8637
J . Phys. Chem. 1990, 94, 8637-8647
FEATURE ARTICLE Reactions in Molecular Clusters following Photoionization Bernhard Brutschy Freie Universirur Berlin, Insrirur fur Physikalische und Theoretische Chemie, Takustrasse 3, Dl 000 Berlin 33, Germany (Hrceiced: March 21. 19901
This article presents a review of our recent work on photoionization mass spectrometry of molecular aggregates, with both onc- and two-photon ionization. The energetics and dynamics of solvated ions have been studied. Charge transfer and chemical reactions have been observed, exhibiting pronounced size effects. The reactive behavior of mixed clusters has in some cases bccn found to deviate strongly from that observed in gas-phase ion-molecule reactions.
1. Introduction Molecular clusters have recently aroused considerable interest in physical chemistry and chemical physics.'-' The main reason for their impact on today's research in these fields stems from their mediating role between the gas and condensed phases. They are sometimes considered to represent a fourth state of matter. Molecular clusters are used as model systems to study the homogeneous and heterogeneous phases and the influence of intermo1ecul:ir interaction on molecular energetics and dynamics from a microscopic point of view. They are of great importance in different fields of application research, for instance, in catalysis, solvation chemistry, studies of nucleation phenomena, transport processes, and reaction The field of cluster research may be roughly divided according to the nature of the intermolecular bonds into the field of metal clusters and inorganic or organic van der Waals (vdW) clusters. This feature article deals only with the latter species. Clusters are generally produced in adiabatic expansions with different sizes and structures and in the case of mixed clusters in different mixing ratios. In detailed studies they have to be characterized according to these properties. In their neutral state they are relatively fragile due to the weak intermolecular forces that hold them together, and most of their chemical and physical properties depend only weakly on their size. This is normally determined by mass spectrometry via ionization by electron or photon impact. There is a major problem inherent in this method. Simultaneously with the ionization process, fragmentation very often takes place due to the energy released as the clusters change from their neutral to their ionic equilibrium structure. The ionic clusters may disperse excess energy by evaporation of neutral ( I ) Lcvy. D. H . Adr. Chem. Phys. 1981, 47, 323. (2) Ng. C. Y. Ad[.. Chem. Phys. 1983, 52, 152. (3) Mark. T. D.;Castleman Jr.. A. W. Ado. A t . Mol. Phys. 1985, 20, 65. (4) Jortner. J.; Pullmann. A.: Pullmann. B. Large Finite Systems; Reidel: Dordrccht. 1987. (5) Scolch. G.. Ed. Atontic and Molecular Methods; Oxford University Prcss: Ncw York. 1988: Vol. I . (6) Haynnm. C. A.: Mortimer. C.; Young, L.: Levy, D. H. J . Phys. Chem. 1987. 91, 2526. (7) Lctokhov. V . S. Laser Photoionization Spectroscopy: Academic Press: Ncw York, 1987. (8) Reichhardt. C . Soli-ent Effecis in Organic Chemistry. Monographs in Modern Chemistry: ELXI, H. F..Ed.; Verlag Chemie: Weinheim, 1979; Vol. 3.
(9) Hagcna. 0. Molccular Bcams and Low Density Gas Dynamics. Gus dynanrics; Wcgcncr. P. P.. Ed.; Marcel Dekker: New York, 1974; Vol. 3. ( I O ) Experiments on Clusters, Discussion Meeting in Konigstein. Oct. 1983; Ber. Bunsen-Ges. Phys. Chem. 1984, 88. ( I I ) Lcvinc. R . D.: Bernstein. R. B. Molecular Reaction Dynamics and Chenrical Renclirity: Oxford University Press: Oxford, 1987. (12) Pcifcr. W . R.: Garvey. J . F. J . Phys. Chem. 1989. 93, 5906.
0022-3654/90/2094-8637$02.50/0
subunit^,'^ a process we call vdW fragmentation (vdWF). Hence the size distribution in the mass spectrum does not represent that of the neutral beam, making the characterization of the size of neutral clusters a difficult task. The characterization of their structure is an even more challenging problem that generally can be solved only with spectroscopic methods. Both types of information are necessary for a microscopic description of what goes on in clusters after an excitation or ionization process. Therefore an energy-selective method is mandatory. On the other hand, the vdW interaction is generally so weak that the molecular subunits retain their individuality, allowing one to selectively excite or ionize them. The methods discussed in the following are photoionization mass spectrometry with synchrotron (PIMS) or laser radiation (R2PI). Each method has its intrinsic merits and drawbacks, and each can give answers to specific questions. Examples are presented in which both are applied to study the energetics and dynamics of ionic species. One-photon ionization with synchrotron radiation (SR) is used to study the size dependence of the ionization (IP) and fragmentation appearance potentials (FAP) of clusters thereby allowing the calculation of lower bounds to formation enthalpies, dissociation energies, and proton affinities. With laser-induced resonant two-photon ionization, this is also possible although at greater expense. Aside from the energetic quantities, which are of importance for a consistent set of data in thermochemistry, the product ions may also reveal the intracluster ion chemistry and its dependence on solvation. Cluster ions are models for transition states in ion-molecule reactions in the sense that their fragmentations represent the "half-collision" analogues to the latter. The main difference to real collisions is that the intracluster reacting particles are often intermolecularly trapped and hence exhibit only a limited range of trajectories. Ion-molecule reactions in the gas phase have been studied during the past two decades by many groups mainly by high-pressure and ICR mass s p e c t r ~ m e t r y . ' ~ Much ~ ' ~ valuable information on solvation energetics has evolved from these studies. However it should be noted that while at low pressures these gas-phase reactions are bimolecular, at high pressures the experimental parameters are not easily controlled. The ion-molecule reactions in condensed phases are not only bimolecular but may also be termolecular. For example, a solvent shell surrounding an ion may absorb excess energy and serve as a heat bath. Or i t may react in several consecutive steps with the primary formed ion. Therefore clusters are more realistic models to investigate ~~
~~
(13) Buck, IJ. J . Phys. Chem. 1988, 92, 1023. ( 14) Kebarle. P. Annu. Rer. Phys. Chem. 1977, 28, 445. ( 1 5) Aue, D. H.; Bowers, M. T. Gas Phase Ion Chemistry; Bowers, M. T.. Ed.; Academic Press: New York. 1979; Vol. 2.
0 1990 American Chemical Society
8638
p’T; :T-n
The Journal of Physical Chemistry. Vol. 94, No. 24, 1990
Brutschy
1
I
SI 5
4
7
6
h”,
h”,
n
so
s o
a)
-
50
100
150
200
m I z
Figure 1 . Photoionization mass spectrum of methyl fluoride clusters (Po = 3 bar. no seed gas, To = 300 K, d = 80 Wm. undispersed SR).
h Y)
.e Y
complex reaction behavior as often observed in the condensed phase. On thc other hand, ion-molecule reactions studied under equilibrium conditions at high pressures seem to give better adiabatic values of the solvation energetics than those derived from vertical ionization transitions in clusters, as will be discussed below. Typical aspects of the energetics and dynamics of ionizing clusters pertain to the following questions: ( I ) How do the energetics of ionization and fragmentation of a molecule change on homogcncous or heterogeneous solvation? (2) What relaxation proccsscs are typical after ionization and how do they depend on thc cluster size? (3) Are reactions in small clusters representative for those in larger ones and how do they compare with those in the gas or condensed phases? I n trying to answer these questions, we should admit that we arc only at the beginning of a fast growing but wide field. In the following m i i c rcccnt rcsults from !he author’s group shall illustratc thc methods applied and their power and limitations for studying thcsc qucstions. 2 . Experimental Setup Today vdW clustcrs are mainly produced by expanding a gas undcr high prcssurc into vacuum, by which a supersonic beam is formcd.I6 By adiabatic cooling translational temperatures of only few degrees kelvin are achieved. The intramolecular degrees of frccdom of rotation and vibration are cooled to a lesser extent. Howcvcr ncarly all molecules are in their energetic ground state. In this p r o w s of cooling formation of clusters also sets in due to spontancous conden~ation.~ After the expansion the molecules and clusters in the beam are in free molecular flow with a common velocity cqual to thc flow velocity of the beam. Therefore the neutral clustcrs cannot be separated with velocity selectors. Figurc 1 shows a typical mass spectrum of a pure expansion of mcthyl fluoride, ionized with undispersed synchrotron radiation. Onc clcarly observes a quasi-exponential intensity decrease with the clustcr sizc. In thc casc of largc molecules with many degrees of freedom, a purc expansion is not very efficient for cooling. In this case the samplc gas is diluted in a carrier gas normally He, which acts as a cooling bath. By this “seeded beam” technique cold clusters of largc niolcculcs arc casily produced. Depending on the paramctcrs of the expansion, namely, stagnation pressure (no),nozzle diamctcr ( d ) , gas temperature ( T o ) ,and mixing r a t io (ps!pHe), diffcrcnt clustcr size distribution may be produced. By diluting a second samplc gas, mixed clusters are synthesized with a size distribution depending also on the relative mixing ratio of both samplc gascs. Normally a narrow distribution is favorable for ionization spectroscopy, because then fragmentation of larger clusters is diminished. Such distributions are achieved at mixing ratios of 0.1-0.01. Photoionization of molecular clusters with photons of variable energy affords synchrotron radiation from the vacuum ultraviolet (16) Andcrson, J . €3. Molecular Beams and Low Density Gas Dynamics. Gas Dynamics; Wegener. P. P.,Ed.; Marcel Dekker: New York, 1974; Vol.
3.
b)
Figure 2. Energy scheme of resonant two photon ionization (a) with photons of different color (2C-R2PI), and (b) with photons of one color (IC -R 2P I). With aromatics the intermediate state is the S,.
160
1
3
120
.0
80
v
z
,=
40
0
I
96
196
296 396
mass scale ( a.
496
U. )
Figure 3. TOF mass spectrum for a mixture of C 6 H S F / C H , 0 H / H e after one laser shot (rpulre< IO ns). T h e size distribution was not optimized for small clusters (n:m clusters = FB,.ME,, AN = anisole).
region (hv = 7-25 eV). With laser radiation a multiphoton ionization scheme is applicable with photons typically in the UV region. This is efficiently accomplished a t relatively moderate intensities ( I C IO6 W/cm2) by resonant two-photon ionization via an electronically excited state (Figure 2). From a practical point of view both ionization methods are complementary. With monochromatized SR, supplied in the author’s case by the Berlin electron storage ring BESSY, broad tunability (8-28 eV) and good energy resolution ( A E / E = 500-1 000) allows one to measure the ionization potentials (IPS) of clusters very easily. However the relatively low intensity ( 109-10’0photons/(A s)) limits the studies to small clusters. A further limitation in ionizing with only one photon is the loss of ionization selectivity in mixed clusters if the photon energy is larger than the IPS of the individual molecules. Lasers, on the other hand, allow via the resonant absorption step the selective ionization of specific molecules (chromophores) in the clusters.18 In addition, by choice of an appropriate excitation transition the internal energy deposited in the ion may be controlled within narrow limits.” By combination of supersonic beams, i.e., cold molecules exhibiting a reduced spectral complexity, with the high spectral resolution (0.5 cm-I), wide tunability, and high intensity (106-108 W/cmZ) of pulsed dye lasers, the selectivity is further enhanced.I8J9 This allows selective detection of spurious quantities of similar molecular aggregates with high efficiency. Meanwhile a rapidly growing number of groups are working with this method in the cluster fi~ld.~*~~ (17) Anderson, S. L.; Rider, D. M.; Zare, R. N . Cfiem. Pfiys. Lett. 1982, 93, 1 I . (18) Boesl, U.; Neusser, H.-J.;Schlag, E. W . Z.Nuturforscfi. A 1978, 33, 1546. (19) Boesl, U.; Neusser, H.-J.;Schlag, E. W. Luser-inducedProcesses in Molecules; Smith, J . D., Kompa, K. L., Eds.; Springer: Berlin, 1979; Vol. 6.
(20) Hopkins, J . B.; Powers, D. E.; Smalley, R . E. J . Pfiys. Cfiem. 1981, 85. 3739. ( 2 1 ) Leutwvler. S.: Even. U.: Jortner. J. Cfiem. Pfivs. Lett. 1982.86. 439.
(22j Gonohk, N.; Shimizu,-A.; Abe, H.;Mikami, N.; ito, M. Cfidm. Pfiys. Lett. 1984. 107. 22. 4871. ( 2 3 ) La’w. K: S:; Schauer, M.; Bernstein, E. R . J . Cfiem. Pfiys. 1984,8/,
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8639
Feature Article
The typical experimental setups for both ionization methods are depicted in Figure 4. In photoionization with continuous S R single pulse counting is applied. With pulsed lasers up to several thousand ions are produced within about I O ns. Hence only fast transient digitizers are suitable for recording the mass spectms6 3. Theoretical Considerations MAIN CHAMBER
-%?-
b) Figure 4, Schcmatical vicw of thc cxpcriment (a) with synchrotron radiotion ( S M . T M = spherical, toroidal mirror, L i F = lithium fluoride cutoff filter, W = photon converter, P M T = photomultiplier), ( b ) with R2PI (SHG = second harmonic generator crystal, R E T O F MS = reflcctron T O F m:iss spectrometer. pyro det = pyroelectric energy probe).
Thc lascr tcchniquc offcrs other benefits as well. The wavelength dcpcndcncc of the ion yield, measured by tuning the excitation luscr in IC- or 2C-R2PI, represents the absorption spcctrum of the neutral precursor cluster (R2PI spectrum). Hence additional information on cluster size and structure is available by the changes induccd in the spectrum of the chromophore by its molecular pnrtncrs. Thcse changes are band shifts and additional blinds due to vdW vibrations or due to structural isomers. If these "optical fingerprints" can be assigned by model calculations or rotationally rcsolvcd by high-resolution spectroscopy, they further allow the characterization of the structure of the complex. This "optical mass s p e c t r ~ m c t r y "has ~ ~a high detection efficiency, particularly if combined with high-resolution T O F mass spcctromctry as has been available since the advent of reflectrons.26 This is illustrated in Figure 3 by the T O F mass spectrum ( T O F MS) of ;I cluster beam of fluorobenzene (FB) and methanol (ME) diluted in helium and ionizcd with a single laser shot. The chicf drawbacks of R2PI. aside from the price of the laser equipmciit. is the requirement that the resonant intermediate state must not exhibit ultrafast relaxation processes, which would destroy the selectivity. Of course the excited state must not necessarily be reached by absorption of only one photon. In the case of small molecules with energetically high lying electronically excited or ionic states a nonresonant two-photon excitation may be applicable with conventional UV lasers allowing a 2 + 1 ionization schcmc. In the examples presented in the following, we studied aromatics as chromophorcs. Their first electronically excited state S, normally lies halfway between the neutral and ionic ground state, thus allowing :I simplc 1 C-R2PI ionization scheme. (24) Dao. P. D.: Castleman Jr.. A. W. J . Chem. Phys. 1986, 84, 1435. (25) Letokhov, V. S.Phys. Toduy 1977, Muy, 23. (26) &SI, U.; Neusser, H. J.; Weinkauf. R.; Schlag, E. W. J . Phys. Chem. 1982, 86, 4857.
T o interpret the absorption spectra of the clusters, it is useful to make some model calculations concerning the structure and binding energy of the neutral clusters. For large molecules a b initio quantum mechanical calculations of vdW forces are not yet available with good accuracy. To have an approximate guide in interpreting the experimental data, it is hence reasonable to work with semiempirical intermolecular potentials, preferably of the atom-atom pair potential type. One model potential among several others, which incorporates electrostatic, repulsive, dispersive, and hydrogen bonding interaction is that proposed by Scheraga et aL2' It was used for the first time by Bernstein et a1.28for modeling neutral clusters. Applying it to evaluate cluster binding energies by energy minimiza5on procedures often gives quite reasonable results. From the intermolecular interaction potential vdW modes can be calculated by the G F formalism in the harmonic oscillator approximation. An alternative method applied by us, which does not use the latter assumption, is molecular dynamic calculation^.^^ Although the frequencies derived from this model, are those of the neutral ground state of a cluster, they may be compared with the experimental values measured in the R2PI spectra for the electronically excited state. I f the hypersurfaces of both states are not very different, a condition often met in heterogeneous complexes, this may be a tolerable compromise. Thus model calculations may help us to get a rough idea of the structure and binding energy of a complex. 4. Results and Discussion While with synchrotron radiation we were mainly interested in the energetics of ion formation in homogeneous clusters with R2P1, we have studied processes in mixed, ionized clusters, eonsisting of aromatic chromophores and polar and nonpolar solvent molecules. The processes studied hitherto are electron- and proton-transfer and chemical substitution reaction^.^'-^^ Before going into details it should be noted that only results with small complexes (microclusters) consisting of up to five molecules are studied. This might appear to be a devastating limitation. From the studies presented here we conclude that even small clusters show behavior that is often very different from gas-phase behavior. In addition, the fragmentation pattern of small clusters is often identical with that of larger ones, an observation supporting the importance of small reactive subunits in larger complexes. I . Photoionization Mass Spectrometry of Molecular Clusters rzYth Synchrotron Radiation. Polar molecules: Polar molecules, particularly those intermolecularly bound by hydrogen bonds, most often undergo a proton-transfer reaction (PT) after ionization near threshold.30 This may be illustrated by the complexes of ammonia, The ammonia studied in a pioneering work in Lee's dimer, for example, decays according to (NH,), h v - (NH4)+ NH2' + e(1)
+
+
at an energy of 50 meV above the ionization limit of the parent (27) Nemethy, G.;Pottle, M. S.; Scheraga, H. A . J . Phys. Chem. 1983, 87, 1883. ( 2 8 ) Menapace, J. A.; Bernstein, E. R. J . Phys. Chem. 1987, 9 / , 2533. (29) Janes. C.; Wassermann, B.; Brutschy, B.; Baumgargel, H., to be published. (30) Brutschy, B.; Biding, P.; Riihl, E.; Baumgargel, H. Z.Phys. D 1987, 5 , 217. (3 I) Dimopoulou-Rademann, Ou.;Rademann, K.; Biding, P.; Brutschy, B.; Baumgartel, H. Bey. Bunsen-Ges. Phys. Chem. 1984, 88, 215. (32) Brutschy, B.; Janes, C.; Eggert, E. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 435. (33) Brutschy. B.; Janes, C.; Eggert, J . Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 74. (34) Baumgartel, H.; Brutschy, B.; Riihl, E. Phys. Scr., in press. (35) Ng. C. Y.; Trevor, D. J.; Tiedemann, P.W.; Ceyer. S . T.; Kronebusch, P. L.; Mahan, B. H.; Lee, Y . T. J . Chem. Phys. 1977, 67, 4236.
8640
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The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 M
:.>
TABLE I: Absolute Proton Affinities (kJ/mol) of the Alkylamine Cluster System M' PA(M)' PA(M)* PA(M)IhW' PA(M,)* PA(M,)b MeNH2 896 930 92339 970 975 EtNH, 908 940 980 995 Me,NH 923 955 990 995 Et2NH 945 965 1000 Me3N 942 900 920 Et,N 972 930 960
+(M-H)'+H++~-
f PA(M
D.I
I
'Lias et
* P I M S f 1 5 kJ/mol. 'Me = CH,, Et
+ CH,CH,.
TABLE 11: Ionization Potentials IP(M,) (eV) of the Alkylamine Clusters Ma.' IP(M)' IP(M)* IP(M,)* IP(M,)* MeNH, 8.97 f 0.02 8.97 f 0.05 8.1 i 0.2 7.9 f 0.2 8.86 f 0.02 8.82 f 0.05 8.1 f 0.2 7.9 f 0.2 EtNH, Me2NH 8.24 f 0.03 8.22 f 0.05 7.8 i 0.2 7.7 f 0.2 Et,NH 8.01 i 0.01 8.01 f 0.05 7.5 f 0.2 7.82 f 0.02 7.87 f 0.05 Me3N Et3N 7.50 f 0.02 7.48 f 0.05
"Reference 40. bReference 37. < M e = CH3. Et = CHlCH2 D (Mn.3
- M)
TABLE 111: Association Energies AE,, of the Neutral Alkylamine Clusters M, (kJ/mol)" and Dissociation Energies of the Ionic Alkylamine Clusters (kJ/mol) M AE," A E , AEd D ( M t - M)* D(M,+- M)* MeNH, 15.7 (14) 43 71 100 50 EtNH, I7 (15) 46 75 90 50 Me2NH 1 4 ( 1 3 ) 39 65 50 35 I 5 (14) 42 69 60 Et,NH 2 (2) 6 Me,N Et3N 3 (3) 7
'Values in bracket from ref 42. *f20 kJ/mol.
10
15 ENERGY
20 :eV)
25
Figure 6. Photoion yield curves of the benzene dimer (BZ,) at different stagnation pressure? The arrows indicate the higher ionization potentials bBz = 60 Torr)
dimer ion. Calculations by T ~ m o d reveal a ~ ~ that the ionized dimer exhibits in its energy minimum the structure (NH4)+--NH2'. With additional cncrgy ammonium ions are formed by dissociation of thc radical. Thc P T takes place from the primarily ionized molecule to its neutral partner via the H bond. I f PT is observed for one cluster size it is generally observed for largcr clusters, too. In experiments with many different polar molecules we found that PT is a typical fragmentation pathway evcn if thc moleculcs were not hydrogen bonded in their neutral ground state. From the energetics of this intracluster acid-base reaction, Figurc Sa. lower bounds to the absolute proton affinity (PA) of the clustcrs may be deduced by a Born-Haber cycle.37 These (36) Tomoda, S. Chem. Phys. 1986, 110, 431. (37) Biding, P. G . F.: Ruhl. E.; Brutschy. B.; Baumgartel, H. J . Phys. Chenr 1987. 91. 4310.
values are useful for establishing an absolute proton affinity scale3* by providing values for calibrating the relative proton affinity ladder. The values for some alkylamines in Table I may illustrate this. The most important result is the increase of the PA values with the cluster size, due to the enhanced stabilization of a proton with increasing number of solvent molecules. On the other hand if one compares the IPS of the unprotonated parent clusters in Table 11, they show a decrease with cluster size. This is similarly due to the additional stabilization of a cluster ion by polarization or exchange forces as compared to its neutral ground state. From this reduction of the IP of M, relative to M,,, lower bounds to the ionic dissociation energy D(M,,+ - M) of a monomer in an n-mer can be calculated by a Born-Haber cycle (Figure 5b) giving the following relation: D(M,,+ - M) = IP(M,,) - IP(M,) + D(M,I - M) (2) with D(M,, - M) as the binding energy of a monomer to a neutral Mn-, cluster. In Table 111 some values of the ionic dissociation energy D( M,+ - M) are listed, derived by applying eq 2. Usually these values are smaller than those derived from equilibrium measurement^,^^ indicating that the vertical IPS of clusters measured in PIMS may differ considerably from the adiabatic ones due to vibrational excitation of the ions after a vertical ionization transition. This also reflects the different equilibrium distance in a neutral and an ionized cluster. The neutral binding (38) Lias, S. G.; Liebman, J . F.; Levin, 1984, 13, 695.
R. D. J . Phys. Chem. ReJ Data
(39) Del Bene, J. E.; Frisch, M. J.; Raghavacharl, K.;Pople, J. A. J . Phys. Chem. 1982, 86, 1529. (40) Watanabe, K.; Nakayama, T.; Mottl, J. J . Quanr. Specfrosc. Radial. Transfer 1962, 2, 369. (41) Bisling, P . G . F. Doctor Thesis, Freie Universitat Berlin, 1987. (42) Lambert, J . D.; Strong, E. D. T. Proc. R. Soc. London, A 1950,200, 566. (43) Ruhl, E.; Bisling, P. G. F.; Brutschy, B.; Baumgartel, H. J . Electron. Spectrosc. Relat. Phenom. 1986, 41, 41 I . (44) Baumgartel, H.; Jochims, H.-W.; Brutschy, B. Z . Phys. Chem. 1987, 154. I .
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8641
Feature Article encrgics D(M,, - M) in eq 2 may be deduced from fragmentation appearance potentials (FAP, Figure 5b) according to the following relation: FAP(M,I+)
+ D(M,-I
= IP(M,I)
- M)
(3) The sum of thc ncutral dissociation energies of a cluster defines its vdW association cncrgy AEn. CH3F: As an cxamplc of clusters of polar molecules with no hydrogcn bonds in thc ncutral-state methyl fluoride is of some interest. Its ion chemistry was studied in ion-molecule reactions by M c A ~ k i l and l ~ ~ Beauchamp et aLa some years ago. Bernstein and G a r ~ c yrecently ~ ~ studied its cluster ion mass spectrum by elcctron impact ionization with energetically undispersed electrons. Photoionization with monochromatized SR allows one additionally to dcducc thc formation enthalpies of the product ions from the appcarancc potcntials. Thus thcir ionic structure may often be assigned.43 The mass spectrum of thc cluster system is depicted in Figure I . Surprisingly cvcn at the thresh.wI only protonated clusters appcar. Whilc thc monomer mainly fragments to CH2F+ and CH3+. the clusters favorably fragment by proton transfer or by loss of H F if the protonated species are the primary ions formed upon ionimtion. From the energetics and pressure dependence of the product ions one may assign them to intracluster ionmolecule reactions ( I M R ) in which the primarily formed ions rearrange in strongly exothermic reactions with the surroundings by cjcction of small, ncutral molecules like H2 and HF. This behavior is similar to that observed in gas-phase ion-molecule reactions. However, one also observes product ions like C2H7F2+, which do not appear in IMRs. Most probably they are formed in trinicrs by thc following cascade reactions: (CH,F)3 + hv [(CH3F)3+] + eC2H7F2++ CH2F' + e- (4) 4
+
and may partially decay according to C*H,F2+
-+
C2H6F+
+ HF
-+
C2H4F' i- H2
+ HF
(5)
The corrcsponding association reaction although cxothcrmic by 134 kJ/mol does not take place in bimolecular collisions. Thc fragmcntation behavior of these cluster ions clearly demonstrates that intracluster reactions are similar to but not identical with thosc obscrvcd in the gas phase. The energetics of the ions also unveils the dominant role of protonated dimers in the intraclustcr fragmcntation pattcrn. The product ions in the mass range above the trimer are, aside from additional solvation of neutral subunits, identical with those just discussed, indicating that their fragmentation dynamics is most probably determined by dimeric or trimeric reaction centers. Nonpolar molecules: For nonpolar or weakly polar molecules PT most often is a negligible process in the ionization threshold region. For example, benzene and its derivatives show no chemical fragmentation. On the other hand clear evidence is found that thcir clusters may vdW fragment after IC-RZPI via the S, So transition.*O As an example,48the photoion efficiency (PIE) curve of the benzene dimer (Figure 6) exhibits a t higher energies a pronounced dependence on the stagnation pressure, while the value of the threshold remains constant. This clearly reflects fragmentation of larger clusters which are favorably produced at higher stagnation pressure. From eq 3 follows, with D(M,, - M) being positive FAP(M,-,+) > IP(M,,) +-
-
11 5
11 I 11.9 PHOTON ENERGY / e V
Figure 7. PIE curves of heteroclusters (a) C6H6*Arand (b) C6HSF.Ar near the atomic resonances of Ar. indicated by arrows.
unfragmented M,,+ ions only some tens of meV above the ionization limit. The fragmentation process may be induced by excitation of intramolecular vibrations in the solvated ion, which may couple to the intermolecular vdW modes with concurrent predissociation particularly in intermolecularly "hot" ions. In eq 3 it was assumed that the vertical ionizing transition is not far from the adiabatic limit. This is most often the case as otherwise the thresholds should be shifted contrary to the observation a t higher stagnation pressure. The given example also demonstrates the limits of ionization mass spectroscopy without energy dispersion. In summary it is mandatory in interpreting data from mass-analyzed cluster beams to characterize the neutral clusters in size and if possible in structure with an energy-selective method. 2. Infraclusfer Penning Ionization. If in mixed clusters a molecular subunit is electronically excited with an excitation energy larger than the ionization potential of one of its molecular partners, a charge transfer may take place by which the partner is ionized: A*.B
-
A.B+
+ e-
(7)
This autoionization process is generally called Penning ionization (PEI). While it has been studied in the past two decades by many groups in gas-phase collisions experiment^,^^^^^ it was observed in clusters only recentIy.24,5'*52 lntracluster PEI may be illustrated by the benzeneargon (BZaAr) mixed cluster. If in the Ar atom a 3p6 3ps 4s dipole transition is excited a t about 11.6 eV, the benzene moiety may be ionized. The PIE curve of the complex (Figure 7a) therefore exhibits in this energy region the resonances of the atomic transition in the associated Ar atom. Note that in comparison to the resonances for a bare Ar atom in the cluster they are shifted to lower energy by 70 meV. Hence the Ar atom is bound to the aromatic and is not produced in a collision by associative Penning ionization:
-
Ar*
+ BZ
-
Ar-BZ+
+ e-
(8)
The shift to lower energies reflects the increased attraction forces between FB and Ar* as compared to unexcited Ar. A simple
Hence fragment ions MFI+ contribute to the ion yield curve of (45) McAskill, N . A. Aust. J . Chem. 1970, 23. 2301. (46) Beauchamp. J . L.; Holtz, D.;Woodgate, D. S.; Patt, S. L. J . Am. Chem. SOC.1972, 94, 2798. (47) Garvey, J . F.; Bernstein, R. B. J . Am. Chem. SOC.1987, 109, 1921. (48) Ruhl, E.; Brutschy. B.; Baumgartel, H. Chem. Phys. Lett. 1989, 1-77, 379.
(49) Hotop, H.; Niehaus, A . 2.Phys. 1969, 228, 68. (50) Fujisawa, S.;Ohno, K.; Masuda, S.;Harada, Y . J . Am. Chem. Soc. 1986, 108, 6505. (51) Kamke, W.; Kamke, B.; Kiefl, H. U.; Hertel, I . V. Chem. Phys. Lett. 1985, 122, 356. (52) Ruhl, E.; Bisling, P.; Brutschy, B.; Beckmann, K.; Leisin, 0.; Morgner, H. Chem. Phys. Lett. 1986, 128, 512.
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Thr Journal of Physical Chemistry. Vol. 94, No. 24, 1990
-
TABLE IV: Shifts ( A D = Do - D , ) of the Ar Autoionization 4s P3,2]in Mixed Clusters with the Resonances IAr 3p6 .4roinatic M 101 ucnc
Dnh 1.85
bc.n/cnc Iluorubcn/c.nc 1)-difluorobcnicnc
1.35 1.6 4 73
\I
n+b 17.12 11.51
9.17 I .O?
txpcriincntai v a l u c ~i n incV ( * S k.l/ mul. In me\'
.I,hmrD
-127.4 -84.8 -47.4
.I*""Da
-95 -70 -40
38
explanation mal be given for the red-shift. When in Ar an electron is promoted to the 4s orbital, it is about I2 8, away from the atomic core. Thcrcforc the interaction energy between the aromatic molcculc and the Ar*. which is located 3.5 8, above the aromatic ring. i\ determined by the attractive ion-induced dipole interaction. This is for highly polarizable aromatics considerably stronger than the di\persivc forccs in thc ground state of the cluster. b ' i t h other substituted benzenes the shifts depend on the substituent> electron-withdrawing power and the dipole moment ;is ciin be \ccn froiii Figurc 7b and Table IV. respcctivcly. This siiiiplc electrostatic picture was verified in a niodcl calculation. The thcoreticalll derived shifts, enlisted i n Table I V , represent thc diffcrcncc in the binding energy of a ncutral (Do) and ;I vcrtic;\lly ioni7ed (D+)Ar atom (columns I and 2 in Table I V ) . Both energies were calculated for the geometry of the neutral complex ;IS determined by the Scheraga potential. The agreement w i t h thc experimentally observed shifts is quite satisfactory. confirming t hc electrostatic model. The Penning resonances also depend on the number of Ar atom\.53 Sharp resonances are observed for clusters with up to thrcc ' l r :itom\. From 1 :3 clusters onward, broad blue-shifted resonance:, appear due to larger Ar clusters. At high partial prc\\urca of A r the PIE curve of benzene+ shows broad Penning rcsonanccs that arc blue-shifted relative to the atomic resonances bq about 120 mcV. Similnr resonances are observed in large Ar cluster^'^ iind migncd to surface excitons. Hence exciton-induced disaoci:itivc Penning ioni7ation of ben7ene molecules aggregated a t tlic surf:icc of Iargc .Ar,, clusters ( n = 20-50) may explain the obxrv:i t ions. 3. K m ) r i o i i t Two-Photon Ionization of Mixed Clusters. With R2PI hiivc lookcd niorc into the details of intracluster reactions, utiliiing ita .;pcci:il tlbility for characterizing the primarily formed cations. Siiiiilar to one-photon ionization, vdW fragmentation is ;IISO irwitablc \ r i t h this method but can normally be distinguished a h H i l l be discussed below. Particularly in one-color RZPI. vdW fragmentation is observed as the cncrgl drpositcd in the ion is not as freely variablc as in two-color R?PI (Figure I ) . where fragmentation may be diminished b) tuning the ionizing laser onto the ionization threshold region. Tu st'irt n i t h ;I vcrl siiiiplc example for IC-RZPI, Figure 8a shuns the ion !,icld spcctrii of fluorobenzene+ (FB) and FB.Ar,+ (ti = 1-3) c n t i o n ~ . It ~ ~illustrates the main changes in a chromupilore'\ spectrum surrounded by solvent molecules. The resonant excitation step is the S I So vibrationless transition (0;) in tlic FB iiioict\. Thc band is shifted additively to the red for u p t u t h o a t o n i a and to thc blue for three atoms. The phenomenon of spectroscopic shifts induced by solvation has biiicc long been known in condensed phase and is called soIV;itocIiroi)iisiii.X The red shift is generally due to a stronger intcriiioleculiir binding of the cluster with the chromophore in the S l ( n * ) a s compared to thc S o ( x ) state due to a n increased polari/.abilit!, Red shifts ;ire rather general for substituted aromatics
-
( 5 3 ) Kamke. W.: Kamke. B.: Wang. 2 . : Riihl. E.: Brutschy. B. J . Chem. Phy.y. 1987. 86. 2 5 2 5 . (54) Wurmer. J : Gurielski. V.: Stapelfeldt. J.: Zimmerer. G.: Moller. T. Ph.i.\. Scr.. in prew ( 5 5 ) Wnltcrr. E ,A : Grover. J . R.: White. M . G . : Hui. E. T.J . P h y . C'he,,r. 198s. 89. 38 14. ( 5 6 ) Rndcm;inn. K : Brutschy. B.: Baumgartel. H. Chem. Phys. 1983, 80. I29
hi FB Ar'
iiicV). hThcoretic;il v;iIucs i n
dl FB.Ar,'
RELATIVE FREQUENCY 1 cm' i Figure 8. Ion yield curves of (a) FB+ and (FBAr,)' ( n = 1-3) excited i n thc vicinity o f the 0: transition of FB. T h e hatched bands a r e due to \,dW fragmentation ( * : vdW vibration).
and nonpolar solvent molecules. In the following, however, some examples will be given of how polar solvent molecules may induce a blue shift which reflects the influence of their dipole moment. One reason for this may be that during a vertical excitation transition the charge distribution of the aromatic is changed with the solvent's dipole being unable to accommodate instantaneously to the energetically most favorable geometry. This may result in a destabilization of the excited cluster resulting in a blue shift of the transition wavelength. The additivity of the shift for up to two argon atoms may be explained by geometrically equivalent sites on opposite sides of the aromatic ring. A similar effect was observed by Amirav et 211.~' and Levy et al.58in the fluorescence spectra of clusters. The blue shift for the 1.3 complex must be due to the association of an Ar dimer and is not well understood. I n addition to spectroscopically shifted 0: bands each spectrum in Figure 7 shows small satellite bands, which increase in intensity a t higher stagnation pressure. They are assigned to vdW frag(m iiicntation. As a rule, if two small clusters AB,' and AB,,,,' = 1-3) possess identical spectral fingerprints in their R2P1 spectra with the bands of AB,' exhibiting a pronounced pressure dependence relative to other bands, they are assigned as fragment bands of an AB,+,,+ precursor. Band * in Figure 8b is unchanged by an increase of po and is hence attributed to a vdW mode with a frequency of 40 cm-'. In ; I model calculation we determined for a FBeAr cluster in the So state a vdW stretching mode of Ar along the z axis vertical to the ring plane of 41 cm-I. With this method, clusters reveal an individuality that is not observed in one-photon ionization spectroscopy. These "optical fingerprints" allow us to selectively ionize small clusters or to retrieve the neutral precursors of product ions. The method is restricted to small aggregates since the fast increase of vibrational and structural degrees of freedom in larger complexes leads to broad spectrally congested absorption bands. Although the size selectivity is then lost, the ionization selectivity is still conserved in many cases. ( 5 7 ) Amirav. A.; Even, ti.; Jortner, J . J . Chem. Phys. 1981. 75. 2489. ( 5 8 ) Haynam. C.A.: Brumbaugh. D. V.: Levy, D. H. J . Chem. Phys. 1984, 80. 2 2 5 6 .
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8643
Feature Article ,
10 -
EIeV
8-
9.199
6n
Y
4.688 hv
4 -
-5
= 37816 cm
2-
4 104
.g
0 n (II
86-
$!
(FB-Ar) +
v
: *
.2
P
h
.2
500 J4 -!;
0 .m
iJ
I
i
41 h
.r(
J,
E: .r-i
I
56250
36350
I
I
I
I
,
400
-
300 -
I
36450
hv 2 (cm-')
- 100
Figure 9. Ion yield curvc of' FB+ and ( FB-Ar)' measured with 2C-R2PI
according to the depicted excitation scheme under field-free conditions.
-50
0
50
100 150
relative frequency (cm-' Figure 10. R2PI spectra of some ions from a FB/dioxane/He expansion
near the 0; transition of' FB. 1:l complex (x, y, 2 direction). In a two-color R2PI experiment (Figure 1 ) the wavelength of the ionizing laser niay be tuned while the exciting laser is fixed to ;I cluster-specific absorption band, giving size-selective ionization t hrcs holds ;I nd fragnicn ta tion appearance potentials ( FA P). A + + -D - I\ t I Figure 9 shows the results of such an experiment with F B s A ~ . ~ ~ While the threshold of the monomer is very sharp, that for the AtD+ + e mixed I : I cluster is shifted by 27 nicV to lower energies and exhibits a slower rise of the threshold mainly due to a change in the equilibrium distances. By this method highly accurate IPS and FAPs ( A E = 2-5 meV) can be determined. In the following Ex 2 wc report the application of R2PI to the study of charge-transfer and chemical reaction processes. 4. Electrori Transfir in Heterogeneous cd W Complexes after K P I . I f :i specific molecule in a cluster is ionized. it may be neutralized by the surrounding solvent molecules by electron transfer (ET). This redox reaction is summarized by intermolecular distance ..
+ P
. ' ; :
A+.B,
-L(A.B,,+)*
2
A
+ B,+
Figure I I . Schematic potential diagram for dissociative electron transfer (9) ( A = acceptor, D = donor). If the dissociation step 2 docs not take place, ET is not detectable by mass spcctromctry. Thcrcforc we call the reaction dissocinticv more qualitative discussion of dET with the help of a schematic elc~ctrotttran.?fcr(dET). The precursor A+.B, of a possible product potential diagram in Figure 1 1 . We have chosen as reaction B,+ is retrievable from its R2PI fingerprints. For example, Figure coordinate the intermolecular distance between both molecules. I O shows thc R2PI spectra of some ions from an expansion of FB ET takes place if the curves of the ingoing-FB+ DX-and and dioxnnc (DX = C4H,02). with FB being the c h r o m o p h ~ r e . ~ ' * ~ ~outgoing-FB + DX+-ionic channels intersect each other. If The spectrum of thc 1 : l coniplcx shows instead of the 0: band both have the same symmetry, then instead of crossing each other of the bare monomer an equally spaced progression ( 6 v = 18 cm-I) thc adiabatic potentials are split by an amount depending on the starting 32 cm-' to the red. In the mass spectrum one also observes coupling of these two states (avoided crossing). After a vertical DX+ ions. Their R2P1 spectrum nearly perfectly matches that transition, the starting point on the ionic surface is expected to of the 1 : 1 coiiiplcxcs, giving direct evidence for an intracluster lie outside the equilibrium distance RM. Hence both molecules dET process ;IS formulated in eq 9. The appearance of dET and should vibrate around R M . At each passage of the intersection the excitation of :I vdW mode in the 1 : l complex niay be qualregion at R, the system either stays trapped in the adiabatic well itatively explained from a molecular point of view by considering or follows the diabatic curve in an ET process. It dissociates from the silndwich structure in the inscrt in Figure I O as calculated this new state if its total energy is above the asymptotic dissociation with thc Scheraga potential. The orbitals involved in the transfer limit. Due to energy conservation, the following relation must arc thc electron hole in the 7r-orbital of FB' and the lone-pair hold for d E T to take place: HOMO of onc of thc ncnrcst-neighbor oxygen atom in DX. After IP(A) > IP(B,) (10) a SI Sotransition the molecule vibrates in the z direction (hptheor = 22 cm-I). After ionization this vibration is expected to be even The transition probability may be described by Landau-Zener stronger due to incrcascd polarization forces. Thereby ET may theory. As ET is a vertical ionization transition, the Francktake place. Condon factors in the donor must be nonzero for the recombination The generally accepted theory for ET is that of Marcus.60 Due energy E(R,). For small differences in the IPS of the donor and to the unknown potential curves we shall confine ourselves to a acceptor molecule the crossing point is expected at a large intermolecular distance, resulting in a narrow splitting, which is (59) Radcmann. K.; Brutschy, B.; Baumgiirtel, H. X l l l ICPEAC Berlin, favorable for a transition. If, on the other hand, the velocity with Book of Abstracts; Eichler. et al.. Eds.; North Holland: Amsterdam, 1983; which the system passes through the avoided crossing is small, p 73. (60) Marcus, R. A. Electrochim. Acta 1968, 13. 995. the probability for ET in one passage should be low. But since
+
-
8644
The Journal of Physical Chemistry. Vol. 94, No. 24, 1990
Brutschy
the donor niolccular is trapped, ET may takc placc after many vi brat ion\. DEP is ;I rathcr general phenomenon in clusters. It is observed for many systems for which eq IO holds. Sometimes it sets in only in larger I :n complexes due to the size dependence of the ionization potential of the solvent cluster as was also reported by Castleman I f energetically allowed, it always dominates other ct competing processes such as proton-transfer or substitution rcactions as will be shown below.. From chemical common sense this is not uncxpcctcd because ET generally rcquircs no complicated rearrangement of the nuclei (transition state). 5 . Prutoir Tru/i.sJor i/i Misod Clusters. Another elementary reaction that proceeds i n clusters is proton transfer, which after Bronsted is the elementary process in an acid-base reaction. We havc studicd i t by preparing a cationic aromatic acid and have looked fur the following dissocioriw proton-transfer (dPT) rea cI i on : R'H+.(B,) -!+ (R'-B,H+)*
2
R'
+ B,H+
(1 I)
Again the R2Pl spectrum of the protonated solvent cluster B,H+ musi revcal thc fingerprints of the neutral precursor RH.B,. As discussed above, the proton affinity of a cluster generally increases with its !,i/c. Hence onc cxpccts that dPT should depend on the sizc of the solvcnt cluster B,, as the proton transfer mainly depends on the rchtivc proton affinities of the conjugate base R' and the B, clustcr. respectively. As a rule the latter is not clearly defined for nonpolar molecules as thcy may surround thc aromatic on different sidcs and with stronger bonds to the chromophore than to each othcr. For polar molecules the situation scems to bc different particuliirly if they form hydrogen bonds. Some evidence for thc cxistcncc of "solvent clusters" B, in I:n aggregates is pruvidcd from thc R2PI spcctrn, as discussed belo\*. I n the mixed clusters we mainly used toluene' (TO') a s thc cationic acid. Proton transfer reactions with cations of methylbenrcncs arc of grcnt importance in the oxidation of hydrocarbons and take placc cvcn i n rathcr acidic solutions. The general rcaciion schcnic is the following: TO+.B,
-
-
benzyl'.B,H+
benzyl'
+ B,H+
(1 2)
With aiiiiiiunia as solvent B this reaction already sets in for the 1 : I complcx.33 With methanol it starts only in 1:2 complexes. Still more iiiolcculcs arc necessary for dPT with water. The smallest protonated waicr complex observed in the T O F MS of an expansion of toluene with D 2 0 is the protonated trimer. The R2PI spectra of the dominant ions are displayed in Figure 12. Thc intensity of the (D20)3H+complex (Figure 12f) is relatively small. Its fingerprint band (16) reappears with higher intensity in thc spectrum of the 1 :3 precursor and as a fragmentation band in thc spcctrum of thc next two smaller clusters. I n contrast to this the signal of the protonated tetramer is a factor of IO more intense whilc no 1:4 precursor ions are observed in the mass spcctrum. The fingerprints in Figure 12e show two bands ( 1 7 . 18) which reappear as fragmentation bands in the spectrum of thc I :3 complex. From the intensities we calculate the following branching ratio for the two competing processes, i.e., vdW fragmcntiition (vdWF) and dPT. in the 1.3 and 1.4 clusters: (1:3)+-
365?
(I:3)+
- + - + - + - + - + 359
13'1:
I69
( I :4)+ 16'4
(1:2)+
(l:l)+
DzO (vdWF)
(DzO)? (vdWF)
( D20)3H+ 747
( 1 :3)+
(Dz0)4H+
ben7yl' (dPT) DzO (vdW F)
benzyl' (dPT)
(61) Breen. J . J.: T7eng. W . B.: Kolgore. K.: Keesee, R . G.:Castleman, Jr.. A . W . J . Chcwr. Phj,s. 1989. 90. 19.
.Y
Lo
C
61
-t
2
t?
5 n
60
.
o
50
100
no
200
relative frequency ( cm-') Figure 12. R2Pl spectra of some ions from \ion
d
t o l u e n e / D 2 0 / H e expan-
Direct conclusions from the branching ratio in these clusters, which arc also supported by results of other systems, are the following: ( I ) dPT and vdW fragmentation are competing processes. (2) With larger solvent clusters dPT becomes dominant and the probability of vdW fragmentation is reduced. I n discussing the observed size dependence of dPT we have assumed that the solvent molecules are forming a solvent cluster stabilized by hydrogen bonds. This assumption is supported by the analysis of the spectra in which the fingerprints of the protonated species always reappear in the spectra of the mixed clusters with identical number of molecules. If the cluster contained molecules on different sides of the chromophore, the dissociating protonated moiety would be expected to contain fewer solvent molecules than its precursor. The excess energy in the dPT channel can be derived by Born-Haber cycles similar to that in Figure 5a. From these one deduces that with increasing number of solvent molecules dPT is becoming exothermic due to an increase of the proton affinity of the solvent cluster. On the other hand, with increasing size of the complex the number of degrees of freedom increases and the excess energy stored in the complex after IC-RZPI-typically some hundreds meV-may dissipate into the intra- and intermolecular vibrational modes. Hence we expect from statistical theory a reduction of vdW fragmentation in larger solvent clusters. Both conclusions are supported by the experimental findings. From the energetics we calculated the minimum cluster sizes necessary for dPT to take place and found for all systems excellent agreement with the observations by using thermochemical data. I n summary, the intracluster proton transfer (protolysis) seems
The Journal of Physical Chemistry, Vol. 94, No. 24. 1990 8645
Feature Article
TABLE V: Appearance Energies (eV) and Neutral Prccursors of Some ions M + in Fieure 13 Measured bv Excitation of Band X M IP(M) FAP( M+) band X precursor FB 9.200 f 0.005 0 FB*ME 9.057 f 0.01 3 FB*ME 9.2S1 i 0.01 5 1 :2 anisole 9.112io.01 5 I :2 80 n
I
40
'I
.u
v)
o C 40
-
3
g
v
30 20 10
h u . iJj 0
.+ 5
.r
8 6
C
.-0
4 2
0
-50
0
50
100
150
200
relative frequency ( cm-' Figure 13. R2Pl spectra of some ions from a FBlmethanol-d,/He ex-
pansion. to depend only on the relative proton affinities of donor and acceptor. No indication of substantial activation barriers for PT was found. 6. Nircleophilic Ipso Substitution Reactions in Clusters. In addition to these charge-transfer processes chemical reactions may also take place between an ion and its solvent As an example Figure 3 shows the T O F spectrum measured for an cxpansion of FB and deuterated methanol (ME). A product ion A N is clearly seen with the mass of (phenyl methyl-d3 ether)' (anisole'). Evidence for an intracluster nucleophilic substitution rcxtion (SN) can be deduced from the R2Pl spectra in Figure 13. Thc fingerprints of thc 1 : l complex in Figure 13b are blue shiftcd and exhibit two groups of bands. The bands 2 and 3 renppar in the monomer as fragmentation satellites (see the insert in Figurc 13a). Bands 4-6 do not show such fragmentation bands but corrcspond to identical resonances in the spectrum of anisole' in Figurc l3c. Hcncc this group is called "reactive", i.e., it is assigned to ;I rc;ictivc prccursor. Since no such bands appear in the spectra of larger 1 : / I complcxcs, one would at first assign it to ;I I : I coniplcx and postulatc two different isomers, one of which reacts and one of which fragments. Nibbcringh3 w;is not able to find thc analogous ion-nioleculc reaction in a n ICR mass spectrometer, although according to thcrniochcmicnl dntn i t should be exothermic by 100 kJ/mol. Hence there must be an activation barrier for the substitution renction. Rcmcasuring the spectra of the 1 : 1 complex under dil'l'crcnt cxpansion conditions and with 2C-R2P1 resulted in the assignment of a 1 :2 reactive precursor. which should completely fragment cithcr by a reaction giving anisole' or by vdW fragmentation giving a 1 : I cluster ion. Further fingerprints for anisole' (binds 8 lind 9 in Figurc I3c) and anisole+.methanol (Figure 13d) (62) Cook. K . D.; Tajlor, J . W. Itit. J . .Mass. Spertrom. Ion Phys. 1980. 115. 159.
( 6 3 ) Nibbcring. N . Amsterdam. private
communication.
relative frequency ( c m - ' ) Figure 14. R2PI spcctra of some ions from ;i FB/ammonia/He expansion.
reveal that similar processes take place in 1.3 clusters. Therefore thc reactions may be summarized as follows: ( 1 : 2 ) + 3( l : l ) +
495
(anisole)+ + D F
(l:3)+-
22%
5 1 '1
+ ME
27%
(vdWF)
+ ME
(1:2)++ ME
(15)
(S,)
(16)
(vdWF)
(17)
+ DF + ME (S,) (anisole)+ + ME2 + D F (Sh,vdWF) (anisole)+-ME
(18)
The appearance energies of anisole' and of the 1 : I cluster fragmcnt, dctcrmined by 2C-R2P1,64 are summarized in Table V . From these data additional cvidencc is provided that the assignment in the eq 15 is correct and that the nucleophilic ~~~
~
(64) Richn. Ch.; Avdiew. Y . ; Wassermann,
H . Ber. Bunsen-Ge.r. Phys. Chem.. in press.
B.: Brutschy, B.; Bauingartel,
8646
Brutschy
The Journal of Physical Chemistry, Vol. 94, No. 24. 1990
substitution reaction is solvent catalyzed. With amnionia ;is nucleophile a similar SNreaction takes place giving aniline'.32 In addition in the T O F M S protonated and unprotonated ammonia clusters appear, which must be produced by dET. The R2PI spectra measured for the dominant ions of this cluster system are shown in Figure 14. Bands 3 of aniline' (Figure 14c) reappears ;is a vdW fragmentation band in the spectrum of 1 : l complex and as a dET band in the spectrum of (NH,),'. This provides direct evidence for a 1:2 precursor with three competing exit channels. Obviously dET starts only in the 1 :2 complcx since no N H,' is found. This can be rationalized by thc energy condition in cq I O since it is only satisfied by the IPS of the ammonia clusters larger than the dimer (IP(FB) = 9.2 eV; IP((NH,),) = 9.02 e V 9 . Again the spectrum of the detected 1 :2 complcx docs not show a similar band due to complete fragmentation of the 1 :2 complex. Very recently Tholmann et al.69 reported that no substitution reaction takes place in the gas phase between FB' and ammonia in accordance with our observations for the 1 : l complex. Using mono-, di-, and trimethylamine, with rapidly decreasing I P but with growing nucleophilicity analogous chemical reactions are expected but not observed. Charge transfer is the only exit channel, starting already in the 1 :1 complexes. The dominance of dET may be rationalized by the larger exothermicity of dET. Surprisingly, chlorobenzene' (CLBZ) may react with ammonia already in the 1 :1 complex, giving protonated aniline (anilinium).32366*68Both ions exhibit identical optical fingerprints in the corresponding R2PI spectra, from which we assign the 1 :1 complex as precursor for the following substitution reaction:
(ID=F
@
=H
@=o
0
=c
Figure 15. Gcomctry of the FB.(methanol), complex a s calculated with the Schcraga potential. In the energy minimum a H-bonded methanol dimer is aggregated to the chromophore.
(19) The analogous gas-phase reaction was observed by Van der Hart et and Tholmann and G r u t z m a ~ h e r confirming ,~~ our hypothesis mentioned earlier that reactions in 1 :1 complexes should also be observed in bimolecular collisions. A similar substitution reaction takes place with methylamine, both in the cluster66and in ion-molecule collision^.^^ For di- and trimethylamine dET is again the only exit channel. With p-chlorofluorobcnzcne and ammonia as nucleophile, similar behavior is observed. I n the 1:l complex only chlorine is substituted, while in the 1.2 aggregate both halogens may be substituted. In summary, the following conclusions can be drawn from these data: In ipso substitution reactions of the halobenzenes fluorine is substituted only in 1 :2 complexes with H F as the leaving group, while chlorine is substituted in 1:l aggregates with CI' as the leaving group. These results suggest a number of questions which need to be answered to assign the observed product ions to intracluster SN reactions. First, what evidence do we have that the reactions indeed take place in the ionized cluster? From 2C-R2PI-measurements, from the laser power dependence of the ion signal, and from the fluorescence spectra we can exclude photoreactions with FB in the SI state.66 Second, what may be the reason for their remarkable size dependence'? Two different reaction mechanism seem to be necessary to account for the differences in the leaving group in chlorine and fluorine substitution. In trying to answer these questions, the structure of the FB. (methanol)? precursor deserves some consideration. Using the Scheraga potential, we have calculated the geometry of this complex by minimizing its intermolecular binding energy. We found the deepest minimum for a structure in which a H-bonded dimer is aggregated to the chromophore as depicted in Figure 15. Note that in this geometry the dipole moment of one methanol is approximately antiparallel to that of the C-F group, indicating a considerable contribution of dipole forces. The second molecule
may form a weak H bond to the fluorine for which the strength is not well-known but which does not change the structure very much. From this geometry the following qualitative explanation for the SN reaction may be given. After ionization of FB the positive partial charge of the C, carbon is enhanced. Therefore one expects strong Coulombic attraction of the negatively polarized nearest-neighbor oxygen atom of the methanol dimer. While no ET should take place at greater internuclear distance between this and the radical cation due to the differences in the IPS (IP(FB) = 9.2 eV and IP((CH,OH),) = 9.8 eV), at close contact a reduction of the nucleophile's IP is expected due to exchange and polarization forces (similar to Figure 1 I ) . If an electron transfer from the lone-pair orbital of oxygen to the a system of the ring then takes place, a proton transfer to the second methanol molecule should immediately follow. The methoxy radical thus produced may attack the neutralized FB in the ipso position, forming a temporary addition intermediate (a complex). The protonated methanol molecule in turn may transfer a proton to the fluorine and break the C-F bond with formation of H F (DF), ME, and anisole'. The ipso attack of substituted aromatics by a radical has also been discussed in the Iiterature'O as an addition-elimination mechanism, i.e., by the generally accepted mechanism for homolyt ic aromatic substitution. In the proposed SN mechanism the size dependence of the process is due to a double function of the second methanol molecule: ( I ) It lowers the barrier for electron transfer and initiates the formation of a radical by proton transfer. (2) After protonation it enables the elimination of the fluoride. For the ET and the nucleophilic substitution to start, it is necessary for the molecules to approach sufficiently closely so that chemical forces will operate and the atomic rearrangement can take place. I f the electron transfer is exothermic, the complex may dissociate without formation of the intermediate. This would explain the suppression of the substitution with the methylamines and the competition of dET and SN with ammonia. Recently there has been a controversy in the literature about the question, Can a radical cation be directly attacked by nucleophiles'? Pross7' has postulated high-energy transition states, arguing with the configuration mixing model. I n the condensed phase the reactivity of many radical cations with nucleophiles is indeed very These arguments may probably explain the kinetic hindrance of the otherwise exothermic reaction with fluorobenzene'. The reaction of ammonia with CLBZ' on the other hand proves that SNreactions are not generally forbidden in bimolecular collisions. Other cases have also been reported in the 1 i t ~ r a t u r - e . ~ ~ In the ipso substitution of CLBZ' by ammonia the cluster geometry as calculated with the model potential resembles a a-complex, as postulated for aromatic substitution reactions, with the ammonia's hydrogen atoms pointing toward the ring. An ET
(65) Kamke, W., private communication. (66) Eggcrt, J . Doctor Thcsis, Frcic Univcrsitat Berlin, 1990. (67) Van der Hart. W.; Luijten, W.; Thuijl, T. Org. Mass. Spectrom. 1980, IS. 463. (68) Macyania, T.; Mikami, N . J . Am. Chem. SOC.1988, 110, 7238. (69) Thdmann. D.: Griitzmacher. H.-F. Chem. Phys. L e f t . 1989, 163, 225.
(70) Tiecco, M. Acc. Chem. Res. 1980, 13, 51. (71) Pross, A. J . Am. Chem. SOC.1986, 108. 3537. (72) Eberson, L.; Blum. Z.; Helgee, B.; Nyberg. K. Tetrahedron 1978, 34, 731. (73) Drewello, T.; Heinrich, N.; Maas. W.; Nibbering, N.; Weiske, T.; Schwarz. H. J . Am. Chem. SOC.1987, 109, 4810.
C6H5C!'*NH,
+.C6HSNH3'
4-
c!'
Feature Article should already proceed in the I :1 complex because the asymptotic IPS of both molecules (IP(CLBZ) = 9.08, IP(NH3) = 10.16)differ by only 1.08 cV. Thcrcforc if thc NH3+ radical cation forms a cr-intermediate at close contact, only a chlorine radical may be ejcctcd giving thc Linilinium ion. The small difference in the IPS of FB and CLBZ may not explain the difference of their reactivity with aininonio. A possible reason for this may be that the C-CI bond is more than 1 CV wcakcr than the C-F bond, making the pulling xtion of ;t second protonated solvent molecule unnecessary. The stronger ncgativc polarization of a fluorinc as compared to a chlorine substituent may further rationalize the differences in thc leaving group. In summary for both types of substitution reactions w c proposc an addition-elimination mechanism. Recently Thdmann and G r i i t ~ m a c h e rhave ~ ~ given a similar interpretation for thc ipso substitution of CLBZ’ with ammonia and supported it by M N D O calculations. 5. Conclusion and Future Directions The examples presented here demonstrate complex reaction behavior for ionized organic clusters dcpcnding strongly on the cluster size. Only by the optical fingerprints in the R2PI spectra and their pressure dcpendence may the neutral precursors of the product ions be assigned, allowing the study of intracluster reactions from a microscopic level. The findings demonstrate non-gas-phase behavior in intracluster reactions even for microclusters. Hence they are interesting model systems on the way to a microscopic understanding of reactions in condensed phases. The methods applied may be further improved. For instance, in photoionization mass spectrometry with synchrotron radiation il better characterization of the primary formed ion in a cluster moy be achieved by applying photoion-photoelectron coincidence (PIPECO) tcchniquc~.’~From the breakdown curves of product ions (energetic fingerprints of the ion) neutral precursor complexes may be assigned. (74) Gantcfijr. G.:Broker. G.: Holub-Krappe. E.; Ding, A. Z . Phys. D 1988. 10. 319.
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8647 With R2PI, which is even more versatile than PIMS, many ionic reactions may be studied by using clusters as natural “test tubes”. From the cluster size dependence of competing processes a direct insight into complex kinetic reaction schemes is possible. The catalytic action of solvation may be studied on a molecular level. I n addition. if the structural information underlying the optical fingerprints are fully interpreted by model calculations, the structural aspects of the reaction behavior may be revealed. By measuring the appearance potentials of the product ions with ZC-RZPI, upper values to the energy barriers for the formation of intermediates may be derived. Whilc in the results presented above R2PI was applied only to processes in the ion, it may also be applied to study the size dependence of solvation-induced processes in electronically excited chromophores such as intramolecular charge t r a n ~ f e r ’or~ exciplex f~rmation.’~Another example is the solvent-induced formation of T l C T state^,^^.^* which are probably of importance in photosynthesis. In summary, the size-selective study of “tailor-made” model clusters may help to bridge the gap in the understanding of reaction behavior in gas and condensed phases. Thus some riddles of “real” chemistry may hopefully be tackled with clusters in the near future.
Acknowledgment. I a m greatly indebted to Prof. Dr. H. Baumgartel for his interest, support, and fruitful discussions. I further express my gratitude to several colleagues, who have contributed to the results presented: J . Awdiew, P. Biding, J . Eggert, C. Janes, E. Riihl, K . and 0. Rademann, B. Wassermann, and Ch. Riehn. Financial support by the BMFT (Project 05-413 FXB9) and by the Deutsche Forschungsgemeinschaft (SFB 337) is also gratefully acknowledged. (75) Cheshnovsky, 0.;Leutwyler, S. Chem. Phys. Lett. 1985, 121, I . (76) Anner, 0.;Haas, Y. C‘hem. Phys. Lett. 1985, 119. 199. (77) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (78) Grassian. V . H.: Warren, J. A.; Bernstein, E. R . J . Chem. Phys. 1989, 90. 3994.
