9
METHYLENE PRODUCED BY VACUUM-ULTRAVIOLET PHOTOLYSIS rate constant. Most of the Hg(Tz), if formed, is converted to the ("1) state by collisions1s with nitrogen. Callear and Wood18 estimated that the ratio of forma~ 4:l. The evidence, tion rates of Hg(3P1,2)and 3 Pwas thus, is for a total quenching rate constant 2 6 X 1013 cm3 mol-' sec-l. The largest potential source for error in our experiments is the [Hg]. The rate of reaction of K2(A38,+) with NO was measured in the manner just described for Hg. The major is formation of NO(A2Z+), which reaction has a lifetime of 2.2 X lo-' sec and is not quenched under our experimental conditions. I n this case both emissions are band systems and the relative band areas, corrected for monochromator-detector sensitivity, are satisfactory for eq 12. Concentrations of NO were used such that no detectable removal of N2(A3Zu+)was observed. The value measured from eq 12 was 6.7 X
1013cm3 mol-' sec-l for a 2.1-sec radiative lifetime for N2(A3Z,+). Two other studies have reported total quenching rate constants for NO, ours is just for formation of NO(A2Z+), and the values of Table I are basically in satisfactory agreement. It is that a very small amount of NO(B) is formed by the reaction, but it is too small to affect our rate constant measurement, The NO(a411) state, if formed a t all, makes a negligible contribution.1s Young and St. Johnsb also reported a rate constant for formation of just NO(A) as well as the total quenching rate constant of Table I. If the new value for the lifetime of M2(A) is used, their data, which were based upon relative emission intensities, give a rate constant for NO(A) formation of 1 X l O I 4 cm3mol-' sec-l. (34) D, H. Stedman and D. W. Setser, Chem. Phys. Lett., 2, 542 (1968).
Methylene Produced by Vacuum-Ultraviolet Photolysis. IV. Energy Distribution for the Reaction C,H,
+ hv
(123.6 nm)
= CH, + C,H,
by R. D. Koob Department of Chemistry, North Dakota State University, Fargo, North Dakota
58103 (Received June 9, 19?'1)
Publication costs assisted by the National Science Foundation
A study of the pressure dependence of ethane and the methylene insertion products formed in the photolysis of propane at 123.6 nm is reported. The cofragments formed in the primary photoreaction (1) C3H8 hv (123.6 nm) + CZH~ CHZ( A H = 98 kcal/mol) must carry away 133 kcal of excess energy. The energy of each of these fragments has been estimated by observing rates of their subsequent reactions. As measured relative to an external standard, the yield of ethane does not change in the pressure range 2-760 Torr. Failure to observe any decomposition at pressures as low as 2 Torr suggests that ethane carries less than 70 kcal on the average. Isobutane formed by insertion of singlet methylene into propane decomposes with the rate constant (high pressure limit) k d = 1.3 & 0.1 x lo8 sec-I. Based on a RRKM calculation, this rate corresponds to an average of 31 kcal being supplied to the insertion reaction by the methylene. The energy limits placed on the cofragments of reaction 1, E*(C2He) < 70 kcal and E*(CHz) > 31 kcal are not consistent with the excess energy associated with reaction 1 being statistically partitioned. Either a CHz('A1) in a higher vibra-
+
+
tional state or the production of the electronically excited CHz('B1) would account for this discrepancy. Circunistantial evidence favors the latter, though any choice with the available data is speculative.
Introduction Despite the simplicity of the molecule, the photolysis of propane has been demonstrated to give a com-
plex product mixture. Not only does one find products rationalized on the basis of elimination of free radicals and small molecules from propane, but also a number of products which would appear to arise from secondary decomposition of the fragments produced in the primary photochemical process.
To trace successfully the course of events commencing with the absorption of a photon and terminating with a collection of thermalized products, it is necessary to have some knowledge of the nature of the excited state(s) involved in the process. Propane is known to have a strong absorption spectrum in the vacuum ultraviolet.' The order of magnitude of the (1) H. Okabe and D. A. Becker, J . Chem. Phys., 39, 2549 (1963).
The Journal of Physical Chemistry, Vol. 76, N o . 1, 1972
10
extinction coefficients involved are characteristic of “allowed” transitions, and the propane excited state has been suggested to be of a u,u* typeU2 Such a characterization is of little use in describing the structure and properties of the excited state, and there has been little agreement as to what structural changes actually O C C U ~ . ~The relatively little structure found in the absorption spectrum suggests an excited state of short lifetime. That the excited state is negligibly depopulated by fluorescence has been shown by Hirayama and L i p ~ k y . ~ Presumably, a short lifetime is the result of a rapidly dissociating excited state. It is interesting, in this regard, that a most important feature of propane photolysis is molecular elimination rather than simple bond cleavage. These very fast molecular rearrangements are, to say the least, unusual when compared with the bulk of known chemical reactions, and the excited state producing them is worthy of more study than it has presently received. Because insufficient information about the structure of this excited state is available from direct spectral studies, we have chosen to examine in some detail the fragments it produces. In this paper we report studies of the way in which energy is distributed among two products, methylene and ethane, of a primary photolytic reaction of propane.
E. D. KOOB
W 0
I
z . w
LT
CHq(t), S A M P L E The yields of methane, arbitrary units, from the sample and reference cells of the apparat& described in the Experimental Section. The slope of this plot is the ratio of light intensity entering each of the two cells and is equal to 0.55 f 0.03. The absolute value of methane yield increases with photolysis time, but the ratio remains the same. Similarly the ratio of yields is pressure independent. n,20 Torr; A, 100 Torr; 0, 300 Torr.
Experimental Section
Materials. Phillips research grade propane was used. After purification by gas chromatography using a silica gel column, impurity levels were below 5 ppm. The purified propane was dried over Drierite and vacuum distilled to a storage bulb. The oxygen was Linde CP and the helium was Air Reduction Co. medical grade. Both were used without further purification. Lamps and Cells. A krypton resonance lamp, similar to those described by Ausloos and Lias15was used for the photolysis. The lamp was filled on a mercuryfree vacuum line capable of achieving pressures less than 1 X 10-6 Torr (Veeco discharge gauge). The lamp was gettered with a titanium gettering assembly. The chromatic purity was greater than 98% in the region between 105 and 165 nm (McPherson 0.3-m vacuum monochromater) , MgF2 windows were used for the lamp, The light intensity, calculated from the yield of acetylene from the photolysis of ethylene,6 was 1.9 f 0.1 X l O I 4 quanta/sec. A “T” shaped lamp with windows at each end of the crossbar was used to study the photolysis. Each window looked into individual sample cells. Each cell had a 2.5 cm i d . with a path length of 2.5 cm. The ratio of light intensities entering each of the two cells was constant over the course of the investigation. Thus, one cell with constant sample conditions was used as an external standard to which runs made in the other cell could be compared. The Journal of Physical Chemistry, Vol. 76, No. 1, 1978
The yield of methane from the photolysis of oxygenscavenged propane at a pressure of 20 Torr was used as the external standard. Figure 1 demonstrates the reliability of the technique under a variety of experimental conditions. All photolyses were conducted with 5y0 oxygen added to scavenge free radicals and triplet methylene. No products which could be ascribed to these species were found. In all experiments, photolysis was carried to less than 0.1% conversion of parent into product. All analyses were done by gas chromatography (FID) on a 25 ft, 0.25 in. o.d., 35% (w/w) squalane column maintained a t room temperature. Results The pressure dependence of the relative amounts of ethane, isobutane, and n-butane is given in Table I. The yields are relative to the external standard. Each value in Table I is an average of three or more separate experiments. An important feature is the lack of any pressure effect on the yield of ethane. Within ex(2) J. Calvert and J. Pitts, Jr., “Photochemistry,” Wiley, New York, N. Y., 1966, p 493. (3) (a) E. N. Lassettre, A. Slrerbele, and M.A. Dillon, J . Chem. Phys., 49, 2382 (1968); (b) J. W.Raymonda and W. T. Simpson, ibid., 47, 430 (1967). (4) F. Hirayama and S. Lipsky, ibid., 51, 3616 (1969). (5) P. Ausloos and S.G. Lias, Radiat. Rea. Rev.,1, 75 (1968). (6) S. G. Lias, G. J. Collin, R. E. Rebbert, and P. Ausloos, J . Chem. Phys., 52, 1841 (1970).
11
METHYLENE PRODUCED BY VACUUM-ULTRAVIOLET PHOTOLYSIS perimental error, the ratio of ethane to external standard is constant over the pressure range 2 to 760 Torr. In contrast, the yields of isobutane and n-butane increase quite strongly with increasing pressure.
Table I: Variation of Relative Product Yields with Total Pressurea
I
0
2.26 2.16 2.30 2.29 2.25 2.26 2.28 2.25
2 10 20 206 50 100 380 760
Photolysis of propane a t 123.6 nm. Nd, not determined. a
G
Ndc 0.206 0.299 0,274 0.331 0.364 0.403 0.382 Av 2.24 =k 0.04 b
Nd 0.645 0.827 0.715 0.860 0.904 0.966 0.970
0-1
1 atm of He added.
Methylene is produced in the primary photochemical process
+ CH2 (AH = 98 Ircal/mol)
x
108
0.4
1
0.5
sec
Figure 2. cbl,/[butane yield] us. W-1: 0, isobutane; a, n-butane; and W, collision frequency. The collision frequency of CaHlo*,W, has been calcukated with the following collision 5.87 A; and CaH,, 5.24 A. diameters: i-CaHlo, 5.82 A; TL-CPHIO,
Discussion
+ hv
0.3
0.2
0.1
that the yield of a butane will follow the kinetic rate equation, d[C,H~o]/dt = +Iaw/(u k4). 4 is the apparent quantum yield, I , is absorbed light intensity, calculated from the yield of standards, and w is the rate of collisional deactivation of C4H10*, assuming unit efficiency and the collisional cross sections given in Figure 2. This may be rearranged to give
+
+I,/yield = 1
+ k*/w
(6)
A plot of #Ja/yield lis. 0-l would be expected to be linear with a slope equal to the observed unimolecular The only ethanes obtained in a mixture of C3Ds rate constant. Figure 2 is a plot of 41a/[i-C4H10] and C3H8 NO photolyzed at 147.0 and 123.6 nm are +I&/[n-CJllo] vs. u-l. A least-squares treatment of Cd& CzH6.' This is interpreted as indicating that the data gives a unimolecular rate constant of 1.3 =k reaction 1 is the only source of ethane in the photolysis 0.1 X lo8 sec-l for isobutane and 6.6 =t 0.4 X lo7 of scavenged propane. The mass balance of eq 1 has sec-' for n-butane. been discussed extensively in a previous paper.s Even I n general, a t low pressures deviations from linearity if all methylenes produced in reaction 1 are singlet, for plots of data according to eq 6 may be found. This collisional conversion into triplet methylene is comdeviation is predicted by RRKM theory and may petitive with insertion and a number of the methylenes reflect either the distribution of energies in the diswill not be observed as insertion product. However, sociating molecules or inefficiency in energy removal as triplet methylene will be removed by the ~ x y g e n , ~ from a reactive molecule by a colliding partner.l0 At one need consider only the reactions of singlet methhigher pressures, the apparent rate constant (experiylene. The insertion reactions of singlet methylene mental value IC*) is equal to the mean of microscopic are given by reactions 2 and 3. An asterisk denotes rate constants for the reaction.1° It is this mean vibrational excitation. which is of interest to us in this case. If the low pressure point, 10 Torr, of Figure 2 is not included, a least'CH, CaHs +i-CdHio* squares treatment of the remaining data points gives (AH = -101 kcal/mol) (2) values for IGq which fall within the error range quoted 'CH2 C3Hg +n-C4Hlo* above for the slope determined using all of the points. The significance of the lowest pressure point, then, is (AH = -99 kcal/mol) (3) that it indicates that deviations from linearity are not The butanes formed in reactions 2 and 3 can undergo important yet a t 10 Torr. It is, however, the higher unimolecular decomposition via reaction 4 or collisional stabilization via reaction 5 CaHa
= C%H6
(1)
+
+ +
+ +
(7) P. Ausloos, S. G. Lias, and I. B. Sandoval, Discuss. Faraday
SOC.,36, 66 (1963).
k
C4H1O4 +decomposition products
(4)
C4Hio* C,Hio (5) Assuming stea,dy-state conditions, it is easily shown
(8) A. K. Dhingra and R. D. Koob, J . Phys. Chem., 74, 4490 (1970). (9) R. L. Russell and F. 8 . Rowland, J. Amer. Chem. SOC.,90, 1671 (1968). (10) B. S. Rabinovitch and D. W. Setser, Advan. Photochem., 3, 1 (1964).
The Journal of Physical Chemistry, Vol. Y6,No. 1, 1078
12
pressure points which determine the value of the apparent rate constants used in the following discussion. The total amount of energy to be distributed among the degrees of freedom of the products of reaction 1 is the sum of the absorbed light energy and the thermal energy of the propane less the endothermicity of reaction 1. The heats of formation of propane and ethane have been established a t -24.82 and -20.24 kcal/mol, respectively, at 298OK’l The heat of formation of methylene has been estimated to be 93.7 kcal/mol at 298”1
