J . Phys. Chem. 1986, 90, 931-936 whether or where charge resonance related absorption in retinyl dimer cations is present. Relative to the location of the lowest absorption of aromatic monomer cations," the corresponding dimers show absorption bands that are blue-shifted. The situation with retinyl cations is comparable to this behavior only up to the extent that there is redistribution of relative vibronic intensities on going from monomer to dimer (Figures 2 and 4) while the general location of the band system remains practically unchanged. This is not surprising because the geometric arrangement of polyene moieties in their dimer cations is not necessarily the same as that of aromatic moieties in the related dimer cations. In particular, with retinoic acid, one would expect intermolecular H bonding to be a dominant force leading to a tail-to-tail arrangement in the dimer. Since the spectral changes for retinal and methyl retinoate are quite similar to those for the acid, similar linear arrangement may also be proposed for them. The fact that the propensity for dimerization is rather small for retinol and retinyl acetate (in acetone) suggests that ion-dipole interaction is also an important factor causing aggregation in heteroatomended polyenes. The equilibrium constant for dimerization of retinoic acid in the ground state is 1100 M-' in CCl, (at 28 OC)., Noting that DCE as a solvent is not too different from CC14,one could consider the corresponding equilibrium constant for the retinoic acid radical cation to be much higher (3700 M-I at 22 "C). This underlines the importance of the ion-dipole interaction in radical cation aggregation. Interestingly, on going from acetone to DCE, K values become larger by almost one order of magnitude (Le. AGO'S become more negative by 1 kcal mol-'). This solvent effect can
-
931
be understood by the fact in the polar solvent (acetone) the monomer cation is more stabilized relative to the dimeric form because of increased hydrogen bonding or ion-dipole electrostatic interaction (solvation) involving the smaller-size monomer cation. A similar trend has been found by Arai et al.loafor pyrene; the monomer e dimer equilibrium constant in this case decreases by a factor of two on going from acetone to the more polar solvent, benzonitrile. Radical cation dimerization has long been known9-I3 for aromatics and olefins (including dienes). Also, for a decade or so, considerable interest has been ~ h o w n ' ~ in - ' ~the properties of radical ions of long-chain polyenes. However, as far as we know, this is the first time results are being reported that strongly suggest associative trends in radical cations of long-chain polyene systems in fluid solutions. In acetone, K values (Table I) for retinal and retinoic acid are nearly as large as the K value for pyrene (550 M-' at 20 OC).I0. Our results indicate that cation monomer/dimer equilibrium is specially favored by polyene systems ending with heteroatoms and particularly when these substrates are in a relatively nonpolar environment. This associative behavior may be critical in the role of r-systems as charge mediatorsz4along bilayer membranes and in organized assemblies. Registry No. Retinal, 116-31-4;retinoic acid, 302-79-4;methyl retinoate, 339-16-2;acetone, 67-64-1;biphenyl, 92-52-4; DCE,107-06-2. (24) (a) Berns, D. S. Photochem. Photobiol. 1976, 24, 117-139. (b) Schadt, M. Biochem. Biophys. Acta 1973, 323, 351-366. (c) Kobamoto, N.; Tien, H. T. Ibid. 1971,241, 129-146. (d) Hong, F. T. Photochem. Photobiol. 1976, 24, 155-169, and references therein.
Gas-Phase Reactions with Ethylene and Hydrogen Sulfide of Tritium Atoms Thermalized after Nuclear Recoil Nun-Yii Wang, R. Subramonia Iyer, and F. S . Rowland* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: August 2, 1985)
The tritium atoms formed with high kinetic energy in the ,He(n,p)T nuclear reaction have been adapted for experimental studies of gas-phase thermal atomic reactions through multiple collisions with either CF4 or Kr as nonreactive moderator. One-fourth of the hot tritium atoms released in CF4 react while still energetic to form TF, and an additional 0.3% substitute for F to make CTF,. The remaining three-quarters of the tritium atoms are thermalized and are available for reaction with other substrates present in low mole fraction. Krypton is not as efficient as CF4 in removing the kinetic energy of tritium but furnishes a useful alternative for confirmation of mechanisms and reaction rates measured with CF4 moderator. The ratio of rate constants, k l / k z ,for thermal tritium has been determined as 1.17 & 0.04 at 295 K in CF, moderator by measurement of the yields of HT from T + H2S HT + HS (1) and CHT=CH2 and CH2TCH3from T + CH,=CH2 CH2TCH2* (2). The CH2TCH2*radicals formed in (2) decompose by loss of H with a rate constant equivalent to collisional stabilization at 22 f 2 torr in CF4. The small yield of CHT=CH2 observed from hot reactions of tritium with CH2=CH2 is linear with the mole fraction of CHz=CHz, corresponding to 0.3% of the reacting tritium for a C2H4 mole fraction of 0.0030. The ratio of rate constants, k , / k 2 ,is 1.32 f 0.06 when measured with krypton as the moderator gas, prior to correction of the direct observations to eliminate the contributions of hot reactions of T* with H2S from these thermal tritium atom studies. An average value of 1.20 & 0.04 for k l / k 2with both Kr and CF4 moderators is consistent with the corresponding observations for H atoms, after allowance for the H / T isotope effect in the addition reaction with ethylene.
-
Introduction Radioactive tracer techniques can be readily adapted to the study of the chemical reactions of atomic hydrogen through the use of tritium ( t l 1 2= 12.3 years) but have been extensively applied only for reactions occurring with "hot" atoms possessing abundant extra kinetic energy at the time of chemical reaction.' The usual nuclear sources of these energetic atoms are the thermal neutron-induced reactions ,He(n,p)T and 6Li(n,cu)T, which release ~
~~
(1) F. S. Rowland, MTP Int. Reu. Sci. Phys. Chem., Ser. One, 9, 109 (1972).
0022-3654/86/2090-0931$01.50/0
-
tritium with initial energies of 1.92 X lo5 and 2.7 X lo6 eV per atom, respectively. In comparable experiments with radioactive atomic fluorine (18F)2,3and chlorine (38C1)4Jstudies have been conducted both on chemical reactions initiated by "hot" halogen atoms and also by halogen atoms brought down to thermal energies (2) F. S. Rowland, F. Rust, and J. P. Frank, ACS Symp. Ser., 66, 26
(1978).
(3) J. W. Root, C. A. Mathis, R. Gurvis, K. D. Knierim, and S.-H. Mo, Adu. Chem., No. 197, 207 (1981). (4) F. S. C. Lee and F. S. Rowland, J . Phys. Chem., 81, 1229 (1977). ( 5 ) L . D. Spicer, A h . Chem., No. 197, 123 (1981).
0 1986 American Chemical Society
932 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 by numerous nonbond-forming collisions with nonreactive moderator molecules in the same system. Studies from this and other laboratories which have used such thermalized radioactive halogen atoms have provided nseful information about the chemical reaction mechanisms and relative reaction rates of such species with a variety of We have now applied similar techniques for the moderation of high kinetic energy tritium atoms down to thermal energies and used such thermal tritium atoms to study the chemical reaction of T atoms with ethylene and with hydrogen sulfide. Energetic tritium atoms are readily formed in very high yield with gaseous )He in a thermal neutron flux, because of its 5330 barn cross section, and can be satisfactorily obtained from 10-20 torr of 3He in a IO-mL bulb in a few minutes of irradiation in a nuclear reactor. Most of the 192000 eV of initial kinetic energy of such tritium atoms is lost in collisions which cannot possibly result in bond formation for the tritium atom because of its extremely high forward momentum, and bond forming reactions are essentially limited to collisions involving atoms with perhaps 50-eV kinetic energy or less.’ The analytical technique of radio gas chromatography12separates the individual molecular species and then assays the amount of radioactivity found in each compound. Because the proportional counter detector registers only the actual decay of radioactive atoms, the molecules formed in all collisions for the tritium atom except the last, i.e. the interaction which results in bond formation, are automatically excluded from detection. The basic technique of thermalization of nuclear recoil species, as developed with the radioactive halogen atom^,^-^ relies on a nonreactive moderator gas in great excess such that most recoiling atoms pass through the energy range in which hot atom reactions are feasible without encountering a reactive substrate molecule. After moderation to thermal energies, the tritium atoms can then continue to collide until eventually they find and interact with the reactive substrates present in low mole fraction. Reactions initiated by hot atoms can be detected and progressively eliminated by extrapolation to zero mole fraction of reactive substrate of the measurements made with smaller and smaller mole fractions of target molecule. The choice for a suitable moderator gas can be either a noble gas incapable of forming a stable chemical bond to a single atom, or a polyatomic molecule found by experiment not to be reactive toward atomic hydrogen. Nonreactive polyatomic molecules have been the usual choice for thermalization of energetic ISF(SF6,’ CF4,233*11 NF3, etc.) or 38CI (CC1F3,, CC12F2, etc.). In those systems, however, the moderator molecules serve a dual purpose because they also contain the stable isotopes (I9For 37Cl)needed as targets for the neutron-induced nuclear reactions. Our experiments with tritium atoms from nuclear recoil have been carried out with two different moderator gases: krypton gas as the noble gas, and carbon tetrafluoride as the inert polyatomic molecule. Both have proven to be reasonably successful as moderators in our experiments, although each has some limitations. The very large inequality of 3 vs. 84 in the masses for T-Kr collisions places a severe limit on the maximum fraction of kinetic energy which can be lost in a single collision, so that many collisions with krypton are required by the tritium atom for passage without chemical bond formation through the energetic chemical reaction range. Lighter noble gases which offer more favorable relative mass ratios for collisions with tritium atoms have not been tested so far in order to avoid another possible difficulty-failure of neutralization for energetic T+ ions moving through a noble (6) M. Kikuchi, J. A. Cramer, J. P. Frank, R. S. Iyer, and F. S. Rowland, J . Phys. Chem., 86, 2677 (1982). (7) P. Rogers, D. C. Montague, J. P. Frank, S. C. Tyler, and F. S. Rowland, Chem. Phys. Left., 89. 9 (1982). (8) F. S. C. Lee and F. S. Rowland, J . Phys. Chem., 84, 1876 (1980). (9) M. Kikuchi, F. S. C. Lee, and F. S. Rowland, J . Phys. Chem., 85,84 (1981). (10) J. W. Root and R. G. Manning, Adu. Chem. No. 197, 79 (1981). (1 1) S. H. Mo, E. R. Grant, F. E. Little, R. G. Manning, C. A. Mathis, G. S. Werre, and J. W. Root, ACS Symp. Ser., 66, 59 (1978). (12) J. K. Lee, E. K. C. Lee, B. Musgrave, J. W. Root, Y.-N. Tang, and F. S. Rowland, Anal. Chem., 34, 741 (1962).
Wang et al. gas with an ionization potential greater than that of hydrogen atoms. The near-equality of ionization potentials for Kr (14.00 eV) and T (13.60 eV) greatly reduces the possibility of complications from ionic T+ reactions mixed in with the thermal T reactions. The chief difficulty with CF4 as an “inert” polyatomic moderator gas has been the experimental determination that it is not as inert toward energetic tritium atoms as it is toward kinetically hot F or C1 atoms. The major cause of this difference in reactivity is the exothermicity of F atom abstraction because of the very strong H-F (or T-F) bond. Approximately ‘/, of the energetic tritium atoms released in a mixture of 3He and CF, fail to appear among the reaction products from thermal tritium reactions with minor substrates such as CH2=CH2 or HIS, from which we infer that most of the missing T atoms have abstracted F from CF, prior to reaching thermal energies. A trace of CTF3 is found from the hot T/F substitution reaction with CF4. Despite the loss of about 1/4 of the T atoms to other processes, mostly hot reactions with CF,, the remaining 3/4 of the initially energetic tritium atoms are thermalized by collisions with CF, and are available for thermal reactions with any substrate added in minor mole fraction. One thermal tritium atom reaction expected with H2S is the abstraction of H, as in (l), while the chief reaction with CH2= T + H2S H T H S (1)
T
+
+ CHZwCH2
CH2TCH2*
+
CH,TCH2* CH2TCH2*
M
+ H2S T + H2S
CHZTCH2
+
CH2TCH2*
(2)
+H CH,=CH2 + T CH2TCH2 + M CHZTCH, + H S HST + H
(3)
+
CHT=CH,
+
-+
+
(3’) (4) (5)
(6)
CH, is addition, as in (2). The CH2TCH2*radicals formed by addition of a thermal tritium atom are initially excited by the formation of the C-T bond and are capable of undergoing decomposition (D) by loss of H in ( 3 ) , or by the isotopic variant of loss of T by (3’). The other competitive channel open to the CH2TCH2*radicals from (2) is stabilization (S) of the excited radical by collision with the moderator gas, as in (4), leaving a thermal CH2TCH2radical. In the presence of a molecule such as HIS which can readily react with such alkyl radicals by H donation, these stabilized radicals are converted into CH2TCH3 by (5). An alternative reaction to (1) with H2S is the isotopic hydrogen exchange reaction in (6). Linear pressure dependence of this (D/S) ratio, Le. [CHT= CH2]/[CH2TCH3],in excess CF4 or Kr demonstrates that the excited CH2TCH2*radicals are almost uniformly monoenergetic, and all react with essentially the same value for k,. Monoenergetic radicals are expected if the excitation energy is furnished only by the exothermicity of the addition of T to the double bond, with a negligible energetic contribution from the kinetic energy of the T atom. The relative rate constants for the two competing thermal tritium atom reactions, k l / k 2 ,can be measured through observation of the yield ratio of H T to the sum of (CHT=CH, plus CH2TCH3)vs. the substrate ratio [H2S]/[CH2=CH2]. Absolute and relative reaction rates for the H atom analogues of (1) and (2) have been measured several times previously by using H atoms formed by the photolysis of H2S,’3-’4with some discrepancies among the results. The corresponding ratio of rate constants for tritium atom reactions has not been previously measured but can be usefully compared to these H atom measurements. Experimental Section Tritium atoms were formed by the 3He(n,p)T nuclear reaction initiated by thermal neutrons in the UCI Mark I TRIGA nuclear reactor. All experiments were carried out by using the standard techniques for sample preparation, irradiation, and radio gas (13) B. deB. Darwent and R. Roberts, Discuss. Faraday SOC.,14, 5 5 (1953). (14) G. R. Wooley and R. J. Cvetanovic, J . Chem. Phys., 50,4697 (1969).
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 933
T Reactions with CH2=CH2 and H2S TABLE I: Volatile Tritium-Labeled Products from the Reactions of Tritium Atoms in Thermal Neutron Irradiated Mixtures Containing ”e. CH,=CH,. and H S in a 2/1 Ratio, Plus CFd as Moderator Gas
CHz=CH2 mole fraction 0.0030 0.0030 0.0029 0.0030 0.0030 0.0030 0.0052 0.0055 0.0055 0.0055 0.0055 0.0101 0.0100 0.0099 0.0100 0.0240 0.0248 0.0242 0.0247
total press., torr 606 1212 1357 1560 1954 2093 833 1033 1597 1710 2073 673 1010 1652 1990 738 1004 1647 1944
HT 29.3 28.5 27.4 29.1 26.4 25.9 27.5 31.4 26.3 26.8 24.4 29.1 28.6 27.7 27.7 30.3 27.9 26.0 27.7
product yield, % CHT=CH2 CHITCHJ 1.70 1.01 1.03 0.93 0.84 0.56 1.69 1.61 1.12 0.94 0.88 2.58 2.05 1.47 1.50 4.26 4.35 3.08 3.52
41.0 44.0 43.0 45.9 43.3 42.3 42.6 50.2 44.3 44.9 42.2 47.4 47.8 47.4 45.9 50.0 46.9 44.8 47.8
D/S
CTF? 0.27 0.24 0.33 0.37 0.37 0.26 0.20 0.19 0.22 0.25 0.28 0.17 0.30 0.26 0.28 0.28 0.20 0.24 0.34
chromatography developed for studying the energetic reactions of recoil tritium atoms.’ Samples with total pressures in the range from 600 to 2500 torr were prepared containing about 15 torr of ,He and varying amounts of CH2=CH2, H2S, and CF, or Kr in 10-mL bulbs (2 cm diameter) made from Pyrex 1720 glass. The samples were irradiated for 8 min with a neutron flux of 3 X IO1] neutrons cm-’ in the rotating specimen rack of the reactor. The temperature during irradiation was 295 K, as controlled by the temperature of the reactor water moderator. Two monitor bulbs containing n-C4Hlo,H2S, and ,He were included in each set of irradiated samples to permit precise calibration of the neutron flux and of the absolute yields of tritium per sample. Previous experiments have shown that samples irradiated simultaneously in the rotating rack are exposed to the same neutron flux within *I%. The amounts of tritium activity expected in the experimental samples are reduced by losses of energetic atoms into the ampule walls. These recoil losses are in the 6% range near 2000 torr and 20% around 700 torr for samples containing CH2=CH2.” The volatile tritium-labeled contents of each ampule were analyzed with standard radio gas chromatographic techniques.” No measurements were made of readily exchangeable tritium activity originally present in molecular forms such as T F or HST. The yields of these products can be estimated approximately by difference vs. the total activity expected from the monitor bulbs. The samples containing n-C4HIowere analyzed with a 50-ft dimethylsulfolane (DMS) column, while those with CH2=CH2 present were separated with a 50-ft column of propylene carbonate coated on alumina (PCA). The observed radioactivities for tritium-labeled products are expressed in percentage yields compared with the activities calculated from the monitor samples after correction for the recoil loss appropriate to the total pressure in each sample. Research grade CH2=CH2 was purchased from Phillips, ,He from Monsanto Research Corp. and krypton from Spectra Gas Inc. Hydrogen sulfide, CF4, and n-C4Hlowere all obtained from Matheson. Each gas (except ,He) was purified by several cycles of degassing at liquid nitrogen temperature, and then used without further purification.
Results and Discussion Mixtures Containing CH2=CH2, H2S, and ,He with CF4 as Moderator. A series of experiments were carried out with samples (15) B. DeB. Darwent, R. L. Wadlinger, and M. J. Allard, J. Phys. Chem.
71, 2346 (1967).
(16) B. G . Dzantiev and A. V. Shishkov, Khim. Vys. Energ., 1, 192 (1967). (17) J. W. Root and F. S. Rowland, Radiochim. Acta, 10, 104 (1968).
I 0.001 I 0.0 0.2 0.4
I
I
I
1
1
I
I
0.6
0.8
1.0
1.2
1.4
1.6
1.8
( I / P ) ~lo3 torr-’ Figure 1.
.,
Decomposition/stabilization yield ratio [(CHT=CH,)/
(CH2TCH,)] vs. reciprocal pressure for tritium atom reactions in CF4-moderatedmixtures of C2H4and H2S. Mole ratio of [C2H4]/[H2S] = 2.0. Mole fraction of CZH4: A, 0.0100; 0.0055; 0 , 0.0030. which contained CH2=CH2 and H2S in a ratio of 2.0/1 over the total pressure range from 600 to 2100 torr. The tritium-labeled products which were identified and measured quantitatively included HT, CHT=CH2, CH2TCH3,and CTF,. The absolute yields of these observed products summed to about 75 f 5%, as shown in Table I. The yield of HT from the thermal abstraction reaction with H2S in (1) is 26-30% and is essentially independent both of the mole fraction of CH2=CH2 and of the pressure in these high CF,/CH2=CH2 ratios. The yield of CH2TCH3represents the contribution from the sequence of reactions 2,4, and 5 , while that of CHT=CH2 is primarily from the sequence of reactions 2 and 3. The summation of these yields originating with the addition reaction 2 represents 43-51% of the total tritium formed in the system. The missing tritium yield needed to sum to 100% is assumed to be in the form of T F from the abstraction reaction of T with CF4 in reaction 7 or as HST from reaction 6
+ CF4 -,T F + CF, T + CF4 CTF3 + F T
-
(7)
(8)
and was not measured by the analytical techniques used. The observed yield of CTF, is about 0.3% from all samples and originates in the hot substitution of T-for-F in CF, by reaction 8. The precision of measurement for CTF, is about &0.07%, and we attach no significance to any of the apparent variations in its yield in Table I. The yield ratios of CHT=CH2 to CH2TCH3 from Table I exhibit a linear dependence vs. the reciprocal of the total pressure for fixed mole fractions of CH2=CH2, as shown in Figure 1 for mole fractions of 0.0030,0.0055, and 0.0100. Each of these data sets can be readily fitted with a straight line which extrapolates to a nonzero intercept at infinite pressure. Linearity is expected for CH2TCH2*radicals all of which have essentially the same excitation energy from the newly formed C-T bond. The significance of an intercept greater than zero is that CHT=CH2 can be formed by some process other than the decomposition in (3) of these monoenergetic CH2TCH2*radicals formed by the addition of thermal tritium atoms to ethylene. The three straight lines in Figure 1 are calculated with the same slope for each, and with different intercepts each proportional to the mole fraction of CH2=CH2 in that set of experiments. The common slope for each data set corresponds to the assumption that most of the tritium atoms react by the pressure-dependent competition between (3) and (4), i.e. as thermalized tritium atoms. The intercepts proportional to mole fraction are consistent with a reaction mechanism involving CH2=CH2 and tritium atoms still possessing extra kinetic energy from nuclear recoil. One possible mechanism for these hot atom reactions is simply the analogue of (2) with additional excitation energy for the radical
934 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Wang et al.
TABLE 11: Volatile Radioactive Products from Reactions of Tritium Atoms with Thermal Neutron Irradiated Mixtures of 3He, CF,, CHz=CHz, and HzS at Total Pressures of 2500 torr, and with the Mole Fraction of CHZ=CH, Fixed at 0.0060
product yield, % corrected
[H2S1/
[CIH,]
HT
CHT=CHZ
CHZTCH3
CTF,
cza
0.26 0.54 1.06 1.92
16.9 27.4 33.5 40.4
0.90 0.99 0.70 0.47
54.3 44.1 28.4 17.8
0.29 0.36 0.26 0.35
54.7 44.6 28.6 17.8
“Sum of yields of (CHT=CH, action to form CHT=CH,.
+ CH2TCH3)- 0.5% for “hot” re-
from the kinetic energy of the tritium atom, creating a very highly excited CH2TCH2**radical. This extra excitation energy is then postulated to be sufficient to cause decomposition of most of such CH2TCH2**radicals for all of the pressures used in our experiments. The needed additional energy for 90% or more decomposition of the CH2TCH2**radicals is only about 10 kcal/mol and much more than that is readily available. We have estimated from the D/S intercept of 0.006 in Figure 1 that the hot atom contribution to the formation of CHT=CH2 is about 0.27% of the total tritium for a mole fraction of CH,=CH2 of 0.0030 and is linearly proportional for higher mole fractions. If no H/T isotope effect were involved between H and T in (3) and (39, only z/3 of the decomposition product (D) from CH2TCH2*would produce C H T = C H , by (3), with the other causing the loss of T by (3)’-with the latter reaction leaving no measurable molecule to indicate that the addition reaction ever occurred. However, the isotope effect for the decomposition of partially deuterated CH2DCH2*radicals has been calculated to favor the formation of CHD=CHz by a factor of 7.3 to 1.’* The loss of T by (3’) is expected to be so much less likely than H by (3) that most of the CH,TCH2* radicals decompose to leave CHT=CH,, and the actual measurement of the [CHT= CH2]/[CH2TCH3]ratio is probably a good approximation to the total D/S ratio. The lower limit of the pressure for stabilization of half of the CH2TCH,* radicals (PI,,) is given by the slope of the straight lines in Figure 1, which correspond to stabilization and decomposition being equivalent at 22 2 torr. The lifetime of CH2TCH2*radicals in CF4 bath gas is thus about 5 X
*
S.
The abstraction of hydrogen from H2S and the addition to CH2=CH2are two of the reactions competing for thermal tritium atoms through (1) and (2). Other possible thermal reactions can include the exchange reaction of (6) and the abstraction reaction of (7). The thermal abstraction of F from CF4 is exothermic, but its reaction rate is unknown at 295 K. The exchange reaction 6 probably occurs and results in a small diminution in observed thermal yield. Experimentally, the yields from these two unmeasured reactions cannot be more than minor routes because of the high percentage yields found in the measured products. A series of experiments was therefore carried out for the evaluation of k , / k z through observation of the change in the ratio of HT to the sum of CHT=CH, and CH2TCH3as the reactant ratio [H,S]/ [CH,=CH2] is altered. For this calculation, the yields corresponding to thermal T reactions with CH2=CH2 are corrected for hot reactions by subtraction of the appropriate intercept at infinite pressure. No correction has been made for decomposition of CH2TCH2*by (3’), on the assumption that it much less favored than the statistical factor of 1 in 3, as discussed above. The competitive reactions were studied at 0.0060 mole fraction of CH,===CH2, for which the hot correction is about 0.012 times the sum of CHT=CH, and CH,TCH,, or 0.5% in the 2/1 mixtures of Figure 1. In principle, the hot correction for 0.0060 mole fraction of CH2=CH2 should be independent of the H,S concentration because the competition for hot T*atoms is between reaction with CHz=CH2 and deexcitation by collisions with CF4 (18) J. A. Cowfer and J. V. Michael, J . Chem. Phys., 62, 3504 (1975).
0.0 0.0
0.4
0.8
CH2S]
1.2
1.6
2.0
/ [%%I
Figure 2. Competitive yields for tritium atom reactions with H,S (-HT) and C2H4 (-+ CHT=CH, plus CH2TCH3) in CF4-moderated mixtures at 2500 torr total pressure. Mole fraction of C2H4= 0.0060.
Data corrected as in Table 11. TABLE 111: Volatile Radioactive Products from Reactions of Tritium Atoms in Thermal Neutron Irradiated 3He/CHZ=CHz/Kr/HzS Mixtures with CHz=CHz/HzS Ratios of 2.0/1
CH2=CH2 mole fraction
total press., torr
product yield, % HT
0.0030 0.0032 0.0027 0.0029 0.0030 0.0030
571 620 1038 1045 1613 1950
35.8 33.9 35.1 33.4 36.6 35.8
5.69 5.91 4.37 5.28 4.19 3.82
40.3 36.8 41.2 43.4 49.3 45.4
0.0055 0.0050 0.0055 0.0055 0.0055
618 650 973 1632 1944
37.2 34.2 35.3 37.2 35.2
7.77 7.09 6.84 5.38 6.01
46.2 40.9 48.1 55.8 54.6
0.01 14 0.0 100 0.0100
599 1563 2063
34.9 29.8 31.4
10.8 6.36 6.60
46.1 37.0 40.9
CHT=CH,
CH,TCH1
(or Kr). However, the indicated 0.5% correction to the CHT= CH, yield for hot reaction is slightly too large for the last entry in Table 11. It is quite possible that a higher fraction of C2H4T** radicals is found as CH2TCH3when the concentration of H2S is much higher, and we have chosen to make a standard “hot” reaction correction of 0.5% to the summed C2 yields for all of the data in Table 11. We have then graphed the yield ratio of H T to the corrected total C2 yields vs. the [H2S]/ [CH2=CH2] ratio in Figure 2. The slope of the line in Figure provides an estimate that k , / k 2 = 1.17 f 0.04. Krypton Moderated Mixtures of CHz=CH2, H,S, and ,He. The experimental results from a series of samples containing varying mole fractions of CH2=CH, and varying total pressures with Kr as the moderator are summarized in Table 111. The observed tritium-labeled products from these experiments were the expected HT, CHT=CH2, and CH2TCH3from reactions 1-5. The total measured yield approaches 100% at the higher pressures and would not include products such as HST from the exchange reaction 6 with H2S. The pressure dependence of the ratio of [CHT=CH,]/[CH,TCH,] is plotted in Figure 3, which shows behavior for Kr moderator similar to that exhibited in Figure 1 with CF4 as the moderator gas. The linear relationship vs. reciprocal total pressure again indicates that most of the CH2TCH2* radicals possess essentially the same excitation energy, corresponding to the addition of thermal tritium to CH2=CH2 in (2). The pressure for half-stabilization of CH2TCH2*radicals from Figure 3 is 67 f 8 torr in Kr, approximately three times greater than the p l 1 2for CH2TCH2*with CF4. The requirement for 3
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 935
T Reactions with CH2=CH2 and H2S
D/S
CH T I
A’
0.241
0.001
0.0
I
0.2
I
0.4
I
0.6
I
0.8
I
1.0
I
1.2
I
1.4
I
1.6
1
1.8
( I / P ) X lo3 torr-’ Figure 3.
Decomposition/stabilization yield ratio [(CHT=CH,)/
(CH,TCH,)] vs. reciprocal pressure for tritium atom reactions in krypton-moderated mixtures of C2H4 and H2S. Mole ratio of [C2H4]/[H2S] = 2.0. Mole fraction of C2H4: A, 0.0100; B, 0.0055; 0 , 0.0030.
Figure 4. Competitive yields for tritium atom reactions with H2S
TABLE I V Volatile Radioactive Products from Reactions of Tritium Atoms in Thermal Neutron Irradiated 3He/CH2=CH2/Kr/H2S Mixtures with 0.0060 Mole Fraction of CH2=CH2 and Total Pressure of 1800 torr product vield. 7% W2SI / sum [CH2=CH2] H T CHT=CH2 CH2TCH, of C2 corrected‘
for the formation of such “hot” HT. A plausible “hot” H T correction from H2S equivalent to that for hot reaction with CH2=CH2 would lower this estimate of kl/k2 from 1.32 to 1.25. Previous experiments with unmoderated energetic tritium atoms showed the formation of a small hot abstraction yield from CH2=CH2, as in reaction 9 in a ratio of 0.223 f 0.004 vs. the
0.21 0.74 1.02 1.52 2.00
11.9 42.0 50.8 58.7 62.3
5.55 5.17 5.54 4.28 4.53
60.1 41.3 33.0 25.9 21.1
65.7 46.5 38.5 30.2 25.6
63.9 44.7 36.7 28.4 23.8
a The experiments in Table 111 indicate nonnegligible contributions by hot reactions to the total yield of CHT=CH2 and CH2TCH3 when the mole fraction of CH2=CH2 is 0.006. The summed C 2 yields have been reduced by 1.8% in allowance for these contributions from energetic reactions.
times as much krypton as CF, to provide equivalent stabilization for CH2TCH2*radicals is consistent with hypotheses either that much less energy is removed per collision with krypton than with CF, and/or that the effective collision radius for krypton is much smaller than for CF,. Our experiments do not provide any means for distinguishing the relative importance of these two contributions. Calculations of the molecular size for Kr and CF4 suggest that both factors are probably contributing. The data of Table I11 and Figure 3, as with Table I and Figure 1, can also be fitted with linear equations having the same slope for all mole fraction ratios and nonzero intercepts proportional to the mole fraction of CH2=CH2. The contribution from hot processes even at mole fractions as low as 0.0030 is considerably larger (Le. D/S = 0.038) with krypton as moderator, in contrast to the D/S = 0.006 found with CF, moderator. The nonzero intercept in krypton is about 6 times larger than in CF,, consistent with less efficient moderation of the extra kinetic energy of the tritium atom by krypton than by CF,. The relative rate constants, kl/k2, were also studied in the krypton moderator system, with the results summarized in Table IV at a mole fraction of 0.0060 for CH2=CH2. A more substantial correction is required for hot processes with CH2=CH2 in krypton moderator and has been estimated by us to be 1.8% of the total tritium, as applied to the data in Table IV. The corrected data are graphed in Figure 4, from which the slope provides an additional estimate of the relative rate constant ratio, k,/k2 = 1.32 f 0.06. However, the poorer moderator characteristics of krypton are likely t o permit hot reactions by T* with H2S as well as with CH2=CH2, with the consequence that the actual thermal yield of HT is overestimated by failure to correct
(4
HT) and C2H4 (4CHT=CH2 plus CH2TCH3)in krypton-moderated mixtures at 1800 torr total pressure. Mole fraction of C2H4 = 0.0060.
T*
+ CH,=CH,
+
HT
+ CH=CH2
(9)
yield from the hot addition r e a ~ t i 0 n . l ~The yield from such a reaction can be expected to be very small in CF4 moderated experiments but would tend to reduce the krypton moderator estimate of k l / k 2 toward the value found in CF,. Our measurements with CF4 and with Kr moderators are consistent with one another at a rate constant ratio for k1/k2of about 1.20 f 0.04. The measurements with CF4 provide a more accurate estimate of k1/k2,providing that the residual 25% of T atoms, presumed to have formed T F or HST, do not react further in some manner which would interfere with the yield measurements used in the ratio calculation. The moderator experiments with krypton serve to validate the CF, data by confirming that a value of kl/k2 = 1.2 is also observed with a noble gas moderator. The corresponding values for the rate constant ratio measured with H atoms released by the photolysis of H2S have varied from 1.6 to 0.83.I3-l6 The absolute reaction rate constant for the H atom analogue to reaction 1 has been given as 7.3 X cm3 molecule-’ s - ’ . ~ O Measurements of the absolute reaction rate constants for the H atom analogue to (2) have varied widely, with a value of 11.3 X cm3 molecule-’ s-l most representative of the recent res u l t ~ . ~ The ~ - ~ratio ~ of the rate constants (kIH/klH) can be estimated as 0.64 from these two absolute rate constants, in contrast to our observed value of 1.20 for (klT/k2T). Ab initio calculations for D atom addition to CH2=CH26 have indicated (19) J . W. Root, W. Breckenridge, and F. S. Rowland, J . Chem. Phys., 43, 3694 (1965).
(20) M. J. Kurylo, N. C. Peterson, and W. Braun, J . Chem. Phys., 54, 943 (1971). (21) D. Mihelcic, V. Schubert, F. Hoefler, and P. Potzinger, Ber. Bunsenges. Phys. Chem., 79, 1230 (1975). (22) J. H . Lee, J. V. Michael, W. A. Payne, and L. J. Stief, J . Chem. Phys., 68, 1817 (1978). (23) Y. Ishikawa, M. Yamabe, A. Noda, and S. Sato, Bull. Chem. SOC. Jpn., 51, 2488 (1978). (24) J. V. Michael, D. T. Osborne, and G. N. Suess, J . Chem. Phys., 58, 2800 (1973). ( 2 5 ) M. J . Kurylo, N. C. Peterson, and W. Braun, J . Chem. Phys., 53, 2776.(1970). ,-- , (26) S. Nagase, T. Fueno, and K. Morokuma, J . Am. Chem. Soc., 101, 5849 (1979).
936
J. Phys. Chem. 1986, 90, 936-941
*
that addition of D should be about 1.3 times slower than for H atom addition at 295 K and have been confirmed by experimental observations.21 In contrast, the rate constants for H abstraction from H2Sby H and D have been measured to be about the same.27 Application of these isotopic observations to an estimate of k 1 / k 2 for D atoms should increase the H atom ratio by a factor of 1.3, i.e. from 0.64 to about 0.84. Isotopic correction for T atoms should increase this ratio even more, putting the estimate in reasonable
agreement with the value of k l / k 2= 1.20 0.04 obtained from our experiments. The most important general conclusion to be drawn from the plausibility of the measured value of k l / k i nour experiments is that the technique of using CF, to moderate energetic tritium atoms provides a very useful alternative approach for study of the thermal reactions of atomic hydrogen.
(27) D. Mihelcic and R. N. Schindler, Ber. Bunsenges. Phys. Chem., 74, 1280 (1 970).
Registry No. T, 15086-10-9; C2H,, 74-85-1; H2S, 7783-06-4; Kr, 7439-90-9; CF,, 75-73-0.
Acknowledgment. This research has been supported by Department of Energy Contract No. DE-AT03-76ER-70126.
-
Absolute Rate Coefficients for Methyl Radical Reactions by Laser Photolysis, HX CD,H 4- X (X = Br, I) Time-Resolved Infrared Chemiluminescence: CD,
+
D. J. Donaldson and Stephen R. Leone* Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: August 19, 1985)
-
Absolute rate coefficients are reported for the room temperature reactions of deuterated methyl radicals with HI and HBr, CD3 + HI (HBr) CD3H + I (Br). Excimer laser photolysis of CD3I is used to generate methyl radicals, and time-resolved infrared chemiluminescencefrom the CH stretch of the CD3H products is detected to follow the time evolution of the reaction. The rate constants obtained in this manner are (7.7 0.7) X IO-’2 cm3 molecule-’ s-l for CD3 + HI and (4.7 f 0.4) X cm3 molecule-’ s-I for CD, + HBr. These rate constants are considerably greater than earlier, indirectly measured values and indicate that the activation energy for these light-atom transfer reactions is lower than previously believed.
*
Introduction In order to gain a complete understanding of combustion systems, both kinetic data and a knowledge of the energy disposal in reactive and inelastic processes are required. Due to their simplicity and importance in many combustion systems, the reactions of methyl radicals have been extensively studied and a vast body of kinetic data has been established.’+2 In spite of this wealth of information, considerable discrepancies still exist in the literature. One reason for this may lie in the fact that indirect methods have been used to make many of the kinetic measurements. Rate parameters have been extracted from ratios of rate constants, either with reference to some “standard” reaction (e.g. 2CH3 C2&) or as one term in an equilibrium expression. The extraction of the various rate constants is often made more difficult by the fact that there are several simultaneous reactions occurring in the system under study. The relative simplicity of methyl radicals and their reactions makes them useful as testing grounds for various kinetic theories. The relative reactivities of C H 3 vs. CF, radicals have been extensively explored in order to investigate the effects of highly electronegative groups on reactivity.2 Several studies have been aimed at the rate differences between reactions of CH3 and CD3 in an effort to understand better the secondary kinetic isotope In many cases this effect leads to an enhancement of the CD, reaction rate over that of CH,.6.7 The kinetics of the H-atom abstraction reactions of methyl radicals with H X has been thoroughly CH, + HX (X = CI, Br, I) CH4 X Much of the interest in these particular reactions is motivated by the need for thermodynamic properties of the methyl radical ( e g ref 12, 16, and 17). These properties (heats of formation, en-
-
-
+
* Staff member, Quantum Physics Division, National Bureau of Standards, Boulder. CO 80309. 0022-3654/86/2090-0936$01.50/0 , I
!
tropies, etc.) can often be obtained for free radicals from kinetic measurements (e.g. ref 21-23), especially on reactions of the type: R + HX R H + X. An accurate determination of the activation energy and preexponential factor is used in conjunction with known bond strengths or heats of formation to derive the unknown quantities of interest. In studies of this kind the kinetics are
-
~~~
~
(1) “Handbook of Bimolecular and Termolecular Gas Reactions”, Vol. 1, Kerr, J. A., Moss, S. J., Eds.; CRC Press: Boca Raton, 1981. (2) Kerr, J. A. In “Comprehensive Chemical Kinetics”, Vol. 18, Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1976. (3) Arthur, N. L.; Donchi, K. F.; McDonnel, J. A. J . Chem. Soc., Faraday Tram. J 1975, 71, 1407, and references therein. (4) Schatz, G. C.; Wagner, A. F.; Dunning, Th. H., Jr. J . Phys. Chem. 1984, 88, 221. ( 5 ) Ting, C-T.; Weston, R. E., Jr. J . Phys. Chem. 1973, 77, 2257. (6) Canadell, E.; Olivella, S.; Poblet, J. M. J . Phys. Chem. 1984,88, 3545. (7) Canadell, E.; Poblet, J. M.; Olivella, S. J. Phys. Chem. 1983, 87, 424. (8) Williams, R. R., Jr.; Ogg, R. A., Jr. J . Chem. Phys. 1947, 15, 696. (9) Cvetanovic, R. J.; Steacie, E. W. R. Can. J . Chem. 1953, 31, 158. (10) Eckling, R.; Goldfinger, P.; Huybrechts, G.;Martens, G.; Meyers, L.; Smoes, S. Chem. Ber. 1960, 93, 3014. (1 1) (a) Pohjonen, M-L.; Koskikallio, J. Acta. Chem. Scand., Sect. A 1979, 33, 449. (b) Pohjonen, M-L. Acta Chem. Scand., Sect. A 1980, 34, 597. (12) (a) Baghal-Vayjooee, M. H.; Colussi, A. J.; Benson, S. W. Inr. J . Chem. Kinet. 1979, 11, 147. (b) Heneghan, S. P.; Knoot, P. A,; Benson, S. W. Int. J . Chem. Kinet. 1981, 13, 677. (13) Andersen, H. C.; Kistiakowsky, G. B. J . Chem. Phys. 1943, 11, 6. (14) Kistiakowsky, G. B.; Van Artsdalen, E. R. J . Chem. Phys. 1944, 12, 469. (15) Fettes, G. C.; Trotman-Dickenson, A. F. J . Chem. SOC.1961, 3077. (16) Batt, L.; Cruickshank, F. R. J . Phys: Chem. 1967, 71, 1836. (17) Gac, N. A.; Golden, D. M.; Benson, S.W. J . Am. Chem. SOC.1969, 91, 3091. (18) Benson, S . W.; O’Neal, E. J . Chem. Phys. 1961, 34, 5 14. (19) O’Neal, E.; Benson, S. W. J . Chem. Phys. 1962, 36, 2196. (20) Flowers, M. C.; Benson, S . W., J . Chem. Phys. 1963, 38, 882. (21) Islam, T. S . A,; Benson, S. W. I n t . J . Chem. Kinet. 1984, 16, 995. (22) Rossi, M. J.; Golden, D. M. Int. J . Chem. Kinet. 1983, 15, 1283. (23) Rossi, M. J.; Golden, D. M. J . Am. Chem. SOC.1979, ZOI, 1230.
0 1986 American Chemical Society
