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Reactions and energy distribution in dissociative electron capture processes in sulfuryl halides
International Elsevier
Journal of Mass Speciromet7y
Scientific
REACTIONS ELECTRON
JIA-SHEN
Publishing
(Received
36 (1980)
and Ion Physics,
Amsterdam
-
Printed
in The
233-247
233
Netherlands
AND ENERGY DISTRIBUTION IN DISSOCIATIVE CAPTURE PROCESSES IN SULFURYL HALIDES
* and J.L.
WANG
Department
Company,
of Chemisfry. 21 March
FRANKLIN
Rice
Univekty,
Houston.
Texas
77001
(U.S.A.)
1980)
ABSTRACT The electron
sulfur-y1 capture.
halides, SOsFs, The ionization
SOzCl,, efficiency
and SOsClF form several ions curves are determined for each
by dissociative process. Where
the ion intensity was great enough, the kinetic energy of the ion was measured over the greater part of the electron energy of the resonance. In most instances the kinetic energy of the ion increased linearly with electron energy over the range of the resonance. Extrapolation to zero kinetic energy release gives the electron energy corresponding to the heat of the dissociation reaction_ The kinetic energy released in the formation of F: and Cl5
was only slightly greater than that predicted by the quasi-equilibrium theory, as would be expected from the fact that the molecule had to change its configuration during dissociation_ Essentially al1 the excess energy was released in translation when SOzFwas formed. The ions Ffrom SOsF2 and Clfrom SOsCl+ are formed in two separate resonances, and the kinetic energy lines extrapolate to thermal appearance potentials (mr) corresponding to the formation of SO2 and the halogen atom. The m= values for these reactions as well as that for Cl5 at their higher energy resonances appear to correspond to an electronic transition for SO2 of 2.5 eV. INTRODUCTION
In a study of negative ion-molecule
reactions in the sulfuryl halides [ 1 ] ions were formed with considerable The amount of translational energy of the ion together with the accompanying intemd energy, if the ion is polyatomic, in some instances might alter the interpretation of the mechanism of reaction. Consequently, we undertook a study of the energy distribution in the principal dissociative resonance capture processes of S02F2, SO&I, and S02ClF. This paper reports the results of that study. it
became
evident that several translation&l energy.
of
the
primary
EXPERIMENTAL
These studies were made with a Bendix Model 14-107 Time-of-Flight mass spectrometer and a data collection system which was made possible by several predecessors to improve the sensitivity and resolution. In this system *Present
adress:
93
Shadow
OOZO-7381/80/0000-0000/$02_25
Ridge
Rd., @
Stanford,
1980
Elsevier
CT
06905,
Scientific
U.S.A. Publishing
Company
234
the clock, 1 MHz, of a BNC Model 7020 digital delay generator, is modified by a frequency divider into 10 kHz. This pulse is used to start the mass spectrometer and also is fed into the delay circuit of the digital delay generator to produce a reference pulse. An Ortec model 467 Time-to-Pulse-Height Converter (TPHC) converts the difference in time between the reference pulse and the data pulse into a pulse whose height is proportional to that time difference. The data pulse is the ion signal collected in the mass spectrometer and a-mplified to a level that can be accepted by the stop input of the TPHC. In order to store the signal in a Packard Model 901-1024 channels computer memory, the output of the TPHC is transformed into digital form by a Packard Model 960 Analog-to-Digital Converter. The accumulated data (the mass spectrum) in the computer memory can then be recorded by the regular recording system. The beginning time is set by the digitill delay generator which has a working range from 0.1 ps to 1 m F -.ith a 0.1.ps increment and a jitter of 0.1 ns. The time domain of a spe&rum is set by the TPHC which has a range of 0.05-800 ps. For a single mass peak, the range of 0.2 ps has very often been used. By using a 1024-channel analyzer, this range gives a resolution of 0.2 ns. Therefore, in this system, if all the other conditions are ideal, the limiting factor to the actual resolution will be a few tenths of a nanosecond. This is much superior to the previous gating system which had a resolution of 20 ns [ 21. The other advantage of the present system is that a large signal-to-noise ratio for a weak signal may be obtained by accumulating data for long periods while in the previous system the signal-to-noise ratio was limited to a fairly low value. The translational energies of the ions were measured by the method [2,3] described in detail previously. In this method, the translational energy distribution of an assemblage of ions was determined from the width of the mass peak in the time-of-flight mass spectrometer. Experimentally, a calibration curve of ion peak width at half-height (W,,,) versus the square root of the ionic weight (u) was constructed by measuring the value of W,,* for several ions known to be formed with thermal energy. The WI,, value of the ion being investigated was then converted into an apparent ionic mass, ma_, and the translational ener%T of the ion, 6 is calculated from .~i = ~RT(m,,,/mi)
(1)
where mi is the actual mass of the ionic species. In Fig. 1 typical calibration curves are given for the instrument employing the present system and the previous one employing the gate. The advantage of the present method, especially with the lighter ions, is obvious. From the conservation of momentum, one obtains the following relation between i and the translational energy in the center of mass, Z;:
235 1
I 0
X
30-
FROMCOUNTING SYSTEM FROM
lOI 5
GATING
SYSTEM
I
I
IO
15
(ION Fig. 1. Kinetic
where m, molecule,
MASS)
energy calibration
I
20
“2
curves.
and M are the mass of the neutral fragment respectively.
The
electron
energy
was
calibrated
and of the parent with
O-/SO2
[4],
the first peak maxima occurring at 4.9 eV. The sulfuryl chloride and sulfuryl chlorofluoride were obtained from the Aldrich Chemical Company; sulfuryl fluoride from the Matheson Company. The samples were used without further purification; however, for each run, the hesh sample was introduced into the sample inlet system several tunes before data were taken. RESULTS
It has been shown in several studies [ 3--51 that the translational energy of an ion formed by dissociative electron capture will usually increase linearly with increasing excess energy over the greater part of the resonance. In such instances, it is possible to extrapolate eqn. (2) to Ft = 0 at which point 6 = mi/~~(~RT), the contribution of the thermal energy of the precursor to the transiational energy of the fragment ion. The corresponding electron energy will be the energy of the dissociation reaction. It has been shown [Z] that such linear dependence of kinetic energy release, Et, upon electron energy can be expressed by the quasi-theoretical equation (3) where E:., is the excess vibrational and translational energy above the dissociation asymptote_ For the process in question, N is the number of vibrational modes in the ion prior to dissociation and cy is an empirical constant. If FYis plotted against E’, the slope of the line is l/oCnr and is the fraction of
236
the excess energy (above thermal) released in translation. The smaller a! is, the greater the lirzction of excess energy going into translation.
_4t low electron energies three ions, F;, S02F-, SOzF2. These ions will be discussed separately.
and F- are formed
from
The intensity and kinetic energy of F; as a function of electron energy are given in Fig. 2. The width of the resonance curve at half-height is 1.5 eV, whereas the FWHM of SF; was about 0.5 eV in our ;Jlstrument. The intensity of F; is relatively small so that satisfactory measurements of translational energy could be made only near %e -maximum of the resonance curve. Nevertheless, a linear dependence on electrcln energy is observed with a slope (l/&V) of 0.204 and an cz value of 0.54. When the kinetic energy line is extrapolated to the thermal value of 0.34 kcal mol-’ the electron energy corresponding to AEI, is found to be 1.61 eV or 37 kcal mol-’ for the reaction SOzIp + e + F; + SO* Using 1 one value tained
(4) this value and heats of formation of SOzFz and SO* taken from Table obtains ANr(F;) = -73 kcal mol- l_ This is in fair agreement with the of 71 kcal mol-1 obtained by Chupka et al. [7], and with values obby other experimenters.
SOIF-
The S02Fion is formed in the same electron energy region and with a resonance curve of the same FWHM as F,. The intensities of the S02F- ion are 2-2.5 times greater than those for F;. Figure 3 gives the resonance and
1.0
r 8-
0
I
2
4
ELECTRON
Fig. 2. Ionization
efficiency
ENERGY,
5
6
eV
and kinetic energy curves f&r Fs from S02Fz.
237 TABLE Heats
1 of formation
employed
Substance
Nf*9,
F F-
(kcai mol-’ )
Ref.
18.88 -60.03 -71.0 -181.3 28.68 -54.64 -54.9 -84.9 -133 -70.336 -93.4 -13
F2
SOzFz
Cl clCl; so2c12 S02ClF SO, so; CIF
6 6 : 6 6 7 8 9 6 6 6
kinetic energy curves. it will be observed that the kinetic energy rises linearly at electron energies above 3 eV, i.e. just below the resonance maximum. At lower energies the kinetic energy remains nearly constant; we attribute this to the high energy tail of the electron energy distribution. The rising portio:n of the kinetic energy curve extrapolates to a thermal energy of 0.73 kcal mol-’ at an electron energy of 2.5 eV, giving a heat of reaction of 57.7 k&l mol-’ for the reaction S02Fz
+ e --t S02F-
+ F
(5a)
From this one computes AHf(S02F-) = -142.3 kcal mol-I. From the slope of the kinetic energy curve in Fig. 3,l/oN = 0.79 and a = 0.14, so that a huge fraction of the excess energy in this process appears in
I
2
ELECTRON Fig.
3. Ionization
efficiency
3
4
ENERGY, and
kinetic
5
6
eV energy
curves
for
SOzF-
from
SOzF2.
238
translation. The large fraction of excess energy released in translation suggests that very little change in the arrangement of the remaining atoms occurs as the electron replaces one F atom, F-
Two resonances corresponding to two different electronic states are observed in Fig. 4 for the formation of F- ion. The ion intensities are m lch greater in the lower than in the higher energy resonance. The translational energy of F- under both resonances rises linearly with increasing elect Ton energy. A linear decrease in energy occurs in the region where the two st: rtes overlap, as would be expected_ Extrapolation of the translational energy line for the state of lower energy reaches a thermal value at 2.8 eV. This agrees within 0.1 eV with heat of the reaction
(6) -, F- + F f SO* it is tempting to think that this reaction accounts for the formation of Fover the whole range of the first resonance. However, the simultaneous separation of F- and F is considered to be improbable, and so we doubt that this mechanism accounts for the formation of F-. Further, the small increase in kinetic energy of F- in this energy range suggests that a different mechanism is involved. Note in Fig. 4 that the translational energy line for F’- starts to increase at 3.1 eV. This point of increase corresponds approximately to the eiectron energy at which the kinetic energy of SO*F- begins to decrease. If this energy, 3.7 eV, is +aken as the energy at which S02F- has just enough vibrational energy to dissociate, one finds, for the formation of S02F- by reaction (5a), the total excess energy Ez., to be 28 kcaI mol-I, Et to be 22 kcal mol-’ and E,. the vibrational energy of SO*F-, to be 6 kcal mol-‘. SO,F,
2
3
4
5
ELECTRON Fig.
4
Ionization
efficiency
and
kinetic
6
ENERGY, energy
7
9
a
eV curves
for
F-
fT>v
S02F2.
239
With this amount of vibrational released by the reaction S02F-
energy in SO*F-
it is assumed that F- is just
--f F- + SO,
(5b) The value of AHI(S02F-) is then calculated to be -139 kcal mol-‘. This is in fair agreement with AH,(SO,F-) = -142 kcal mol-’ obtained from the extrapolation to its thermal value of the kinetic energy line for SO,F- in Fig. 3. Although it is not possible to calculate, the exact kinetic energy line for F- on the basis of this mechanism, a reasonable assumpt.ion that (11= 1 for the dissociation of SO*F- results in fair agreement of calculated and experimental values for Ei( F-) as shown in the following tabulation: Electron
energy
(eV)
q(F-)
5.5 4.5 4.0 3.5 It
(kcal
mol-‘)
talc.
meas.
4.3 3.0 2.3 1.4
5.2 3.4 2.4 1.4
is conceivable that F- might be formed by the folIowing mechanism
SOzF2 + + F; + SOz
(7a)
F;+F-+F
(7b)
Reaction (7a) will involve 24 kcal mol-’ excess energy at the extrapolated onset of 2.8 eV for F-. Using the measured value of l/crN = 0.204 determined above it is found that g = 4.9 kcal mol-’ and the vibrational energy T; = 19 kcal mol-‘. Reaction (7b) is endoergic by 27 kcal mol-’ , SO even in the improbable case of all 19 kcal mol-’ going to excite the vibration of F;, the reaction would still lack 8 kcal mol-’ to proceed. Further, if reactions (7a) and (7b) are considered at several higher energies one finds eV
pi
Gil F-1
3.7 4.0 4.5
6.5 5.4 8.8
1.9 2.4 3.4
If this mechanism prevails and if one assumes that, at 3.7 eV, F? receives just enough vibrational energy to dissociate, Ei(F-) should be one-half of ci(F;) or 3.25 kcal mol- I_ This is clearly more than the measured value. At higher electron energies the kinetic energy of F- would be greater than 0-5fi(F;) by one-half the amount of vibrational energy in F; converted to translation. Thus, the kinetic energy of F- formed by reactions (?a) and (7b) would be considerably greater than the measured values, and it is concluded that Fis not formed by this mechanism. We conclude, therefore, that F- is probably formed by reactions (5a) and
2+0
(5b), but we cannot exclude Figure 4 shows a second
the possibility resonance for
of its formation by reaction (6). the formation of F- and a Linear
dependence of kinetic energy upon electron energy, with an extrapolated thermal onset at 5.24 eV. The ion kinetic energy cwve is very nearly parallel to that at lower energy resonance. Behavior of this kind is characteristic of a process yielding the same product in both resonances but with one of the products in the higher energy resonance being electronically excited. The difference in thermal energies at onset is 2.44 eV, and this can be taken as the eIectron.ic transition energy. This cannot occur with F, and it seems highly improbable that F- would have an electronic state at this level. It seems probable then that SOS is electronically excited. Herzberg [lo] reports the lowest excited state of SO* to be Z(3B,) at 3.2 eV above the ground x( *A 1) state. Our value does not agree well with this, but we know of no other explanation of our results.
Dissociative electron capture processes occur in SO&l2 with resonance maxima at about l-l.2 eV and 4.5-5.1 eV. In both resonances Cl- and Cl; ions are observed, Cl- being a major ion and Cl; a minor one in both. In addition, SO; occurs in great abundance at the higher energy state_ The resonance and kinetic energy curves for Cl; are shown in Fig. 5. The lower energy resonance curve shows a peak width at half maximum of about one electron volt and thus 13 characteristic of many dissociative resonance capture processes. The resonance at the higher energies is much broader as is sometimes observed when two or more close-lying electronic states occur in the energy range in question. The kinetic energy of Cl; is quite large and exhibits a linear dependence on electron energy in both resonances with a decrease in kinetic energy cu l.O-
I
G
zG? z p
0.8 0.6-
z L?_I
0.4-
>
F
“J
0.2-
ILI
cc
o-
-_-I
0
I
ELECTRON Fig. 5. Ionization
efficiency
-2
THERMAL 3
ENERGY,
ION 4
ENERGY 5
1 6
eV
and kinetic energy curves for Cl5 from SO2CIz.
241
where the two resonances appear to overlap. The extrapolated thermal at -1.37 eV and +0.86 eV. The onset for the reaction SO&l2
+ e + Cl; + SO,
curves
are (8)
in the lower resonance results in a value of AH,(Cl;) of 46 kcal mol-‘. This discrepancy from the accepted value of -55 kcal mol-l is disappointingly large; however, we can account for it only as error resulting from the quite long extrapolation. The extrapolated onset of the kinetic energy curve in the second resonance involves 2.65 eV excess energy. Since there is no excess translational energy and presumably no excess vibrational energy at this extrapolational onset, this excess energy must be electronic. This is not likely to reprosent ex.Aation of Cl;, so we conclude that it must represent excitation of SO,. It will be recalled that the onset of the second resonance for the F- ion in SOzFz was interpreted as involving excitation of SO2 with an electronic transition of 2.44 eV, While the agreement of the electronic transitions in the two processes is not nearly so precise as one would expect of spectroscopic measurements, the agreement does indicate that the same states are involved
in the two
processes.
CIFigure 6 shows two resonances for Cl- with m axima at 1.2 eV and 5.1 eV. The curve for the higher energy resonance is very broad and may include more than one electronic state. The ion kinetic energy rises linearly in both resonances with a change in slope in the region where the two resonances overlap. Extrapolation yields zero kinetic energy release at -0.5 eV and +f.8 eV. The -0.5-eV intercept is concordant with the reaction SOZCIZ + e -+ Cl- + Cl + SO* where Cl-, l.O‘c-l 0 g OB- g 0 1”
0.6-
2 W
30
-
b
20
& 0.2-
d nf
40
I0.4
2 z
Cl and SOZ separate simultaneously.
.
ci E
(9) Presumably reaction (9) will
E z
o-
‘O 0 -I
0
I
2
ELECTRON Fig. 6. Ionization
efficiency
and
3
4
ENERGY. kinetic
energy
5
6
7
eV curves
for
Cl-
from
SO~CI~.
242
occur over the greater part of the resonance, but since it is a three-body process it is not possible to compute the total kinetic energy released as a function of electron energy. Conceivably Cl- might be formed from the dissociation of either Cl; or SO&l-. The latter is not observed as a primary ion and reasonable calculation of the kinetic energy of Cl- formed through this intermediate is not possi’ole. Calculation of the kinetic energy of Cl- from Cl; as an intermediate yields much smaller values than those obtained experimentally_ Consequently, we conclude that Cl- is formed by reaction (9). The kinetic energy of Cl- rises much more rapidly in the higher energy region than in the lower; in fact, about one-half of the added electron energy appears as translational energy of Cl-. Although it is not possible to determine the distribution of the remaining energy, it seems reasonable that essentially all of the additional energy must be released in translation_ It also seems reasonable to assume that Cl- is formed in this higher energy region by a reaction similar to reaction (9) but with one of the products electronically excited. At the 1.8-eV thermal onset, reaction (9) involves 56 kcal mol- I or 2.4 eV excess energy which we take to be electronic excitation of S&. This is in good agreement with the other values given above. SO,
This ion occurs only in the higher energy resonance where it is the most intense of the three primary ions formed in that energy range. It is noteworthy as is seen in Fig, ‘7 that SO; is formed with very large kinetic energy. The ion formed at 4.1 eV, where the intensity is the smallest that will permit a satisfactory measurement of translational energy, is seen to have a translational energy of about 37 kcal mol- I. Although such large kinetic energies cannot be measured accurately by our method, the translational energy rises linearly with increasing electron energy and extrapolates to a thermal value at 0.66 eV.
0
I
2 ELECTRON
Fig.
7.
ionization
efficiency
3
4 ENERGY.
5
6
eV
and kinetic energy curves for SO:
from SOzC12.
243
It appears reasonable reactions:
that SO;
may be formed
by one of the following
SO&l,
+ so;
+ 2 Cl
(10)
SO&l~
+ so;
+ Cl*
(11)
The former is endoergic by 49 kcal mol-’ so could not occur below about 2.2 eV. At 4.5 eV the reaction is exoergic by 54 kcal mol-I. At that electron energy
the
t.ranslational
energy
of
SO,
is 44
kcal
mol-I,
and
it is difficult
to imagine a mode of separation in which the two chlorine atoms would receive only 10 kcal mol-*. Thus, SO; does not appear to be formed by reaction (10) Reaction (11) is exoergic by 8 kcal mol- *_ The linear kinetic energy line extrapolates to 0.45 eV rather than to -0.4 kcal mol-I. This discrepancy could, of course, result from inaccuracies in the measurements, but we think that any errors will be considerably less than one electron volt. We doubt that there would be this much energy remaining in vibration and rotation at onset tronic
where no energy is released in translation_ There remains only elecexcitation to account for the excess energy_ The slope of the kinetic energy line when multiplied by M/m, is almost precisely unity, which is in keeping with reaction (11) if all the excess (non-electronic) energy is released in translation. The first excited state of CI,(AZTI’,_,) is 2.26 eV above the ground state and thus cannot account for the excess energy. Conceivably SO; might be electronically excited; there is no other evidence for this. The neutral species NF2 and C102 are isoelectronic with SO; and are known to have their first excited states at 4.5 eV and 2.6 eV, respectively [lo]. These are, of course, much greater than the measured excess energy but do indicate that an excited state of SO; might exist. We must conclude, then, that either the extrapolated onset is in error or that a previously unknown excited state of SO: exists. We are unable to make a firm choice between these possibilities.
This compound forms four ions by dissociative resonance capture, namely, F-, Cl-, SO*F-, and ClF-. All are formed in the same energy region, a resonance having a maximum at 2.5-2.6 eV. The F- and Cl- ions are formed in large abundance and S02Fin relatively small abundance. The intensity of ClF- ion was too small to yield useful data,
aThe
ionization efficiency and translationaI enew curves for Cl- from SO&IF are given in Fig. 8. The thermal onset resulting from extrapolation of the kinetic energy curve occurs at 1.01 eV and is in close accordance with
244 1.0 ‘-
100
l-
v
z
08
5 PO6 zy G =
0.4
> kg
02
-I
!
20
i+ a
o!
w
ELECTRON
ENERGY,
400 t
eV
CORRECTED
Fig. 8. Ionization
efficiency
imd kinetic energy curves
Fig. 9. Ionization
efficiency
and kinetic energy curves for SOIF-
the
known thermochemistry
SO&!F
ELECTRON
ENERGY,
V
for Cl- from S02CIF. and F- from S02CIF.
of the reaction
+ e + Cl- +- F + SO,
(12)
for which the experimental and calculated heats of reaction are 23 and 22 kcal mol-’ , respectively. The kinetic energy of Cl- is quite large, and while it is not possible to compute the kinetic energies of the other fragments or the vibrational energy of the SO2 product, we think it probable that the neutral fragments also are formed with considerable kinetic energy. If this is true, then a large fraction of the excess energy is released in translation. SO,F_ This ion was formed in small abundance so that accurate measurement of its kinetic ener,9 was not possible. As shown in Fig. 9, a few values of kinetic
energy
were
obtained.
There
was
considerable
scatter
of
these
data
points so a quantitative evaluation was not possible. It is clear, however, that a larger fraction of the excess energy is released 2.5 eV the kinetic energy of S02Fdrops sharply.
in translation.
Above
about
FThis is the most abundant ion in the mass spectrum of S02ClF. As shown in Fig_ 9, the kinetic energy of F- is relatively small. At the thermal onset the calculated and measured heats of the reaction SO&lF
+ F- + Cl + SO2
(13)
are 32 and 49 kcal mol-‘, respectively, so this reaction does not account for the formation of F:. If the neutral fragment is SO&l, it is not possible to compute the heat of reaction since AHz(S02Cl) is not known; however, if
245
one uses the measured heat of reaction, AH,(SO,Cl) is calculated to be -22 kcal mol-‘. Since this is 20 kcal mol-’ greater than that of the possible dissociation products, SO2 + Cl, it is expected that SO&l would dissociate spontaneously. Thus, the neutral fragment formed along with F- is not SO*Cl.
Since it is known that SO*Fis formed in small amounts and in the same electron energy range, we assume it t- he an intermediate in forming F- and hence write the mechanism SO&IF SOIF-
+ e + +
S02F-
F- + SOz
+ Cl
(14a) (14b)
It will be seen in Fig. 9 that the kinetic energy curve for F- begins to increase at the same electron energy (2.5 eV) at which that of S02Fdecreases rapidly. We take this to mean that only a small fraction of the excess energy appears in vibration of S02Fat electron energies below 2.5 eV, but that at about 2.5-2.6 eV the distribution of energy changes so that a larger fraction occurs in vibration and a smaller fraction occurs in translation. Suppose one assumes that SO*Fhas just enough vibrational energy (9 kcal mol-‘) at 2.5 eV for reaction (14b) to occur. It is found that at that electron energy the excess energy for reaction (14a) is 35 kczil mol-‘, and the energy released in translation is 29 kcal mol-’ _ From this one calculates the This presumably is the kinetic kinetic energy of SO*Fto be 8.6 kcal mol-‘. energy of only a portion of the S02Fions formed at this electron energy. If now it is assumed that there is no kinetic energy released in reaction (14b), the translational energy of F- would result only from the share Freceives from the translational energy of SO*F-, i.e. (19/83) X 8.6 = 2 kcal mol-*. This is somewhat greater than the measured value of 1.4 kcal mol-‘. Although one could wish for better agreement, this approximate agreement is good enough to support the mechanism expressed by reactions (14a) and (14b). DISCUSSION The results of this investigation are summarized in Table 2. Several points of interest are noted. In the formation of F; from SOzFz the fraction of excess energy released in translation is only 0.20. While this is greater than the 0.11 predicted by the quasi-equilibrium theory, the majority of the excess energy is released in vibration. On the other hand, with formation of SOzFion from S02F2, 0.79 of the excess energy is released in translation. Presumably, energy must be redistributed among the various bonds to cause enough vibration and distortion for the F; to be formed. On the other hand, the separation of SO*Fand F would require only elongation of the bond in question and little energy redistribution would be required. Consequently, one would expect relatively little energy to be released in translation in the
246
TABLE 2 Thermochemical
values
determined
Reaction
A&
SOzFz + F2 + SO2 SOzFz + SC&F + F S02Fz +F +F+SOa SO*F- --f F- + F + SO*’ SO&12 --f Cl* + so* SO&i* y$ 2 SO2Cl2
-> Cl, + Cl2
-i- so2 * + Cl + so*
+a-+a+.so2 --f
*
so2
S02CIF --f Cl-
37 58
+
+
a2
F + SO2
1E -31.5 20 -12 4.1 15 23
ft
AHr (ion) (kcal mol-’ )
0.20 0.79
-73 -142
0.39
2.44 eV (elect. trans.) -46
0.59
2.65
eV
(elect.
trans.)
2.45 1.05
eV eV
(elect. (elect.
trans.) traris.)
--55 1.0 -58
formation of F; and a large amount to be released in translation in the formation of SaF-. We were regrettably unable to make a similar comparison of ions formed from SO&l, since we did not observe any SO,CI- ions. The value found for AHf(S02F-) of -142 kcaI mol-’ is presumably much greater than the value of >-177 kcal mol-’ deduced by Sullivan and Beauchamp [ll] from the non-endoergicity requirement of ion-molecule reactions. Further, Robbiani and Franklin [l] in Fig. 10 of this reference show a characteristic rapid decrease in intensity of the primary F- ion and a corresponding increase in SO,F- with increasing pressure of SO,ClF, as would be accounted for by the reaction F- + SO,FCl
--r SO,F-
+ FCl
(15)
Since F- is formed with very little kinetic energy, reaction (15) would be endoergic by some 40 kcal mol-1 if our value of A&(SO,F-) is correct but would be very nearly exoergic if Sullivan and Beauchamp’s [11] value is correct. We are unable to reach a definite conclusion for the proper heat of formation of S02F-. Possibly the AHf(S02F-) value that we folund represent an electioniczlly excited state and a ground state of this ion occurs some l-5---2 eV below that which we measured. This would account for the occurrence of reaction (151, but we have no other evidence for this hypothetical state. It is possible to account qualitatively for the decay of .F- and increase in SO,F- intensity observed by Robbiani and Franklin Cl] if one assumes that SO*F- is formed in a vibrationally excited state almost iustantancously by election impact:The excited SO,F- ion can then dissociate to form F-; however, with increased pressure, deactivating collisions can occur which result in stabilization of the SO,Fion and reduction’ of the intensity of F-. Fair agreement with experiment can be obtained by using the rate equations for such a mechanism. Whether this is actually the mechanism by which F-.dis-
247
appears and S02F- increases with increashg pressure of SO&IF is not established, but it seems the most reasonable explanation in view of the apparent value obtained in this study for 4Hf(S02F-). ACKNOWLEDGEMENT
We express our gratitude to the Robert of this investigation.
A. Welch Foundation
for support
REFERENCES 1 R. Robbiani and J.L. Franklin, J. Am. Chem. Sot., 101 (1979) 3709. 2 MA. Haney and J.L. Franklin, J. Chem. Phys., 48 (1968) 4093. 3 R.E. Pabst, S.L. Bennett, J.L. Margrave and J.L. Franklin, J. Chom. Phys., 65 (1976) 1550. 4 P.W. Hadand, J.L. Franklin and D.E. Carter, J. Chem. Phys.. 58 (1973) 1430. 5 P-W. Harland and J.L. Franklin, J. Chem. Phys., 61 (1974) 1621. 6 H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, (1977) Suppl. 1. 7 W.A. Chupka, J. Berkowitz and D. Gutunan, J. Cbem. Phys., 55 (1971) 2724. 8 D.R. Stull and H. Prophet, JANAF Thermochemical Tables, Natl. Bur. Stand. Ref. Data Serv., Natl. Bur. Stand., Washington, DC, 1971. 9 D.R. Stull and H. Prophet, JANAF Thermochemical Tables Supplement, Natl. Stand. Ref. Data Serv., Nati. Bur. Stand., Washington, DC, June 30, 1971. 10 G. Henberg, Molecular Spectra and Molecular Structure III. Electronic Spectra and Molecular Structure of Polyatomic Molecules, Van Nostrand, Princeton, NJ, 1966. 11 S.A. Sullivan and J.L. Beauchamp, Int. J. Mass Spcctrom. Ion Phys., 28 (1978) 69.
Journal of Mass Speciromet7y
Scientific
REACTIONS ELECTRON
JIA-SHEN
Publishing
(Received
36 (1980)
and Ion Physics,
Amsterdam
-
Printed
in The
233-247
233
Netherlands
AND ENERGY DISTRIBUTION IN DISSOCIATIVE CAPTURE PROCESSES IN SULFURYL HALIDES
* and J.L.
WANG
Department
Company,
of Chemisfry. 21 March
FRANKLIN
Rice
Univekty,
Houston.
Texas
77001
(U.S.A.)
1980)
ABSTRACT The electron
sulfur-y1 capture.
halides, SOsFs, The ionization
SOzCl,, efficiency
and SOsClF form several ions curves are determined for each
by dissociative process. Where
the ion intensity was great enough, the kinetic energy of the ion was measured over the greater part of the electron energy of the resonance. In most instances the kinetic energy of the ion increased linearly with electron energy over the range of the resonance. Extrapolation to zero kinetic energy release gives the electron energy corresponding to the heat of the dissociation reaction_ The kinetic energy released in the formation of F: and Cl5
was only slightly greater than that predicted by the quasi-equilibrium theory, as would be expected from the fact that the molecule had to change its configuration during dissociation_ Essentially al1 the excess energy was released in translation when SOzFwas formed. The ions Ffrom SOsF2 and Clfrom SOsCl+ are formed in two separate resonances, and the kinetic energy lines extrapolate to thermal appearance potentials (mr) corresponding to the formation of SO2 and the halogen atom. The m= values for these reactions as well as that for Cl5 at their higher energy resonances appear to correspond to an electronic transition for SO2 of 2.5 eV. INTRODUCTION
In a study of negative ion-molecule
reactions in the sulfuryl halides [ 1 ] ions were formed with considerable The amount of translational energy of the ion together with the accompanying intemd energy, if the ion is polyatomic, in some instances might alter the interpretation of the mechanism of reaction. Consequently, we undertook a study of the energy distribution in the principal dissociative resonance capture processes of S02F2, SO&I, and S02ClF. This paper reports the results of that study. it
became
evident that several translation&l energy.
of
the
primary
EXPERIMENTAL
These studies were made with a Bendix Model 14-107 Time-of-Flight mass spectrometer and a data collection system which was made possible by several predecessors to improve the sensitivity and resolution. In this system *Present
adress:
93
Shadow
OOZO-7381/80/0000-0000/$02_25
Ridge
Rd., @
Stanford,
1980
Elsevier
CT
06905,
Scientific
U.S.A. Publishing
Company
234
the clock, 1 MHz, of a BNC Model 7020 digital delay generator, is modified by a frequency divider into 10 kHz. This pulse is used to start the mass spectrometer and also is fed into the delay circuit of the digital delay generator to produce a reference pulse. An Ortec model 467 Time-to-Pulse-Height Converter (TPHC) converts the difference in time between the reference pulse and the data pulse into a pulse whose height is proportional to that time difference. The data pulse is the ion signal collected in the mass spectrometer and a-mplified to a level that can be accepted by the stop input of the TPHC. In order to store the signal in a Packard Model 901-1024 channels computer memory, the output of the TPHC is transformed into digital form by a Packard Model 960 Analog-to-Digital Converter. The accumulated data (the mass spectrum) in the computer memory can then be recorded by the regular recording system. The beginning time is set by the digitill delay generator which has a working range from 0.1 ps to 1 m F -.ith a 0.1.ps increment and a jitter of 0.1 ns. The time domain of a spe&rum is set by the TPHC which has a range of 0.05-800 ps. For a single mass peak, the range of 0.2 ps has very often been used. By using a 1024-channel analyzer, this range gives a resolution of 0.2 ns. Therefore, in this system, if all the other conditions are ideal, the limiting factor to the actual resolution will be a few tenths of a nanosecond. This is much superior to the previous gating system which had a resolution of 20 ns [ 21. The other advantage of the present system is that a large signal-to-noise ratio for a weak signal may be obtained by accumulating data for long periods while in the previous system the signal-to-noise ratio was limited to a fairly low value. The translational energies of the ions were measured by the method [2,3] described in detail previously. In this method, the translational energy distribution of an assemblage of ions was determined from the width of the mass peak in the time-of-flight mass spectrometer. Experimentally, a calibration curve of ion peak width at half-height (W,,,) versus the square root of the ionic weight (u) was constructed by measuring the value of W,,* for several ions known to be formed with thermal energy. The WI,, value of the ion being investigated was then converted into an apparent ionic mass, ma_, and the translational ener%T of the ion, 6 is calculated from .~i = ~RT(m,,,/mi)
(1)
where mi is the actual mass of the ionic species. In Fig. 1 typical calibration curves are given for the instrument employing the present system and the previous one employing the gate. The advantage of the present method, especially with the lighter ions, is obvious. From the conservation of momentum, one obtains the following relation between i and the translational energy in the center of mass, Z;:
235 1
I 0
X
30-
FROMCOUNTING SYSTEM FROM
lOI 5
GATING
SYSTEM
I
I
IO
15
(ION Fig. 1. Kinetic
where m, molecule,
MASS)
energy calibration
I
20
“2
curves.
and M are the mass of the neutral fragment respectively.
The
electron
energy
was
calibrated
and of the parent with
O-/SO2
[4],
the first peak maxima occurring at 4.9 eV. The sulfuryl chloride and sulfuryl chlorofluoride were obtained from the Aldrich Chemical Company; sulfuryl fluoride from the Matheson Company. The samples were used without further purification; however, for each run, the hesh sample was introduced into the sample inlet system several tunes before data were taken. RESULTS
It has been shown in several studies [ 3--51 that the translational energy of an ion formed by dissociative electron capture will usually increase linearly with increasing excess energy over the greater part of the resonance. In such instances, it is possible to extrapolate eqn. (2) to Ft = 0 at which point 6 = mi/~~(~RT), the contribution of the thermal energy of the precursor to the transiational energy of the fragment ion. The corresponding electron energy will be the energy of the dissociation reaction. It has been shown [Z] that such linear dependence of kinetic energy release, Et, upon electron energy can be expressed by the quasi-theoretical equation (3) where E:., is the excess vibrational and translational energy above the dissociation asymptote_ For the process in question, N is the number of vibrational modes in the ion prior to dissociation and cy is an empirical constant. If FYis plotted against E’, the slope of the line is l/oCnr and is the fraction of
236
the excess energy (above thermal) released in translation. The smaller a! is, the greater the lirzction of excess energy going into translation.
_4t low electron energies three ions, F;, S02F-, SOzF2. These ions will be discussed separately.
and F- are formed
from
The intensity and kinetic energy of F; as a function of electron energy are given in Fig. 2. The width of the resonance curve at half-height is 1.5 eV, whereas the FWHM of SF; was about 0.5 eV in our ;Jlstrument. The intensity of F; is relatively small so that satisfactory measurements of translational energy could be made only near %e -maximum of the resonance curve. Nevertheless, a linear dependence on electrcln energy is observed with a slope (l/&V) of 0.204 and an cz value of 0.54. When the kinetic energy line is extrapolated to the thermal value of 0.34 kcal mol-’ the electron energy corresponding to AEI, is found to be 1.61 eV or 37 kcal mol-’ for the reaction SOzIp + e + F; + SO* Using 1 one value tained
(4) this value and heats of formation of SOzFz and SO* taken from Table obtains ANr(F;) = -73 kcal mol- l_ This is in fair agreement with the of 71 kcal mol-1 obtained by Chupka et al. [7], and with values obby other experimenters.
SOIF-
The S02Fion is formed in the same electron energy region and with a resonance curve of the same FWHM as F,. The intensities of the S02F- ion are 2-2.5 times greater than those for F;. Figure 3 gives the resonance and
1.0
r 8-
0
I
2
4
ELECTRON
Fig. 2. Ionization
efficiency
ENERGY,
5
6
eV
and kinetic energy curves f&r Fs from S02Fz.
237 TABLE Heats
1 of formation
employed
Substance
Nf*9,
F F-
(kcai mol-’ )
Ref.
18.88 -60.03 -71.0 -181.3 28.68 -54.64 -54.9 -84.9 -133 -70.336 -93.4 -13
F2
SOzFz
Cl clCl; so2c12 S02ClF SO, so; CIF
6 6 : 6 6 7 8 9 6 6 6
kinetic energy curves. it will be observed that the kinetic energy rises linearly at electron energies above 3 eV, i.e. just below the resonance maximum. At lower energies the kinetic energy remains nearly constant; we attribute this to the high energy tail of the electron energy distribution. The rising portio:n of the kinetic energy curve extrapolates to a thermal energy of 0.73 kcal mol-’ at an electron energy of 2.5 eV, giving a heat of reaction of 57.7 k&l mol-’ for the reaction S02Fz
+ e --t S02F-
+ F
(5a)
From this one computes AHf(S02F-) = -142.3 kcal mol-I. From the slope of the kinetic energy curve in Fig. 3,l/oN = 0.79 and a = 0.14, so that a huge fraction of the excess energy in this process appears in
I
2
ELECTRON Fig.
3. Ionization
efficiency
3
4
ENERGY, and
kinetic
5
6
eV energy
curves
for
SOzF-
from
SOzF2.
238
translation. The large fraction of excess energy released in translation suggests that very little change in the arrangement of the remaining atoms occurs as the electron replaces one F atom, F-
Two resonances corresponding to two different electronic states are observed in Fig. 4 for the formation of F- ion. The ion intensities are m lch greater in the lower than in the higher energy resonance. The translational energy of F- under both resonances rises linearly with increasing elect Ton energy. A linear decrease in energy occurs in the region where the two st: rtes overlap, as would be expected_ Extrapolation of the translational energy line for the state of lower energy reaches a thermal value at 2.8 eV. This agrees within 0.1 eV with heat of the reaction
(6) -, F- + F f SO* it is tempting to think that this reaction accounts for the formation of Fover the whole range of the first resonance. However, the simultaneous separation of F- and F is considered to be improbable, and so we doubt that this mechanism accounts for the formation of F-. Further, the small increase in kinetic energy of F- in this energy range suggests that a different mechanism is involved. Note in Fig. 4 that the translational energy line for F’- starts to increase at 3.1 eV. This point of increase corresponds approximately to the eiectron energy at which the kinetic energy of SO*F- begins to decrease. If this energy, 3.7 eV, is +aken as the energy at which S02F- has just enough vibrational energy to dissociate, one finds, for the formation of S02F- by reaction (5a), the total excess energy Ez., to be 28 kcaI mol-I, Et to be 22 kcal mol-’ and E,. the vibrational energy of SO*F-, to be 6 kcal mol-‘. SO,F,
2
3
4
5
ELECTRON Fig.
4
Ionization
efficiency
and
kinetic
6
ENERGY, energy
7
9
a
eV curves
for
F-
fT>v
S02F2.
239
With this amount of vibrational released by the reaction S02F-
energy in SO*F-
it is assumed that F- is just
--f F- + SO,
(5b) The value of AHI(S02F-) is then calculated to be -139 kcal mol-‘. This is in fair agreement with AH,(SO,F-) = -142 kcal mol-’ obtained from the extrapolation to its thermal value of the kinetic energy line for SO,F- in Fig. 3. Although it is not possible to calculate, the exact kinetic energy line for F- on the basis of this mechanism, a reasonable assumpt.ion that (11= 1 for the dissociation of SO*F- results in fair agreement of calculated and experimental values for Ei( F-) as shown in the following tabulation: Electron
energy
(eV)
q(F-)
5.5 4.5 4.0 3.5 It
(kcal
mol-‘)
talc.
meas.
4.3 3.0 2.3 1.4
5.2 3.4 2.4 1.4
is conceivable that F- might be formed by the folIowing mechanism
SOzF2 + + F; + SOz
(7a)
F;+F-+F
(7b)
Reaction (7a) will involve 24 kcal mol-’ excess energy at the extrapolated onset of 2.8 eV for F-. Using the measured value of l/crN = 0.204 determined above it is found that g = 4.9 kcal mol-’ and the vibrational energy T; = 19 kcal mol-‘. Reaction (7b) is endoergic by 27 kcal mol-’ , SO even in the improbable case of all 19 kcal mol-’ going to excite the vibration of F;, the reaction would still lack 8 kcal mol-’ to proceed. Further, if reactions (7a) and (7b) are considered at several higher energies one finds eV
pi
Gil F-1
3.7 4.0 4.5
6.5 5.4 8.8
1.9 2.4 3.4
If this mechanism prevails and if one assumes that, at 3.7 eV, F? receives just enough vibrational energy to dissociate, Ei(F-) should be one-half of ci(F;) or 3.25 kcal mol- I_ This is clearly more than the measured value. At higher electron energies the kinetic energy of F- would be greater than 0-5fi(F;) by one-half the amount of vibrational energy in F; converted to translation. Thus, the kinetic energy of F- formed by reactions (?a) and (7b) would be considerably greater than the measured values, and it is concluded that Fis not formed by this mechanism. We conclude, therefore, that F- is probably formed by reactions (5a) and
2+0
(5b), but we cannot exclude Figure 4 shows a second
the possibility resonance for
of its formation by reaction (6). the formation of F- and a Linear
dependence of kinetic energy upon electron energy, with an extrapolated thermal onset at 5.24 eV. The ion kinetic energy cwve is very nearly parallel to that at lower energy resonance. Behavior of this kind is characteristic of a process yielding the same product in both resonances but with one of the products in the higher energy resonance being electronically excited. The difference in thermal energies at onset is 2.44 eV, and this can be taken as the eIectron.ic transition energy. This cannot occur with F, and it seems highly improbable that F- would have an electronic state at this level. It seems probable then that SOS is electronically excited. Herzberg [lo] reports the lowest excited state of SO* to be Z(3B,) at 3.2 eV above the ground x( *A 1) state. Our value does not agree well with this, but we know of no other explanation of our results.
Dissociative electron capture processes occur in SO&l2 with resonance maxima at about l-l.2 eV and 4.5-5.1 eV. In both resonances Cl- and Cl; ions are observed, Cl- being a major ion and Cl; a minor one in both. In addition, SO; occurs in great abundance at the higher energy state_ The resonance and kinetic energy curves for Cl; are shown in Fig. 5. The lower energy resonance curve shows a peak width at half maximum of about one electron volt and thus 13 characteristic of many dissociative resonance capture processes. The resonance at the higher energies is much broader as is sometimes observed when two or more close-lying electronic states occur in the energy range in question. The kinetic energy of Cl; is quite large and exhibits a linear dependence on electron energy in both resonances with a decrease in kinetic energy cu l.O-
I
G
zG? z p
0.8 0.6-
z L?_I
0.4-
>
F
“J
0.2-
ILI
cc
o-
-_-I
0
I
ELECTRON Fig. 5. Ionization
efficiency
-2
THERMAL 3
ENERGY,
ION 4
ENERGY 5
1 6
eV
and kinetic energy curves for Cl5 from SO2CIz.
241
where the two resonances appear to overlap. The extrapolated thermal at -1.37 eV and +0.86 eV. The onset for the reaction SO&l2
+ e + Cl; + SO,
curves
are (8)
in the lower resonance results in a value of AH,(Cl;) of 46 kcal mol-‘. This discrepancy from the accepted value of -55 kcal mol-l is disappointingly large; however, we can account for it only as error resulting from the quite long extrapolation. The extrapolated onset of the kinetic energy curve in the second resonance involves 2.65 eV excess energy. Since there is no excess translational energy and presumably no excess vibrational energy at this extrapolational onset, this excess energy must be electronic. This is not likely to reprosent ex.Aation of Cl;, so we conclude that it must represent excitation of SO,. It will be recalled that the onset of the second resonance for the F- ion in SOzFz was interpreted as involving excitation of SO2 with an electronic transition of 2.44 eV, While the agreement of the electronic transitions in the two processes is not nearly so precise as one would expect of spectroscopic measurements, the agreement does indicate that the same states are involved
in the two
processes.
CIFigure 6 shows two resonances for Cl- with m axima at 1.2 eV and 5.1 eV. The curve for the higher energy resonance is very broad and may include more than one electronic state. The ion kinetic energy rises linearly in both resonances with a change in slope in the region where the two resonances overlap. Extrapolation yields zero kinetic energy release at -0.5 eV and +f.8 eV. The -0.5-eV intercept is concordant with the reaction SOZCIZ + e -+ Cl- + Cl + SO* where Cl-, l.O‘c-l 0 g OB- g 0 1”
0.6-
2 W
30
-
b
20
& 0.2-
d nf
40
I0.4
2 z
Cl and SOZ separate simultaneously.
.
ci E
(9) Presumably reaction (9) will
E z
o-
‘O 0 -I
0
I
2
ELECTRON Fig. 6. Ionization
efficiency
and
3
4
ENERGY. kinetic
energy
5
6
7
eV curves
for
Cl-
from
SO~CI~.
242
occur over the greater part of the resonance, but since it is a three-body process it is not possible to compute the total kinetic energy released as a function of electron energy. Conceivably Cl- might be formed from the dissociation of either Cl; or SO&l-. The latter is not observed as a primary ion and reasonable calculation of the kinetic energy of Cl- formed through this intermediate is not possi’ole. Calculation of the kinetic energy of Cl- from Cl; as an intermediate yields much smaller values than those obtained experimentally_ Consequently, we conclude that Cl- is formed by reaction (9). The kinetic energy of Cl- rises much more rapidly in the higher energy region than in the lower; in fact, about one-half of the added electron energy appears as translational energy of Cl-. Although it is not possible to determine the distribution of the remaining energy, it seems reasonable that essentially all of the additional energy must be released in translation_ It also seems reasonable to assume that Cl- is formed in this higher energy region by a reaction similar to reaction (9) but with one of the products electronically excited. At the 1.8-eV thermal onset, reaction (9) involves 56 kcal mol- I or 2.4 eV excess energy which we take to be electronic excitation of S&. This is in good agreement with the other values given above. SO,
This ion occurs only in the higher energy resonance where it is the most intense of the three primary ions formed in that energy range. It is noteworthy as is seen in Fig, ‘7 that SO; is formed with very large kinetic energy. The ion formed at 4.1 eV, where the intensity is the smallest that will permit a satisfactory measurement of translational energy, is seen to have a translational energy of about 37 kcal mol- I. Although such large kinetic energies cannot be measured accurately by our method, the translational energy rises linearly with increasing electron energy and extrapolates to a thermal value at 0.66 eV.
0
I
2 ELECTRON
Fig.
7.
ionization
efficiency
3
4 ENERGY.
5
6
eV
and kinetic energy curves for SO:
from SOzC12.
243
It appears reasonable reactions:
that SO;
may be formed
by one of the following
SO&l,
+ so;
+ 2 Cl
(10)
SO&l~
+ so;
+ Cl*
(11)
The former is endoergic by 49 kcal mol-’ so could not occur below about 2.2 eV. At 4.5 eV the reaction is exoergic by 54 kcal mol-I. At that electron energy
the
t.ranslational
energy
of
SO,
is 44
kcal
mol-I,
and
it is difficult
to imagine a mode of separation in which the two chlorine atoms would receive only 10 kcal mol-*. Thus, SO; does not appear to be formed by reaction (10) Reaction (11) is exoergic by 8 kcal mol- *_ The linear kinetic energy line extrapolates to 0.45 eV rather than to -0.4 kcal mol-I. This discrepancy could, of course, result from inaccuracies in the measurements, but we think that any errors will be considerably less than one electron volt. We doubt that there would be this much energy remaining in vibration and rotation at onset tronic
where no energy is released in translation_ There remains only elecexcitation to account for the excess energy_ The slope of the kinetic energy line when multiplied by M/m, is almost precisely unity, which is in keeping with reaction (11) if all the excess (non-electronic) energy is released in translation. The first excited state of CI,(AZTI’,_,) is 2.26 eV above the ground state and thus cannot account for the excess energy. Conceivably SO; might be electronically excited; there is no other evidence for this. The neutral species NF2 and C102 are isoelectronic with SO; and are known to have their first excited states at 4.5 eV and 2.6 eV, respectively [lo]. These are, of course, much greater than the measured excess energy but do indicate that an excited state of SO; might exist. We must conclude, then, that either the extrapolated onset is in error or that a previously unknown excited state of SO: exists. We are unable to make a firm choice between these possibilities.
This compound forms four ions by dissociative resonance capture, namely, F-, Cl-, SO*F-, and ClF-. All are formed in the same energy region, a resonance having a maximum at 2.5-2.6 eV. The F- and Cl- ions are formed in large abundance and S02Fin relatively small abundance. The intensity of ClF- ion was too small to yield useful data,
aThe
ionization efficiency and translationaI enew curves for Cl- from SO&IF are given in Fig. 8. The thermal onset resulting from extrapolation of the kinetic energy curve occurs at 1.01 eV and is in close accordance with
244 1.0 ‘-
100
l-
v
z
08
5 PO6 zy G =
0.4
> kg
02
-I
!
20
i+ a
o!
w
ELECTRON
ENERGY,
400 t
eV
CORRECTED
Fig. 8. Ionization
efficiency
imd kinetic energy curves
Fig. 9. Ionization
efficiency
and kinetic energy curves for SOIF-
the
known thermochemistry
SO&!F
ELECTRON
ENERGY,
V
for Cl- from S02CIF. and F- from S02CIF.
of the reaction
+ e + Cl- +- F + SO,
(12)
for which the experimental and calculated heats of reaction are 23 and 22 kcal mol-’ , respectively. The kinetic energy of Cl- is quite large, and while it is not possible to compute the kinetic energies of the other fragments or the vibrational energy of the SO2 product, we think it probable that the neutral fragments also are formed with considerable kinetic energy. If this is true, then a large fraction of the excess energy is released in translation. SO,F_ This ion was formed in small abundance so that accurate measurement of its kinetic ener,9 was not possible. As shown in Fig. 9, a few values of kinetic
energy
were
obtained.
There
was
considerable
scatter
of
these
data
points so a quantitative evaluation was not possible. It is clear, however, that a larger fraction of the excess energy is released 2.5 eV the kinetic energy of S02Fdrops sharply.
in translation.
Above
about
FThis is the most abundant ion in the mass spectrum of S02ClF. As shown in Fig_ 9, the kinetic energy of F- is relatively small. At the thermal onset the calculated and measured heats of the reaction SO&lF
+ F- + Cl + SO2
(13)
are 32 and 49 kcal mol-‘, respectively, so this reaction does not account for the formation of F:. If the neutral fragment is SO&l, it is not possible to compute the heat of reaction since AHz(S02Cl) is not known; however, if
245
one uses the measured heat of reaction, AH,(SO,Cl) is calculated to be -22 kcal mol-‘. Since this is 20 kcal mol-’ greater than that of the possible dissociation products, SO2 + Cl, it is expected that SO&l would dissociate spontaneously. Thus, the neutral fragment formed along with F- is not SO*Cl.
Since it is known that SO*Fis formed in small amounts and in the same electron energy range, we assume it t- he an intermediate in forming F- and hence write the mechanism SO&IF SOIF-
+ e + +
S02F-
F- + SOz
+ Cl
(14a) (14b)
It will be seen in Fig. 9 that the kinetic energy curve for F- begins to increase at the same electron energy (2.5 eV) at which that of S02Fdecreases rapidly. We take this to mean that only a small fraction of the excess energy appears in vibration of S02Fat electron energies below 2.5 eV, but that at about 2.5-2.6 eV the distribution of energy changes so that a larger fraction occurs in vibration and a smaller fraction occurs in translation. Suppose one assumes that SO*Fhas just enough vibrational energy (9 kcal mol-‘) at 2.5 eV for reaction (14b) to occur. It is found that at that electron energy the excess energy for reaction (14a) is 35 kczil mol-‘, and the energy released in translation is 29 kcal mol-’ _ From this one calculates the This presumably is the kinetic kinetic energy of SO*Fto be 8.6 kcal mol-‘. energy of only a portion of the S02Fions formed at this electron energy. If now it is assumed that there is no kinetic energy released in reaction (14b), the translational energy of F- would result only from the share Freceives from the translational energy of SO*F-, i.e. (19/83) X 8.6 = 2 kcal mol-*. This is somewhat greater than the measured value of 1.4 kcal mol-‘. Although one could wish for better agreement, this approximate agreement is good enough to support the mechanism expressed by reactions (14a) and (14b). DISCUSSION The results of this investigation are summarized in Table 2. Several points of interest are noted. In the formation of F; from SOzFz the fraction of excess energy released in translation is only 0.20. While this is greater than the 0.11 predicted by the quasi-equilibrium theory, the majority of the excess energy is released in vibration. On the other hand, with formation of SOzFion from S02F2, 0.79 of the excess energy is released in translation. Presumably, energy must be redistributed among the various bonds to cause enough vibration and distortion for the F; to be formed. On the other hand, the separation of SO*Fand F would require only elongation of the bond in question and little energy redistribution would be required. Consequently, one would expect relatively little energy to be released in translation in the
246
TABLE 2 Thermochemical
values
determined
Reaction
A&
SOzFz + F2 + SO2 SOzFz + SC&F + F S02Fz +F +F+SOa SO*F- --f F- + F + SO*’ SO&12 --f Cl* + so* SO&i* y$ 2 SO2Cl2
-> Cl, + Cl2
-i- so2 * + Cl + so*
+a-+a+.so2 --f
*
so2
S02CIF --f Cl-
37 58
+
+
a2
F + SO2
1E -31.5 20 -12 4.1 15 23
ft
AHr (ion) (kcal mol-’ )
0.20 0.79
-73 -142
0.39
2.44 eV (elect. trans.) -46
0.59
2.65
eV
(elect.
trans.)
2.45 1.05
eV eV
(elect. (elect.
trans.) traris.)
--55 1.0 -58
formation of F; and a large amount to be released in translation in the formation of SaF-. We were regrettably unable to make a similar comparison of ions formed from SO&l, since we did not observe any SO,CI- ions. The value found for AHf(S02F-) of -142 kcaI mol-’ is presumably much greater than the value of >-177 kcal mol-’ deduced by Sullivan and Beauchamp [ll] from the non-endoergicity requirement of ion-molecule reactions. Further, Robbiani and Franklin [l] in Fig. 10 of this reference show a characteristic rapid decrease in intensity of the primary F- ion and a corresponding increase in SO,F- with increasing pressure of SO,ClF, as would be accounted for by the reaction F- + SO,FCl
--r SO,F-
+ FCl
(15)
Since F- is formed with very little kinetic energy, reaction (15) would be endoergic by some 40 kcal mol-1 if our value of A&(SO,F-) is correct but would be very nearly exoergic if Sullivan and Beauchamp’s [11] value is correct. We are unable to reach a definite conclusion for the proper heat of formation of S02F-. Possibly the AHf(S02F-) value that we folund represent an electioniczlly excited state and a ground state of this ion occurs some l-5---2 eV below that which we measured. This would account for the occurrence of reaction (151, but we have no other evidence for this hypothetical state. It is possible to account qualitatively for the decay of .F- and increase in SO,F- intensity observed by Robbiani and Franklin Cl] if one assumes that SO*F- is formed in a vibrationally excited state almost iustantancously by election impact:The excited SO,F- ion can then dissociate to form F-; however, with increased pressure, deactivating collisions can occur which result in stabilization of the SO,Fion and reduction’ of the intensity of F-. Fair agreement with experiment can be obtained by using the rate equations for such a mechanism. Whether this is actually the mechanism by which F-.dis-
247
appears and S02F- increases with increashg pressure of SO&IF is not established, but it seems the most reasonable explanation in view of the apparent value obtained in this study for 4Hf(S02F-). ACKNOWLEDGEMENT
We express our gratitude to the Robert of this investigation.
A. Welch Foundation
for support
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