10
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
2. Diaz and R. D. Doepker
(22) M. Ackerman in “Mesospheric Models and Related Experiments”, G. Fiocco, Ed., D. Reidel Publishing Co., Dordrecht, Holland, 1971, pp 149-159. (23) D. J. Wuebbles and F. Luther, unpublished calculations. (24) M. T. Leu, C. L. Lin, and W. B. Demore, J . Phys. Chem., 81, 190
(1977). (25) W. S. Smith, C. C. Chou, and F. S. Rowland, Geophys. Res. Lett., submitted for publication; presented at the 173rd National Meeting of the American Chemical Society, New Orleans, La., March, 1977. (26) L. T. Molina and M. J. Moiina, Geophys. Res. Lett., 4, 83 (1977).
Gas-Phase Photolysis of Tetrahydrofuran at 147.0 and 123.6 nm Zalda Dlaz and Richard D. Doepker” Department of Chemistty, University of Miami, Coral Gables, Fiorida 33 124 (Received May 3 1, 1977) Publication costs assisted by the University of Miami
The vacuum-ultraviolet photolysis of tetrahydrofuran, 1:1 mixtures of tetrahydr0furan:tetrahydrofuran-d8, and tetrahydrofuran-3,3,4,4-d4 was investigated using xenon (147.0 nm) and krypton (123.6 nm) resonance radiation. Nitrogen oxide and hydrogen iodide were extensively employed as a means of determining radical yields. Mass spectral analysis of major components contributed to the identification of radical species, intermolecular processes, and intramolecular rearrangements. Major product of the photolysis include ethylene, propylene, cyclopropane, carbon monoxide, and hydrogen. Evidence is presented for the occurrence of seven primary reaction channels to which quantum yields have been assigned.
Introduction In contrast to the large volume of work that has been reported on the photochemistry of hydrocarbons, the photochemistry of simple heterocyclic compounds containing oxygen atoms in the ring has received relatively little attention. The only investigation of the vacuumultraviolet photolysis of a cyclic ether that has been reported in the literature is that of ethylene 0xide.l The thermal vapor phase decomposition of tetrahydrofuran (THF) was investigated by Klute and Waltem2 The major products found were ethylene, carbon monoxide, and methane, and acetaldehyde and formaldehyde were identified as intermediates. The photolysis of T H F with a full mercury arc was studied by Roquitte3v4 in the pressure range 10-165 Torr, with a variation of temperature from 30 to 120 “C. The major products were carbon monoxide, hydrogen, methane, ethane, ethylene, propylene, propane, cyclopropane, and formaldehyde. The author suggested the following decomposition scheme THFt
b-+[p]* +
c-C,H, t HCHO
-+
CH,CH,CH, t CO C,H, t CH, t H t CO
--f
(1) (2) (3)
The cyclopropane formed in reaction 1 is vibrationally excited and its yield increases with pressure. This increase is accompanied by a decrease of the yield of propylene with increasing total pressure, which agrees with the observation that thermally5 and chemically produced6,7 excited cyclopropane isomerizes to propylene. This study represents an attempt to examine the molecular decomposition processes of the T H F molecule at 147 and 123.6 nm. Deuterium labeling of the parent compound has been used to explore the molecular rearrangements leading to product formation, and the efficiency of each primary channel has been estimated at both wavelengths.
Experimental Section Materials. THF (Aldrich), THF-d8 (Merck Sharp and Dohme), and THF-3,3,4,4-d4 (Merck Sharp and Dohme) 0022-3654/78/2082-0010$01 .OO/O
were purified by vapor chromatography using a 6-ft Chromosorb 101 column maintained at 60 “C. The purity of the resulting materials was better than 99.8%. Mass spectrometric analysis revealed the presence of 6.4% C4HD70 in the THF-dg, and 5.8% C4H5D30 in the THF-d4. Purification of other materials has been described elsewhereb8 Irradiation and Analysis. The vacuum-ultraviolet photolysis of T H F was investigated at room temperature in a standard static system using a 600-cm3reaction vessel and a “gettered” xenon (147 nm) or krypton (123.6 nm) resonance lamp.gJO Analysis was performed by vapor chromatography (25-ft Squalane column at 60 “C and 6-ft Chromosorb 101 column at 75 “C) and mass spectrometry (C.E.C. 21-103 C) as reported earlier.g The chemical = actinometers used a t 147 nm were cyclobutene (@czHz 0.61)’O and cyclopentene (@c& = 0.23),11while at 123.6 nm = O.19)I1 was employed. only cyclopentene (@c~H~
Results The quantum yields of the major products of the photolysis of T H F at 147 and 123.6 nm are reported in Tables I and 11, respectively. Hz and CO yields a t both wavelengths are collected in Table 111. In addition to the results presented in these tables the following observations should be reported. (1)A careful search was made for possible formation of HCHO, CH,CHO, furans, and dihydrofurans in this system, but with negative results. However, the presence of small amounts of formaldehyde would not be disclosed by the chromatographic analysis, which is about a factor of 7 more sensitive to methane than it is to formaldehyde. (2) In this study HI has been found to be a more effective radical interceptor than H2S, as suggested by Ausloos.l* Increasing the concentration of HI from 3 to 15% revealed that the optimum concentration of HI for radical interception in this system is 5%. This corresponds to the maximum in the radical yield vs. HI concentration curve. When THF was photolyzed in the presence of 5% HI the quantum yields of CH4, CzH4, and C3Hsincreased, indicating the presence of CH3 (or CHz), CzH3, and C3H6 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
Gas-Phase Photolysis of Tetrahydrofuran
11
TABLE I: Quantum Yields at 147.0 nm for the Photolysis of Tetrahydrofuran THF, Torr Add, Torr
0.165 None
10 None
1.0 0.1 NO
10 1.0 NO
2.0 0.2 NO 100 N,
1.0 0.05 HI
10 0.5 HI
CH'l CZH,
0.06 0.02 0.37 0.06 0.14 0.01 0.02 0.02 0.06
0.05 0.02 0.36 0.04 0.18
0.02 0.02 0.27
nda 0.01 0.28
nd 0.02 0.28
0.34 0.01 0.38 0.01 0.29
C2H4
a
C,H, C;Hi C3H4b C.HAd C-*c3H, l-C,H, Not determined.
0.12
0.16
0.19
0.35 0.02 0.39 0.01 0.26
tC
t
t
t
t
0.01 0.03 0.04
0.02 0.02 0.01
0.01 0.03 nd
0.01 0.04 0.01
0.02 0.02 0.01
0.01 (0.03) 0.01
1.0 0.05 HI 0.33 0.04 0.42 0.04 0.38 0.01 0.02 0.07 0.01
10 0.5 HI 0.37 0.01 0.51 0.05 0.51 0.01 0.01 (0.16) 0.01
Methylacetylene.
Trace.
t
Allene.
TABLE 11: Quantum Yields at 123.6 nm for the Photolysis of Tetrahydrofuran THF, Torr Add, Torr
a
Methylacetylene.
0.5 None 0.04 0.03 0.28 0.04 0.18 0.01 0.01 0.05 0.05 Trace.
10 None 0.05 0.02 0.45 0.05 0.36 0.02 0.01 0.13 0.07
0.5 0.05 NO 0.02 0.03 0.29
2.0 0.2NO 0.03 0.03 0.44
0.19
0.31 t 0.02 0.09 0.01
tb
0.02 0.06 0.01
147.0 147.0 147.0 147.0 123.6 123.6 123.6 123.6 a
2.0 2.0 10.0 10.0 2.0 2.0 10.0 10.0
50 5.0 NO 0.02 0.01 0.36 0.28
0.36
t
t
0.01 0.16 0.01
0.01 (0.21) 0.01
Allene.
TABLE 111: H, and CO Quantum Yields for the Photolysis of Tetrahydrofuran Wavelength, THF, nm Torr
10 1.0 NO 0.02 0.02 0.4 5
Add, Torr
H,
co
None NO(0.2) None NO(1.0) None NO(0.2) None NO(1.0)
0.57 0.34 0.44 0.23 0.64 0.32 0.44 0.28
0.29 0.29 0.20 0.19 0.45 0.46 nda 0.32
Not determined.
radicals. Absolute rac :a1 yields have been estimateG .,y assuming that the titration of these radicals with HI is quantitative. (3) Increasing the concentration of NO from 5 to 15% had no noticeable effect on the product distribution. Increasing the concentration of CHJ from 5 to 10% had no effect on the relative product yields, except for increasing the yields of methane and ethane. (4) Relative product yields in the presence of HI and CH31 were insensitive to irradiation time. Normal conversions were maintained below 0.1 % when additives were present. (5) The quantum yields of ethylene and propylene at 123.6 nm appear to go through a maximum as the pressure is increased, and at the high pressure limit attain constant values of 0.35 and 0.29, respectively. (6) The quantum yield of vinyl radicals as determined by HI titration at 123.6 nm decreases with pressure, and no vinyl radicals can be detected at pressures above 15 Torr. (7) Since the ionization potential of THF (9.55 eV)13 is lower than the photon energy at 123.6 nm (10.0 eV) the possibility of photoionization was examined. The ionization efficiency was measured, and a value of 0.02 was obtained. At such a low ionization efficiency ion-molecule reactions were considered unimportant, and were not investigated.
(8) Assuming an experimental ratio H / D = 1.00, the isotopic composition of the hydrogens formed in the photolysis of 1:l THFZTHF-da mixtures in the absence of NO correspond to 53% atomic and 47% molecular hydrogen detachment at 147 nm, and 55% atomic and 45% molecular hydrogen elimination at 123.6 nm. In the presence of NO the results are 48% atomic and 52% molecular hydrogen detachment a t both wavelengths. (9) Irradiation of 1:l mixtures of THF-THF-da revealed that memthane, ethane, l-butene, and a fractionof the ethylene and propylene arise as a result of bimolecular reactions. The ethylene formed in these experiments a t 147 nm consisted of 43% CzH4, 9% C2H3D, 8% C2HD3, and 40% C2D4. (10) The isotopic distribution of the hydrogen fractions formed in the photolysis of THF-3,3,4,4-d4in the presence of NO was 63% H2, 34% HD, and 3% D2 at 147 nm, and 60% Hz, 37% HD, and 3% D2 at 123.6 nm. The ethylene formed in these experiments consisted of 55% CzH2D2and 45% C2D4 at 147 nm, and 62% C2H2Dzand 38% CzD4a t 123.6 nm. The acetylene, propylene, and cyclopropane consisted entirely of C2D2,C3H2D4, and c-C3H2D4,respectively. (11) The methane fraction formed in the krypton photolysis of THF-d4 in the presence of H2S contained 50% CH4, 13% CH3D, 31% CH2D2, 5% CHD,, and 1% CD4. (12) 93% of the methane produced in the photolysis of THF-da-HZS mixtures at both wavelengths consisted of CD3H. The remaining 7% was CD4. The ethylene and propylene fractions consisted mainly of the perdeuterated compounds, with contributions (ca. 15%) of CzD3H and C3D5H, respectively. Discussion The photolysis of THF a t 147 nm results in the formation of neutral excited molecules, which fragment into molecular products and free radicals. At 123.6 nm a small number of parent ions is also formed. Formation of Ethylene. The formation of ethylene may perhaps be best represented in the following manner
12
The Journal of Physical Chemistry, Vol. 82, No. 7, 7978
[*e ]' [--e--]" THF + hv
+
-+
.+
C,H, t CH,
+ HCO
2. Diaz and R. D. Doepker
(4a)
C,H, t CH, t HCO
The C-0 bond in T H F is of the order of 84.1-85.2 kcall mol,I4so that the initially formed biradical would be highly excited and could rearrange and decompose into smaller fragments. The lifetime of the biradical is probably shorter than the collision interval at the pressures used, since the quantum yield of ethylene does not decrease with increasing pressure. Reactions 4a and 4b would be indistinguishable in nonlabeled experiments, but in the photolysis of THF-3,3,4,4-d4reaction 4a would result in the formation of C2HzD2and CHD2,whereas reaction 4b would produce C2D4 and CH3. If most of the excess energy of reactions 4 is located on the ethylene molecule further fragmentation might o ~ c u r ~ ~ - l ' C,H,' C,H, t H, (5) -+
-+
C,H, t H
(6)
C2"l
(7)
M +
A second channel for the formation of ethylene might involve fragmentation of a T H F molecule into ethylene and atomic oxygen
0+ I'
0
hv
-+
2C,H, t 0
