Oct. 5, 1964
MERCURY-PHOTOSENSITIZED DECOMPOSITION
orientations are not related b y hexagonal (or trigonal) symmetry operations and therefore the crystals are not hexagonal urea inclusion compounds. Sebacic acidurea crystals, on the other hand, are hexagonal and the e.s.r. spectra indicate t h a t the sebacic acid radical is undergoing large amplitude oscillations. l 9 This motion removes the a-proton anisotropy in a plane perpendicular to the hexagonal needle axis, and produces an easily recognizable isotropic spectrum in this plane. Similar results are obtained for 1,12-dodecanedioic acid, and crystals of sebacic acid-urea and 1,12-dodecanedioic acid-urea are apparently urea inclusion compounds. Therefore, there is a transition from hexagonal urea inclusion crystals to crystals of lower symmetry as the saturated dicarboxylic acid chain length is shortened.26 From the e.s.r. data i t is evident t h a t ( 2 6 ) This transition was first noted i n t h e series of acids: succinic acid, adipic acid, a n d sebacic acid by Schlenk" from X - r a y powder diffraction
[ CONTRIBUTIOX FROM
THE
394 1
some of the short-chain length acid molecules have similar orientations in the nonhexagonal crystals. In particular i t is interesting to note t h a t the orientations of the acid radicals are the same in the two crystals, succinic acid-urea and fumaric acid-urea The positions of the urea molecules cannot be, determined from e.s.r. data since urea does not produce stable paramagnetic damage sites a t mom temperature. Acknowledgments.-\Ye wish to thank Professor J. Holmes Sturdivant for his advice and help concerning this work, especially in relation to the Laue X-ray photography. We are also greatly indebted to Professor Harden hf. hlcconnell for the generous use of his laboratory facilities. d a t a . Of course, neither t h e powder diffraction d a t a nor t h e single crystal e.s.r. d a t a are a substitute for a (single crystal) X - r a y crystallographic investigation. However, e.s.r. does provide a method f o r investigating t h e s t r u c t u r e , positions, a n d molecular motion of t h e X-ray produced free radicals in t h e crystals.
RADIATION LABORATORY, COXTINEXTAL OIL
COMPAKY, POXCA CITY, OKLAHOMA]
Mercury-Photosensitized Decomposition and Collision Broadening of Resonance Line BY KANGYANG RECEIVEDMARCH 25. 1964 A low-pressure mercury lamp commonly employed in photochemical investigations emits 2537 resonance radiation with a Doppler half-breadth 4.35 times t h a t of the absorption line. This mismatch in the two line shapes drastically reduces light absorption in spite of the high absorption coefficient; thus, a 4.8-cm. long cell with mercury a t 25" absorbs only 5670 of the incident light. T h e addition of foreign gases sharply increases the light absorption, because the absorption line becomes collision broadened. From the dependence of the light absorption on foreign gas pressures, the Lorentz half-breadths for various gases are determined under experimental conditions commonly used in photosensitized decompositions. Experimental broadening cross sections estimated from the Lorentz half-breadths agree well with theoretical ones if it is assumed t h a t the line broadening occurs by the formation of a loosely bound activated complex. T h e quantum yield of hydrogen in the photosensitized decompositions of ethane, propane, n-butane, isobutane, and neopentane increases with bp-l even after making appropriate corrections for the line broadening. increasing pressure; 4-l = # m - l The high pressure limit of quantum yield, 4m, is found to be approximately unity for all paraffins. I n the decomposition of ethylene, the correction due t o the line broadening does not alter the already known rate equation, @-'I2 = 40-'/~ b ' p . This correction, however, significantly increases the estimated lifetime of excited ethylene. The only important primary process in ethylene decomposition is the formation of molecular hydrogen, because the low pressure limit of the quantum yield, +o,is found to be unity.
+
+
Introduction light by ground-state merT h e absorption of 2337 cury atoms results in the following transition Hg('So)
+ hu +H g ( 3 P ~ )
(1)
Resulting excited atoms have been extensively employed t o photosensitize various gas phase reactions. ' Elucidation of the mechanism of these reactions often demands detailed information on the dependence of the absorption coefficient, k ( u ) on various experimental parameters. Among them, the pressure of foreign gases is of prime importance. Most gases employed in the photosensitized reactions do not absorb a t 2537 A. ; nevertheless, they profoundly affect k ( u ) , because k ~. i ~ becomes ) pressure broadened.2 Though itsAimportance has been repeatedly emphathe variation of k ( v ) with foreign gas pressures is usually neglected in mechanism discussions. An ap~
(1) For example, see ( a ) W. A. Noyes, Jr., a n d P. A. Leighton, "The Photochemistry of Gases," Reinhold Publishing Corp., New York, N. Y . , 1941; (b) E. W. R. Steacie, "Atomic and Free Radical Reactions," Reinhold Publishing Corp., New York, N. Y., 1954. ( 2 ) For example, see A. C. G. Mitchell and M . W. Zemansky, "Resonance Radiation and Excited Atoms," T h e Cambridge University Press, Cambridge, 1961.
parent justification seems to be that, due to high K ( Y ) , the light is completely absorbed ; and any variation in the total amount of light absorbed is governed by the resonance lamp itself and not by possible changes in k ( u), This is true, provided one considers onlv a sinple frequency, Y O , a t which k ( u ) is maximum. ForhxamGe, the fraction of light passing unabsorbed through a 4.8cm. long cell with mercury reservoir a t 25' can be estimated to be only exp(-38.6). More properly, however, the whole frequency range contributing to the resonance transition should be considered. Let E(u) be the frequency distribution of the emission line; then the transmitted light between u and u dv a t a path length of 1 cm. is E i u ) exp(-k(u)l); hence the absorption, A , becomes
+
A = l -
transmitted light incident light
KANGYANG
3942
Thus, -4critically depends on how well the emission line matches the absorption line (a plot of k ( u ) v s . u ) . Usually the emission line is much broader than the absorption line, and I-1 is significantly less than unity. A preliminary experimental investigation indicated that I-1 N 0.56 instead of 1 - exp(-38.6) in the above example. The error in -1arising from the neglect of the line broadening could thus amount to 79% in this case. \\-hen the temperature of the mercury reservoir is 25" or less and foreign gas pressure does not exceed a few atmospheres, the most important process affecting k ( u ) is the Lorentz collision b r ~ a d e n i n g . " ~The absorption line in this case is expressible in terms of Lorentz half-breadth AVL,which increases with increasing foreign gas pressure. The present paper reports these ~ V values L for various foreign gases determined under conditions commonly encountered in photosensitized decompositions. Still another object of the present paper is to reinvestigate the photosensitized decomposition of gaseous paraffins and ethylene with appropriate corrections for the line broadening. T h e results for the pressure dependence of hydrogen yield indicate that this correction does not alter qualitative conclusions about the mechanism, but the estimated rate constants change significantly. Experimental A . Material.-All hydrocarbons were Phillips research grade. The ethane contained ethylene as the major impurity. This was removed as follows. A H*SOa-P206 mixture was degassed b y sweeping with Linde helium. The helium employed here as well as in other degassings was freed from possible impurities, such as oxygen, by passing through a spiral molecular sieve column maintained at liquid nitrogen temperature. Prior to use, all sieve columns were pumped for 12 hr. a t 400". The ethane was bubbled through the degassed H2S04-P205 mixture very slowly. X KOH trap and a molecular sieve trap, both a t room temperature, then followed. T h e resulting ethane was swept through with helium for 45 min., during which the ethane was kept in liquid phase by occasional dipping in liquid nitrogen. Thorough degassing by repeated evacuation followed. S o attempt was made t o determine olefin content of the other paraffins; but t o make sure, all the paraffins were subjected t o the same treatment as the ethane. The ethylene was degassed by repeated evacuation and subjected t o one stage trap-to-trap distillation, retaining the middle third. Matheson's assayed reagent rare gases and hydrogen were used without purification. B. Determination of the Absorption.--An absorption cell was made by fusing two optically flat quartz windows of 5-cm. diameters a t both ends of a 4.8-cm. long Yycor cylinder. The outlet t o the mercury reservoir had a large diameter of 2 c m . , which ensured a rapid equilibration of mercury vapor. The light source was a Hanovia low-pressure mercuryolamp (94.4-1) made of Vycor, which does not transmit a t 1849 A . This linear lamp had an over-all length of about 47 c m . , b u t only the middle 5 cm. were employed in the experiments. The operating condition was very mild (575 v . and 0.120 a m p . ) . Though n o precaution was taken to cool it, the effective emitting zone had a wall temperature of 3 2 " . The intensity of the light passing through the cell was first reduced 130 times by inserting two layers of nickel screen. The reduced light then passed a round opening of 1-cm. diameter and was projected on the phototube inlet of a Beckman D U spectrophotometer. Precautions were taken t o avoid measuring scattered light.4 The resonance lamp also emits light not involved in the resonance transition. T o estimate necessary corrections, a few experiments were carried out in which the same cell was placed in the spectrophotometer sample compartment while the same lamp occupied the light source. The wave length selector was positioned a t 2537 Is., and the absorption experiments were repeated. ~
~~
(3) For a recent review, see S . Chen and ?VI. Takeo, Rev. M o d . P h y s . , 29,
20 ( 1 9 5 7 ) . 14) h f . W. Zemansky, P h y s . Rev., 36, 8 1 8 ( 1 8 3 0 ) .
Vol. 86
Transmission, T ' , measured with all wave lengths is
T' =
ci ~
+A
Vo 4- A where UOand U Ldenote the resonance light a t a cell length of 0 and Z(4.8 cm.) and A indicates the light not involved in the transition; hence, .4as defined in ( 2 ) is
The A determined with 2537 A. in the presence of 600 mm. propane was 0.978. Corresponding T' was 0.175, hence ( 1 A/L'o) = 1.185. This factor was used throughout the experiments. Table I summarizes a typical result on the determination of A .
+
TABLE I ABSORPTIOX AT DIFFEREXT ETHYLENE PRESSURES A = 1 - (transmitted light/incident light) Pressure, mm.
A
0 10 20 30 50 70 100 150
0 0 0 0 0 0 0 0 0 0 0 0
200
300 400
600
556 752 806 856 914 939 957 968 971 974 979 980
C. Photosensitized Decomposition.-The photosensitized decomposition of ethylene was carried out simultaneously with the determination of A . This procedure was not convenient in paraffin decompositions where it was necessary to carry out the photosensitized decompositions at low intensity and low conversion. Here, a larger Vycor cell (140-cc. cyclinder, 5-Cm. diameter) was employed with the reasonable assumption t h a t the dependence of A on paraffin pressures was the same in both cells. Hydrogen, the only product analyzed, was determined gas chromatographically with a homemade unit. Some precautions were needed to improve sensitivity and reproducibility. A thermistor in the sensing side of the detector was covered with a layer of nickel screen t o reduce background noise. Two separate argon tanks were provided, one for the sensing side and the other for the reference side. This precaution together with the use of a large surge tank reduced pressure disturbances arising from sample injections. The signal from the thermistor was fed to a recorder through a d.c. stabilized amplifier. All power lines were controlled. Other details have been described b e f ~ r e . Photon ~ input rate was determined by assuming that the high-pressure limit of hydrogen quantum yield in propane decomposition was unity. All experiments were carried out a t 25 i. 1O.
Results and Discussion discuss the A. Lorentz Collision Broadening.-To pressure dependence of A in terms of line broadening, i t is necessary to know the explicit expressions for emission and absorption line shapes. The former is taken to be a Doppler line
where C is a constant and
cz = A U E A U D
Here, A v , is the Doppler half-breadth of the absorption line and A u E denotes the half-breadth of the emission ( 5 ) K Yang a n d P. L. G a n t , J. P h y s Chem
6 5 , 1861 (1961)
Oct. 5 , 1964
”
3943
MERCURY-PHOTOSENSITIZED DECOMPOSITION
line. T h e absorption line shape is usually derived b y supposing t h a t Lorentz collision broadening superimposes on natural damping and Doppler T h e absorption coefficient then takes the following form
2.8
2.4
where k ( v 0 ) is the absorption coefficient a t the maximum absorption and
+
AVN AVL dlx AVD
a =
(6)
Here, AvS denotes natural damping half-breadth. Within t h e present pressure range, A V L>> A U K ; hence the latter is neglected when the collision broadening occurs. From (2), (3))and (3)
/c2n4
-
2 0-
1.8 1.8-
0.8
-
A = l I
P (mm)
Fig. 1.-Dependence
(7) As shown in Appendix I , a is determined b y Zemansky’s method4 and found to be 4.35. In the present experiment, k(uoj2 = 38.6. With these data, (7) is numerically integrated by using an IBM 7090 computer. T h e resulting A values at various a are summarized in Table 11. Using these data, a (hence A V L ABSORPTIOS AT
of Lorentz half-breadth on foreign gas pressures.
the origin which has been attributed to the neglect of line asymmetry. * Table I11 summarizes ( T L for ~ various foreign gases deTABLE I11 COLLISION BROADENING CROSSSECTIOX’S FOR VARIOUS FOREIGN GASES 0 2 , A.2
TABLE I1 DIFFERENT L O R E X THALF-BREADTHS Z
37 54 108 135 37 127 194 209 142 223
k(vo)l = 38.6; a = 4.35 (transmitted light/incident light)
a = AvL-/AvD;
A
=
1
-
a
A
a
A
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
0.4868 0 5821 0.6216 0,6578 0.6891 0.7159 0 ,7390 0.7590 0.7764 0,7918
0.50 1.00 1,50 2.00 2.50 3.00 3 50 4.00 4.50 5 00
0.8055 0.8864 0.9219 0.9403 0.9504 0.9558 0.9580 0,9578 0.9556 0.9518
also) can be estimated from experimentally determined A . 4 This procedure is found to be unreliable when a foreign gas pressure exceeds about 100 mm. where small ‘-1 accompanies too much Aa. In Fig. I , some a values are plotted against foreign gas pressures. T h e linear increase is consistent with the Lorentz theory,2 which demands that AVL =
1 -
[no. of collisions~sec./atorn]
7r
1
= -
(8)
gL2n[87rRT,!p]’”
x
where UL? is the broadening cross section, p signifies reduced mass, and n is the number of foreign gas molecules per cc. T h e lines, however, do not pass through (6) V . Weisskorf, Z . P h y s i k , ‘76, 287 (1932)
termined from the slope of straight lines, such as shown in Fig. 1. Using an ideal resonance lamp emitting a sharp line with A v E / A v ~= 1.21, Zemansky4 reported t h a t u ~ * ( H 2 )= 24.5 A . 2 , while the use of a cooled discharge tube gave u ~ ~ ( H 2=) 53.1 A,* for H g Z o 2species.78 T h e present result of 37 is within this range. According to the absolute rate theory, reactions involving no activation energy proceed by the formation of a loosely bound activated complex in which internal degrees of freedom of original reactants hardly change. I t is reasonable to suppose such a complex in the broadening process. When the potential energy between an excited mercury atom and a foreign gas molecule is taken as
V(r) = -C;rS
s>2
(9)
(7) K. R. Osborne, C. C. McDonald, and H . E . Gunning, J . Chem. P h y s . , 26, 124 (1957). (8) A good review is available on the role of line hroadening in the photochemical separation of mercury isotopes: H. E. Gunning and 0. P Strausz. “Advances in Photochemistry,” W . A Noyes, J r . , G . S Hammond, and J , N . Pitts, Jr., E d . . Interscience Publishers. Inc., S e w York, S. Y . , 1963. (9) S. Glasstone. K. Laidler, and H. Eyring, “ T h e Theory of R a t e Processes,’’ hZlcGraw-Hill Book Co., Inc., New York, N. Y., 1941, pp. 220-222.
3944
2
KANGYANG
240
-
200
-
Vol. 86
-
160-
Theo.
0
x
"E
120-
0,70!
0
N
b
80
-
40
-
o'60
t 100
200
400
300
500
600
700
800
P Lmm)
0
0
2
,
4
Fig. 3.-Absorption 6
I
with increasing AVL (hence with pressure)2
CT ( e r g 5 cm*) Fig. 2.-Effect
of intermolecular potential ( -C/re) collision broadening cross section.
on the
where /3 is a dimensionless constant defined as =
T1/z2(3S-4)/2S
S(S -
2jC2-S)iS
r(2 - 2/s) (11)
Equating (11) with kinetic theory rate constant, i t can readily be seen t h a t q , 2
=
@(ST)-1/Z(kT)- l / y y n
(12)
Thus, should increase with the '/3 power of C. The values of C can be estimated from the relationsll C(Hg-AI)
=
[C(Hg-Hg)C(R.I-M)]"'
C(M-h!I)
= 4€U6
A ys _ A VL
then the rate constant for the collisional interruption of light emission becomeslo
p
a t different ethane pressures.
10x10-20
8
(13)
(14)
where e and u are the force constant of Lennard-Jones potential and M denotes a foreign gas; E and u estimated from viscosity data are used, while C(Hg-Hg) is erg-cm6. taken to be 255 X I n Fig. 2, UL* is plotted against C1l3. Apparently, the line does not pass through the origin, which is not explainable by the present theory. The slope of the line to which more significance should be attached is found to be (2.1 f 0.5) X lo6. T h e theoretical value estimated from (12) with van der Waal's dispersion force is 3.1 X lo5. The agreement is quite good. I n principle, information on AvL suffices to calculate .4 a t different foreign gas pressures. Figure 3 shows such an attempt with ethane as a foreign gas. Calculated L4 is lower throughout. This discrepancy in the region of pressures lower than 200 mm. comes from taking q2= (constant) X p instead of uL2 = constant X p f constant, as indicated in Fig. 1. When this is properly considered, agreement is quite good u p to 200 mm. ; but the discrepancy a t higher pressures still exists. This is attributable to the neglect of line asymmetry. Collision broadening is always accompanied with a line shift, A V S .whose absolute value increases (10) K . Yang and T . Ree. J . Chem. P h y s . , 36, 588 (1961). (11) J . 0 . Hirschfelder, C . F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids," John Wileg and Sons, Inc., New York, N. Y . . 1954, pp. 968-468, 1110-1112.
- -1 tan 2
K ~
S-1
where S is a potential parameter in (9). A similar asymmetry is likely to be present in E ( v ) ,and additional light absorption should occur as AVS in k ( v ) approaches that in E ( v). Equation 7 is useful in estimating an intensity distribution within a cell. This information is indispensable in treating some photolysis mechanisms, such as the diffusion of H atoms to a cell wall. Such a calculation is performed on the system with 100 mm. ethane, The result indicates that only 0.65 em. is needed to reduce the intensity one-half. This drastic variation must always be considered in discussing intensity-dependent reactions. l 2 B. Photosensitized Decomposition of Paraffins.The only important primary process in the mercury photosensitized decomposition of paraffins is C-H bond rupture13 Hg*(3Pi)
+ M +Hg('So) + H + R
(16)
T h e quantum yield of H atoms in the decomposition of methane increases with increasing pressure.14 A slight increase is also reported in the decomposition of n-butane.I6 Undoubtedly, the line ' broadening contributes to this increase. We demonstrate below, however, t h a t there exists an increase in paraffin decompositions even after making appropriate corrections for the line broadening. The quantum yield of hydrogen atoms, 4(H), is usually determined by measuring the quantum yield of hydrogen molecules H + M ---+Hz + R (17) In doing so, extra care must be taken to avoid internal ~cavenging'~"J~J~ R+R+S(=
olefin)
+paraffin H+S-+R*
(18) (19)
(20)
(12) G. J. Mains and H . Niki, a paper presented a t t h e 145th National Meeting of t h e American Chemical Society, New York, N. Y . , September, 1963. (13) For example, see: ( a ) R. A. Back, Can. J . Chem., 37, 1834 (1959); (b) R. A . Holroyd and G. W. Klein, J . Phys. Chem., 67, 2273 (1963). (14) R . A . Back and D. van der Auwera, Can. J . Chem., 40, 2339 (1962). (15) K. R. Jennings and R. J. Cvetanovif, J . Chem. P h y s . , 36, 1233 (1961).
3945
MERCURY-PHOTOSENSITIZED DECOMPOSITION
Oct. 5, 1964 16.6-'
14.0
-
12.0
-
c
-. 0 0
u)
10.0-
0
E i
I
0 -
8.0-
X
n
-
6.0
4.0
-
2.0-
Fig. 4.-The
TIME (rnin.1 rate curves in ethane decomposition; the numbers in each curve indicate ethane pressures in mm
When internal scavenging occurs, +(Hz)decreases with increasing conversion. In the decompositions of propane, n-butane, and isobutane, no difficulty is encountered in determining initial +(Hz). In the decomposition of ethane and neopentane, however, the quantum yields are significantly affected by conversions as low as 1 X as shown for ethane in Fig. 4. The rate equation which is derived in Appendix I1 is well suited for the determination of initial +(Hz) from low conversion data such as in Fig. 4 t =
tx
- ~ (1 e-fx)
(21)
where t is irradiation time, x = [H2]/[M],and t , 7, and { are constants. When x is small, (21) may be expanded to give
where (' = (l/r18[M])(k20/k17)f, ri and ki denote the rate and the rate constant of reaction i, and f represents the fraction of alkyl radical ending up as olefin. The two constants, r16 and t ' , are estimated by leastsquares treatment. In doing so, the third-order term in (22) is always retained; and those data showing significant contribution from this term are all discarded. This practice limited the rate study to the range of ethane pressure larger than 100 mm. Figure 4 shows how well calculated solid curves fit the experimental data. Table IV summarizes the rate data in the decomposition of ethane. With increasing pressure, r16undoubtedly increases. From t ' , f(k20/k17) may be estimated. Large scattering in the fourth column comes from the fact that f ( & / k 1 7 ) is estimated from very small corrections in the rate curves; nevertheless, the magnitude
TABLE IV PRESSURE DEPENDENCE OF Hz YIELDIS ETHANE PHOTOLYSIS Pressure, mm.
nfi0
f(kdk17)
E'" X 10-6 X lo-' Q(Hz), uncor. +(Hz), COT. m 100 5.32 23.6 0.554 0.581 9 0.601 0 630 128 5.77 22 8 150 5.93 17.8 8 0.618 0 638 16.8 12 0 716, 0 732 200 6.87 300 7.13 5.13 6 0 743 0.752 500 7.90 7.32 16 0.823 0.827 a Concentration in Mmoles/cc. and time in min. X 106
is reasonable. Ethane radiolysis data6 indicate that f = 0.4, while photolysis d a t a give f = 0.1.13a Taking f to be 0.3, use of an average value ( = 10) for f(k20/k17) gives k20/k17 = 3 X lo4. Reported literature v a l u e ~ ~ range , ' ~ ~ ,(2.5-4.0) ~~ X lo4. As shown in fifth and sixth columns in Table IV, the correction due to the line broadening significantly alters +(H2). A few experiments have also been carried out a t an intensity of pmole of photons/cc./min., which is about 9.00 X 100 times less than that employed to obtain the data in Table V. At this intensity, r16 a t the ethane pressure TABLE V PRESSURE DEPENDENCE OF THE HYDROGEN QUANTUM YIELDSIPS PARAFFIN PHOTOLYSIS Compd.
CzHe CBHB n-C&a i-C1Hio neo-C6H12 neo-CsHlz, uncor.
Q,
0.92 f 0.04 1 . 0 0 f 0.00 1.00 f 0.01 1 . 0 0 f 0 01 0.86 f 0.02 0 . 8 7 f 0 02
b, mm.
64 f 6 1 2 4 i 0 4 2.9 f 0 . 3 2.1 f0.4 87 i 3 98 f 4
of 100 mm. was 5.24 X mole/cc./min.; thus the ratio, + (low intensity)/+ (high intensity), is 1.05, (16) K . Yang, J . P h y s . Chem.. 67, 562 (1963).
3946
KANGYANG
role in reducing 4(Hp). Neopentane behaves very much like ethane, and peculiarities attributed to high symmetry do not exist. Various mechanisms leading to the pressure dependence have been already considered.14 C. Photosensitized Decomposition of Ethylene.Table VI summarizes the rate data in the decomposi-
Neopentane
-
2.00
1.80
Vol. 86
-
TABLE1.1 HYDROGEN FORMATIOS IS ETHYLENE DECOMPOSITION Pressure, mm. Time, sec. Hz,moles/cc., X 10' 10 60 4.08 10 90 6 15 8 16 10 120 100 100 100
(6,81 i 0.01 X lo-" mole/cc. sec.) 60 1.19 90 1,75 120 2.41 (1.99 f 0.02 X IO-" mole/cc. sec.)
r
1.04
1.00
tion of ethylene. The rate is independent of conversion but decreases with increasing pressure as reported before.Ig B n attempt was made to determine ~ ( H z ) a t 13 mm. of ethylene. The rate of ethylene decomposition was 0.436 times t h a t in propane decomposition a t 600 mm. At 600 mm., 4(H2) in propane decomposition was 0.980; hence ~ ( H zin ) ethylene decomposition with no correction for the line broadening becomes 0.447. LVith the correction for the line broadening, this value increases to 0.570. This clearly indicates t h a t the line broadening must be always considered in using secondary dosimeters. The present ~ ( H z ) of 0.570 a t 13 mm. is higher than C$(CZHZ) of 0.37 a t the same pressure.1g The discrepancy, however, may not be real, since no correction due to the line broadening has been made in the latter value. Table VI1 summarizes 4(H2) a t different ethylene
1 2
4
6 8 I / P (mrn-')
IO
12
14x10-~
Fig. 5.-Pressure dependence of the quantum yield of hydrogen in the decomposition of neopentane and propane.
which indicates t h a t $(Hz) is essentially intensity independent. The nature of the olefin formed in neopentane decomposition is uncertain. In addition, the reaction H
+ S -+
Hz
+ allyl
may occur, thus complicating the rate e q ~ a t i 0 n . l Be~ cause of these uncertainties, no attempt is made to use the rate equation. Instead, rI6 is determined from the visual extrapolation of rate curves to zero time ; hence the neopentane data to be reported below are most inaccurate. In all paraffin decompositions, reciprocal quantum yield is found to increase linearly with reciprocal pressure
4-1
=
4m-'
+ bp-1
TABLE VI1 PRESSURE DEPEVDESCE OF +(H*)IX ETHYLESEDECOMPOSITIOY Pressure, mm
(23)
where 4- denotes the high pressure limit of quantum yield and b represents an empirical constant. Two such examples are shown in Fig. 3. Table V summarizes C$= and b estimated by least-squares treatment. Error ranges here are standard deviations. Since 4m is close to unity, the standard deviation in 4mis about the same as t h a t in q5m-1. Somewhat lower 4- ( = 0.86) in neopentane decomposition" may partially be due to inaccuracy in initial rates; otherwise the present data strongly indicate t h a t C-H bond rupture occurs with quantum yield close to unity. X previous reportla on the very low yield C$(Hz)( = 0.0043) in neopentane deconiposition is not confirmed. Internal scavenging and radical recombination could have played the major (17) D r H. E Gunning (University of .4lberta) kindly informed us t h a t t h e d, v a l u e of 0 86 is in good agreement with t h e result recently obtained in his laboratory. (18) B d e B. Ilarwent and E. \T' R . Steacle, C a n . J . Res., 27B,181 (1949).
O W z ) , cor
~ ( H z uncor ),
0 0 0 0 0 0 0 0 0 0 0 0
10 13 20 30 50 70 100 150 200 300 400 600
0 0 0 0 0 0 0 0 0 0 0
584 570 527 453 337 253 171 100 0659 0345 0197 0099
594 570 502 391 283 206 137 0795 0521 0271 0154 0 0077
pressures. Both corrected and uncorrected +(H2) are found to obey the empirical relationz0 $-1/2
=
+
40-11z b'p
(24)
Least-squares treatment gives for corrected ~ ( H z ) =
+-1/2
(1.04 + 0.03)
+ (1.T2
f
0.01) X 10-zp
and for uncorrected #J(Hz) 4-'/z
=
(1.03
f
0.04)
+
(1.50
*
0.02) X 10-2.b
Significantly, the low-pressure limit of +(Hz)is close to (19) D. J. LeRoy and E. W. R. Steacie, J . Chem. P h y s . , 9, 829 (1941) (20) A. B. Callear and R.J. Cvetanovib, ibid., 24, 873 (1'356).
Oct. 5 , 1964
3947
MERCURY-PHOTOSENSITIZED DECOMPOSITION
unity in agreement with the mass spectrometric data, indicating t h a t the only primary process in ethylene decomposition is the formation of molecular hydrogen.21 Equation 24 is consistent with the following mechanismzO Hg*
+ CzH4 +Hg + CzHa’
(25)
CzH4* +CZH4**
(26)
+ C2H4 +2CzH4 CZH4** +Hz + CZHZ C2H4** + CZH4 +~ C Z H ~ CzH4*
(27) (28) (29)
provided k 2 6 / k 2 7 = kZ8!kz9. According to the above mechanism, b’ in (24) is given as k29/(k28+01”). Tak(molecules/cc.)-’ set.-', the mean ing k29 = 1 x life of excited ethylene estimated from corrected +(H2) is 5.1 x sec., while t h a t resulting from uncorrected +(H2) is 12% lower (4.5 X sec.). We thus conclude t h a t , in the mercury photosensitized decomposition of paraffins and ethylene, the correction due the resonance line broadening does not alter qualitative conclusions; nevertheless, it must be taken into account in quantitative discussion of the mechanism. Acknowledgment.-Dr. F. H . Dickey and Dr. L. 0. Morgan offered valuable discussion; Mr. J. Paden and M r . C. L. Hassell developed the computer program for estimating A ; Dr. D . Cooper offered the computer program used in the linear and nonlinear least-squares treatment; and Mr. J. D . Reedy helped with experimental work. The author gratefully acknowledges this assistance. Appendix I I n t h e present work, the emission line is presumed to be a Doppler line
E(v)=
ce-(”/“)’
in this expression can be estimated by considering A in the absence of any foreign gas; then AVL = 0, and a given in (6) is equal to A v s z / F 2 / A v ~= 0.0012. Using ( 7 ) , A can now be calculated a t different a. Some numerical results are given CY
a:
A
3.0 0.7036
3.5 0.6425
4.0 0.5882
4.5 0.5440
In the present work, A = 0.556 which corresponds t o the CY value of 4.35. In the Hanovia lamp, some selfreversal is likely to be present; b u t (3) does not take into account this fact. T h e self-reversal most drastically modifies the center portion of the emission line. In most photochemical experiments, K ( u 0 ) l is quite large, say above 10, and this portion is likely t o becompletely absorbed; hence the explicit form of E(u) there is probably unimportant. In this connection, it is illuminating t o draw the emission and absorption lines, as shown in Fig. 6. Regardless of the nature of broadening processes which affect k(v), the area under the absorption curve remains constant and is equal to *’IQ, because
-$.O
-3.0
-2.0
-1.0
1.0
0.0
Fig. 6.-The
=
(&2)
(21) F. P. Lossing, D . G. H . Marden, and J. B. Farmer, Can. J. Chem., S4,
A, =
+
Appendix I1 Rat& equation 21 is derived below. Referring to reactions 16-20, i t can readily be seen t h a t d
-
dt
[&I
=
716
1
+ kzo[Sl//ki7[Ml
T o express [SIas a function of [ H z ]and t, we note t h a t [SI remaining after t is equal to [S]r formed in this period minus [ s ] d consumed in the same period. [SI = [Slf -
[Sld
T h e formation of S always involves alkyl radicals. Let R be the total alkyl radicals formed in t and f be the fraction of alkyl radicals ending up as S. Similarly, we define R* and f* as the vibrationally excited alkyl radicals formed in (20) ; then material balance demands t h a t [Sir = f R = f(7iet
+ f*R* 4- [HzI) -k f * ( M - [Hzl)
and [ S l d = 716t
Thus 701 (1956).
4.0
C in (3) is taken t o be 1/a so t h a t the area under both curves becomes equal. The two curves do not match well, and significant fraction of the emission line lies in the region where k ( v ) is nearly zero. It should also be noted t h a t an explicit form of E ( v ) in the range -1 < o < 1 is unimportant because the light there is nearly completely absorbed.
K(VO)AVD
and w is related to u by (4). In drawing the emission line,
3.0
shape of absorption and emission lines; absorption line, A Z = emission line.
‘/%
k(v)dv
2.0
-W-
- [Hz]
3948
SANG
UP C H O I A N D NORMAN
+
kI7[h1])(1 f - f * ) and h = (r1&20/ f*) We make the reasonable approximation t h a t f a n d j * are independent of t; then integration of the above expression gives (21), where { = (PI1 Y 1 6 ) j l J - j*)(1 - - f*)!7 = (kli[MI'k*@. (1 - f - f * ) ? , and j- = (kro k17)(1 - f - f*). Y16)(2f) The low conversion approximation ( 2 2 ) does not contain f* This indicates t h a t , to an approximation correct up to [HI]?,the hot radical effect can be neglected Equawhere g = k17[11])(1 -
f
+
-
s
[CONTRIBUTION F R O M
THE
Vol. 86
h'. LICHTIN
tion 22 can also be derived from the eq '7 of a paper by Cvetanovib, Falconer, and JenningsZ2 with the approximations that 01; = 0 and k7 = 0 In this connection, it should be noted t h a t f = ("*) [ k 1 J ( k 1 ~ k19)] Here the numerical factor, '2, is necessary, because only one-half of the alkyl radicals undergoing disproportionation reaction finally ends up as olefins
+
(22) R J CvetanoviC W E Falconer, and K R Jennings, J ChPm P h y s , 35, 1225 (1961)
DEPARTMEXT O F CHEMISTRY, BOSTON USIVERSITY, BOSTON, MASSACHUSETTS]
The Radiolysis of Methanol and Methanolic Solutions. 111. The Effect of Oxygen on the Radiolysis of Liquid Methanol by 6oCo7-Rays and by 1°B(n,cr)'Li Recoils1,2 BY SANGUP CHOIA X D NORMAN N. LICHTIN RECEIVED .kPRIL 30, 1964 \.apor phase chromatography was used to determine yields of gaseous products.
Yields of products per
l I i 0 e.v absorbed obtained by -,-radiolysis in t'ucz~oare: H2, 5.0; CH,, 0.43; CZHG, 0.006; CgH4, 0.004; CO, 0.06; C H 2 0 , 2 2 ; and (CH20H)z,3.2. Yields per 100 e.v. obtained by recoil radiolysis in Z ~ C U Oare: HP, 5 . 5 ;
CH,, 0.66; C?Hs,0.04; C2HI, 0.01; CO, 1 . 0 ; C H 2 0 , 3 . 0 ; and (CHrOH),, 0.87. In the presence of
G(Hz), b u t not in its absence, include C o r , HCO,H, H 2 0 a ,and probably CHaOOH. G ( C 0 ) from ?-radiolysis is increased by O? but is decreased in the case of recoil radiolysis. Limiting yields of products are obtained a t concentrations mole I . -' or less. Important limiting yields per 100 e.v. from y-radiolysis include: of the order of 1 x of 0,. H:, 1.9: CH,, 0.18; CO, 0.09; peroxide, 3.1; CHnO, 8 . 7 ; (CHnOH)?,0.1; and HCO?H, ca. 1.5. Important limiting yields per 100 e.v. from recoil radiolysis include: Ha, 2.4; CH?, 0.6; CO, 0 . 8 ; peroxide, 1.5: CHPO. :3.9: (CH:OH)?, 0.4; and HCO:H, ca. 1.0. The -,-radiolysis data are consistent with a mechanism which assumes that only molecular products in unreactive states and free radicals diffuse into bulk solution from the spurs and which does not involve chain autoxidation of methanol. Radical and "molecular" yields relating to the proposed mechanisnl can be estimated. In contrast, the recoil radiolysis data cannot be explained entirely in terms of free radicals and unreactive molecular products. The data can he rationalized by assuming that metastable, excited methanol molecules also diffuse into hulk solution. The various relevant "molecular" and radical yields can also be estimated for recoil radiolysis.
G(CH?OHI?, and G( CH,) decrease, while G( CH20) increases. Products formed in the presence of
Introduction -1study of the radiolysis by X-rays and by 6@Co y-rays of methanol saturated with air and with oxygen, respectively, a t 1 a t m . has been reported.3 The extent of variation of oxygen concentration is not sufficient in
this work to permit other than qualitative interpretation. In the present study, oxygen concentration was varied systematically, and both T o y-rays and log(n,a)'Li recoils were employed in radiolyses. The unsatisfactory reproducibility of product yields from the y-radiolysis of "pure" liquid methanol is well known. Ailthoughthis lack of reproducibility cannot be correlated with the methods of analysis employed, itiiprovemen t in analytical reliability was sought in the present case through vapor phase chromatographic determination of gaseous products. In addition, further attention was directed toward achieving the complete degassing of radiolyzed methanol, in accord with the experience of Theard and Burton.5
Experimental Materials.-Methanol f Eastman Organic, Spectrograde) was purified as described previously.4sfi Oxygen (Matheson) was passed through a column of Ascarite and silica gel, followed by a ( 1 ) Research carried o u t under t h e auspices of t h e U S . Atomic Energy Commission under Contract XT(RO-I)2383. '2) €'re.;ented a t t h e 146th National Sleeting o f t h e American Chemical S o c i p l y , Ilrnvei-. Colo , Jan , 1964. 0 )E Hayon and J J. Weiss, .I. Chrm S o c , 3970 11961). 84) Sf.I m n r n u r a , S. U. Choi, and N . .V. Lichtin, J . A m . C h r m . Soc., 86, 3S65 Ili(6R) ( 5 ) I.. R I Theard and SI Burton. J . P h y s . C h e m . , 67, J9 (1963). ' 6 ) T h e present preparation corresponds t o methanol C of ref. 4 .
0 2 ,
0 2 ,
trap cooled by Dry Ice, before it was introduced into the vacuum line. Methyl borate (Matheson) employed in recoil radiolysis was rectified on a Todd column. The middle fraction of distillate was further rectified under vacuum after transfer t o the vacuum line. The middle third from the second distillation was stored on the vacuum line. Iliquots were transferred by distillation. Preparation of Oxygenated Solutions.-The amount of oxygen transferred to a cell (containing methanol a t Dry Ice temperature) was determined manometrically by difference, with the aid of a calibrated volume. T h e concentration of oxygen in solution a t the temperature of irradiation (taken as 25") was then calculated from the methanol volume and the k n ~ w nsolubility. ~,~ Irradiation.-Pyrex glass cells used for ?-irradiation were prepared, cleaned, and attached to the vacuum line, and methanol was transferred under vacuum from the storage system to irradiation cells, all as described p r e v i ~ u s l y . ~The irradiation cells were sealed off from the line under vacuum and the amounts of methanol in the cells determined by weighing.@ -,-Irradiations were carried out a t about 20' in a Schwarz-Allen source'o for 9 t o 35 min. Irradiation times as long as 70 type min. were employed occasionally to facilitate analyses for minor gaseous products. Dose rates were determined by means of the aerated acid (0.8 IV sulfuric acid) ferrous ammonium sulfate (10-3 mole/l.) d ~ s i m e t e r . ~ The dose rate to methanol was about 1.8 X 10': e.v.-l ml.-I min.-'. T h e volume of methanol was 10-20 ml. in most cases. The ratio of the volume of vapor t o t h a t of the liquid was about 1 : 2 . Recoil radiolyses were carried out in the Brookhaven thermal neutron facility., Quartz irradiation cells and procedures employed in filling the cells, and in irradiating and determining doses, were those reported previously., Concentrations of 17) "International Critical Tables," Vol. 111, McCraw-Hill Book Co , Inc.. Kew E'ork, 5 Y . , 1928, p. 262. (8) C . B. Kretschrner, J . Nowakowska, and R Wiebe, 1 n d E ~ Chem K , 38. 506 (1946). (9) N . N . Lichtin, J . P h y s . C h e m . , 63, 1449 (1959). (10) H. A . Schwarz and A . 0 . Allen, .\*ucleonics, 11, 58 (19.54).
