J . Phys. Chem. 1984,88, 3329-3333 parameter, k is the Boltzmann constant, p is the permanent dipole moment, and /3 is the angle of the dipole moment p to the long molecular axis, respectively. Among the variables in the eq 1, the angle of the dipole moment to the long molecular axis p plays the most important part in determining the magnitude of the dielectric anisotropy. If is small, the dielectric anisotropy ta is strongly positive, whereas if p is sufficiently large, t, may be negative. Schadtis measured the dielectric anisotropy of N-(4-ethoxybenzylidene)-4-aminobenzonitrile which is oriented by the magnetic field and reported that the dielectric constant along the long axis t i is extensively large and the dielectric constant perpendicular to the long axis t 2 is smaller than the one in the isotropic phase. The increase in t 1 is interpreted mainly as being due to the dipole moment of the cyano group parallel to the long axis of molecules. The above discussion indicates that the dielectric constant of the system increases when the molecules take a conformation to make p minimum and orient in the direction of the electric field. Though the conformation of the molecules is random in the isotropic (normal) liquid, the molecules have a specific conformation in the meso phase. Methyl acrylate consists of the planar vinyl group, the planar group I, and the free rotative methyl group. When the geometry between the two planar groups is appropriate, the molecular conformation which makes p minimum is probable. If the methyl acrylate molecules align and have the conformation (15) Schadt, M. J. Chem. Phys. 1972, 56,
1494.
3329
to make p minimum at high pressure, the capacitance may increase more than that in the isotropic phase at low frequency. This discussion is significant only at the low frequency at which the cluster of aligned molecules follow the alternative electric field. In the high-frequency region, el and e2 are averaged and the measured dielectric constant should show the value in the isotropic phase. The dielectric dispersion shown in Figure 3 occurs at lower frequency compared with the data of Schadt. This means that the cluster of aligned molecules caused by pressurization can be oriented to the direction of the electric field only at very low frequency. Consequently, we concluded that the sharply discontinuous capacitance change at the low-frequency region is brought about by the molecular alignment at high pressure. The discontinuous capacitance changes of the compounds shown in Figure 6 are interpreted by the same reasons. The magnitudes, and the onset pressures of the discontinuous capacitance change differ from each other. These are considered to be a reflection of the molecular structure such as a steric hindrance in rotation and a magnitude of permanent dipole. The pressure giving the sharp increase in the capacitance of these compounds corresponded with the breaking pressure in the P-'Visotherms.1.2,8.12This fact and the absence of characteristic dielectric dispersion in the compounds which do not have group I indicate that the planar group I plays an important role for the pressure-induced alignment. Registry No. Methyl acrylate, 96-33-3; methyl methacrylate, 80-62-6; vinyl acetate, 108-05-4;hexane, 110-54-3;methyl acetate, 79-20-9; ethyl acetate, 141-78-6;n-butyl vinyl ether, 11 1-34-2.
Reaction of SO:, with Water Clusters and the Formation of H2S04 R. Hofmann-Sievertt and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: June 20, 1983; In Final Form: December 14, 1983)
The gas-phase reaction of SO3 with water clusters was studied by using a molecular beam reaction technique combined with an electric deflection field and mass spectrometer analyzer-detector. Electric deflection experiments gave identical results for both the products of the reaction and for H2S04,from which it is concluded that the adduct (S03.H20)rapidly isomerizes to sulfuric acid. This is borne out by observation of identical fragmentation patterns for the products and H2S04. The results are found to be in agreement with predictions based on the RRKM theory, from which it is deduced that the maximum barrier for isomerization is about 13 kcal/mol.
Introduction The gas-phase reaction of SO3with water was reported' as early as 1934, but the molecular details are still not well-known. Interest stems in part due to the belief that SO3plays a role in the production of H2S04,2which is an important pollutant as well as a nucleation agent in the a t m ~ s p h e r e . ~The , ~ clustering of water with H2S04is considered in various nucleation model^,^,^ but there is still a lack of knowledge of the details of the mechanism of H2SO4-aerosol production. Some evidence7has supported the suggestion' that the reaction of SO3and water is fast. Since direct reaction to form H2S04 would have to proceed via a four-center mechanism, the findings have been rati~nalzed'-~by speculation that an adduct is first formed. Thereafter, the adduct may be expected to rearrange to form the H2SO4 m o l e c ~ l ethe ; ~ lower the energy barrier, the more facile is expected to be the rearrangement. Using semiquantitative quantum-mechanical calculations, we predicted the energy barrier for rearrangement to be about 3.3 kcal/mol. The aim of the present study is to investigate the mechanism of the t Present address: Ciba-Geigy A.G., P.O.Box CH-4002, Basel, Switzer-
land.
reaction of SO3 with water and water clusters and to determine the molecular nature of the products.
Experimental Section The detailed experimental setup is described elsewhere.1° Briefly, it is comprised of a high-pressure nozzle reaction source, three intermediate stages of pumping with the third containing (1) C. F. Goodeve, A. S . Eastman, and A. Dooley, Trans. Faraday SOC., 30. 1127 (1937). - - , ~~(2) R. P. Turco, P. Hamill, 0.B. Toon, and R. C. Whitten, J. Armos. Sci., 36,699 (1979). ( 3 ) R. H. Heist and H. Reiss, J. Chem. Phys., 61, 573 (1974). (4) (a) C. S . Kiang and P.Middleton, Geophys. Res. Lett., 4, 17 (1977); (b) C. S . Kiang and R. D. Cadle, J . Armos. Sci., 34, 150 (1977). (5) W. Roedel, J. Aerosol SOC.,10, 375 (1979). (6) K. Suzuki and V. Mohnen, J . Aerosol Sci., 12, 61 (1981). (7) A. W. Castleman, Jr., R. E. Davis, H. R. Munkelwitz, I. N. Tang, and W. P. Wood, Inr. J . Chem. Kinet., 7 (Symp. No. l), 629 (1975). (8) J. G. Calvert, private communication. (9) P. M. Holland and A. W. Castleman, Jr., Chem. Phys. Lett., 56, 51 1 \ - -
I -
(1978).
(10) R. Sievert, I. Cadez, J. Van Doren, and A. W. Castleman, Jr., J . Phys. Chem., in press.
0022-3654/84/2088-3329$01.50/00 1984 American Chemical Society
3330 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 an electrostatic focusing field, and a final detector chamber housing a quadrupole mass spectrometer. Two reaction source configurations were used: one where SO, is admixed through a metal capillary bent as a ring containing a series of extremely small orifices shot into it by a pulsed laser and the other one where SO3 is introduced through a coaxial glass tube that is fitted around the inner capillary so that the ring opening between the two glass tubes is as small as possible. In the latter case, the area of the annular opening was about 5-6 times larger than the area of the inner capillary hole. In the case of experiments made with the coaxial nozzle, the SO, and H20 reaction takes place during the coexpansion. Experiments made with the SO3added through the metal capillary lead to the interaction of the species late in the expansion. In both arrangements, only the products of gas-phase reactions are sampled. For preparation of an experiment, all connectors were thoroughly pumped out, dried, and thoroughly outgassed. SO, was supplied from a thermostated bath of 99% stabilized liquid in both mixing systems. No polymers of SO3were detected by the mass spectrometer nor was any HzSO4 found to be present in the beam of pure SO,. During the course of an experiment, the SO3flow was adjusted by setting the temperature of the bath which thereby fixed the vapor pressure. Most experiments were performed with the coaxial glass system since it was more resistive to the extremely corrosive SO3. In order to investigate the products of the reaction with water, vapor was expanded into a vacuum of about lo-, mbar from a source of about 1-atm total pressure through a glass nozzle of 100-pm i.d. The typical average flow was about 1 cm3/s (NTP). The nozzle was heated to prevent condensation of the water vapor. In some experiments neat vapor was expanded, while in others water vapor was seeded with argon. By varying the expansion conditions, we obtained widely differing water-cluster distributions. The water-cluster distribution was checked before each experiment. The nozzle was aligned with the axis of the beam apparatus and the skimmer (aperture 1 mm) that separated the nozzle exhaust from a differential pumping region of 10” mbar. Alignment was achieved by optimizing the mass spectrometric signal of the water trimer at mass/charge equal to 37; the trimer is detected as the protonated dimer formed upon rearrangement following electron bombardment. The molecular beam entered the focusing region containing the quadruple rods through a hole; the on-axis beam could be blocked by a mechanical beam stopper which enabled a determination of the contribution of background to the detected signal. The background pressure in this region was typically about lo-’ mbar. A pressure of 5 X 10-9-10-8 mbar was maintained in the chamber which housed the molecular-beam mass specrometer detection system. The beam was ionized by electron impact; thereafter, the ions were separated by a quadrupole mass filter and amplified by a channeltron. Further amplification was achieved through an integral preamplifier-amplifier-discriminator, and the signals were counted and stored in a multichannel analyzer. For cluster distribution measurements sweeps of the whole mass range were done, whereas for the deflection experiments the mass spectrometer was set to a single mass peak and the signal was averaged. Results Cluster Distributions. A typical product distribution is shown in Figure 1. Part (a) is the pure water or deuterium oxide distribution in the seeded beam with Ar; part (b) is an averaged distribution of the S03(H20), clusters. T h e y axis is the relative intensity, that is, the signal of the masses corresponding to the ions H+(H20), or H+(S03)(H20), relative to H+(H20)detected at m / e 19 or HS03+at m / e 81, respectively. Typical stagnation chamber pressures were around 600 mbar, while typical bath temperatures for H,O were 353 K with the nozzle temperatures set a few degrees higher. The water monomer in the beam was detected as H20+or D 2 0 f . In comparison to the water clusters, the overall intensity of the mixed S03(H20), cluster ions was invariably very low as indicated by the HS03+signal shown for
Hofmann-Sievert and Castleman 3.
n
IO1
Figure 1. (a) Typical water-cluster distribution; water seeded in Ar. Ordinate: mass spectrometric signal of the cluster ion H+(H,O), relative to the mass spectral signal of the cluster ion of the dimer Ht(H20) ( m / e 19). Abscissa: n, number of water molecules in the cluster minus 1. I deontes error bars. (b) Typical product distribution of S03(H20), clusters under comparable conditions. Ordinate: signal of the (S03)(H20),Ht ion relative to the mass spectral signal corresponding to HtS03 ( m / e 81). Abscissa: n, number of water molecules in the cluster minus 1.
comparison in part (a). This was manifested by the large error bars for the S03(H,0), cluster-ion distribution. No masses corresponding to more than one SO, molecule in a cluster were detected, but S03(H20),H+ clusters were detected up to mass 225. This observation shows that neutral species having at least nine water clusters were formed. The mixed clusters have mass spectral fragmentation similar to that of other hydrogen-bond bound clusters,” where a dominant channel is (HzO),S03
+ e-
-
H+(H20),S03 + OH
+26
But, since the proton affinities of the SO, (594 kJ/mo112) and H2S04(71 1 kJ/mol14) are lower than that of H 2 0 (723 kJ/mol13), ionization may also proceed via loss of a sulfur-containing unit and lead to the formation of a proton clustered with pure water. This could make the overall intensity of the products appear very low. Such clusters would be indistinguishable from the watercluster reactants and would only be detectable if they produced new features in the water-cluster distribution as in the case of hydrochloric acid-water clusters.” Experiments were also made with D20 that enabled contribution of the mass spectral background of normal water to be distinguished. This procedure was only of value for certain cluster sizes. Since one SO, molecule has the same mass as four D20 molecules, there is a strong interference between reactants and products for certain combinations of mixed clusters. Electric Dejlection Experiments. Electric deflection experiments were made to assist in determining the products of the reaction. Polar molecules refocus in a quadrupole field, and molecular-beam electric deflection in a quadrupole field has proved to be a useful technique for studying molecular geometries.10,16 (1 1) V. Hermann, B. D. Kay, and A. W. Castleman, Jr., submitted for publication in Chem. Phys. (12) B. Munson, D. Smith, and C. Polley, In?. J . Muss Spectrom. Ion. Phys., 25, 323 (1977). (13) D. H. Aue and M. T. Bowers, “Gas Phase Ion Chemistry”, Vol. 2, Academic Press, New York, 1971. (14) R.Walder and J. L. Franklin, Int. J. Muss Spectrom. Zon.Phys., 36, 85 (1980). (15) B. D. Kay, Ph.D. Thesis, University of Colorado, Boulder, CO 1981.
T he Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3331
Reaction of SO3 with Water Clusters 50
I
I
I
I
I
6
o '
1 0
I
i
from SO, t H20 from SO, t HO , H,SO,+ from H,SO, o HNO: from HNO,
HSO: H,SO:
A
0
A
30
t-AH/kJ mol"
W
a
5001
A
0
0.00
I
I
I
I
I
I
I
I
2.40 3.60 4.80 APPLIED VOLTAGE (KV) SECOND-ORDER STARK FOCUSING
1.20
I
6.00
Figure 2. Electrostatic focusing experiments employing the second-order Stark effect. Data for the functions of HN03 are also shown for comparison.
The force exerted on a molecule having a permanent dipole moment as it passes through an inhomogeneous electric field is proportional to the negative gradient of the field energy. Therefore, the deflections give information about the sign and magnitude of the molecular Stark effect. In the case of the electric deflection experiments performed during the course of this work, a beam obstacle was positioned to block any straight-through trajectories from directly entering the mass spectrometer detection chamber. Deflections and refocusing were accomplished with the application of voltage to the quadrupole rods. The main aim of the present study was to determine whether the adduct of the reaction of SO3 with water is long-lived or whether isomerization to sulfuric acid is very fast. An S03.H20 adduct is expected to have a significantly different deflection behaviorgthat should be discernible even with low signal intensities. In the present experiments the deflection behavior and the mass spectral fragmentation were calibrated with concentrated H2S04 (azeotrope 98%). It has been demonstrated before that H2SO4 is a strong focuser.I6 At elevated temperature an appreciable amount of decomposition of the acid was found. The mass spectral fragmentation of H2S04for the fragments HS03+(81) and H2SO4' (98) (81:98) is 1.3 f 0.2, the ratio of 81:98 in the products was 1.3 f 0.3. The percentage of the refocused beam of the unknown product of the SO3-water cluster reaction and of the H,S04 was measured at m / e 81 and 98 a function of high voltage. Within experimental errors the behavior or calibrant and of the product were found to be the same. See Figure 2 . The electric deflection experiments show that the fragments at mass 81 (HS03+)and at mass 98 (HzSO4') were both strong focusers, but all the higher clusters that could be examined, e.g., (H20)2S03,(H20)3S03,and (H20)4S03,were found to defocus. This is similar to the situation with water clusters and nitric acid-water clustersi5 in contrast to results with acetic acid, where large clusters were found to have permanent dipole moments.i0 Discussion There are three observations which provide evidence that under the present experimental conditions the adduct has already isomerized to sulfuric acid: (1) the ratio of the fragments at masses 81 and 98 stays constant over a wide range of stagnation chamber conditions; (2) the ratio is the same as in pure HZSO4; and (3) the electric deflection experiments show the same deflection for masses 81 and 98 for the reaction product as well as for the calibrant HzSO4. Experiments with D 2 0 and with a defocused beam were also performed. The results of these clearly established that the products originated in the beam and did not arise due to reaction with background H 2 0 present in the gas phase or on the reactor walls. (16) A. Buchler, J. L. Stauffer, and W. Klemperer, J. Chem. Phys., 46, 605 (1967).
Figure 3. Energetics of the reaction of SO,
+ H 2 0 at 0 and 298 K.
Considerations of Reaction Dynamics. The reaction of SO3 with water clusters leads to chemically activated (S03)(H20), clusters that according to unimolecular rate theory can redissociate or decompose to other products, become stabilized by collisions, or undergo isomerization to sulfuric acid-water clusters. The energetics for the possible reactions are shown in Figure 3. If the forward-reaction rate constant for the reaction of SO3 with the H 2 0 monomer and with the H 2 0 dimer were about the same and if this governed the reaction dynamics, the ratio of the products S 0 3 ( H 2 0 ) / S 0 3 ( H 2 0 ) would 2 be the same as the ratio of H,O/(H,O),, namely 8:l. As expected, this is not the case, and the experimentally measured ratio is 3: 1. If a decomposition reaction like SO3 (H20), S03(H20),* S03(H20),-, + nHzO
+
-
-
would be the dominant reaction channel, a shifted distribution of S03(H20), corresponding to (H20), would result. The ratio of water dimer to trimer is 1:1, showing that a simple displacement reaction was not being observed for n = 1; in fact, no general trend for any value of n was discernible from the data. There are several possible factors which could influence the ratio. First, mass separation in the beam might possibly concentrate the higher clusters on the beam axis, but the mass difference between S 0 3 ( H 2 0 )and S03(H20)2is too small to account for a large effect. The reaction cross section for the water monomers might be smaller than the cross section for the dimers. But, although no ionizing cross sections for the clusters are known, they are expected to vary smoothly with cluster size. Unimolecular reaction dynamics provide the plausible reason for the experimental observations. The activated [S03.H20]* complex that is produced by the reaction of water monomer with SO3has a smaller lifetime than the complex [S03(H,0),]* from the reaction with the dimer as nearly the same reaction enthalpy is distributed over more oscillators. So, there will be more reverse reaction in the first case. The effect should be largest between monomer and dimer, and this explains why the higher clusters show the expected smooth decline. The general energy scheme for the reaction of SO3with water is shown in Figure 3. Except for the reverse reaction, namely the decomposition of S03.H,0 back to SO3 and water, there is no fragmentation channel energetically open. The energy of the least endothermic reaction, the decomposition to SO2and H,Oz, is about 200 kJ/mol higher than the back-reaction." Energies of the decomposition reactions to form O H + H2S03,H + HS04, and H, + SO4were calculated from values given in ref 18 but are even more endothermic. Although the structure of H2S04is foundlg (17) D.R.Stull and H. Prophet, Nutl. Stund. Re5 Data Ser. (US., Nutl. Bur. Stand.), NSRDS-NBS 37 (1971). (18) S. W. Benson, Chem. Rev., 78,23 (1978).
3332 The Journal of Physical Chemistry, Vol, 88, No. 15, 1984
Hofmann-Sievert and Castleman
TABLE I: Calculation of k ( E ) for Cases Shown in Table I1 case I values' EZt,cm-' E , cm-' log s,l,,*b log d",b T, K k ( E ) , s-'
-S(OH), 7804.58 8590.3 39.8 43.909 298 1.4 X lo',
case I1
-OH 7804.58 8590.3 39.16 43.909 298 7 x 10'2
-S(OH), 7977.5 8590.3 41.19 43.909 298 1.4 X IO"
case 111
-S(OH)2 7743.48 8590.3 40.43 43.909 298 4.6 X IO"
-OH 7977.5 8590.3 40.77 43.909 298 3.8 X 10"
-S(OH), 7743.48 8062.1 40.43 43.909 0 3.4 x 10"
-OH 7743.48 8590.3 39.81 43.909 298 2.1 x 1012
' E z = 8492.5 cm-I; Eo = 391 1.53 cm-I. bFrequencies given in Table 111.
to be slightly different from that postulated in ref 9, no appreciable differences are expected regarding the estimated energy of the adduct and the barrier to isomerization. The following analysis was made using the requisite values taken from ref 9 without changes. The only reasonable reactions for the activated (S03.HzO)* are the reverse reaction, the stabilization, or the isomerization to sulfuric acid according to the following scheme:
TABLE 11: List of Cases Considered for Calculations Given in Table I case I :
* H.
0-H
x I
-
'2
s=o
-
s=o
loose complex frequencies q* as SO,, H,O, and SO,.H,O; see Table 111 case I1 : internal hydrogen bonds * Y
ks1M
S03*H20
The stabilization of the (S03.H20)* adduct, k, = ycZ[M] (yc is the collision efficiency, Z is the Lennard-Jones collision frequency, and [MI is the density), compared to its isomerization to sulfuric acid, k(E), can be approximately assessed by using the RRKM theory. To determine whether a stabilized S03.Hz0 adduct is to be expected under our experimental conditions, the specific rate constant, k(E), for the isomerization of chemically activated (S03.HzO)* to sulfuric acid was estimated as was the back-reaction, isomerization of chemically activated H2S04to S03.H20adduct. If the zero-point energies of the activated complex E> and of the ground state E,, the vibrational frequencies vi and vi*, and the barrier Eoare known, k(P) can be calculated according to ref 20. In making the present estimate, we used the frequencies and thermodynamic data for sulfuric acid from ref 17. Three sets of frequenices were estimated for the one for a rigid complex with internal hydrogen bonds approximated by the frequencies of liquid HZS04,one for a loose complex approximated by the frequencies of waterz4and so3," and one for an approximation suggested by Astholz et aLZ6for a similar isomerization reaction. Eowas taken from ref 9. The temperature of the beam, especially the vibrational temperature, was not very well-defined. Therefore, the calculation was also done for 0 K as this is the lowest limit for k ( E ) . The estimated k(E) values for Eo = 3.3 kcal/mol from ref 9 were in the range of 1 X 1Oll-7 X 10l2SKI.The actual values for the range of parameters considered are given in Table I; the cases considered are given in Table 11, and the estimated frequencies in Table 111. The competition between isomerization, k ( E ) , and collisional stabilization (deactivation), k, = y,[M], gives the amount of the stabilized (S03.H20) adduct. In order to estimate the minimum amount of stabilized S03.H20,the smallest k ( E ) = 5 X 10" s-l was compared to the collision deactivation which is about 5 X lo5 s-l under the conditions in the expansion. Since exact knowledge (19)R.L. Kuczkowski, R. D. Suenram, and F. J. Lovas, J . Am. Chem. (1981). (20)J. Troe, J . Phys. Chem., 83, 114 (1979). (21)P. A. Giguere and R. Savoie, J . Am. Chem. SOC.,85, 287 (1963). (22)S.Chackalackal and F. E. Stafford, J . Am. SOC.,88, 723 (1966). (23)K.Stopperka and F. Kilz, Z . Anorg. Allg. Chem., 370, 49 (1969). (24)T. Shimanouchi, Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.), (NSRDS-NBS 17) (1968). (25)R. W. Lovejoy, J. H. Colwell, D. F. Eggers, Jr., and G. D. Halsey, Jr., J . Chem. Phys., 36, 612 (1961). (26)D. C. Astholz, J. Troe, and W. Wieters, J . Chem. Phys., 70, 5017 (1979). (27)M.Quack and J. Troe, Int. Reu. Phys. Chem., 1, 97 (1981). Soc., 103, 2561
frequencies vi* in liquid H,SO, rigid complex case III? H
" 0 '
frequencies vi* as in referenced paper; vi* = 0.911~ a
Based on suggestions in ref 26.
TABLE III: Frequencies (v) Employed in Calculations Presented in Table I Y , cm-'
HZSO, SO2, str, asym SO2 str, sym SO, rock SO, bend OH-str, sym OH-str, asym torsion S(OH)* str, SYm
S(OH), str, asym S-OH bend, asym S-OH bend, SYm S-(OH), bend S-(OH)2 rock OH wag HO-H bend
S 0 3 W and
HzO(g) 1390 (SO,) 1069 (SO,) 530.18 (SO,) 497.55(SO,) 3656.5 (H,O) 3755.8 (H,O)
loose (SO,.H,O) 1390 1069 530.18 497.5 3656.6 3755.8 390 800
g
1
1450 1220 568 550 3500 3600 390 834
1368 1195 623 563 2450 2970 392 910
883
973
800
1159
1240
800
1138
1137
800
380 400 450
392 392 675
300 300 260 1594.59 (H,O)
of beam densities and collision efficiencies were unavailable, the strong collision approximation and an average ( A E ) of energy transfer of 4 kJ/rnolz0 were chosen to ensure again that an upper limit of stabilization was obtained. In order to compete, k ( E ) and k, have to be of the same order of magnitude. The above procedure predicts that a pressure of lo3 bar is necessary in the
J. Phys. Chem. 1984, 88, 3333-3337 I
1012 -
2 w
I
* lo”
-
10”
-
IO’ -
t/
I 0
5
IO
* 15 E-Eo/Kcol
mor‘ Figure 4. Specific rate constant k(E) as a function of E - Eo ( E = 15.1 5 kcal/mol). Eo varies in steps of 1 kcal/mol. The arrow indicates the barrier from ref 9.
beam to stabilize some part of the (SO3-H2O)*adduct, whereas the actual pressure in the experiments was between lo-’ and mbar (although higer “effective” pressures may exist in the expanding beam). Since the barrier Eo has been calculated in ref 9 under restricted assumptions, calculations of k(E) with diffferent energy barriers, Eo were performed. The results are plotted in Figure 4. The arrow indicates the barrier from ref 9. The curve is calculated for the case of no barrier and for values up to a barrier height of E - Eo < 1 kcal. It should be noted that, if the energy is only slightly higher than the approximations used to calculate the density of states are no longer valid. The values plotted in Figure 4 were determined for the lower limit for k ( E ) , namely a t 0 K. The S-(OH)2 stretching frequency was taken out, and ~~ for vi*, the mixed model (case 111, Table 11) with vi* = 0 . 9 was employed. The vi values were chosen for H2S04(g). In the case of SO3-H20,the frequencies were estimated for ones similar to S03(g) and H20(g), with the other required ones being taken as similar to H2S04(g). As expected, the results were found not to
3333
depend very strongly on the choices made. Figure 4 shows that k, = lo6 s-I competes with k ( E ) < lo8 s-l for barriers E,, > 13 kcal/mol, which seems unreasonably high. As no adduct is found in the experiment, and hence all products apparently isomerize to H2SO4, Eo < 13 kcal/mol is an upper limit for the actual energy barrier. The aim of the estimation given above is to give an upper limit for the amount of stabilized (S03.H20)adduct. Corrections to the simplified treatment of k(E) and k, such as including rotational barriers, exact calculations of density of states, or weak collision deactivation will not change the conclusions from the (estimations made for the reactions under consideration. The above considerations do leave open the question of the actual fraction of H2S04 which is stable enough to survive and be detected. Without a collision to remove some internal energy, the lifetime against redissociation would not be expected to be very long. It may be that some of the energy of reaction is removed by collisions in the expanding jet. Alternatively, the reaction may proceed by interaction of SO3 with water clusters but still leave enough internal energy for isomerization.
Conclusion Theory as well as experiment confirms that the S03.H,0 adduct cannot be stabilized under the beam conditions of the present experiment. The product formed in the reaction of SO3with water and water clusters is, in fact, sulfuric acid, at least after the few hundred microseconds of transit time following formation and before being subjected to the electric deflection analyzer. The barrier to isomerization is a maximum of 13 kcal/mol. Acknowledgment. This work was made possible due to a scholarship (R.S.) of the Committee for Sciences of NATO through DAAD awarded to Dr. Rita Hofmann-Sievert. Support of the National Aeronautics and Space Administration under Grant NAG 2-176 and of the US.Department of Energy under Grant DE-AC02-82-ER60055 is gratefully acknowledged. Dr. Manfred Hofmann’s help in performing some of the experiments is gratefully acknowledged. We also thank Dr. Robert Keesee for helpful discussions. Registry No. SO3, 7446-11-9.
Surface Characteristics of Thin Films Prepared by Plasma and Electrochemical Polymerizations R. Hernandez, A. F. Diaz,* R. Waltman, and J. Bargon IBM Advanced Technology, San Jose, California 951 93 (Received: June 23, 1983; In Final Form: October 10, 1983)
A comparison is made of the surface characteristics of thin films prepared by plasma and electrochemical polymerizations. Films prepared by the two methods have similar surface energies and composition. The films produced by the plasma process are much smoother, although this difference does not effect the surface energy. The electrochemicallyprepared films, which are known to be electroactive, show a small change in their wettability when the film is switched from the neutral to the ionic form.
Introduction The stoichiometric electrochemical polymerization reaction of heterocyclic aromatic compounds which was first reported with is proving to be a fairly general reaction for the preparation of thin film coatings on electrode surface^.^-^ As a (1) A. Diaz, K. K. Kanazawa, and G. P. Gardini, J. Chem. Soc., Chem. Commun., 635 (1979). (2) A. Diaz, Chem. Scr., 17, 145 (1981).
0022-3654/84/2088-3333$01.50/0
preparative method, it is somewhat similar to the gas-phase po1ymeriZatiOn reactions which generate film Coatings in a glow discharge.bs With both reactions, a thin film coating is produced (3) A. Diaz, J. Bargon, and R. Waltman, J . Electrochem. Soc., 83, 332 (1982). (4) R. Waltman, J. Bargon, and A. F. Diaz, J . Phys. Chem., 87, 1459 (1983).
( 5 ) J. Bargon, S.Mohmand, and R. J. Waltman, I E M J . Res. Devel., 27, 330 (1983).
0 1984 American Chemical Society
