1505
J. Phys. Chem. 1986, 90, 1505-1506
Collisional Electron Transfer to CH,CN Clusters from High-Rydberg Krypton Atoms Koichiro Mitsuke, Tamotsu Kondow,* and Kozo Kuchitsu Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan (Received: February 7 , 1986)
Negative cluster ions, (CH,CN),- ( n 2 13), were produced by collisional electron transfer from high-Rydberg krypton atoms to neutral clusters, (CH3CN),, formed in a supersonic beam. It was found by mass spectrometry that evaporation from the cluster ions was negligible, i.e. m = n, the excess energy generated by the electron attachment being partitioned among the intermolecular vibrational degrees of freedom. The threshold size, nth = 13, thus shows that the critical size beyond which the vertical electron affinity of the CH3CN cluster becomes positive is also 13.
Introduction Negative-ion states of van der Waals clusters have received much attention in relation to the bound excess-electron states in the liquid phase such as the states of a solvated electron.'$* Experimental results have shown that stable negative ions can be produced by electron attachment to clusters composed of molecules whose vertical electron affinities (EA) are This is because the EA values of clusters increase with cluster size. Recently, we have reported the production of negative cluster ions by collisional electron transfer from high-Rydberg krypton atoms, Kr**, to neutral cluster^.^^^ The characteristic feature of this method is to utilize the very slow outermost electron of Kr** (the Rydberg electron) for attachment, from which information on the vertical EA of a cluster and the mechanism of relaxation after the electron transfer can be obtained. In the present Letter, this method is applied to acetonitrile clusters, (CH,CN),. In the gas phase the rate constant for thermal electron attachment to the 1.24 X cm3 s-I molecule-') CH3CN molecule is very small (I On the other hand, stable because the vertical EA is negative-ion states of CH3CN have been reported in the liquid and solid phases.'0*'' This suggests that the EA value for (CH3CN), increases with m and that the cross section for electron attachment to (CH,CN),,, becomes appreciable beyond a certain critical cluster size. Experimental Section Acetonitrile vapor seeded in He was expanded through a nozzle of 50 pm diameter, and (CH3CN), was generated in this supersonic beam. The mole fraction of CH,CN was maintained at about 0.04 in the stagnation region. The skimmed cluster beam was introduced into a concentric triple-grid ion s o ~ r c e Negative .~ ions were formed in this region by collisional electron transfer from the Kr** atoms with principal quantum numbers np = 25 - 35. In the outer region of the ion source, Kr** was prepared by excitation of Kr gas by electron impact. The ions were massanalyzed by a quadrupole mass filter. The mass-to-charge ratios, m l z , were calibrated by the known fragmentation pattern of the negative ions produced from perfluorokerosene (PFK) by electron impact in the ion source.'* The mass resolution, m l A m , was no (1) Haberland, H.; Ludewigt, C.; Schindler, H.-G.; Worsnop, D. R. J . Chem. Phys. 1984, 81, 3742. Surf. Sci. 1985, 156, 157. (2) Kestner, N. R.; Joitner, J. J . Phys. Chem. 1984, 88, 3818. (3) Klots, C. E.; Compton, R . N . J. Chem. Phys. 1978, 69, 1636. (4) Knapp, M.; Kreisle, D.; Echt, 0.;Sattler, K.: Recknagel, E. Surf. Sci. 1985, 156, 313.
(5) Stamatovic, A.; Leiter, K.; Ritter, W.; Stephan, K.; Mark, T. D. J . Chem. Phys. 1985, 83, 2942. (6) Kondow, T.; Mitsuke, K. J. Chem. Phys. 1985, 83, 2612. (7) Mitsuke, K.; Kondow, T.; Kuchitsu, K. J. Phys. Chem., in press. ( 8 ) Stockdale, J. A,; Davis, F. J.; Compton, R. N.; Klots, C . E. J . Chem. Phys. 1974, 60, 4279. (9) Compton, R. N.; Reinhardt, P.W.; Cooper, C. D. J . Chem. Phys. 1978, 68, 4360. (IO) Williams, F.; Sprague, E. D. Arc. Chem. Res. 1982, 15, 408. ( 1 1) Bell, I. P.; Rodgers, M. A. J.: Burrows, H. D. J . Chem. SOC.,Faraday Trans. I 1977, 73, 315.
0022-3654/86/2090-1505$01.50/0
less than 150 at m l z 400, so that the m l z numbers of the detected ions were determined with an accuracy of f 1.
Results Figure 1A shows the mass spectrum of the negative ions produced from (CH,CN), in collision with Kr**. The following experimental results are obtained from the mass spectra under different stagnation pressures. (1) All the observed negative ions can be assigned to (CH,CN); ( n L 13). (2) The threshold size, nth, is 13, being independent of the stagnation pressure, Po, in the range of 1000-2600 Torr. (3) At Po = 1250 Torr, the size distribution curve for (CH,CN),- is such that the ion intensity increases rapidly with n to a maximum at n 20, beyond which it falls off monotonically 40. to n (4) At Po = 2500 Torr, the size distribution curve increases gradually with n to n N 30 and levels off thereafter. The following additional information is obtained by the mass spectra of (CH3CN),IH+ produced by impact of electrons having an energy of 50 eV (see Figure 1B). (5) Protonated cluster ions, (CH3CN),_,H+ ( n L 2 ) , are observed. The size distribution curve for (CH,CN),_,H+ ( n L 11) is scarcely affected by a change in the stagnation pressure. Discussion As described in previous papers, the following three-step mechanism can be applied to the formation of ( C H , C N ) L . ~ ~ ~ (a) The Rydberg electron is transferred to (CH,CN),, and a transient negative-ion state is formed: (CH,CN), Kr** (CH,CN),- + Kr+ (1)
+
-
(b) The nuclear configuration of the cluster is reorganized in the presence of the captured electron. (c) The excess energy generated by this rearrangement may, in general, be released by evaporation of molecules: (CH,CN),-
-+
(CH3CN);
+ ( m - n)CH,CN
(2)
In order that a transient ion, (CH,CN),-, can be formed in step a, the vertical EA of (CH,CN), should be nonnegative from the requirement of the energetics set in process 1, where the small kinetic energy (10-20 meV) of the Rydberg electron can be disregarded. The vertical EA of a CH,CN monomer has been estimated to be -2.84 eV; this energy corresponds to that of the shape-resonant state of CH3CN- (*TI),where an incoming electron is temporarily trapped in the unoccupied T * orbital of CH3CN.13 The negative-ion state of (CH,CN),- tends to be stabilized by the charge-induced dipole interaction as the cluster size is i n c r e a ~ e d . ~ J ~ The vertical EA is eventually switched from negative to positive at a certain critical size and then approaches that of liquid (12) Gohlke, R . S.; Thompson, L. H . Anal. Cfiem. 1968, 40, 1004 (13) Jordan, K. D.; Burrow, P. D. Acc. Cfiem. Res. 1978, 11, 341. (14) Jortner, J. Ber. Bunsenges. Phys. Chem. 1984, 88, 188.
0 1986 American Chemical Society
Letters
1506 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
electric field produced by the trapped electron polarizes the surrounding CH3CN molecules. The nuclear configuration of (CH,CN),- is then reorganized as stated in step b; these CH3CN molecules are oriented with their CH3 ends directed toward the dimeric anion component. The effective vibrational temperature of (CH3CN),- is increased, because the excess energy, Eex(m), generated by this rearrangement is dissipated to at least (6m 6) intermolecular vibrational modes. If the energy is partitioned statistically among these modes, the temperature increment, AT(rn), can be approximated by'
A T ( m ) = E e x ( m ) / ( 6 m- 6 ) k B L
m Y
ldoo ljoo M A S S NUMBER ( m l z ) Figure 1. Mass spectra of the cluster ions produced by impact of highRydberg krypton atoms [A] and electrons having an average kinetic energy of 50 eV [B] on the CHJN clustcrs formed in a supersonic nozzle beam a t a stagnation pressure of 1250 Torr. T h e numbers above the ion peaks indicate the sizes of the cluster ions: (CH,CN),- [A] and (CH,CN),_,H+ [B].
CH3CN, which is reported to be about 1.85 eV.]' Finding ( 2 ) shows that this critical size corresponds to the observed threshold size, nth = 13, because no significant evaporation (process 2) is expected in this case, as discussed later. An ESR measurement for the y-irradiated crystal of CH3CN shows that dimeric radical anions are stable entities in the CH3CN crystal.I0 Hence, it is likely that the transferred electron in process 1 is trapped in the cluster as a dimeric anion, where the two molecules are in an antiparallel orientation with each other.16 The (15) Jones, J.; Hart, D. J.; Ballard, R. E. J . Mol. Srruct. 1982. 80. 213. (16) Several experimental studies and ab initio calculations have argued that a Rydberg electron can be trapped in the dipole field of a highly polar molecule such as CH,CN (the vertical EA e 0.5 meV) [Jordan, K. D.; Wendoloski, J. J. Chem. Phys. 1977, 21, 145].8,9However, it is unlikely that such a dipole-dominated anion component is formed in (CH,CN);, because this anion center, if produced, should be destabilized in the cluster on account of a very diffuse character of the u* orbital in which the electron is trapped.I0
( m 1 3)
(3)
where k B is the Boltzmann constant. Since the A T ( m ) value, estimated to be about 240 K for m 2 13, is lower than the boiling temperature of liquid CH3CN (355 K), no substantial evaporation from (CH,CN),- (step c) is expected; namely, it is highly probable that m - n = 0 or m = n." It is concluded, therefore, that the size distribution curve of (CH,CN),- ( n 2 13) is simply determined by a product of the size distribution of the neutral clusters, (CH,CN),, in the supersonic beam and that of the cross sections for attachment of the Rydberg electron to (CH,CN),. This prediction is supported by findings (3) and (4) that the size distribution curve for (CH3CN),- is strongly influenced by the stagnation conditions, because the size distribution of the neutral clusters depends critically on the stagnation condition^.^ In contrast, the size distribution of the positive cluster ions, (CH3CN),!Hf, by electron impact is insensitive to a change in the stagnation conditions (finding (5)). This implies that substantial evaporation takes place immediately after the formation of positive cluster ions.
Acknowledgment. The authors thank Professor T. Shida for provision of useful information on the ESR data of solid CH3CN. The present work has been supported by a Grant-in-Aid from the Japanese Ministry of Education, Science and Culture. (17) The excess energy, E,,(m), is estimated by E,,(m) = EA,
+ E,(2) + ESol(m- 2) - Ebo(m)
( m t 3) ( R I )
Here, EA, is the adiabatic EA of CH,CN estimated from an ab initio calculation to be about -0.8 eV [unpublished data]; E,(2) is the reported binding energy of a dimer anion, (CH,CN);, of 0.36 eV;" E,,(m - 2) is the solvation energy produced by the configurational change of ( m - 2) CH,CN molecules in step b; E,(m) is the binding energy of a neutral (CHJN),. For a typical case of m = 13, E,,(13) can be estimated to be about 1.6 eV by assuming 3.2 and 1.2 eV for Esol(l1) and E,(13), respectively, where Eso,(l1) is calculated from charge-dipole and charge-induced dipole interactions. For this calculation the radius of the solvation shell in (CH,CN),, is assumed to be nearly equal to that of the first coordination shell in the liquid CH,CN composed of about ten molecules, which surround two CH,CN molecules in an antiparallel orientation with each other [Hsu, c . s.;Chandler, D. Mol. Phys. 1978, 36, 2151. On the other hand, Eb0(13) is derived from the reported binding energy of (CH,CN), [Bohm, H . J.; Ahlrichs, R.; Scharf, P.; Schiffer, H J . Chem. Phys. 1984, 81, 13891.
