Temperature dependence of the hydrogen atom abstraction reactions


Jet Propulsion Laboratory, Pasadena, Callfornla 9 1103 (Received: November 21, 1980). Rate constants are reported for the reactions of C1+ and HC1+ wi...
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J. Phys. Chem. 1981, 85,313-315

313

Temperature Dependence of the Hydrogen Atom Abstraction Reactions of CI+ and HCI’ With Hz R. D. Cates, M. T. Bowers,* Department of Chemistry, Unlversity of California, Santa Barbara, Californh 93 106

and W. T. Huntress, Jr. Jet Propulsion Laboratory, Pasadena, Callfornla 9 1103 (Received: November 21, 1980)

Rate constants are reported for the reactions of C1+ and HC1+ with Hzover the temperature range 150-400 K. The C1+ reaction has a positive temperature dependence that follows the Arrhenius form k l = A exp(-169/T). The reaction of HC1+has a negative temperature dependence that follows the form kz = AF‘.6. The implication of these results for interstellar chemistry is commented on. Schematic potential surfaces for the reactions are suggested.

Introduction The reactions of C1+ and HCl+ ions with Hz are important in the chain of processes which lead to the synthesis of HC1 in interstellar c1ouds.l The chain is initiated by the photoionization of C1 atoms in optically thin interstellar clouds, followed by reactions 1 and 2, and ter-

--

+ H2 HCl+ + Hz C1+

HC1+

+ H - 0.17 eV

HzCl++ H 5 0 eV

(1) (2)

minated by recombination of HzCl+ with thermalized photoelectrons to yield HC1. However, recent measurements reported2 for the column abundances of C1+ and C1, and an upper limit for HCl in the thin cloud {-Ophiuchi, indicate that this model underestimates C1+ and overproduces HCl. The observations and gas-phase chemical model could be made to agree if the rate constant for the initiating process, reaction 1,were smaller at interstellar temperatures (10-100 K) than the value which has been measured3 at 300 K. A temperature dependence on the ~ model is order of kl = A exp(-200/T) is r e q ~ i r e d .The independent of the rate constant for reaction 2 as long as k2 is sufficiently large that chemical equilibrium may be assumed. In this paper we report the temperature dependence of the rate constants of reactions l and 2 measured over the range 150 I T I400 K . Experimental Section The experiments were performed on a temperaturedependent drift-mode ion cyclotron resonance (ICR) spectrometer that has been previously described in the literature.6 The C1+ ions were formed by electron impact on CC14,CHC13,and CF2C12.All three sources of C1+ gave identical values of lz, indicating electronically excited ions were not involved. Most of the work used CFzClzas the source of C1+ because of its low freezing point. The HC1+ ions in reaction 2 were formed by low-energy electron (1)J. H. Black and A. Dalgarno, Astrophys. J . Supp. Ser., 34, 405 (1977). (2) Cited by J. H. Black and D. L. Smith in “Proceedings of the IAU Symposium No. 87 on Interstellar Molecules”, Montreal, Canada, 1979. (3) F. C. Fehsenfeld and E. E. Ferguson, J. Chem. Phys., 60, 5132 (1974). (4) J. H. Black, private communication. ( 5 ) M. T. Bowers, P. V. Neilson, P. R. Kemper, and A. G. Wren, Int. J. Mass Spectrom. Zon. Phys., 25, 103 (1977); see also A. G. Wren, P. Gilbert, and M. T. Bowers, Reu. Sci. Instrum., 49, 531 (1978). 0022-365418112085-0313$01.00/0

impact on HCl. This process forms essentially all of the ions in the ground vibrational state of the ground electronic state of HC1.6 Results and Discussion A plot of the experimental value of kl vs. temperature is given in Figure la. These data clearly indicate that reaction 1 exhibits a positive temperature dependence. Figure l b is a plot of -In k1 vs. TI.The linearity exhibited in Figure l b suggests that the temperature dependence of reaction 1 can be approximated by an Arrhenius-type rate expression

k = [email protected]/RT

(3) where A is a constant and E* is the barrier to reaction using the zero point energy of the reactants as the energy zero. From the slope in Figure l b a barrier of E* = 335 f 35 cal/mol is obtained. Hence, the Arrhenius model suggests kl = A exp(-169/T) which is close to the temperature dependence needed4 to explain the current anomalies in the interstellar abundance of C1+. A prediction of the value of kl vs. T from this Arrhenius equation in the interstellar temperature range is given in Figure 2. Reaction 1has not, to our knowledge, been studied using crossed beam techniques and nothing is known about its low-energy dynamics. However, from heat of formation data it is known that reaction 1 is 0.17-eV exothermic. Further, the H2C1+ion is very stable, the lowest energy dissociation pathway being homolytic bond dissociation, HCl+- H, yielding a well depth of the order of 4-5 eV. It is reasonable to assume that the C1+/Hzreactants sample this potential well during the collision and that the HCl+/H products result by simple C1-H bond cleavage. If this is the case then it is unlikely that the barrier to reaction occurs in the exit channel. The most reasonable alternative is, thus, a barrier in the entrance channel yielding the reaction coordinate diagram given in Figure 3a. The existence of an “early” barrier is also consistent with the fact that a temperature increase is effective in driving the reaction (for the temperature range investigated the primary effect of increasing T is to increase the translational energy of the reactants). ( 6 ) D. W. Turner, C. Baker, A. D. Baker, and C. R. Brundle, “Molecular Photoelectron Spectroscopy”, Wiley-Interscience, London, 1970.

0 1981 American Chemical Society

The Journal of Physical Chemistty, Vol. 85, No. 4, 1981

314

70t

Letters

(a)

0.335 Kcol A

CIttH2-

HCI't

200

1

,

I

,

300

400

500

40

io0

(a) H

319 Kcol HCI't

H

(b) HCI't

TEMPERATURE, "K

H, + H

Flgure 3. (a) Schematic reaction coordinate diagrams for reactlon 1 (a) and reaction 2 (b).

* /

I

rT

2i 5 1

HCI'

T-1

x i03,

0

OK

+

H-,

H2CI+

+H

:

i :i

Flgure 1. (a) A plot of k l vs. T: (0)ICR,this work; (A)ref 3 flowing afterglow. (b) The data in part a plotted as In k l vs. 1/T. I XlC

0 A i x IO I

4 00

200

I 300

Temperature, \

m

-

I

400 O K

Flgure 4. A plot of k l vs. T: (0)drift cell ICR, this work; (0)trapped cell ICR, this work; (A)flowing afterflow, ref 3.

In

5

* *

IXiO

-

x

CIt t H2

I XI0

HCIS t H

EXPT

I XlC

-

k = 9 47 x IO-'Oexp (-i69/T)

I

I

I

I

I

I

100

200

300

400

500

600

Temperature, O Flgure 2. A plot of k, vs. Tusing the form k, = to the value of k, at 300 K given in Figure la.

I

K

normalized

Reaction 2 behaves quite differently. A plot of kt vs.

T is given in Figure 4. In this case the rate constant decreases (essentiallylinearly) with increasing temperature

in the range studied. At the lowest temperatures investigated (2' 150 K) kz N 0.5 kL,where kL is the Langevin collision rate constant. If one assumes a form lzz = AT", then a plot of In kz vs. In T should yield a straight line of slope n. Such a plot of the data in Figure 4 does yield a straight line with n = -0.6. Negative temperature dependencies have been observed for quite a number of ion-molecule reactions8 but to our knowledge this is the first such observation for a hydrogen abstraction reaction. Canonical transition state theory has been used by several authors8l9to rationalize negative temperature exponents. The arguments used by these authors require quenching of internal rotations in the transition state, (7) H. M. Rosenstock,K. Draxl, B. W. Steiner, and J. T. Herron, Phys. Chem. Ref. Data, 6 , Supp 1, (1977). (8) M. Mautner in "GasPhase Ion Chemistry",Vol. I, M. T. Bowers, Ed., Academic Press, New York, 1979, pp 198-271. (9) M. Moet-Ner (Mautner) and F. H. Field, J.Chem. Phys., 64,277 (1976).

J. Phys. Chem. 1981, 85,315-317

however, and thus are probably not suitable for reaction 2. We favor an explanation put forward by Brauman and co-workers,1° exemplified by the reaction coordinate diagram in Figure 3b. As the system proceeds along the reaction coordinate from HC1+/H2 reactants toward H2C1+/Hproducts it must pass through the transition state located in the center of diagram 3b. Since the k2 vs. T data give_nosuggestion of a positive T dependence at any value of T , the energy at the transition state maximum is less than the zero point energy of the reactants. Since a fairly strong negative temperature dependence is observed, a tight transition state must occur yielding an entropically unfavorable situation relative to the HC1+/H2reactants. As energy (Le., T ) increases, the density of states in the

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reactants increases much more than in the transition state resulting in the negative temperature dependence. The value of k2 at interstellar temperatures should approach the collision limit, and hence is sufficiently fast to explain interstellar abundances of HC1. In summary, two seeming similar, simple hydrogen atom abstraction reactions exhibit very different temperature dependencies and hence occur on very different potential surfaces. The results, when extrapolated to interstellar temperatures, yield rate constants that help explain observed abundances of C1+, C1, and HCl in interstellar clouds. Acknowledgment. This research was supported by the National Science Foundation under grant no. CHE7715449, the President’s Fund, California Institute of Technology, and by the National Aeronautics and Space Administration under grant no. NAS7-100 to the Jet Propulsion Laboratory, California Institute of Technology.

(10) W. E. Farneth and J. I. Brauman, J. Am. Chem. Soc., 98, 7891 (1976); W. N. Olmstead and J. I. Brauman, ibid.,99, 4219 (1977); 0. I. Asubioja and J. I. Brauman, ibid., 101, 3715 (1979); J. M. Jasinski and J. I. Brauman, ibid., 102, 2906 (1980).

Sonoluminescence from Aqueous Solutions of Br, and I, C. Sehgal, R. G. Sutherland, and R. E. Verrall” Department of Chemistry and Chemical Engineering, Universw of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0 (Received: April 23, 1980)

Sonoluminescence spectra from aqueous solutions of Br2 and Iz are reported by using single-photon counting techniques. The results show that the origin of the sonoluminescence is thermochemical and occurs largely from excited states of the halogens and their oxides.

Introduction Over the past few decades a number of theories have been proposed on the origin of sonoluminescence. Although there is growing evidence to support the thermal hypothesis of this emission, the question of its precise origin is not unequivocal. On the one hand, according to Srinivasan and Holroyd,’ the adiabatic compression of a cavity gives rise to heating of the cavity contents to incandescence. On the other hand, recent spectroscopic24 and nonspectros~opic~ investigations appear to indicate that the origin of sonoluminescence is largely thermochemical. The high temperatures that arise during the rapid compression of a cavity produce electronicallyexcited molecules and free radicals which either radiate back to the ground state or recombine radiatively to produce sonoluminescence. In this study sonoluminescence from argon-saturated aqueous solutions of iodine and bromine is reported and used to provide evidence in favor of the thermochemical theory. Experimental Section The experimental technique is similar to that previously d e ~ c r i b e d . ~The , ~ solutions were maintained a t 285 f 2 (1) Srinivasan, D.; Holroyd, H. V. J. Appl. Phys. 1961, 32, 446. (2) Sehgal, C.; Sutherland, R. G.; Verrall, R. E. J. Phys. Chem. 1980, 84, 388. (3) Sehgal, C.; Sutherland, R. G.; Verrall, R. E. J. Phys. Chem. 1980, 84, 396. ( 4 ) Taylor, K. J.; Jarman, P. D. Aust. J. Phys. 1970, 23, 319. (5) Saxena, T. K.; Nyborg, W. L. J. Chem. Phys. 1970,53, 1722.

0022-3654/81/2085-03 15$01 .OO/O

K and insonated at 459 f 1kHz. The spectra (uncorrected for absorption of halogens) were recorded after 1 h of insonation. Argon gas was used to enhance cavitation intensity, as its presence increases the specific heat ratio of the cavity contents and raises intracavity temperatures and, hence, sonoluminescence. Such an enhancement of cavitation intensity has been observed during the reduction of I2 by hydrogen in an ultrasonic field.8 Results and Discussion The results show that the sonoluminescence obtained from aqueous solutions of Br2 and I2 extends from the ultraviolet to the near infrared. The emission is weak above 500 nm (Figures 1 and 2). This is probably due to the absorption of sonoluminescence radiation by Br2 and I2 molecules. However, the spectral distributions in the two cases (Figures 1 and 2) differ from one another and from that of argon-saturated water (cf. Figures 1and 2 with Figure 2 of ref 2). A comparison of the sonoluminescence from Br2-saturated solution (Figure 1) with the emissions from Br2 :aporgJOand the gaseous mixture resulting from the action of H2S04on KBr0311shows that the sonoluminescence (6) Mead, E. L.; Sutherland, R. G.; Verrall, R. E. Can. J. Chem. 1976, 54, 1114. (7) Sehgal, C.; Steer, R. P.; Sutherland, R. G.; Verrall, R. E. J. Chem. Phys. 1979, 70, 2242. (8) Henglein, A. Naturwissenschaften 1956, 43, 277. (9) Uchida, Y.; Ota, Y. Jpn. J. Phys. 1928,5, 59. (10) Gaydon, A. G. “The Spectroscopy of Flames”; Wiley: New York, 1974; 2nd ed. (11) Guenebaut, H.; Goudmand, P. C.R. Acad. Sci. 1961,253,1926.

0 1981 American Chemical Society