J. Phys. Chem. 1986, 90, 2315-2319 spatial period on the order of 1000 A. The long-range order of this phase is stabilized by dipole interactions. It seems likely to us that a dipolar mechanism may play a role in the rippled bilayer phases of mixtures of cholesterol and DPPC.14 Finally, we note that only the effects of vertical dipole moments have been considered so far; there are undoubtedly in-plane dipole components present as well. It is clear that the unfavorable repulsive interactions of the vertical dipoles can be somewhat reduced by tilting. Also, it has been found that the tilt direction ~~~~
~~
~
(14) Recktenwald, D. J.; McConnell, H. M. Biochemistry 1981, 20, 4505-4510. Owicki, J. C.; McConnell, H. M. Biophys. J . 1980,30,383-397.
2315
of the phospholipid chains is not uniform across the width of strip domains.’ It may be that this “splay” in the tilt direction is also caused by the repulsion of vertical dipoles, and subsequent creation of some in-plane components. Lastly, some observations have led us to suspect that there are weak attractive interactions between solid domains in addition to the apparently much stronger repulsive interactions. If so, these attractions may be due to in-plane dipole components.
Acknowledgment. This work was supported by N S F Grant DMB 83-13770-A1, DOD Grants N00014-84-G-0210 and DAA029-83-G-0095, and an N I H postdoctoral fellowship to D.J.K.
Reactivity of Ionic Silicon Clusters with Methylsilane Studied by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry M. L. Mandich,* W. D. Reents, Jr.,* and V. E. Bondybey* AT & T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: March 18, 1986)
and The reactivity of with methylsilane is investigated by Fourier transform ion cyclotron resonance mass depend strongly on cluster size; no reactions are seen for Si6-,’ and spectrometry. Product distributions for The reaction rates of Si,-5+ decrease with increasing size which is correlated with an increased delocalization of positive charge on the larger clusters. A reaction mechanism is proposed involving the lone pair, singlet coupled electrons on divalent silicon atoms in the clusters. Extra stability of “magic number” clusters has minor effects on their reaction rate and is not reflected in their product spectra.
Introduction Investigations of gas-phase clusters are plagued by the confusion which results from having to study a mixture of cluster sizes. Nowhere are the difficulties so apparent as for measurements of cluster chemistry. For example, neutral cluster reaction studies have been performed using photoionization mass spectrometry for detection of products.’ Interpretation of these results requires knowing whether fragmentation of the nascent cluster distribution has occurred during reaction or photoionization, as well as the ionization cross sections for reactants and products. No wonder, then, that determination of product distributions has been mostly guesswork. In principle, the situation should be far easier for studies of ionic clusters where mass filtering prior to reaction can be used directly. The practice, however, has been less than ideal. A few reactions of sputtered cluster ions have been performed under highly nonthermal condition^.^,^ Under mild conditions, the largest cluster ion studied to date is C O ~ ’ . ~ We have been able to purify our cluster reagents prior to reaction using a straightforward and versatile method. Positively and negatively charged cluster ions are formed by laser evaporation of an appropriate solid and introduced into the ion trap of a Fourier transform ion cyclotron mass resonance spectrometer (FTMS).5 Double resonance ejection is used to eliminate all but a selected cluster mass. The resulting cluster ion population can be stored at low energy for several seconds, and its exothermic bimolecular reactions studied as a function of time. The superior mass accuracy and resolution of the FTMS technique relative to time(1) (a) Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Surf. Sci. 1985, 156, 8. (b) Richtsmeier, S. C.; Parks, E. K.; Liu, K.; Pobo, L. B.; Riley, S. J. J . Chem. Phys. 1985,82, 3659. (c) Morse, M. D.; Geusic, M. E.; Heath, J. R.; Smalley, R. E. J. Chem. Phys. 1985, 83, 2293. (2) Freas, R. B.; Campana, J. E. J . Am. Chem. SOC.1985, 107, 107. (3) Fayet, P.; Woste, L. Surf. Sci. 1985, 156, 134. (4) Jacobson, D. B.; Freiser, B. S . J . Am. Chem. SOC.1986, 108, 27. (5) Reents, W . D.; Bondybey, V. E. Chem. Phys. Left.,in press.
0022-3654/86/2090-2315$01.50/0
of-flight or quadrupole mass spectrometry permit verification of the elemental composition of reactant and product ions by accurate mass measurement to within 20 ppm. In this paper, we report kinetics and product distributions of the bimolecular reactions of silicon cluster ions with methylsilane, CH3SiH3. Of all of the different clusters studied to date, perhaps the most thermodynamic and electronic structure information is known for the silicon clusters.613 Methylsilane was chosen for our first studies as it is an easily handled, quintessential silane reagent whose solution-phase chemistry with silicon-containing compounds had been well e~tab1ished.l~ We use this arsenal of information to explain two important aspects of the reactivity that we observe for these silicon clusters. First, we show that the degree of charge delocalization in the clusters is the determining factor which causes the reaction rate to decrease with increasing cluster size. The thermodynamic stability presumed for “magic number” silicon clusters has only secondary effects on these rates. Second, the rates and product distributions are consistent with a reaction mechanism involving the divalent silicon atoms in the clusters. This is concluded from ab initio calculations of the cluster electronic structures and by (6) Honig, R. E. J . Chem. Phys. 1954, 22, 1610. (7) Richter, C.-E.; Trapp, M. In?. J. Muss Spectrom. Ion Phys. 1981, 38, 21. (8) (a) Tsong,T. T.Phys. Rev. B 1984,30,4946. (b) Tsong, T. T. Appl. Phys. Lett. 1984, 45, 1149. (9) Furstenau, N.; Hillenkamp, F. In?. J . Muss Spectrom. Ion Phys. 1981, 37, 135. (10) (a) Bloomfield, L. A,; Freeman, R. R.; Brown, W. L. Phys. Rev. Lett. 1985,54,2246. (b) Bloomfield, L. A,; Geusic, M. E.: Freeman, R. R. Chem. Phys. Lett. 1985, 121, 33. (1 1) Heath, J. R.; Liu, Y.;OBrien, S . C.; Zhang, Q-L.; Curl, R. F.; Tittel, F. K.; Smalley, R. E. J. Chem. Phys. 1985, 83, 5520. (12) (a) Raghavachari, K.; Logovinsky, V. Phys. Rev. Left. 1985,55,2853.
(b) Raghavachari, K. J . Chem. Phys., in press. (13) Phillips, J. C. J. Chem. Phys. 1985, 83, 3330. (14) Eaborn, C. Organosilicon Compounds; Butterworths: London, 1960.
0 1986 American Chemical Society
2316 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Letters
Si2-
-SI>-
t
m
2 W
-
c
z
w
-
5
-
1
c
i'i
50-
-
I
j
2
5
I
SI2+ I
MASS (A.M. U 1
+ 0-
0
40
>-
t
VI
z W
100 120 140 MASS ( A . M. U.)
160
180
200
-
ISi4+
80
Figure 2. Positive silicon cluster ion mass spectrum taken 300 ms after firing the vaporization laser with 9 X IO-' Torr of CH3SiH3present in the cell. The trapping potential was +0.06 V. Mass 28 (Si') was ejected immediately after the laser pulse in order to suppress its products and enhance the signals of the products of the cluster ions. The signal/noise ratio in this mass spectrum is 1000. Note that this spectrum shows all products formed in the reactions of the silicon clusters with CH,SiH,; however, the paper discusses only the primary products of the bare Si,+.
15.8 S13+
60
z
W
I
t
-I
W
cc 0 150
il
I
1
I
200
MASS ( A M U.)
1
TABLE I: Cluster Size Dependence of the Rate Constants for the Reaction of Si,' with CH3SiH3and Comparisons with Cluster Binding Energies and Fragmentation Energies"
cluster ion 0-
40
60
80
100 120 140 MASS ( A . M . U.)
160
180
200
Figure 1. Typical mass spectra of trapped positive and negative silicon cluster ions 5 ms after firing the vaporization laser. Trapping potentials are +0.06 and -0.06 V, respectively. The isotopic pattern pertains to singly charged silicon cluster ions formed according to their natural abundance. The signal/noise level is about 2000 for the positive ions and 7000 for the negative ions relative to the intensity of mass 56. The magnitude of the monomer ions relative to the cluster ions is particularly sensitive to laser power, trapping voltage, and cleanliness of the sample surface. Although the monomer signal intensity may vary somewhat, the heights of the cluster peaks are more consistent with respect to each other. In the positive ion mass spectrum, the peak at mass 39 results from K+.
comparison with the known reactivity of other silicon-containing species. Experimental Method
A pulsed Nd:YAG (typical powers of 5-20 mJ/pulse, IO-ns fwhm, operating at either 1064 or 532 nm) is used to ablate polycrystalline silicon (99.999% pure, Metron Inc., Allemuchy, NJ) just outside of the trapping cell of a modified Nicolet FT/ MS- 1000 Fourier transform mass spectrometer. The instrument has a differentially pumped dual-cell configuration similar to the Nicolet FT/MS-2000. Typical conditions were as follows: magnetic field strength, 2.96 T; cell dimensions, 5 1 X 51 X 101 mm; trapping voltages, +0.6 V for Si,' and -0.6 V for Si,-; 16K data points; mass range, 26-400 amu; trapping times, 0-400 ms with pressures of CH3SiH3(obtained from PCR, Inc. and used and 3 X without further purification) between 0.3 X Torr. All reaction pathways observed were checked by standard double resonance technique^'^ and elemental compositions of reactant and product ions were verified by accurate mass measurement to within 20 ppm. Results and Discussion Figure 1 depicts typical mass spectra of the trapped Si,' and Si; obtained in our experiments. The isotopic ratios in these data (15) Beauchamp, J. L. Annu. Reu. Phys. Chem. 1971, 22,527.
reaction rate,b cm3 molecule-' s-I
Si+ 17 X IO-" Siz+ 5 f 1 x 10-" Si,' 7.6 f 0.7 X lo-" 1.6 f 0.2 X IO-'' Si4+ 3.0 f 0.3 X lo-" Si5+ Si6+ 5 1 . 2 x io-" Si7+ 5 7 . 3 x 1O-I)
cluster binding fragmentation energy," energy,O (eV), for eV/atom Si,' Si + Si,_,+
-
1.9
2.6 3.3 3.38 3.73 (3.76)
3.7 4.0 5.5
3.4 5.2 4.1
"From ab initio electronic structure calculations in ref 12a of the energy required to separate a Si,+ cluster into ( n - 1)Si' + Si. In this work, the binding energy for Si7+was approximated. bRate constants were determined from slopes of In [I(Si,+)] vs. time; all semilog plots were single exponential. Decay rate measurements were duplicated at differing pressures for (Pressures measured with the BayardAlpert ionization gauge were corrected for instrumentation effects and sensitivity to CH3SIH3 relative to CH4.35*36)The decay rates were measured several different times for The range of values for the rate constant for these runs was used to determine the tabulated errors. Note that the listed errors do not account for possible error in the pressure correction factor which may be off by up to a factor of three. This would change the rate constants of the various clusters in absolute value but leave them unchanged relative to each other. indicate that these clusters are all singly charged ions. Following introduction of CH3SiH3into the cell, a number of new peaks appear in the positive ion mass spectrum indicating that exothermic reactions with Si,+ have occurred (Figure 2). Comparison of the positive ion spectra before and after reaction (Figures l a and 2) shows immediately that the Si,+ clusters do not all react with CH3SiH3at the same rate. For example, Si+, Si,', and Si3+,which are quite prominent initially, have nearly reacted away at 300 ms. In contrast, relative to the total Si,+ ions, Si4+and Si6+are among the less intense cluster ions at 1 ms, but at 300 ms they are the most abundant. No reactions are seen with any of the Si,- clusters. Reaction Kinetics. Kinetic studies were performed to determine the rates at which the various Si,' react with CH,SiH,. Si7+was the largest cluster with an intensity sufficient for a kinetic measurement. Analysis of the time-dependent intensities of the ion signals indicates that their disappearance can be described by a pseudo-first-order rate constant which is linear in CH3SiH3 pressure. The corresponding rate constants are summarized in Table I. Neither Si6+nor Si7+were observed to form any reaction
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2317
Letters
0.28 0.76
0.40
0.70
0.30
0.03
0.19
0.81
Si4CH4+
100
0.16
0.24
7
0
0.47 0.13
0.53
Si+ Si,+ Si3+ Si4+ Si5+
200
300
400
TIME (MSEC)
Figure 3. Time evolution of primary products formed in the reactions of Si3+with 1.3 X IO" Torr of CH3SiH,. Product intensities are normalized to the total ion intensity in the cell as a function of time.
products and the rate constants in Table I are therefore upper bounds for their reaction rates given by the detection limits of the instrument at the highest pressures of CH3SiH3examined (- 1 X Torr). The kinetic studies reveal a dramatic variation of reaction rate with cluster size. Atomic silicon cations are the most reactive with cm3 a rate about one-tenth the Langevin rate of 1.2 X molecule-' s-'.I6 The reaction rate (Table I) then decreases by three orders of magnitude with increasing cluster size up to Si6+. Since the Langevin rates for the Si,+ are inversely proportional to the square root of the reduced mass of the CH3SiH3-Si,' collision pair, the actual reaction probability per collision decreases less steeply with increasing n. The measured decrease in rate does not smoothly vary with cluster size; rather it falls abruptly between Si+and Si2', between Si3+and Si4', and between Si5+and Si6+. The kinetic data also suggest that the reaction rate of a particular size of Si,+ (except for Si7+,which is uncertain) is enhanced if the cluster contains an odd number of atoms. Reaction Products. Table I1 summarizes the exothermic primary reaction products of Si,' with CH3SiH3at a trapping voltage of 0.6 V." Product distribution ratios were determined by ejecting all ions except for a particular Si,+ from the cell and monitoring the reactions with time. Typical data obtained from these experiments are shown in Figure 3 which depicts the reactions of Si3+with 1.3 X 10" Torr of CH3SiH3for trapping times of 0-400 ms. All primary products were subsequently verified as reactions of selected Si,+ by using standard double resonance techniques. The primary product ions also react with CH3SiH3 but these secondary reactions will not be discussed in this paper. Ion-molecule reactions of Si+ with CH3SiH3 have been investigated previously by tandem mass spectrometry.Is In that (16) Su,T.; Bowers, M. T. In Gas Phase Ion Chemisfry,Vol. 1, Bowers, M. T., Ed.; Academic: New York, 1979; pp 84-118. (17) The 0.6-V trapping voltage limits the kinetic energy of the trapped ions to
