8524
J . Phys. Chem. 1991, 95, 8524-8528
of H+ or OH- as a catalyst, even though the reaction is exothermic by 16.7 kcal/mol in the gas phase. The possible reason for this may be due to some reaction barrier for breaking the m N bond. The catalytic effect of H30+ and OH- on the hydrolysis of CH3CN in aqueous solution has been well established to involve a two-step reaction mechanism.23 Namely, CH3CN is attacked first by OHto form the reaction complex intermediate OH-(CH3CN); there are analogous cases involving H30+. This reaction intermediate then reacts with a water molecule to form acetic acid and release the OH- ion. Since the reaction involves a strong ion-molecule interaction that usually does not involve appreciable reaction barriers, reaction 18 is thus catalyzed by OH- or protons. In this work, we studied the solvation effect of water ligands on the reactivity of different anions. Here we concentrate on the OH-(H20), clusters. As discussed above, only a proton-transfer channel was found to be open for OH-(H20),+,. More water ligands cause the reactivity of OH-(H20), to drop dramatically. Actually, the intensity of OH-(H20),,,l ions increases as CH3CN is added into the flow reactor due to the feeding from the reactions of O-(HZO),. From the point of view of the reaction thermodynamics, there are three possibilities for the OH--catalyzed hydrolysis reaction of CH3CN in the gas phase: OH-(H,O), + CH3CN CH$OO-(H20), +NH3 (19.1) +
+
+
C H 3COO-( H 20),(N H 3)
OH-(H20),1
+ CH3CONH2
(19.2) (19.3)
N o products of the forms C H 3 C 0 0 - ( H 2 0 ) , or CH3COO-(H20),(NH3) (of the same mass as OH-(H20),(CH3CN)) were detected in the present experiments, which indicate that the base-catalyzed hydrolysis reactions, reaction 19.1 and 19.2, are unlikely under the conditions studied here. This is possibly due to the short reaction time (about 10 ms) and the short lifetime for the loosely bound reaction intermediate complexes OH-(H20),(CH3CN). ( 2 3 ) Tables of Chemical Kinetics, Circular of NBS 510, National Bureau of Standards, 1951, and references therein.
The possibility of reaction 19.3 cannot be excluded by the present experiments since all the OH-(H20), clusters coexist in the flow reactor and furthermore there are always some 0-(H20), species that can produce OH-(H20), upon reaction with CH3CN. A SIFT experiment might give more conclusive results to verify this possibility.
Conclusion Studies of the reaction kinetics of large hydrated anion clusters X-(H20)n=I-59with CH3CN in the gas phase at different temperatures and pressures reveal the following facts: OH-(H20),.~, react with CH3CN at the gas collision rate via a proton-transfer mechanism. Further hydration greatly reduces their reactivity due to the thermodynamic instability of the reaction products compared to the reactants. However, on the other hand, 0-(H20), clusters react with CH3CN at all cluster sizes, probably dominantly via a hydrogen-transfer mechanism. A new reaction channel is found for the reaction of 0-with CH3CN, in which one hydrogen and one proton are transferred from CH3CN to 0-to form CHCN- and water. CHCN- itself reacts further with CH3CN to form CH2CN and CH2CN-. Very slow association reactions are found for the reaction of O2-(H20), and 0 n. The spectra labeled with an n value, and plots of spectral shift vs n, should therefore not be taken too literally. [See below, for discussion of effects specifically attributable to this.] Benzene-nitrogen clusters have been formed from three-component jets (C6H6:N,:He) in variable ratios, a typical example of which is 1:500:104, formed from ambient benzene at -15 OC in about 10 bar of total pressure. The spectra shown here are representative of several scans taken in each n region and repeated under conditions ranging from no He to 95% He mixtures. A frequency-double dye laser tuned to benzene’s Ai,-Bh 6; transition (38608 cm-I) is focused by a 30-cm cylindrical lens into the ionization region of the time-of-flight mass spectrometer.12 Attempts to record the spectrum at the 0; transition (forbidden in the isolated molecule but weakly observed in condensed phases) yielded noisy spectra unsuitable for further analysis. The cluster-beam conditions (seed ratios, nozzles) are close to those used to form larger benzene-argon clusters, as described previo~sly;~~ electron-impact mass spectrometry on the latter shows that the beam consists primarily of neat argon clusters, followed by singly doped benzene-argon clusters and smaller amounts of clusters containing two or three benzene molecules. Figure 1 shows mass spectra of benzene-nitrogen clusters obtained by resonant-two-photon ionization at two particular wavelengths, the first (Figure la) exhibiting only the B.(N2), series (and B,) and the second (Figure 1 b) showing also some contribution from nitrogen (5) Gu, X. J.; Levandier, D. J.; Zhang, B.; Scoles, G.; Zhang, D. J. Chem. Phys. 1990, 93 (7). 4898. (6) Haynes, D. R.; Helwig, K. R.; Tro, N . J.; George, S. M. J. Chem. Phys. 1990. 93. 2836. (7) Nowak, R.; Menapace, J. A.; Bernstein, E. R. J . Chem. Phys. 1988, 89 (3), 1309. (8) Merrithew, R. B.; Marusak, G. V.; Mount, C. E. J . Mol. Spectros. 1968, 25, 269. See also: Barton, T. J.; Douglas, I. N.; Grinter, R. Mol. Phys. 1975, 30 (a), 1677. (9) Bernstein, E. R.; Lee, J. J. Chem. Phys. 1981. 74 (6). 3159. (IO) Nowak, R.; Bernstein, E. R. J . Chem. Phys. 1987, 86 (9), 4783. ( I 1) Clusius, K.;Sperandio, A.; Piesbergen, U. Z . Nururjorsch. 1959, Ilu, 793. (12) Easter, D. C.; Hahn, M. Y.; El-Shall, M. S.; Whetten, R. L. Chem. Phys. Lett. 1909, 157, 277. (13) Hahn, M. Y.; Whetten, R. L. Phys.Reu.Lett. 1988,61 (IO), 1190. See also: Hahn, M. Y. Ph.D. Thesis, University of California, Lcs Angeles, 1989.
14
16
I
18
Time of Flight (11s)
Figure 1. (a, top) Time-of-flight mass spectrum of B-(N,), (n = 0-70) clusters taken at 259.23 nm. (b, bottom) Time-of-flight mass spectrum taken at 259.20 nm showing three series of benzenenitrogen clusters. The number labels (m,n) indicate the numbers of benzene molecules (m) and nitrogen molecules (n) in B,,,.(N,), clusters. The relative intensity of these three series depends on wavelengths.
clusters containing two and three benzene molecules. These mixed clusters are formed by a larger seed ratio, near 1:100, using room-temperature benzene (100 mbar of vapor pressure) in 10 bar of pressure. Significantly, the ledge a t n = 20 is observed throughout the entire region where the larger clusters are found to absorb. The aluminum-argon cluster beams have been formed and ionized by methods described previo~sly.’~ 111. Results and Analysis
In Figure 2 are shown a series of spectra of benzenenitrogen clusters at the molecule’s 6; band, plotted versus the spectral shift from the molecular resonance. At the smallest sizes (to n = 7 ) , these are seen to agree well with the spectra of Bernstein and co-workers,’ who used the two-color method. For the most part, spectra of larger clusters exhibit a single strong band, with very little fine structure. A second, much weaker band is seen at a shift of +30 cm-’ relative to the main band. In the past, such features have been discussed in terms of an active vibrational/ phonon mode of the moleculesolventshell system.i5 Its strength relative to the main band is a slowly decreasing function of cluster size, as seen also for the argon system. Scans further to the red, Le., in the vicinity of the reported gas-to-solid or gas-to-liquid spectral shifts, revealed negligible additional spectral intensity. Also changing the seed ratio (N,:He) did not alter the spectral shifts in any significant way, although the ability to resolve fine structure on the bands is changed somewhat. Crude measures of the spectral size evolution can be obtained by plotting (against n) the shift corresponding either to the first moment of the spectral line functionI2 or to the maximum of the spectral line-in the case of a symmetrical line, these are equivalent. A plot of the maxima locations vs n is shown in Figure (14) Whetten, R. L.; Schriver, K. E.; Persson, J. L.; Hahn, M. Y. J. Chem. See also: Schriver, K. E. Ph.D. Thesis, University of California, Los Angeles, 1990. (15) Dao, P. D.;Morgan, S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1984, 111. 38.
Soc., Furaduy Truns. I 1990,86 (13), 2375.
Li et al.
8526 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
.m a0 -30 -io i o :
. i i o a ~.m do 9~ -io i o 30
-11040 -70 60 -30 -10 10
I
-110-90
-11040 -70 60 40 -10 10 30
-11040 -70 -5Q -30 -10 10
c_
-11040 -70 60 -30 -10 10
:
D
wavenumber (em-]) Figure 2. Optical absorption spectra of B.(N2), clusters, detected by RZPI time-of-flight mass spectrometry. The 0 corresponds to the 61,transition of the isolated benzene molecule at 38 608 cm-I. The spectra are numbered according to the number of N2 molecules in the cluster.
I
0 1 0
20
40
Size, N
m
Figure 3. Red shifts of the 61,spectral lines of B.(N2)"clusters plotted
as a function of cluster size.
Size. N Figure 4. Spectral width (FWHM)of the 61,spectral lines of B4N2),,
clusters plotted as a function of cluster size.
3. One can see that the spectral shift increases monotonically
with size, within the limits of experimental uncertainty. However, the linear dependence found by Bernstein et al.' applies only to a very limited range. Instead, we find several regions where the shift increases more rapidly than in other regions-the slope is steepest in the regions n = 0-4, n = 10-12, and n = 15-18. A second distinctive feature of Figure 3 is that there is no further red shift above n 20, to as far as n = 60. The asymptotic value observed is 66 cm-I, which is far from any of the bulk values (near 140 cm-' or higher).8-'0 It thus seems that the spectral line of benzene in nitrogen clusters fails to converge to the bulk value, despite an asymptote being reached at n 20 and maintained to the limits of the present measorements. The n = 20 cluster is also a magic number in the abundances observed in Figure 1. In the past, the coincidence of a convergence of the spectral shift and an abundance ledge has been used to argue for the formation of a complete solvation shell a t that size.16 Figure 4 shows the spectral line width (FWHM) of the main band as a function of size, as extracted from spectra like those
-
-
(16) Leutwyler, S.;Bosiger, J. Faraday Discuss. Chem. Soc. 1988,86,225.
in Figure 2. At larger sizes, the widths are relatively sharp, in the 10-1 5-cm-' range, and exhibit reproducible size variations (including the minimum around n = 19-22 and n = 34-38) except for the less certain measurements above 52. The peak widths are comparable to those found in the matrix/crystal at the lowest temperatures (7 K) and are much narrower than that of the liquid or supercritical fluid phases at any density. At smaller sizes, the apparent line width shows maxima at n = 11 and n = 16 (as well as at n = 2-3). These are also the regions where the line maximum's location undergoes rapid changes, so that the line width evolution (Figure 4) has the superficial appearance of being a derivative (or first difference) of the line shift evolution (Figure 3). While the line width is due to a number of effects, generally including a distribution of isomers, these maxima have a particularly simple explanation in terms of fragmentation, namely that fragmentation is invisible in regions where the shift is constant but will appreciably broaden the line in size regions where the shift is changing rapidly. The line width maxima are probably a combined effect of the fragmentation and the difference in spectral shifts at different (neighboring) sizes.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8527
Nonbulk Convergence of Solvent Spectral Shifts
I
t
-110
l
l Q)
l
l
n
l
l
do
l
l
do
l
l
-io
l
l
io
l
l
50
wavenumber (cm-1) Figure 5. Absorption spectra of B-(N,),, clusters ( n = 11-16) reflecting the asymmetry switch of the line shapes.
For instance, the fragments from n = 12 fall to the n = 1 1 mass channel but a t a position shifted from the maximum absorption of n = 11, and at low resolution, the effect is a broadened peak at n = 1 1. The greater the differential shift from size-to-size, the greater the broadening. Direct support for this interpretation comes from careful analysis of the line shapes, particularly their asymmetry. Figure 5 shows that spectral lines in the n = 11-16 region are asymmetric, with n = 11-1 3 skewed to the red and n = 14-16 skewed to the blue. We assume that the intrinsic shape is asymmetric, leaning to the blue as n = 14 does. The fragmentation broadening from heavier clusters makes the line shapes broaden on the red side and therefore switch the assymetry from leaning blue to leaning red. Every jump of the spectral shift (Figure 2) makes the asymmetry switch, while sizes in the gradual-shift region keep their intrinsic line shapes. In any case, the widths shown in Figure 4 must be regarded as conservative upper bounds on the true spectral width. An additional effect can be observed under conditions where clusters contain more than one benzene molecule (Figures 1 and 6). if the mass-spectrometer gates are opened to include contributions from the B2.(N2)" clusters. Now a new feature appears at a red shift approximately 40-50 cm-I from the main B.(N2), features. These were found during attempts to locate any features closer to the bulk benzene-nitrogen shift (near 140 cm-1).8Jo In particular, there was no sign that larger clusters, n > 60, have absorptions abruptly shifted to near the bulk value. The magnitude of this additional shift suggests that the two benzene molecules occupy adjacent sites in the cluster (cf. the B to B2 shift of 43 cm-I).I7 The asymptotic spectral shifts and widths found in this work are combined in Table I with literature value for the bulk systems. Also included are the asymptotic shifts observed for two other systems, benzene-argon and aluminum-argon. Figures 7 and 8 show plots of the spectral shift versus size for these systems. It is apparent that, even though a convergence appears to have been reached in each of these three cases, the bulk values are not being approached. In the final section of this report, we briefly consider the possible explanations for this phenomenon. IV. Discussion and Conclusions In contrast to the pictures presented in earlier paper^,^ the systems discussed here clearly fail to exhibit a convergence to the (17) Bornsen, K. 0.;Selzle, H. L.;Schlag, E. W. J . Phys. Chem. 1988, 92, 5482.
bulk spectral shifts. One similarity among our systems is that the dopant molecule (or atom) is smaller in size-close to the size of the solvent molecules-as contrasted to the more frequently studied large-molecule dopants in raragas and molecular clusters. Thus, in the latter class of systems, the dopant might cause greater disruptions to the surrounding lattice or cluster, thereby dominating the local structure in either case, both because of their volume and the larger total interaction energy. This argument serves as an explanation of why those systems appear to converge so well, not why the ones selected here fail to converge to bulk properties. Structural hypotheses that could explain nonbulk convergence fall into two classes-(i) the dopant molecule is in a surface site, in which case the asymptote has significance as the spectral shift of a (bulk) adsorbate system; and (ii) the dopant is in an internal site that is very distinct from that of the bulk system. The first of these has several liabilities. First, there is no value for the bulk adsorbate state with which to compare. In a relevant study of SiF4.Ar, clusters, Gu et aLs have assigned a peak in their infrared spectrum to a surface site. The location of this peak converges at increasing stagnation pressure to a certain value with which there is no componding bulk value to compare. In this important example, they were able to identify this spectrum as that of a surface site, by comparing the spectra obtained for clusters formed by co-expansion with those obtained using the pick-up method. The second liability arises from comparison with the predictions of Perera and Amar.4 They carried out a systematic study of the solvation behavior for a wide variety of heterogeneous cluster systems and have shown how the preferred structural class (surface or matrix) depends on the interaction parameters (strength t and distance u in the 6-12 potential), for the A-B interaction in BOA, clusters. Their work pointed the benzene-Ar, system in the direction of an internal site. For the benzene.(N2), system, we derived from ref 18 the reduced values of the B-N2 interaction parameters, expressed relative to the N2-N2 interaction parameters, which turned out to be t* = 1.55 and u* = 1.09. These values also lie well within the matrix region on their plot (in E*, u* space), in apparent contradiction to the surface-state hypothesis. A comparison with this theory for the case of AI-Ar is more difficult because of the open-shell nature and scarcity of data on the Al-rare gas interaction. Ongoing ab initio computations and simulations should help to clarify the situation with this system.I9 The alternative explanation (a nonbulklike internal site) also has liabilities: Again, in the case of benzeneN2, the spectral shift in both solid and fluid nitrogen is much larger than the cluster values reported here, but see the discussion below. In the case of fluid nitrogen, the density dependence shows a clear maximum near 140 cm-I in the shift vs density plot. Regarding the liquid nitrogen results, it seems possible that, despite considerable effort to avoid aggregation: the cryogenic liquid spectral shift of -278 cm-' is simply that of solid benzene particles.20 A density sufficiently low to produce the shift found here (66 cm-l) is inconceivable. A much higher (than bulk) density seems more probable, with the free boundary of the cluster allowing for a large collapse around the benzene, Le., an effective pressure (or finite-size contraction) of thousands of atmospheres. Again, there is no way to resolve this a t present. In a very recent paper, Goyal et aL2' have shown that the matrix site occupied by SF6in smaller Ar clusters (ca. 1000 atoms) is different from the bulk, as indicated by its infrared spectrum. At larger sizes, the spectrum is that of the surface site, and only beyond 3000 atoms does the molecule return to the interior and show the annealed-matrix shift. With regard to this hypothesis, it is interesting that the benzene-N, (18) Hill, T. L.An Introduction to Statistical Theromdynamics;Dover: New York, 1986; p 484. (19) Estrin, D.; Singer, S. Private communication. (20) Colson, S.D.; Hanson, D. M.;Kopelman, R.; Robinson, G. W.J. Chem. Phys. 1968,48,2215. At 60 K, the band of solid benzene is centered around -260-cm-' spectral shift. (21) Goyal, S.;Robinson, G. N.; Schutt, D. L.;Scolcs, G. Preprint communicated to R.L.W. (22) Abe, H.; Kolb, D. M. Ber. Bunsenges. Phys. Chem. 1987, 87, 523.
Li et al.
8528 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
do 40 -20 0
.1*1P1000
-1*laDlwQ
do 40 -20 0
-14o-lPlmo do 40 -20 0
-1*1aDlQO*
40 40 40 0
wavenumber (cm-1) Fire 6. Optical spectra of benzenenitrogen clusters including both B.(Nz),, and Bz-(N2),,absorptions. The integer labels on the left (right) in each box are the numbers of nitrogen molecules in the dimer (monomer) series of benzenenitrogen clusters.
M TABLE I: Comparison of Asymptotic Spectral Shifts to Bulk Values
2
so
gas, cm-l B*(Nz),, B*(A,),, AI.(Ar),
8a)
.
700-
--a
2
em-
f
0
CJ 0 0 0
0
,
,
[I
0 0 0
=a-
2 =v)
0 0 ~
o o o o ~
0
0
= - 0
"O I
-100 +
a 0
7
+
,
,
,
,
,
,
,
,
,
,
,
38608 38608 24000
asymptotic spectral shift [width]
solid matrix shift
-66 [I41
-143*b
-35 [30] 1200 I2001
+455OZ2
liquid fluid shift -140"
-1 12*'
observed here corresponds to a similar site in the finite-size system. These conclusions also point out the need to carry out measurements, in some cases, to much larger sizes, toward n = lo3 or greater, as is happening with metals. A third interpretation might be a hybrid of the first two-that a single solvation shell around benzene is not sufficient to produce the bulk spectral shift and that subsequent growth of the cluster occurs only on one side, failing to produce the residual shift. But this goes against the conventional wisdom that a single solvation shell should produce the largest fraction of the bulk shift. Furthermore, there is no evidence obtained here that points specifically to this explanation. In conclusion, we have presented and discussed examples where the spectral line shift of a molecule (or atom) doped into a molecular (or rare-gas) cluster exhibits a convergence to a value other than the bulk value. It is not possible at present to decide among the favorite explanations that could account for nonbulk convergence, but we hope that these results will stimulate further experiments, also on surface adsorbates, and further simulations, as well as continued discussion.
Acknowledgment. This research was supported by the Exxon Educational Foundation and the National Science Foundation PYI Award to R.L.W.and received valuable assistance from R. D. Beck, D. C. Easter, and M. M.Alvarez.
