible to know the exhibit some incipient covalent bond formation in addition to the exact ion temperature and therefore the accuracy of the thersimple electrostatic interactions. As discussed earlier, the silver mochemical data derived. On the basis of other similar systems ion has a closed-shell configuration and its weaker binding is where ion spectra have been measured, though, we believe that consistent with mostly electrostaticcontributions. It is interesting the ions produced for this experiment are subthermal. that the silver ion binds almost as weakly as the non-transition While the absolute accuracy of our charge-transfer method metal, magnesium, while copper binds much more strongly. This remains to be established, it is already clear that it offers some trend is predicted by the theory, and it has been discussed by advantages over other techniques for the determination of disBauschlicher and co-workers.22 Our data and the theory follow sociation energies in metal complexes. Virtually any metal can the same trend. However, our numbers for magnesium, copper, be vaporized, and therefore essentially any metal ion complex can and silver complexes are lower than the theoretical predictions, be synthesized. This includes systems with bonding ranging from while our iron data give a dissociation energy that is somewhat weak van der Waals to strong covalent interactions. Supersonic higher than the theory value. It is important to recall that our cooling allows thermal effects in the dissociation threshold to be values are expected to be upper limits to the actual values. It is reduced or eliminated. Laser energies available make photodistherefore possible that our result for iron is shifted to higher energy sociation possible at energies inaccessible with equilibrium than the actual threshold because the vertical transition accesses methods?* If it is observable, the charge-transfer threshold will a higher region on the potential surface. Bauschlicher has also always lie at higher energy than the threshold for dissociation on suggested that our dissociation may correlate to the slightly higher the ground state. It may therefore be in a more convenient laser asymptote for excited iron (4F) atoms. The magnesium, copper, wavelength region, and its detection is less likely to be limited by and silver data, however, cannot be brought into quantitative the spectroscopic absorption probability. This method seems to agreement with the theory unless we assume that the ions in this be generally applicable, and it should provide data on systems experiment are not completely cooled by the supersonic expansion. which cannot be studied by other methods. The disadvantage in Internally hot ion complexes would have apparent thresholds at the method is the uncertain effect of Franck-Condon factors on lower energy than the actual one. Bismuth has not been studied the threshold. However, it should be possible to combine the with theory, but our data indicate complex binding similar to that method here with kinetic energy analysis of the fragmentsZ5to of silver. obtain better characterization of the system energetics. Table I1 also presents a comparison between the dissociation energies obtained in our experiments and those obtained by other Conclusions methods. As indicated, our numbers are in reasonably good agreement with these other measurements, but there are some A variety of metal ion complexes with benzene have been significant differences. Mg+-benzene, Cu+-benzene, and Bi+studied by laser dissociation as a function of energy. Two limiting forms of dissociation behavior are observed: simple electrostatic benzene have not been studied previously, and so ours is the only data on these systems. Freiser and co-workers have measured bond cleavage and dissociative charge transfer. The branching the Fe+-benzene dissociation energy by visible photodissociation ratios of these processes reflect the energetics of the system TABLE II: Metal Ion-Beazeoe Binding Energies (kcal/mol) Determined by Charge-Transfer Dissociation Thresholds" ion AIP (eV) threshold (nm) Do” (exp) DO,,(theory)* 26.9 30.4 1.59 450 Mg+ Fe+ 1.37 305 62.1 51.1 (56.8) 55 5 c 30.1 36.5 1.66 418 Ag+ c35d 37 f l e 55c cut 1.51 >355 45.7 50.1 35.5 Bit 1.95 >355
*
J. Phys. Chem. 1992,96,9111-9113 bonding, the ionization potential differences, and the relative density of electronic states in the wavelength region of interest. Both processes are expected for all metal-benzene ion complexes and probably for all metal-molecule complexes in general, depending on the excitation energy. The energy threshold for dissociative charge transfer provides a new route for the determination of metal ion-molecular dissociation energies. Dissociation energies are determined for magnesium, iron, and silver ion complexes with benzene. The consensus of both theory and several experiments is that the ironbenzene system is quite strongly bound, while silver and magnesium systems have similar much weaker bonding. The present data base for these and other related complexes is quite limited, and the method described here promises to provide a general source of information for a variety of metal-ligand combinations.
Acknowledgment. We appreciate helpful discussions with Charlie Bauschlicher, Ben Freiser, and Peter Armentrout. This research was supported by the National Science Foundation through Grant CHEM-9008246. References and Notes (1) Winstein, S.;Lucas, H. J. J . Am. Chem. SOC.1938, 60, 836. (2) Andrews, L. J. Chem. Rev. 1954, 54, 713. (3) Smith, H. G.;Rundle, R. E. J . Am. Chem. SOC.1958, 80, 5075. (4) Tranham, J. G.;Olechowski, J. R. J . Am. Chem. SOC.1959,81,571. ( 5 ) Mulliken, R. S.J . Am. Chem. Soc. 1952, 64, 811. (6) (a) Castleman, A. W.; Holland, P. M.;Lindsay, D. M.; Peterson, K. I. J . Am. Chem. Soc. 1978,100,6039. (b) Castleman, A. W. Chem. Phys. Leu. 1978, 53. 560. (c) Holland. P. W.. Castleman. A. W. J. Chem. Phvs. 1982, 76,4195. (d) Gkin, K. L.i Guo, B. C.; Kees&, R. G.; Castleman,'A. W. J. Phys. Chem. 1989,93, 6805. (7) Magnera, T. F.; David, D. E.; Michl, J. J. Am. Chem. SOC.1989,111, 4100. (8) Marinelli, P. J.; Squires, P. R. J . Am. Chem. SOC.1989, 1 1 1 , 4101. (9) (a) Shen, M. H.; Farrar, J. M. J . Phys. Chem. 1989, 93, 4386. (b) Shen, M. H.; Farrar, J. M. J . Chem. Phys. 1991, 94, 3322. (10) (a) Lessen, D. E.; Asher, R. L.; Brucat, P. J. J . Chem. Phys. 1990, 93,6102. (b) Lessen, D. E.; Asher, R. L.; Brucat, P. J. J. Chem. Phys. 1991, 95, 1414.
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Li+ Polarizability Due to Collective Motion of Li+ Ions in Dilithium Salts of 2,6-Bis( (diethyiamino)methyl)phenolate Dl-N-oxides as a Function of the Electron Density at the 0 Atom of the Phenolate Group Bogumil Brzezinski, Hanna Maciejewska, Faculty of Chemistry, A . Mickiewicz University, Grunwaldzka 6, PL-60780 Poznafi, Poland
and Georg Zundel* Physikalisch- Chemisches Institut, Universitiit Munchen, Theresienstrasse 41, 0-8000Mirnchen 2, FRG (Received: January 7, 1992)
Six dilithium aurates of 2,6-bis((diethylamino)methyl)-3,4-R-phenoldi-N-oxides were studied in the far-infrared (far-IR) region as a function of the electron density at the 0 atom of the phenolate group. Continua in the FIR region indicate
