J . Phys. Chem. 1989, 93, 245-251
245
Radiative versus Three-Body Addition in Halide Ion Reactions with BF, and BC13 over the Temperature Range 220-410 K Charles R. Herd+ and Lucia M. Babcock* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 (Received: March 9, 1988)
The boron trihalides are Lewis acids which can add halide ions to form tetrahedral anionic species. We have examined a number of these association reactions for BF3 and BCl, with various halide ions, X- (X = F, C1, Br), as a function of both temperature (219-410 K) and pressure (0.3-0.8 Torr of helium) using a variable-temperature selected-ion flow tube (VT-SIFT). These studies provide compelling evidence for the existence of both radiative and collisional stabilization of the nascent (BX4-)* species to give the stable BX4- product ion. These are the only cases, to our knowledge, where radiative stabilization and collisional stabilization are competitive at relatively high number densities where the time between collisions is on the order of microseconds. Upper limits on radiative lifetimes obtained from manipulation of kinetic data are in the range lod s for all the boron systems examined. This is short for IR radiation and suggests that there may be participation from a low-lying electronic state. Collisional stabilization efficienciesappear to be on the order of 30% in helium. The temperature dependence of the observed rate coefficient is consistent with both radiative and collisional stabilization, and it trends in the manner expected for the various systems. We are able to set limits on rates of unimolecular decomposition and to determine the dependence of these rate coefficients on temperature for each of the anionic BX4- species. Finally, some conclusions about the magnitudes of bond strengths of the X3B-X- bonds formed can be drawn.
Introduction Radiative association in ion-molecule reactions plays a vital role in interstellar however, despite its key importance, there have been few experimental studies that report the observation of radiative association, and most of these are lowpressure and/or low-temperature s t ~ d i e s . ~Barlow, *~ Dunn, and Schauer6 reported the first low-temperature, low-pressure observation of radiative association between CH3+and H2, a most important reaction in interstellar chemistry. In addition, Herbst7-' has carried out a considerable amount of work on theoretical calculations of radiative rate coefficients. We have recently identified a unique and exciting group of ion-molecule association reactions in which both radiative and collisional stabilization occur simultaneously. In a flowing afterglow (FA) examination of halide ion addition to the boron trihalides BF3 and BCl, a t room temperature in the presence of a variety of third bodies, we found compelling evidence for the presence of a radiative stabilization component of the mechanism.12 In a detailed examination of F addition to BF3 in helium as a function of both temperature and pressure, we not only obtained information on the rate coefficients for the elementary steps but also developed a method for analyzing temperature, pressure, and third-body effects without extrapolating outside of our experimental pressure range.13 Using this method, we are able to (1) determine whether radiative stabilization is significant, (2) calculate the experimental temperature dependence of the unimolecular decomposition back to reactants, and (3) place upper limits on both the radiative stabilization and unimolecular decomposition rate coefficients as well as upon the efficiency of collisional stabilization. The boron trihalides, BX3, are trigonal-planar Lewis acids which add halide ions to form tetrahedral anionic products; the general mechanism can be represented as BX3
+ X-
(BX4-)* kd
(2) A BX4-
+
hv
(3)
where k,, kd, k,, and k, are rate coefficients for association, unimolecular decomposition back to reactants, collisional stabilization by some third body M , and radiative stabilization, ret Current address: Department of Space Research, The University of Birmingham, Birmingham, England.
0022-365418912093-0245!$01.50/0
spectively. The overall rate coefficient, koW, for this mechanism is given by (4) The general form of the dependence of kd upon the concentration of third body, (M), is shown graphically in Figure 1. Since the unimolecular decomposition rate coefficient, kd,is by far the most strongly temperature dependent of all the elementary rate coefficients in eq 4, and since it has a positive temperature dependence, then one expects kobd for these reactions to show a negative dependence upon temperature just as in the case of ion-molecule association reactions without a radiative stabilization component. Our prior results for addition of F to BF3 yield the following temperature dependence: kd T2.03 and koW a T1.47, consistent with the above mechanism.13 The upper limit which we have placed on k, for stabilization of (BF4-)* is 2 X lo6 s-l. While earlier theoretical work of Herbst7s9predicted k, values in the range 10-103 s-l for emission of an IR photon, in more recent he has indicated that perhaps this range should be extended by as much as an order of magnitude. A recent interpretation by Bates14 of the work of Barlow et a1.6 suggests that k , for the association of H2 to CH3+ may be as high as 3 X lo4 s-l. Clearly, more detailed information about the radiative stabilization rate coefficients for such processes is necessary. One approach which has been employed is the examination of three-body processes at higher pressures in order to obtain lifetime information about the initially formed excited adduct which is common to both radiative and collisional s t a b i l i z a t i ~ n . ~ This ~ J ~ approach can be used to (1) Black, J. H.; Dalgarno, A. Astrophys. Lert. 1973, 15, 79. (2) Bates, D. R. Mon.Net. R . Astron. SOC.1951, III, 303. (3) Herbst, E.; Klemperer, W. Asrrophys. J . 1973, 185, 505. (4) Woodin, R. L.; Beauchamp, J. L. Chem. Phys. 1979, 41, 1. ( 5 ) Woodin, R. L.; Foster, M. S.; Beauchamp, J. L. J . Chem. Phys. 1980, 72, 4223. (6) Barlow, S.E.; Dunn, G. E.; Schauer, M. Phys. Rev. Lett. 1984,52,902. (7) Herbst, E. Astrophys. J . 1976, 205, 94. (8) Herbst, E.; Schubert, J. G.; Certain, P. R. Astrophys. J . 1977, 213, 696. (9) Herbst, E. Chem. Phys. 1982, 65, 185. (10) Herbst, E. Astrophys. J . 1985, 292, 484. (1 1) Herbst, E. Astrophys. J . 1985, 291, 226. (12) Babcock, L. M.; Streit, G. E. J. Phys. Chem. 1984, 88, 5025. (13) Herd, C. R.; Babcock, L. M. J . Phys. Chem. 1987, 91, 2372. (14) Bates, D. R. J . Chem. Phys. 1986, 85, 2624. (15) Adams, N. G.; Smith, D. Chem. Phys. Lett. 1981, 79, 563.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
246
Herd and Babcock TABLE t: Two-Body Rate Coefficients at 219 K and 0.8 Torr in
Helium Low Pressure
v) I
I
reaction
kow, cm3/ (molecule s)
BCl, + Cl-- BCI, BCI, + Br-- BC1,BrBF, + CI- BF,CIBF, + Br- BF3Br-
1.9 X 1.4 X 2.3 X lo-" 6.8 X
I
u Saturated Regioi
n
E
u
--
v
v)
D
0
J
I
I
0
I I
I
U
(MI, molec ~ r n - ~
Figure 1. Generalized shape of curve for pressure dependence of the apparent two-body rate coefficient, ka, for association with both col-
lisional and radiative stabilization.
address the radiative stabilization question at least in part. The halide ion addition reactions of BF, and BC13 provide the ideal opportunity to examine radiative versus collisional stabilization processes as we have d e m o n ~ t r a t e d . ~In ~ Jthis ~ paper we describe our detailed examination of the addition reactions of C1and Br- to BF3 and to BC13 as a function of both temperature and pressure. These results support the general mechanism given in eq 1-3 and yield more insights into the radiative process that occurs in these boron trihalide addition reactions and into the temperature dependence of ion-molecule association reactions.
Experimental Section All of the experiments presented here were carried out on the variable-temperature selected-ion flow tube (VT-SIFT) at Air Force Geophysics Laboratory, Hanscom AFB, MA. This instrument has been described in detail previous1y.l' Ions were produced in a high-pressure ionization source18 using a heated rhenium filament as the source of electrons. Chloride ions, C1-, and bromide ions, Br-, were produced via dissociative electron attachment to carbon tetrachloride, CCl,, and methyl bromide, CH3Br, respectively. Both CC14 and CH3Br were obtained commerically. The methyl bromide was used without further purficiation, but the reagent grade carbon tetrachloride was purified further by a series of freeze/pump/thaw cycles. Ions were produced, mass-selected by a quadrupole mass fdter, and then injected into the flow tube. Neutral reactant gases BF, and BCl, were added downstream. These reagent gases were also obtained commerically and used without further purification; however, the gas bottles were maintained at low temperatures in a dry ice/ methanol bath during experiments in an effort to condense any impurities before withdrawing the gas into the manifold. Temperatures in the range 219-350 K were maintained by circulating cooled or heated fluids through coils surrounding the flow tube. Temperatures in this range were constant to f 4 O C in the reaction zone. The highest accessible temperature, 410 K, was obtained by using a series of heaters spaced at intervals along the flow tube. In all cases, reaction temperatures were measured with Pt resistance thermometers attached to the outside of the reaction chamber. The carrier gas employed for all of these experiments was helium, which was passed through molecular sieve held at 77 K to remove any condensable impurities. Typical helium flows were in the range 6-8 slm (standard liters per minute), corresponding to pressures of 0.3-0.9 Torr. The helium was brought to the temperature of the flow tube prior to injection by passing it through tubing running along the outer length of the flow tube. All gas flows were measured by linear mass flow controllers calibrated for N2and corrected for use with other gases by using appropriate values of heat capacities. Pressures in the reaction zone were (16) Herbst, E.; Adams, N. G.; Smith, D. Astrophys. J . 1983,209, 329. (17) Miller, T. M.; Wetterskog, R. E.; Paulson, J. F. J . Chem. Phys. 1984, 80, 4922. (18) Paulson, J. F.; Dale, F. J . Chem. Phys. 1982, 77, 4006.
(molecule s)
kL. cm3/
koW/ kL
1.4 X 1.1 X
0.14 0.13
8.8 X lo-'' 7.0 X lo-''
0.01
0.03
measured by a differential capacitance manometer. Kinetic information was obtained from observations of reactant ion intensity as a function of added neutral reactant under conditions of constant reaction distance (45.9 cm)and buffer gas flow velocities. Rate coefficient measurements generally are reproducible to f15%, but this varies somewhat with the particular temperature range being studied. Accuracy of the rate data is presumed to be f25%. Because temperature appears as in the determination of k w , the apparent two-body rate coefficient, and as 7" in the determination of the three-body rate coefficient, any uncertainties in temperature are magnified in the kinetic results obtained.
Results For each of the halide ion addition reactions examined, the sole primary product is the addition product as shown below for each of the systems studied:
---
+ BC13 Br- + BC13 C1- + BF3 C1-
BC14-
(5)
BC1,Br-
(6)
BF3C1-
(7)
Br- + BF3 BF3Br(8) The only reaction for which the energy associated with formation of the new B-X- bond is known is reaction 7. The C1- affinity of BF, has been measured as 26 kcal/mol.19 The temperature dependence information obtained in the studies presented herein provides the opportunity to make quantitative estimates of the halide ion affinities represented by reactions 5-8 as will be discussed subsequently. In any event, the energies corresponding to formation of the new B-X- bond, and thus the energies which need to be dissipated in order to obtain stable product ions, are substantial. The two-body rate coefficients, kobsd,measured for X- (X = C1, Br) addition to BX; (X' = F, Cl) exhibit a positive dependence upon pressure and a negative dependence upon temperature, as seen in Figures 2-5. Even at the lowest temperature and highest pressure accessible for each reaction, none of the two-body rate coefficients measured is equal to the association rate, as is apparent from the data in Table I. Upper limits to association rate coefficients are calculated by using Langevin theory for the determination of the collision rate between an ion and a polarizable neutral without a permanent electric dipole moment;21.22polarizabilities of BF3 and BC13 are 3.31 and 9.47 A3, respecti~ely.~~This same general behavior is what was observed for the addition of F ions to BF,, and therefore the complex process of sorting out the pressure and temperature dependencies and of placing limits on kd and k, which will be employed here is that developed in our previous study.', Briefly, this method involves the use of room-temperature two-body rate coefficients measured in helium and in various other third bodies (when available) to determine the ratio of unimolecular decomposition at room temperature to radiative stabilization, kd,29B/kr; this ratio is determined from the intercept of kOw versus (M) graphs. Agreement of the intercept determined from our flowing afterglow three-body work with that found in low-pressure ICR studies of BF4- formation supports the validity of this approach. Larson, J. W.; McMahon, T. B. J . Am. Chem. Soc. 1985, 107,766. Ruhr, P. S.;Herd, C. R.; Babcock, L. M., submitted for publication. Langevin, P. Ann. Chim. Phys. 1905, 5, 245. Gioumousis, G.; Stevenson, D. P. J. Chem. Phys. 1958, 29, 214. Miller, T. M. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1984. (24) McMahon, T. B., private communication. (19) (20) (21) (22) (23)
-
The Journal of Physical Chemistry, Vol. 93, No. I, 1989 247
Radiative Addition in Halide Ion Reactions 25.0
-
I
BClg t CI- + B C I i
20.0
v)
c
V
-
I-
15.0
TABLE I 1 Radiative and Unimolecular Rate Coefficients for Various Values of 0"; kd(T) = AT" product ion @He A n k,, s-l
BC1i
-
0
6
--E
10.0
X D v) yo
5.0
BC13Br-
-
0.0I 0.0
BF3C11 .o
I 2.0
(He), 10
3.0 16
40
molec ~ r n . ~
Figure 2. Observed rate coefficients for the addition of C1- to BCl, as a function of temperature and pressure; the points are experimental data = 0.46 along with appropriate k, and curves are generated by using
and kd(T ) values. The association rate coefficient, k,, is taken as the Langevin rate, and the collisional stabilization rate coefficient, k,, is taken as the appropriate Langevin rate multiplied by a collisional efficiency factor 0, which is characteristic of the particular third body. Once a value for /3 has been determined, k, then can be calculated from kobd data for two-body rate coefficients measured at room temperature. Assuming that 0 and k, are independent of temperature permits one to obtain values of kd a t the other experimental temperatures studied and thus to obtain kd(T). Over the limited temperature range of our experiments, the assumption of constant 0 is appropriate since both theory2s and e ~ p e r i m e n t ~ ~have .~' demonstrated that the temperature dependence of 8, while not easily predicted, is small. The fact that unimolecular decomposition rate coefficients are functions not only of temperature but also of pressure complicates analysis of association reactions. Because our experimentally accessible range of pressures is narrow (0.3-0.9 Torr in helium), the assumption that kd is independent of pressure is valid. Bowers has shown that the change in kd over such a small range of pressures is expected to be mall.^**^^ The most important aspect of this analysis is the fact that it permits analysis of the temperature and pressure dependence without making the simplifying assumption that k,(M) D0(F3B-Cl-) > Do(F3B-Br-) and Do(C13B-CI-) Do(C13B-Br-). This is consistent with the two energies which are known: Do(F3B-F) = 71 k c a l / m 0 1 ~ ~and . ~ ~Do(F3B-Cl-) = 26 k ~ a l / m o l . ' ~This qualitative trend is not surprising, but it occurred to us that perhaps eq 15 could be used in conjunction with our experimental data and the two known bond energies to yield quantitative estimates of the unknown Do values. If we equate our exponential temperature dependence, n, with the s - 1 of eq 15 and assume that the proportionality constant involved is the same for all of our closely related systems, the resulting equation is
-
kd,l
( TI)"'(D~)~'
kd,2
( T2)n2(Do)n'
-=
(16)
In order to test the validity of this approach, we used eq 16 in conjunction with the BF4- and BF3Cl- data given in Table I11 to calculate D0(F3B-Cl-) using the known value for D0(F3B-F).19332 Using this method, we calculate a value of 21.4 kcal/mol, which compares very well with the measured value of 26 k c a l / m ~ l for '~ D0(F3B-Cl-). It appears that this method yields good quantitative estimates for bond dissociation energies, so it was used to calculate values for the unknown Cl3B-C1- and C1,B-Br- bond dissociation energies. With B F 3 / F as the known system, values of 47 and 52 kcal/mol are found for the chloride and bromide affinities of BCl,, respectively. With BF3/C1- as the basis for the calculations, values of 56 kcal/mol for C1- bonding to BC13 and 61 kcal/mol for Br- bonding to BCl, were determined. These results predict, then, that D0(C13B
