eaq- Scavenging
893
the Scavenging of eaq- and on the Possible Breakdown of ki's Equation at High Concentrations of Solutes Gudeon Czapski" and Emanuel Peled Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel (Received July 7, 1972)
It is shown that in very concentrated aqueous solutions of electron scavengers, the initial decay of eaqwith scavengers by diffusion-controlled reactions may break down due to different reasons. In addition to the known time dependence of these rate constants, it is argued that other effects have to be taken into consideration: (a) the finite lifetime of encounter pairs; (b) the initial formation (not through diffusion) of encounter pairs in concentrated solutions; and (c) possible effects prior to thermal equilibration. These effects may explain some of the anomalies in kinetic behavior and in G(eaq-) in the radiation chemistry of such concentrated solutions without necessarily having to postulate any chemical reactivity with dry electrons.
Several attempts have been made in recent years to study early processes in the radiation chemistry of aqueous solutions in the time range of 10 psec. Very concentrated solutions of electron scavengers had to be used for these studies, in order to reach half-lives of eaq- of the order of 10-lo-lO-l~l sec. In some of these studies the decay of eaq- in the 10-psec range was followed by direct observation1-4 while in olhers product analysis and competition kinetics were the in concern.5-8 Results were compared to those in dilute solution (where lifetinlerr are microseconds or longer) arid a few anomalies were discerned which led to the suggestion that dry electrons ( e d r y - ) may react with scavengers in times shorter than 10 psec (reaction l), in competition with the solvation process ( 2 ) . Hunt was able to show that the overall solvation process is complete in less than 10 psecs and recently a lower limit of 4 psec has been given for this process.10
The principal deviations in the behavior of Concentrated solutions when compared to dilute ones can be summarized as followf,. (a) When two solvated electron scavengers are in competition, it has been shown that the relative rate constants depend, sometimes strongly, on the concentrations.2.5 ( b ) The extrapolated initial yield of eaq- in concentrated solutions gave, for Cd2+, N o s - , etc., values which were much lower than e ~ p e c t e d The . ~ indiS) at t < 10 psec from the rect determination of h(ea,extrapolated yields gave different values than at longer times, and for several scavengers, rate constants as high as 2.5 X 1011 M - sec-1 were indicated. These high values were attributed by Hunt to reactions of edry-2*3by assuming that S and 1 1 2 0 compete for edry- through reactions 1and 2. The purpose 3f this communication is to show that the above-described phienomena may be interpreted without necessarily taking into consideration the reaction of edry-, and also to show that extrapolation of initial yields of ea, - in concentrated solutions is questionable. The time dependence and the concentration dependence of rate constants and their ratios of the reaction&of eaq - with electron wavengers has been discussed by
Schwarz.ll His theoretical treatment accounted only for part of the anomalies found in rate constants of shorter times, both for diffusion-controlled reactions of eaq- (with acetone, for example) and for somewhat slower reactions, eag-. This reaction is about five times such as H+ slower than diffusion controlled. Schwarz considered both the time dependency of diffusion-controlled reactions of neutral scavengers, and those with opposite charged ions, the latter being somewhat slower. The results of Aldrich, et ~ l . show , ~ that there is such a time dependence for various reactions rate constants of eaq- with H202, acetone, as well as with some cations and anions. With the ions, the time dependence is complicated by the effect o f the ionic strength which increases with increased concentration. Correcting for this effect12 the time dependence observed in 1 M solutions shows an increase of 50-100%, which is of the right order of magnitude as predicted.11,13 The reactions of eaq- with scavengers S (generally with S in a big excess over eaq-) obey second-order kinetics. The process can be visualized to occur in two steps. The first one is the transport of S toward eaq- yielding a reactive configuration (an encounter pair (eaq- :S)). The second step involves the transformation of this configuration (encounter pair) into the product S-. In addition, the encounter pair may separate through diffusion without reacting. For fast reactions the overall rate is limited by the transport process by which eaq- and S meet, i e . , it is controlled by diffusion. The typical diffusion-controlled reactions are those where practically every encounter
+
+
(1) M. J. Bronskill, R. K. Wolf, and J. W. Hunt, J. Chem. Phys.. 53, 4201 (1970). (2) R. W. Wolff, M. J. Bronskill, and J. W. Hunt, J. Chem. Phys.. 53, 4211 (1970). (3) J. E. Aldrich, M. J. Bronskill, R. K. Wolf, and J. W. Hunt, 55, 530 (1971). (4) M. J. Bronskill. R. K. Wolf, J. E. Aidwich. and 2 . W. Hunt, J. Phys. Chem., in press. (5) E. Peled and G. Czapski, J. Phys. Chem., 75, 3626 (1971). (6) T. Sawai and W. H. Hamill, J. Phys. Chem., 74,3914 (1970). (7) P. L. T. Bevan and W. H. Hamill, Trans. Faraday SOC., 66, 2533 (1970). ( 8 ) S. Khorana and W. H. Hamill, J. Phys. Chem.. 75, 3081 (1971). (9) M. J. Bronskill, R. K. Wolf, and J. W. Hunt, J. Phys. Chem., 73, 1175 (1969). (10) P. M. Rentzepis, R. P. Jonas, and J. Jortner, to he published, (11) H. A. Schwarz, J. Chem. Phys.. 55, 3647 (1971j. (12) E. Peled and G. Czapski, J. Phys. Chem., 74,2903 (1970). (13) R . M. Noyes, Progr. React. Kinet.. 1,129 (1961). The Journal of Physical Chemistry, Vol 77. No. 7, 1973
Gideon Czapski and Emanuel Peled
If we assume that R3 > r ~ 3P, will be given by
yields a product. eaq"
-+
-
4
S e ( e a q
:S)-kz
S-
(3)
A-I
For these reactions k . 1 C kz (otherwise not every encounter H f , k - l / k z = 5.11 would yield S - ) , while for eaq(Sincie for k o b d is about five times smaller than the diffusion-controlled value would be.) For reactions which are diffusion controlled, T~~ (the encounter pair lifetime) is 1/Ezz -k 1,'k-l which is at most 10 psec. ( l / k - 1 5 10 psec for such reactions at room temperature.14) Theories of diffusion-controlled reactions were developed by Smoluchowski.15 The model assumes that S (uncharged) has to diffuse toward eaq- and, when the distance between their centers reaches a value R, they react. TbEh solution of the diffusion equation with the boundary conditions %r, 0) = SO for T > R
+
w+,
S(r,f) = 0
for
I^
4R
(4)
vields
the time dependent form of times to the stationary form
kobsd
which reduces at longer
FOTcharged particles Debye16 extended the theory taking the interaction iinto account, The solution of the diffusion equations in this case yields an equation similar to (5) but R is replaced by reffgiven by
where U is the coulombic interaction energy. For dilute solutions this integration yields
where Q = Zse2/tkTR (ZSis the charge number of S, e is the electron charge, 6 is the dielectric constant, and k is Boltzmann's constant). R in eq 4-7 i s the distance between eaq- and S a t which they react. R is often taken as the sum of the crystallographic radii, (rt i r(eaq-1 = R ) although there is no proper foundation for this. For example, R may exceed rs r(eaq-) if the reaction occurs via tunneling as suggested by Anbar and Nart17 for these reactions. Furthermore, for severizl reactions lit was found experimentally that R ex?(eaq- 1 by a fador of up to 3, both for anions ceeds rs 2.8 and and neutral moiecules'7 (assuming r(eaq-) n is calculated from kobsd and eq 5). For cations reacting with &aq - it is difficult to estimate R, as the value of kobsd is very insensitive to the value of R. Let us examine whether the assumptions for the diffusion-controlled reaci;ions are valid in very concentrated solutions. The boundary condition in Smoluchowski's equation, S(r,Of = 0 fool. r 5 R , assumes that no eaq- is present a t t = 0 in the spheres of radius R around S. In conoentrated solutions of S, the volume occupied by S molecules may be very appreciable. We can calculw1,e the probability P that n molecuies of S, each of an "encounter volume" u = 4/3~R3, do not cover a random spot in the solution of volume V.
+
+
The Journaiof Physical Chemistry, Vol. 77, No. 7, 1973
-
where [SI is the concentration in moleslliter and 8 is the partial molar volume of S in liters/mole. Table I gives the concentrations of S for various values of R with the respective values of P. Since for diffusion-controlled reactions the average value of R can be taken as -6 A, it follows that in --I M solution of S, P 0, 5 , or 0.5 of eaq- which would be introduced randomly into this solution would be formed initially as encounter pairs (eaq-:S). Thus, if S(r,O) = 0 for r < R, this means that half of ea, - will disappear instantaneously as soon as they are incorporated into encounter pairs. Therefore, any yield of eaq- as extrapolated to t = 0 will account only for the fraction of eaq- which is not initially formed as encounter pairs. This analysis assumes a zero lifetime of the encounter pairs (eaq - :S). The relative yield of eaq-, as given by P in eq 8, has the identical functional form as derived by Wolff, et n1.,2 for a model where dry electrons compete in reactions 1 and 2 . Their experimental results2 show that for acetone, HzOz, NO3-, and Cd2+ eq 8 holds. If we attribute the linear decrease of log [G(eag-)] with [SI, found experimentally2 as due to eq 8, the following values of R are found: R = 6, 6.5, 9, and 9.5 A for HzOz, acetone, Nos-, and Cd2+, respectively. Values of R 6 A are within the average value of R for diffusion-controlled reactions of eaq-.I7 A value of R of 9 or 9.5 A is high, although such values are suggested for several solutes in their reactions with eaq-.17 If, with these values of R, we calculate the rate constants of the reactions of eaq- with these solutes from eq 5-7, the computed values are only about twice the value of the measured one. (For a positive ion such as Cdz+, at low ionic strength the coinputed value of R is changed only by 25% should R be either 3 or 10 A). Thus, the results of Wolff, et a1.,2 seem to be interpreted reasonably well without having to assume any reaction of edry- with these solutes. We will try to visualize the possible meaning of the encounter pair and its lifetime. R as defined in eq 4-7 is the distance between the centers of S and eaq- a t which they react and, as argued earlier, R can exceed rs r(eas-). Obviously, R is some average value as the reaction in all probability will not be a step function with distance. If we assume that the probability function is rather steep, namely, the reaction will occur only if the distance between eaq- and S is not changed by more than an average diffusional jump of 1 A, this would take 10-12 see. With this model, when eag- and S reach the reactive configuration (eaq-:S) the lifetime r of the process (eaq-:S) product cannot exceed 10-12 sec, but may be faster. T may be different for various solutes but their diffusioncontrolled rate constants will not differ, a t least in dilute solutions, as long as for these solutes, T 5 10-12 sec, and R and D are independent of the solutes. The effect of such a behavior in. concentrated solutions will be discussed later.
-
+
-
-
(14) S. W. Benson, "The Foundations of Chemical Kinetics," McGrawHill, New Yqrk, p. Y., 1960, p 496. (15) M. V. Smoluchowski, 2. Phys. Chem.. 92, 129 (1917). (16) P. Debye, Trans. Electrochem. SOC.,82, 265 (1942). (17) E. J. Hart and M. Anbar, "The Hydrated Electron," Willey-lnterscience, New York, N. Y., 1970, p 187.
eaq-
Scavenging
095
TABLE I: Concentration Dependence of the Probabilities of Initial Formation of Encounter Pairs on the Reaction Cross Section (RILE
0.1 M ACETONE t 0.25 M ACID
a
Pb
0.9 3
4
5 6 7
8 9 10 Q
0.8
1.45 3.2 0.41 0.9 0.32 0.18
0.4
0.7
0.41
0.25
0.08 0.17 0.055 0.12 0.04 0.09
0.7
0.6
0.5
0.4
0.3
0.2
0.1
5.07
7.25 1 0 13 17.4 28.3 33 2.85 3.72 4.95 6.64 9.5 1.12 1.59 2.2 2.87 3.83 5.12 7.36 0.64 0.92 1.27 1.66 2.21 2.96 4.24 0.41 0.58 0.8 1.04 1.39 1.86 2.66 0.27 0.39 0.54 0.71 0.94 1.26 1.79 0.19 0.28 0.38 0.44 0.66 0.88 1.27 0.146 0.20 0.276 0.36 0.48 0.64 0.92
?.44 2.07
All concentrations are in molss/liter.
I-\
r‘
‘\,
1.OM ACETONE + 0.25 M ACID
P is defined in eq 8 .
It seems to us also possible that T can last as long as 10 psec which implies that for diffusion-controlled reactions the reaction probability would vary slowly from 0 to 1 (possibly the reaction probability can vary over this range form 0 to a fraction of 1) over a distance of about 3 A. In this case, however, the diffusion theory would find itself in error, but it would seem that the error would not be a large one and the theory giving an average value of R would still be giving a good approximation. If the (eaq-:S) has a finite lifetime ( T < 10-1l or 10-12 sec) the following questions arise. (1) What is the dependence of concentration of the (eaq-:S) on time and on [SI? Obviously. a t sufficient low [SI, the concentration of (eaq-:S) rapidly reaches a very low stationary state. (2) What are the propwties of (eaq-:S)? T h e two extTeme cases would be that either it resembles S- or it will be very similar to eac - ++ S as long as it did not react. This last possibility setems more plausible a t least concerning the absorption spectrum, as it was found that in 5 M solutions of NaClQa. NaQZI, and KF, the spectrum of eaq- is hardly affected, although all the ea,- is present in encounter pairs.29 I* The implications of the above-mentioned points concerning some of the anomalies observed in concentrated solutions of S are as follows. (I) In Hunt’s experiments12 eaq- is observed a t t 10 psec, and the yield olF eaq- is integrated from t = 0. In our opinion this integration is invalid, even if the time dependence of kobsd is taken into account, as was done by Schwarz,Ia since thig integration takes into account only the decay which is due to encounter pairs formed through diffusion and neglecis those which were formed initially. Such a niathematical treatment necessarily gives too low values of G(eas-) depending on the fraction of encounter pairs formed initially. Hence, in concentrated solutions the error may be very large (Table I). To demonstrate the extent of this effect, the results obtained by Wolff, rd a1 ,% concerning the decay of eaq- in 0.25 N acid wiih acetone of 0.1, 1.0, and 3 M , will be anaN
Figure l shows the measured2 dependence of eaq- with time, and the lines (computed by Schwarz,ll taking into account the time dependence of the rate constants. In all these computed lines, eaq- exceeds the measured values and this may ripxggest that edry- must have some sort of role to play in these 6,ystems. We added a third line which is the line computed by SchwarzlJ but corrected also for those eaq- initially formed as (eaq-:S) and assumed to decay away wiikin less than 10 psec. It can be seen that
Figure 1. Optical density (arbitrary units) as a function of time during linac cycle: (. . . . . . .) observed line$ (- - - -) calcucorrecting the lated cooperating time dependence;” (--) above line according to eq 8. (Note that acetone] = 2 M in the experimental curve c where t h e correctioA’ is made for 3 M ) .
this correction is even larger than the one due to time dependence in concentrated solutions. With this additional correction taken into consideration, the computed concentration of eaq- in solution of 2 M acetone is less than the observed one. (This is partially due to the fact that Schwarz’s correction is too high as his was done for 3 M acetone.) For 1 M acetone, the agreement is excellent, far better than the accuracy of both corrections. In principle 10 psec which these corrections could give yields a t t would be too low, if (eaq-:S) would have a lifetime T which would be in the range < T < 10-lI sec. In such a case part of (eaq-:S) would still be present a t t = 10 psec, and should its absorption be similar to that of eaq- the observed yield would stand for the sum of eaqand (eaq-:S), whereas the computed one was only for eaq-. The high values of the decay rate constants as derived by Hunt3 from the fraction of eaqx which decayed during his pulse ( t < 10 psec) and attributed by him to reactions of dry electrons with S, could be, in fact, solely due to the decay of encounter pairs initially formed. Figure 2 shows the relative yield of eaq- ab a function of T , the lifetime of eaq- in the presence of S, as computed by Aldrich, et al.3 The observed yields of eaq- in concentrated solutions of various solutes were much lower than predicted from Figure 2, and the known values of T for the reactions of eaq- with these solutes (Table 11, ref 3). Values of 7 which could have been consistent with the observed yields were up to 20 times shorter than the known values of T for the reaction of eaq- with these same solutes. The line computed by Aldrich, et al.,3 did not incorporate a time dependence of k(eaq- ++ s);a correction to this effect would have shortened T(eaq- ++ S) in concentrated solutions of S by a factor of up to about 2. We
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(18) M. Anbar and E. J. Hart.J. Phys. Chem., 69,1244 (1965).
The Journal of Physical Chemisfry, Vol. 77. No. 7, 1973
Gideon Czapski and Emanuel Peied ICONCENTRATION
2
1
---T---T--T
0.4
0.2
0.t
1
I
M 0.04 I
pected to be much lower than observed.2 (In 5 N acid, the yield drops only by -40%.) This suggest either a lifetime for (eaq-:H+) of several psec, or that R is much smaller. Clearly, T and R are averaged values, since as long as R > (rs i- r(eaq-)), several configurations of (eaq-:S) with various values of R may be formed, and they will have different reaction probabilities or lifetimes. As it was found17 experimentally for diffusion-controlled reactions of eaq- with S, values of R may be much rs). Therefore, i t is not possible to larger than (r(eaq-) decide whether any “normal” reaction of eaq- (for which R (r(eaq-) i- r s ) ) actually occurs on every encounter. If for the normal reactions, the real value of R is larger than rs) and only a part of the encounter pairs (r(eaq-) reacts, the value of k&sd in dilute solutions will be unaffected. In principle one would be able to distinguish between these cases in concentrated solutions, by studying the time dependence of the reaction, which should be different for the two cases. This uncertainty makes it even more difficult to estimate the values of R, and therefore the fraction of initially formed encounter pairs in concentrated solutions cannot be well estimated. (11) Another implication of the direct formation of encounter pairs in concentrated solutions is that the competition kinetics may be affected. Should two scavengers SI and SZ, yielding products PI and Pz, in their reactions with eaq- have the same charges, diffusion coefficients, and values of R, they will have identical values of kobsd in dilute solutions, as calculated from eq 5.In competition studies of ea,- with S1 and SZ at low concentrations of the solutes, normal competition kinet,ics will be observed, with
+
-
+
Figure 2. Relative yic!lds of e - as a function of T or [SI.T is the lifetime of e - C S . f y i s for a reaction with D = 5 X (:m2 s e e - y R = 6 , and k(eaq- 4- S) = 2.5 X 10” M-’ sec-1: ( ) predicted by Wolff, et a/.;* (- - -) correction of Wolff’s2 line with eq 8. predicted by eq
added to Figure 2 the yields of eaq- as predicted by eq 8 and then corrected Aldrich’s3 curve accordingly. The corrected curve predicts much lower yields of eaq- for a given value of r(eaq- -k Si) than did the original3 correction. The time dependency would even lower further the yield of e a q - ~For example, if k(eaq- + S) 2.5 x 1O1O f M - l sec-1, D = 6 X 10-5 ,.J cm2 sec-1, R = 6 A, and [Si = 4 M , the relative yield of eaq- predicted by Aldrich, et d . , 3 is 0.45, while with our correction, it is less than 0.05. When Aldrich’s correction is included as well as ours, the yield is expected to drop, and this drop is accounted for without necessarily assuming that dry electrons react with S and compete through reactions 1and 2. If, on the other hand, (eaq-:S) has a finite lifetime, a similar though somewhat smaller error in extrapolating G(eaq -1 will OCCUI. If r , the lifetime of (eaq - :S), is between 10-12 and 10-11 sec, the decay rate of eaq- as observed in very concentrated solutions may approach the decay rate of (eaq- S) and not that of eaq-. This will occur in solutions where the bulk of eaq- is formed initially as encounter pairs with S, provided that the lifetime of these encounter pairs is long enough. These conditions may lbe fulfilled for reactions which are a little slower than diffusion controlled, where only one out of every few encounters will lead to reaction. The reaction of eaq- with H+ could be such a case, and this may explain the relatively low reactivity of H+ in concentrated solutions. The low relative reactivity of Hf in such solutions was partially accounted for earlier by Schwarzll as due to the time dependence of this reiiction. R for the reaction of H+ with eaq- is about 5 A, in 5 C104 80% of eaq- are formed as (eaq-:H+). If y5 of these encounter pairs will yield H in 10-12 sec, the 4/s escaping their originally formed encounter pairs have a very high probability of forming within one diffusional displacement new encounter pairs, and therefore the expected yield of eaLI-in this solution a t t = 10 psec is ex-
-
-
-
The Journal of Physical Chemistry, Vol. 77, No. 7, 1973
(9) and
If one would keep [SI]/[S2] = 1, in dilute solutions onehalf of eaq- will react with each S1 and Sz. In concentrated solutions of two solutes, eaq- may initially participate simultaneously in a n encounter pair with both SI and Sa. If the reactions of eaq- with both of these solutes are diffusion controlled, both will. react with eaq- on every encounter, or within less than the encounter lifetime. If SI reacts faster than S Z with eaq- within the encounter lifetime, one will expect that in their simultaneous encounter pairs with eaq- the relative reactivity, or the product ratio [P1]/[Pz], will exceed that predicted by eq 9. Therefore, [Pl]/[P2], as measured, will depend on and grow with the concentration. This is in addition to the time dependence predicted by Schwarz.11 At very high concentrations, [lP1]/[Pz] will approach the ratio of the lifetimes of S1 and Sz in their respective encounter pairs. The last point which we shall consider in processes occurring a t times as short as 10-11-10-12 sec after the deposition of radiation is that of the effects concerning the temperature in the reaction zone. M o z u m d e P calculated that on the average, in the spur, in a volume with a radius of 20 A, 30 eV are converted into heat in less than 1 psec., This could correspond to a “temperature increase” of 30” in this region. (If a larger fraction of the energy is deposit(19) A Mozgnder, Advan Radiat C h e m , 1, (1969)
Mass Spectrometric S'tudiesof Gaseous Sulfur Fluorides
a97
-
ed initiatly in the central core of the spur ( r 15 A), the temperature increase may be as high as 70"). Mozumderl9 calculated that the thermal equilibration times in H20 are about 18, 5.85, and 2.95 psec respectively a t 273, 313, and 353°K.20 Any process occurring at times shorter than 10 psec may be influenced by conditions occurring prior to thermal equilibrium. This effect could account for solvation of electrons faster than expected a t room temperature. The solvation time of electrons in various alcohols a t room temperature was found to be less than 20 psec,I although the expected value for some of these alcohols lis much larger.l.19 If we assume that the solvation occurs in this time range and at the higher temperature existing at that time, the solvation process is expected to be much faster.20 In very concentrated solutions, where chemical reactions take place in times as short as 10 psec, the rate of these reactions maybe somewhat faster due to the lack of thermal equilibrium during part of these processes, although this effect will most probably be small it could lower to a small extent the yield of eaq- at t 10 psec. The effects which should be considered in concentrated solutions and studies in the picosecond range may be summarized as follows. (1) A t very short reaction times, the initial temperature is higher than after thermal equilibration and may accelerate solvation and also to some extent chemical reaction rates. (2) In concentrated solutions the rate constants OF ea,- with scavengers will depend on the concentration, due to (a) the time dependence of rate
constants as discussed by Schwarz,ll (b) the temperature effect, (c) the breakdown of Smulochowski's equation in the time range where the encounter's lifetime becomes comparable to the half-life of eaq-, and (d) the direct formation of encounter pairs. The last effect may be the most serious one in concentrated solutions and the extent of it the most difficult to estimate since it i s very sensitive to R, the value of which is hard to evaluate. A similar effect could occur where photochemical cages are studied in concentrated scavenger solutions. These effects may explain most of the anomalies in competition studies of concentrated solutions of eat%scavengers and of the kinetic studies in the picosecond range. They may explain the low G(eaq--) observed in some of the concentrated solutions, and seem to be able to explain the results without having to have recourse to novel chemical species such as dry electrons. This, however, does not rule out the possibility of reactions of dry electrons prior to solvation, but indicates this assumption is not yet a necessity.
-
Acknowledgment. We would like to acknowledge many helpful discussions with Professor I. Z. Steinberg and with Dr. M. S. Matheson during the latter's stay in this laboratory and we are especially grateful to one of the referees (H. A. Schwarz), whose repeated critical refereeing was extremely helpful to us. (20) A. Mozumder,J. Chem. Phys., 50,3153 (1969).
Mass Spectrometric Studies of Some Gaseous Sulfur Fluorides
- L. Hildenbrand' PAcDortnell Douglas Astronautics Company, Huntington Beech, California 92647 (Received November 6, 1972)
The reaction of gaseous SF6 with carbon in a heated Knudsen cell was studied by mass spectrometry, and several new molecular species including SF, SF2, and SCFz were identified and characterized. From electron impact threshold measurements, ionization potentials with an estimated accuracy of 8.18 eV were obtained for these species, while similar measurements on SF4 and SF6 were used to derive AHo" = 146.7 f 3.2 kcal for the process SF6 = SF4 2F and A H f 0 2 9 ~= -180,O f 5.0 for SF4. Gaseous equilibration was observed for several isomolecular reactions, and the resulting equilibrium data were used to derive the standard heats of formation, ANf0298, of 3.2 f 1.2, -71.4 f 2.5, and -75.0 f 3 kcal/mol for SF, SF1, and SCP2, respectively. For SF, this is equivalent to the dissociation energy Do'fSF) = 81.0 f 1.2 k c d or 3.51 0.05 eV. The results can be combined with available literature data to evaluate the six individual bond dissociation energies in the SF6 molecule, yielding values which are in qualitative agreement with the valence state concept of covalent bonding.
+
Introduction Although gaseous SF6 is now being used in several hightemperature applications ( e . g . , as an electron scavenger in plasmas and as a fluorine atom source in chemical lasers), very little is known about the thermochemistry of the lower sulfur fluorides. Since these lower fluorides may play al significant role in the overall chemistry, a thermo-
chemical study was undertaken using the familiar techspectrometry* nique Of high-temperature SF6 itself has been rather thoroughly studied, and the thermochemical properties and the spectroscopic con(1) Present address: Stanford Research Institute, Menlo Park, Calif. 94025. The Journal of Physical Chemistry, Vol. 77, No. 7 , 7973
