J . Phys. Chem. 1984,88, 2076-2082
2076
mation in Ps chemistry. Besides these fundamental aspects, a better knowledge of the Ps quenching reactions appears as a prerequisite for a deeper understanding of the processes observed when Ps is used as an analytical probe in various chemical systems. On this point, this paper also shows the usefulness of the positron annihilation techniques to investigate the behavior of paramagnetic species in solutions. Thus, the very strong influence of the solvent, from methanol or HMPT to water, would indicate some important change in the electron distribution on the NO group of the nitroxyls very likely due to the implication of hydrogen bonds. The change in the spin-conversion rate constant with HTMPO in methanol at about 263 K probably arises from the correlated decreases of the collision time and of the viscosity with increasing temperature. Further experiments on the viscosity effects would be rewarding to better assess the mechanism of the Ps spin interactions with the radicals.
+ (a2 - p2)1/2
(A31
The intensities ZI and Z3, correlated to X1 and X3, respectively, measured in PAL may be expressed as A111 = AX3'
+ DX1' + k',,C(A + D)X~"/(X2' -
Xi)
+
k'wmpC(A + D)Xc/(Xc - X i ) (A41 X3Z3
= BX3O
+ Ehl' + k6,C(B + E)X2"/(X2O kkrnpC(B
- A,) + + E)Xc/(Xc - X 3 )
(A51
where
Appendix The following equations can be d e r i ~ e don ~ ,the ~ basis of reactions 1-111 plus the annihilation reactions. PAL Parameters. Besides and h,, the e,+ and Ps complex decay rate constants, respectively, the mathematical treatment shows that the PAL spectra will include two decay rate constant, X1 and X 3 , referring to the shortest and longest lifetimes, respectively, and identified with XIo and X 3 0 in the absence of solute: 2x1 = a
+ k6,C (or k',ompC)
X3 = X3'
A = Z3C0'(h3- X3'
- k 6 , C - k&,,C)/(X,
- Xi)
(A6)
B
- XI
+ k6,C + k~0mpC9/(X3- Xi)
(A7)
D = Z1'Or(X3 - Xi'
- k 6 , C - k6rnpC)/(h3- XI)
(A8)
- XI
+ k6,C + kLmpC)/(X3- Xi)
(A9)
Z3c0r(X30
E = Z1w'(X1O
For pure oxidation, one has2 ,3cor
= Z3/[1
- k6xc/(xZo - X 3 O ) l
(A101
DBARL Parameters. The intensities associated with p-Ps and 0-Ps measured in DBARL arise from photons effectively emitted by these particles:
ZiD = XI0(D/X1 + E/X3)
('41)
ZjD
+ A3;p2 = 4hlh3 - 3(k',,,,C)2 and AI = A,' + 3k$& + k',,C + k6,C A3 = 13' + k',,,,C + k\,C + k:,,,C
with a = A i
X3'(A/hl
(A1 1)
+ B/X3)
For the Ps complex
:1 = kLmpC[(A+ D ) / h 1
+ (B + E)/h3]
(A13)
Registry No. e+, 12585-85-2; Ps, 12585-87-4; HTMPO, 2226-96-2; TMOPO, 2896-70-0;TMCPO, 2154-68-9;HMPT, 680-31-9;water, 7732-18-5;methanol, 67-56-1.
In the case of pure oxidation (or complex formation), eq A2 reduces to
Reactions of Energetic Carbon-I 1 in Homologous Gas-Phase Halocarbons Richard M. Lambrecht* and Alfred P. Wolf Chemistry Department, Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: June 6, 1983)
The reactions of energetic "C atoms were investigated in 34 halocarbons containing O2scavenger. "CO yields were enhanced and (I1)CzH4yields were dramatically reduced by the presence of a halogen in the reaction mixture. (I1)C2H4yields, like (I1)C2H2 yields, are seemingly influenced by the electron density in the collision complex. Product yields and a kinetic model add further evidence to the mechanism of formation of (11)C2H4via the insertion of energetic "CH into C-H bonds in a collision complex, followed by the decomposition of the complex to products. F, C1, Br, and I appear to divert lIC('D) from forming methyne by halomethyne formation and/or spin conversion of "C('D) to 11C(3P).The correspondingchloroethylene which should arise from chloromethyne insertion into CH3C1or a vinyl halide radical precursor was found in only 0.3% yield.
Introduction Recent attempts to unravel the mechanisms of the reactions of energetic carbon-1 1 produced by nuclear transformations have focused on assessing the effects of excess translational and electronic excitation of the " C atom and the influence of electron density and bond polarization effects on the subsequent chemisEnergetic "C atoms carry a large excess translational (1) See for example: (a) R. A. Ferrieri, A. P. Wolf, and Y.-N. Tang, J . Am. Chem. SOC.,105, 5428 (1983); (b) R. A. Ferrieri, A. P. Wolf, D. A. Baltuskonis, and Y.-N. Tang, Chem. Commun., 1321 (1982); (c) R. M. Lambrecht and A. P. Wolf, J . Phys. Chem., 88, 720 (1984).
0022-3654/84/2088-2076$01.50/0
energy, and secondary processes such as stripping, dissociation, and bimolecular reactions shadow the primary reactions. The chemistry is further complicated since the carbon atom is known to exist in the 3Pground state and the ID and IS excited states with 1.7- and 2.3-eV energies, respectively. Energetic "C is (2) See for example: (a) K. K. Taylor, H. J. Ache, and A. P. Wolf, J . Am. Chem. Soc., 98,7176 (1976);(b) K.K.Taylor, H. J. Ache, and A. P. Wolf, ibid., 97, 5970 (1975);(c) K.K.Taylor, H. J. Ache, and A. P. Wolf, Radiochim. Acta, 22, 148 (1975); (d) A. F. Voigt, G. F. Palino, and R. w. Williams, J. Phys. Chem., 75,2248 (1971);(e) T.L.Rose and C. F. MacKay, J . Am. Chem. SOC.,99,4917 (1977).
0 1984 American Chemical Society
Reactions of Energetic Carbon-1 1
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2077
TABLE I: Principal "C Products as Percent of Volatile Activity in CH,X4.., + 4.5% 0,, where X = H,F, C1, Br, and I, Reaction Systems at 760 m m H g C yields
substrate
av devb
co c2
"2
t0.5
C2H4
c1.5
',HZ
*1
3.7
24.9
29.9
14.0 21.6 25.8 27.0
6.1 1.4 Br > C1> F > H, which is also the order of increasing (")CzH4 yields. Ab initio calculationsz2predicted that ethylene arises primarily from energetic C(lD) through the intermediacy of CH(%) and that the primary reactions of C(lS) do not yield bound products. The abstraction reactions of 11C(3P) and "C('D) could yield "C-methyne, but a significant portion of the "CH arising from C(3P) would be as C(,2-), which was predicted not to undergo insertion reactions to yield ethylene.23 The (ll)C2H2yields in the methyl halides do not vary in a manner directly consistent with the bond energy relationships.Ic Steric parameters do not appear to be a controlling factor. The substituent must be affecting the attack step of the electrophilic (22) T. J. Blint and M. D. Newton, Chem. Phys. Lett., 32, 178 (1975). (23) Husain and Kirsch (D. Husain and L. J. Kirsch, Trans. Faraday Soc., 67,2886 (1971)) found xenon to be a very efficient spin converter (ID ' P ) for near thermal carbon atoms. Finn et al. noted that the (11)C2H4 yields in xenon-moderated alkanes were reduced more so than predicted solely by a kinetic energy-transfer effect: R. D. Finn, H. J. Ache, and A. P. Wolf, Radiochim. Acta, 17, 131 (1972). Subsequently, Taylor et aL3&studied noble-gas-moderated admixtures to unravel the electronic states of nucleogenic carbon atoms undergoing reaction in oxygen-scavenged ethane.
-
"C on the substrate. The decrease in (l1)C2H2yield with increasing halogen substitution is expected since (")CZHzyields are known to vary with the statistical availability of hydrogen.I6 Influence of C1 Substitution Additives. Table I1 presents the yields of "C products obtained in CH4, C&, CH3Cl, CzH,CI, and C,-trisubstituted ethyl halides containing 0, scavenger or additives. The I'C product most influenced by the presence of C1 was (ll)C2H4. The relative reduction was independent of whether C1 was present in the substrate or as CC14 additive. In CHI or CzH6 + 4.5% Oz + 14% CC14 the (ll)C2H4yield was reduced to 6.8% and 10.0%, respectively, as compared to 24.9% and 18.5% in CH4 and CzH6, respectively. Increasing C1 substitution on C, of ethane resulted in a reduction of (")CzH4 in CzH6from 18.5% to 4.7%, to 1.8%,to 0% detected product, respectively. Target substrates containing 4.5% 0, and CH3CF2Cland CH3CCl2Hgave the same yields (1 3%and 1.6%) within experimental error. The yield of (")C2H4 is generally regardedI6 to increase with the presence of methyl groups in a substrate. The presence of one to three atoms on the carbon atom adjacent to the methyl group was sufficient to drastically reduce the (11)C2H4yield. The long-range effect of C1 on the carbon adjacent to the methyl group lends further support to electron density on bond polarization effects controlling the decomposition of the collision complex leading to product. The result of the chloroethanes argues against "C-ethylene production solely via "C insertion into a methyl group. The (")CZHzyields in CH,Cl 4.5% 0, were reduced by an additive of 25% Ne or NO from 27.3% to 19.9% and 18.4%, respectively, whereas the (")CzH4 yields were reduced from 4.0% to 1.1% and 1.5%, respectively. Neither 0-50% CH3C1nor 14% CCI, in CH,-containing O2 scavenger had an appreciable effect on (I')C2H2yields. The (11)C2H2yield was not affected by monochloro substitution but was reduced by a factor of 2 with dior trichloro or difluorochloro substitution. The "CO yields are higher when C1 was present but roughly independent of the number of C1 substituents. This suggests a more important role of spin conversion of "C('D) to 11C(3P)than of the abstraction of C1 to yield "CC1 with its subsequent oxidation to "CO. The "COZ yields were not appreciably different except for fluoro substitution. Polyhalogenated Hydrocarbons. Table I11 compares the "CO and "C02 yields in nine polyhalogenated methanes. Elevated "CO, yields seem to be associated with the presence of fluoro substitution more so than with the other halogens. The ' T O z yield from CH3F was comparable to that from HC1,CF. The "CO, yield was enhanced from 26% in CF4 to 5 1% in ClCF,, but only to 31% in ICF,. The "CO, yield in CCI,H, CCI,Br, and CC14 was only 3.6%, 3%, and 6%, respectively. Substitution of H, F; H, CI; or F, F on >CF2 did not alter the "CO and "COZ yields of 75% and 25%, respectively. Data in Tables I and I11 support the argument that if "CC1 or "CBr intermediates are formed, they are preferentially oxidized to "CO. The higher yields of "CO in BrCCl, than in CC14 lend support to the likely role
+
Lambrecht and Wolf
The Journal of Physical Chemistry, Vol. 88. No. 10, 1984
2080
, PERFLUOROHYDROCARBONS
+
4.5 % O2
J
i 0
p
1
45-
I .$
i
35-
30
0
-
20
10
0
30
40
VOLUME % O2
3.O
2.0
\ .O
ZF L C
4.0
atoms atoms
Figure 4. Structure-activity relationship of "C hot products formed in
perfluorocarbons. of spin-conversion processes. Evidence has been advanced& that "CF intermediates are preferentially oxidized to 11C02.Seven C1-C7 perfluorobenzeneand three protonated fluorobenzeneswere irradiated in the presence of 4.5%02.The yield of hot products (100% "CO) in the perfluorohydrocarbons is predicted by the ratio of the number of fluorine to carbon atoms present in the substrate (Figure 4). Competitive Reaction in Binary CHJ/OT Systems. Oxygen was chosen for the competitive gas-phase studies, since it has been shown to be an efficient radical and thermal carbon atom scavengerlZa and gives rise to "CO as the principal nonthermal competition product. Figure 5 depicts the yields of "CO, "C02, (")C2H2, and (]')C2H4in CH3X (X = F, C1, Br) vs. the volume percent of O2 The O2dependence of "CO, (ll)CzH4,and (l1)CZH2 yields in CH4 described by Stocklin16 was confirmed. The dependence of "COZ formation on the C H 4 / 0 2ratio was found to be 3-4% over the 4 5 5 0 %Ozconcentration range. Examination of Figure 4 indicates that the O2dependence of the product yields is linear, which is taken to mean that only one reaction path seems to be predominant for each product. The (I1)C2H2and (11)C2H4 yields decrease with increasing O2concentration. Conversely, the 'IC0 yields increase with a decrease in the CH3X/02ratio. The COz yields for CH,, CH,Cl, CH3C1, and CH3Br are similar (-3-4%), whereas the "C02 yield in CH3Fwas unchanged from about 14% over the oxygen range studied. The near constancy of the "COz yields suggests its formation is principally by thermal processes. Discussion The kinetic model employed to evaluate the relative reactivity cross sections for the competitive gas-phase reaction of energetic llC with methyl halide/02 systems is an extension of the model describedI6 in eq 4-6 and is consistent with pressure dependence studies of "C recoil reactions in alkanesz1 (eq 4). The energetic ["CI*
+
RH
-
["CRHI*
1
-
[carbene intermediates1
-
"C illlc
"C
H 2,
(thermalized)
+
10
0
IO
20
30
40
50
(4) [RH]*
"C interacts with the substrate to form a collision complex. The
60
70
80
90
VOLUME Y. O2
70
t!-
I
I
I
I
CH,Er l - 1
-zal1 : 40
1
30
10 0
0
10
20 VOLUME %
hot products
( I l ) c 2 H 4 , etc.
-1
2j\.
30
40
50
O2
Figure 5. Yields of principal volatile "C products in binary CH3X/02
systems. collision complex either stabilizes and forms carbene intermediates or dissociates, forming thermalized carbon atoms and excited substrate molecules. The excited carbene, once formed, must be structurally and energetically capable of fragmentation to form
Reactions of Energetic Carbon-1 1
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2081
products such as (")CzH2and (11)C2H4.Thermalized carbon atoms "C(3P) are rapidly scavenged by O2 and principally yield l 1 C 0 (eq 5). Increases in the thermalized l ' C 0 yield may also arise
- -
"C (thermalized)
+ 0,
ki
"CO
+ -0.
(5) from the oxidation of C-X intermediates or other processes. "CF intermediates are thought5 to be oxidized to lLCO, (eq 6). The ['IC]'
+0 2
["c02]
kz
"coho, + -0.
(6)
"CO, yield in all systems studied was roughly independent of the 0, concentration at low radiation dose. However, if carbon-I 1 interacts with oxygen while translationally energetic, it may lead to "CO, as the principal hot product, with k3/(kl + k,) 0.05 and the absolute limit of 51.5% for the energetic "C reaction with O2 to yield "CO, (see supplementary material). The absolute rate constant for the C H + O2 reaction was deducedz4as (3.3 + 0.4) X 10" ~m~.molecule-~-s-', which can be compared to the rate constants of (30 f 10) X 10" ~m~.molecule-l.s-~ for the C H CHI and (2.3 f 0.5) X 10" ~m~.molecule-'.s-~ for the C H Hz reactions.25 Methyne reactions with 0, yieldz5 C O and CO,. The thermal contribution to the "CO yield is determined by extrapolation though the data points to 0% Oz. The observed increases with 0, concentration are designated as 'COhot. The reasonable assumptions inherent in the evaluation of the relative reactivity cross sections are as follows. (i) The acetylene yield is dependent on the concentration of energetic "C as "C('D), and 11C(3P)may interact with substrate to form a collision complex and result in carbene formation. Blint and Newtonz2predicted that C( 'S) leads to dissociation without stable-product formation. The intermediates may be formed by C-H insertion. High-energy stripping reactions leading to (ll)C, and (")C2H and subsequent abstraction reactions that result in (li)C2H2 by intramolecular mechanisms are estimated as
