J. Mol. Evol. 13, 1 5 - 2 6 (1979)
Journal of Molecular Evolution © by Springer-Verlag. 1979
Hydrogen Atom Initiated Chemistry Jane Huey Hong and Ralph S. Becket Department of Chemistry, University of Houston, Houston, Texas 77004, U.S.A.
Summary. H atoms have been created by the photolysis of H2S. These then initiated reactions in mixtures involving acetylene-ammonia-water and ethylene-ammonia-water. In the case of the acetylene system, the products consisted of two amino acids, ethylene and a group of primarily cyclic thio-compounds, but no free sulfur. In the case of the ethylene systems, seven amino acids, including an aromatic one, ethane, free sulfur, and a group of solely linear thio-compounds were produced. Total quantum yields for the production of amino acids were % 3 x 10 -5 and % 2 x 10 -4 with ethylene and acetylene respectively as carbon substrates. Consideration is given of the mechanism for the formation of some of the products and implications regarding planetary atmosphere chemistry, particularly that of Jupiter, are explored.
Key words: H atom reactions - Acetylene - Ethylene - Amino acids - Thiocompounds - Jovian atmosphere. Introduction Some simple hydrocarbons such as methane (CH4), ethane (C2H 6) and acetylene (C2H 2) as well as carbon monoxide (CO) have been identified in the atmosphere of Jupiter (Combes et al., 1974; Ridgeway, 1974). There is also indication that ethylene exists (C2H 4) (Ridgeway, 1974). The presence of C2H6, C2H 2 and CO (as well as C2H 4) is an indication of non-equilibrium processes with irreversible conversion of CH 4 into larger hydrocarbons. A steady state must involve downward movement of the heavier hydrocarbons and upward movement of CH 4 (Hunten, 1969). Also, it is estimated that C2-type hydrocarbons produced from the photolysis of CH 4 have a downward flux of 3 x 10-14g cm "2 s -1 (Strobel, 1973). Based on the photolysis of CH 4 (at 123.6 nm) in the presence of deuterium it has been concluded that the photodissociation CH 4 + hv
-~ CH 3 + H
0022-2844/79/0013/0015/~ 02.40
16
J.H. Hong and R.S. Becker
has an almost zero probability of occurrence (Hellner et al., 1971). Appreciable quantum yields of C2H6, C2H 4 and C2H 2 were obtained. The principal reactions appear to be (Hellner et al., 1971) CH 4 + hv -+ 1CH2 + H 2 3CH2 + 2H -+ CH + H 2 + H In the presence of inert gases such as Ar, the yields of C2H 4 and C2H 2 were increased. The possible contribution of hot and thermalized H atoms to the reactions occurring in the Jovian atmosphere is complex. The most important argument is that any hot H atoms produced by photodissociation would normally be expected to rapidly thermalize by collisions with the large excesses of both H 2 and He which are present (compared to any other components, even CH4). In such a case initially hot H atoms would loose their potential for initiating secondary reactions where higher activation energies are required, such as H abstraction reactions. However, thermalized H atoms can undergo addition reactions where low activation energies are involved. This problem appears to be an important one and we have considered it in our later discussion. Hot and thermalized H atoms are produced in both the atmospheres of Venus and Mars (Ferrin, 1974). Furthermore, several of the molecules included in this study have been identified in interstellar space such as NH 3, H 2 0 and C2H 2 (NH 3 and H 2 0 are also present in the Jovian atmosphere). Of all the simulated syntheses of biologically meaningful organic compounds, the carbon sources were frequently acquired from saturated hydrocarbons such as methane or ethane (Hong et al., 1974; Becket et al., 1974; Becker, 1975; Sagan and Khare, 1971a and b, 1973). Little or no effort has been dedicated to the exploration of unsaturated hydrocarbons as carbon sources. One of the primary objectives of this work is to explore the nature and mechanism of H atom initiated chemistry as related to molecules important in chemical evolution, and present in planetary atmospheres and interstellar space.
Experimental All gases used were c.p. grade (Matheson) and were further purified by low temperature distillation and degassing and collected at dry ice-acetone slush temperature to remove acetone, CO 2 and water. GC-MS analysis indicated that these impurities were removed. Deionized and distilled water was used on all occasions where water was involved. The experimental apparatus used in this study and the detailed procedures for product analysis have been described previously (Hong et al., 1974; Becker et al., 1974; Becker, 1975). The volume of the apparatus was carefully calibrated in order to be able to estimate the quantum yields of some of the photochemical products. For each set of reactions, three individual reactions were carried out. Two reactions involved photodissociation of H2S in the presence of substrates and one was a blank thermal reaction involving H2S plus substrates. None of the products identified in this study were found in the thermal blank reaction. Gas mixtures of C2H4-NH3-H20-H2S (1:1:0.1:2) and C2H2-NH3-H20-H2S (1:1:0.1:2.7), where the number in parenthesis stands for the pressure ratio (at one
Hydrogen A t o m Chemistry
17
atmosphere total pressure and total volume of 7212 ml) were irradated respectively for approximately 150 h with a 500 W high pressure mercury lamp from Illumination Industry Inc. A filter system was used for which the band pass was 2 2 0 - 2 8 0 nm with a maximum transmission of 54% at 252 nm. The product identification was based mainly on mass spectral data obtained from a CG/MS/data system (Hewlett-Packard gas chromatograph Model 57 lOA/mass spectrometer Model 5930A/Data System Model 5933A). The gas chromatograph was modified by the installation of a gas sampling valve (Carle Inst. Co.) with provided for fast transfer time from a loop to a column, prevented introduction of air into the system and increased sensitivity. The identity of the amino acids obtained from a reaction was established by direct comparison of the GC retention times and mass spectra with those of authentic samples measured under the same conditions. All other products except thialdin and the trisulfide, were identified by direct comparison of the mass spectra with those given in the literature (Mass Spectrometry Data Center 1974). Thialdin was identified based on mass spectra, NMR, IR and elemental analysis, vida infra, and the structure of trisulfide was assigned from its mass spectrum. Columns used for gas sample analyses include Porapak P, Q, R, N, and Tenax GC. For derivatized amino acid (N-trifluoroacetyl isopropyl esters) analyses, a 6 ft. EGA (0.65% on AW Chromosorb W) packed glass column was used. Supplementary IR (Perkin Elmer Model 237 B) and NMR (Varian T60) data were used occasionally to assist the identification of comparatively more complicated products. Elemental analysis of a solid product was done by Spang Microanalytical Laboratory. A conventional vacuum system with an oil diffusion pump and greaseless stopcocks was used for gas handling. A Matheson type 316 stainless steel pressure gauge was used for pressure measurement. In general, quantum yields of formation of products are calculated in the following way. Liquid nitrogen was used to trap all products except H 2 (not condensable at 77OK). The pressure of H 2 was measured and assuming the ideal gas law was valid, the amount of H 2 was determined. The amount of H 2 is equal to the number of photons absorbed b y H2S since each photon gives one hot H atom which then abstracts another H atom from a substrate to give H 2. The approximations regarding the sources of H 2 are that the reactions H + R" + M ~ RH + M H + HS" + M ~ H2S + M H+ H+M ~ H2 + M
(i)
where M is a third body are not important. This is an extremely good approximation since the concentration of all of the gases are much, much greater than that of the sum of R', HS', and H and furthermore, three b o d y collisions are highly unlikely; therefore, the rates of the above reactions, (I), are vanishing small by comparison. In addition, knowing the amount of products produced permits calculation of the quantum yields for those products. The amount of an amino acid produced was estimated by the GC peak area (as measured by the half-width times height). Calibration was previously done with an amino acid standard of known concentration. However because of the complication of ethylene and acetylene acting as H atom scavengers, in addition to H2S , certain corrections were necessary as described in the next section on Results.
18
J.H. Hong and R.S. Becker
Because of the large excess of H2S compared to any products formed and the wavelengths of light used, contribution from secondary photolysis of products to produce H 2 can be considered to be negligible. Results
A summary of results obtained from this study together with the results of a saturated hydrocarbon substrate is listed in Table 1. It should be noted that light was absorbed by H2S only (initially producing hot H atoms) in all reactions considered. In the case where ethylene was the carbon source, thio-compounds solely of a linear type were found, eight in all, Table 1. Ethane was also found. There were six aliphatic amino acids and one aromatic amino acid found of which glycine was the most abundant. Others such as aspartic acid, glutamic acid, alanine, valine,/~-alanine and phenylalanine were found in decreasing order of their abundances. The individual quantum yields are listed in Table 1. It is very unique that phenylalanine was formed since it was not detected using saturated hydrocarbon substrate counterparts nor in the acetylene system. The overall quantum yield of amino acids with ethylene as a substrate is estimated to be ~ 3 x 10 "5. The quantum yield estimation was complicated by the fact that the photolysis of H2S in the presence of C2H 4 will reduce the hydrogen yield (Wolley and Cvetanovic, 1969; Dzantiev and Shishkov, 1967; Darwent and Roberts, 1953). The latter can be explained by the competition between reactions (1) and (2) with those of (3) and (4): H* + C2H 4 -+ C2H5"* (v) -+ H + C2H4(v)
(1)
H + C2H 4 -+ C2H5" (v)
(2)
H* + H2S -+ H 2 + HS
(3)
H + H2S -+ H 2 + HS
(4)
where H* is a hot hydrogen atom, H is a thermalized hydrogen atom, and C2H5" (v) and C2H5"* (v) are hot and super hot ethyl radicals respectively. It has been suggested that the only fate of the H* when colliding with C2H4, reaction (1), is thermalization (Penzhorn et al., 1973). At sufficiently high pressures, C2H5" (v) from reaction (2) involving thermal H atom is efficiently deactivated and therefore, the reverse of reaction (2) does not occur (Wolley and Cvetanovic, 1969). The ratio k2/k 4 has been measured (Wolley and Cvetanovic, 1969) as 1.2. Also data (Wolley and Cvetanovic, 1969) suggest that the hot H* atoms exhibit even less discrimination in their reaction with olefins and H2S; therefore, the ratio k l / k 3 is much nearer to unity (Wolley and Cvetanovic, 1969) than is k2/k 4. It has also been reported (Wolley and Cvetanovic, 1969) that H 2 formation in the presence of C2H 4 was reduced to 65% of that of H2S photolyzed alone and remained practically constant up to total pressure of 800 torr under the condition that the pressure ratio of H2S to C2H 4 was 1 to 1. In our system although the initial pressure of H2S was twice as much as that of C2H 4, the NH 3 (equal to the C2H 4 pressure) reacted with one-half of the H2S to form NH4HS, which has a dissociation equilibrium constant of 0.1 (25oc) (Tuller, 1956). Also, NH4HS has a
Hydrogen Atom Chemistry
19
.I---t
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b
v ~
~
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g
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V
~2
I
~9 eq II
~
"~
"o
v
b
b o "O
o
o
e~
i ©
20
J.H. Hong and R.S. Becker
solubility in H 2 0 of 128 g/100 ml at 0 o c (Roberts, 1969). A combination of these two factors brings the ratio of H2S to C2H 4 to approximately one (the H2S pressure is probably greater than that of C2H 4 by approximately 25%-40%). On account of this fact, the total light absorbed by H2S based on the amount of H 2 formed from H2S is subject to a correction. This was done by dividing the total yield o f H 2 obtained b y 0.65 (the quantum yield of H 2 formation in the photolysis of H2S alone is unity) which in turn was used to estimate the quantum yield of formation of the amino acids, Table 1. The quantum yields may then be lower limits because the factor 0.65 could be larger by +25 to +40%. In the case of C2H2, the rate constant for the addition of H is % 1.6 x 10 -13 cc mo1-1 s"1 (Payne and Steif, 1976), which for C2H 4 the value is % 10 x 10 "13 cc mo1-1 s -1 (Lee et al., t 9 7 8 ; Cowfer et al., 1971) at approximately 1 arm total pressure and 298OK (our conditions). Therefore, the H 2 formation in the presence of C2H 2 should be reduced to % 90% of that of H2S photolyzed alone. As in the case of C2H4, the total light absorbed by H2S is subject to a correction by dividing the total H 2 obtained by 0.9, which in turn was used to estimate the quantum yield of formation of the amino acids, Table 1. The total quantum yield of amino acids is % 2 x 10 -4. The results with acetylene as a substrate is interesting by contrast in several ways relative to o t h e r c a r b o n sources we have studied. In the acetylene system there is a strong tendency to form cyclic thio-compounds while the ethylene system forms solely linear thio-compounds, Table 1. In addition with acetylene as a substrate three linear thio-compounds were formed, as well as ethylene and an alcohol, Table 1. Sulfur was not found in the acetylene system but was in the alkane and ethylene cases. Where acetylene was the substrate, only two amino acids were found but these were present in relatively high yields. The most abundant of all the products obtained appeared at first as a viscous oil that covered every cool surface of the reaction vessel. It had a peculiar odor characteristic of a sulfur compound, was sparingly soluble in water and had a density greater than water. The oily material was separated from water by separator funnel onto a molecular Sieve (Type 3A) to remove traces of water. This was followed by vacuum sublimation into a collector which gave white crystals with a m.p. of 42oc. Elemental analysis indicated that the empirical formula was C6H 13NS2 (formula weight of 163). Mass spectra showed the highest mass peak was at 163 with high intensity (94% of the base peak, 44) with weak isotope peaks at 163 and 165. Thus, the formula weight was assumed to be the molecular weight. The infrared spectrum (in chloroform) showed no absorption in the C=C, - O H , and - S H stretching regions. A weak absorption at 3300 cm -1 was attributed to an - N H stretching frequency. CH 3 bending absorptions at 1450 and 1375 cm "1 were observed. NMR (in Chloroform-d) showed two doublets at ~ values of 1.4 and 1.6 and multiplets were centered at ~ = 4.2 (which is the result of overlap of two quarters arising from two different C--H hydrogens) with relative areas of 6 : 3 : 3. The NMR spectrum of 5-methyl-5,6-dihydro-1,3,5, dithiazane (Leonard et al., 1962) which has the moiety
s~s
Hydrogen A t o m Chemistry
21
has multiplets centered at 6 = 3.43. The compound 2,4,6 trimethyl-l,3,5 trithiane (Campaigne et al., 1962) which has the moiety
C.H3
s/l-..s has two doublets at a 6 of 1.5 and 1.4 (Campaigne et al., 1962). Based upon all the various data obtained, the compound is assigned as thialdin with the structure of
c 3 N cH3 Discussion Perhaps the most important impact of this work is relative to potential reactions in the Jovian atmosphere. Of first importance is that acetylene and ethylene would appear to be formed by radical recombination reactions of the type "CH2 + "CH2 -+ C2H 4
(5)
-CH +-CH ~ C2H 2
(6)
The radicals originate from the photolysis of CH 4. There is a large enough excess of particularly H 2 and He and at a sufficiently high pressure that they can act as a third body. The photolysis of methane results in a near zero probability for the formation of CH 3 radical (Hellner et al., 1971). It has been proposed that ethane is produced as follows (in the absence of other gases) (Hellner et al., 1971). C H . + C H 4 ~ C2H 5.
(7)
C2H5 - + CH 4 -~ C2H 6 + CH 3-
(8)
The last reaction has an activation energy estimated at 15-20 kcal mo1-1 (Hellner et al., 1971). The C2H 5 radical formed by insertion of CH into the methane presumably has about 100 kcal tool "1 energy in excess of that required to rupture a C-H bond (Hellner et al., 1971); this can be then considered as a super hot radical. This fact would seem to permit reaction (8) to proceed even in the presence of H 2 and He since
22
J.H. Hong and R.S. Becker
rapid thermalization of the C2H5 radical by the light atoms (H 2 and He) would be relatively inefficient. Also, in view of the large excess energy in C2H 5 from reaction (7), dissociation of the super hot C2H5 radical into C2H 4 and H should certainly be able to occur as it does in reaction (1). This would provide a potential additional path for the formation of C2H4 . The photolysis of CH 4 (at 150 torr) leads to the formation of C2H6, C2H4 and C2H2 (among others) with quantum yields of 0.95, 0.14 and 0.05 respectively (Hellner et al., 1971). It is also possible for CH and CH 2 radicals to react with H 2 ultimately to produce methane (McNesby, 1969; Stroebel, 1969; Hunten, 1969). -CH + H 2 ~ CH 3-
(9)
•CH 3 + H 2 -+ CH 4 + H
(lO)
-CH 2 + H 2 + M
(11)
-+ CH 4 + M
where M is a third body. Presumably some radical recombination of CH 3 radical could occur to produce C2H 6. Despite the inability to be quantitative, C2H2, C2H 4 and C2H6 are produced in the Jovian atmosphere and are available for further reactions at some altitude and temperature. We shall now proceed to consider the significance of our experiments involving C2H 2 and C2H4 as well as H2S. In the lower Jovian atmosphere, it appears that a mixture of several gases including H2, He, CH4, C2H2, C2H4, C2H6, NH3, H2S and H 2 0 exist at total pressures in the 1-several arm pressure range and 250-400OK temperature range. Again H 2 and He are present in large excess compared to the other constituents. Photolysis of H2S will lead to the formation of H and HS radicals. In view of the large excess of H 2 and He over the other constituents, hot H atoms will very likely be thermalized before reaction. Despite this fact, the addition reactions of thermalized H atoms to acetylene and ethylene should be able to proceed since the activation energies for such additions are low. The addition of H (thermalized) to C2H 4 to produce C2H5 has an activation energy of 1.77 kcal mo1-1 and only 0.98 kcal/mol in the presence of H 2 as a third b o d y (Baldwin et al., 1966). The addition of H (thermalized) to C2H 2 to produce C2H3 has an activation energy of 1.5 kcal mo1-1 (Dingle and LeRoy, 1950). The activation energy in t h e presence of a third b o d y would appear to be still lower since the rate constant for the addition is larger in the presence of a third b o d y (Volpi and Zoeehi, 1966). The rate constants for the addition of H to C2H2 and C2H4 are % 1.6 x 10 -13 cc mol-1 s-1 (Payne and Steif, 1976) and "v 10 x 10 "13 cc mo1-1 s-1 (Lee et al., 1978; Cowfer et al., 1971) at approximately 1 atm total pressure and 298OK and are generally considered as fast reactions. Furthermore, in the case of the H + C2H2 reaction, the rate constant is both temperature and pressure dependent (Payne and Steif, 1976) although the pressure dependence tends to be quite small at the lower temperatures (193OK). At the upper temperatures (400OK), the high pressure limiting value is ~, 4 x 10-13 cc tool-1 s-1. At the limiting high pressure value, the rate constant varies from 5-43 x 10-13 cc mol-1 s-1 from 228OK to 400OK. In all cases the diluent gas was He. In the case of the C2H 4 + H reaction, the rate constant is also pressure dependent with a high pressure limiting value of % 10 x 10 -13 cc mo1-1 s-1 at 300OK (Cowfer et al.,
Hydrogen Atom Chemistry
23
1971 ; Lee et al., 1978). The rate constant is somewhat temperature dependent being larger at higher temperature. Although no data could be found regarding the activation energy for the addition of SH to C2H4, it is expected to be low. Furthermore, in the photolysis of H2S in the presence of C2H 4, the formation of C2HsSH was considered as necessary to explain the formation of C2H5-S-S-C2H 5 (Arthur and Bell, 1962). We also expect that the activation energy for the addition of SH to C2H 2 would be low. The irradiation of a mixture of H2S and acetylene at -78oc (photolysis of H2S assumed) led to the formation of vinyl mercaptan with minor yields of the saturated mercaptan, polymer, CS2 and possibly some olefins and dithiols (Straus et al., 1965). In the region of the Jovian atmosphere where principally H2S would exist, H2S would be the species undergoing photochemical decomposition (relative to C2H 2, C2H4, H 2 0 , and NH3). This would occur since the shorter ultraviolet wavelengths would have been removed by upper atmosphere molecules such as He, H2, CH 4 and NH 3 . Also, because of particularly the high pressures of He and H2, any hot H atoms generated would likely be rapidly thermalized before undergoing secondary H atom abstraction reactions with C2H2, C2H4, H 2 0 or NH 3. Thus the principal secondary reactions expected would be the additions of H and SH to C2H2 and C2H4 (or other unsaturated organic molecules). We shall first consider C2H4 . The addition of H and HS would proceed as C2H4 + H ~ C2H5 • C 2 H 4 + S H ~ C2H4SH • followed b y reactions of the type C2H 5. + H2S ~ C2H6 + SHC2H4SH • + H2S -+ C2H5SH + SHOnce this has occurred, photolysis of molecules like C2H5SH could occur such as C2H5SH ~ C2H5 S' + H followed by C2G5 S" + C2H5S" ~ C2G5-S-S-C2H 5 to produce more complex organic thio-compounds. These types of reactions can account for many of the thio-compounds, as well as C2H6, found in the reaction involving the photolysis of H2S in the presence of C2H4, NH3, H 2 0 , see Table 1, reaction II. The trace amount of CH3SH found in interesting since it indicates the presence of CH 3 radical. The biradical reaction of C2H5 and H is well known (Rabinowiteh et al., 1960; Steacie, 1954). C 2 H 5 . + H ~ C2H6" ~ 2CH3"
24
J.H. Hong and R.S. Becker
The mechanism(s) of the formation of the amino acids with C2H4 as the carbon substrated cannot be put forward. Nonetheless, the presence of odd number carbon atom amino acids also indicates that the decomposition of C2H5 occurred. Whether or not the formation of such amino acids could occur in a region of the atmosphere where CH4, NH3, and H 2 0 coexist is uncertain. However, it should be pointed out that it is not necessary for H2S to be present to supply a hot H atom for secondary reactions. Furthermore, it is also worth pointing out that we have observed the formation of a number of amino acids b y the photolysis of H2S (only) in the presence of CH4, H 2 0 and NH 3 (Becker et al., 1974). In any event it is worth repeating that it would appear that the formation of organic thio-compounds in the Jovian atmosphere would primarily occur b y addition reactions to unsaturated organic molecules, vide supra, rather than being initiated by secondary reactions involving the H atom abstraction from a saturated organic molecule. In the case of C2H2, we account for the formation of C2H 4 and C2H5SH by the addition of H and SH to C2H2 . Thiophene was produced in relatively large amounts. Its formation has been reported as a photochemical products from the ultraviolet irradiation of hydrogen sulfide in the presence of acetylene (Tsukada et al., 1972). Although it is known that the reaction of H atoms with acetylene forms benzene as one of the products (Cashion et al., 1954; Mains et al., 1963 ; Tsunashima et al., 19681 Shida et al., 1970), it was not found in this study. A small amount of H2S has been shown to decrease the yield of benzene and increase the yield of thiophene (Tsukada et al., 1972). The high concentration of H2S used in our system may have totally suppressed the formation of benzene. Thialdin has been synthesized by boiling of acetaldehyde and ammonium sulfide in water (Bruni et al., 1955). No CH3CHO was detected as part of our reaction products and it therefore appears that a different path involving free radical gas phase processes is involved in the mechanism. The mechanisms responsible for the formation of all of the cyclic thio-compounds are not clear. Nonetheless, it seems quite apparent that the CH2 = CH: and the vinyl mercaptan (-CH = CH - SH) radicals are involved and probably the C2H5 S. radical. ~-aminobutyric acid and glycine were produced with acetylene as a substrate. No other amino acids were detected. Both amino acids formed possess an even number of carbon atoms (2 and 4 carbons). This is not surprising in view of the strong carbon-carbon triplet bond and the large amount of energy that would be required to break it or double bonded radicals formed from acetylene. The absence of amino acids with higher carbon numbers than 4 may be due to the high chemical reactivity of vinyl radical which can be produced and then polymerized or cyclized to form products other than amino acids. A comparison of the results utilizing C2H6, C2H4, and C2H2 as substrates demonstrate that C2H 4 may be the most interesting hydrocarbon substrate for amino acid production especially based on the variety of amino acids produced and the formation of an aromatic amino acid. Nonetheless, the total quantum yield of the two amino acids produced with C2H 2 as a carbon source is ~ 7 times greater than that for C2H4 . In addition to the amino acids, a large number of thio-compounds are produced being principally cyclic beginning with C2H2 and linear beginning with C2H4 . The majority of the C2H 2 ultimately reacts to form thialdin and thiophene. Perhaps a change in the hot H atom source would prevent the formation of thialdin and thiophene
Hydrogen Atom Chemistry
25
and result in the formation of more variety and possibly higher yields of amino acids including aromatic ones (as well as benzene).
Acknowledgement. This work was supported by the National Aeronautics and Space Administration, grant NGR 44-005-O91. References Arthur, N.L., Bell, T.N. (1962). Photochemical addition of hydrogen sulphide to C 2 Olefins. J. Chem. Soc. 48664870 Baldwin, R.R., Simmons, R.F., Walker, R.W. (1966). Inhibition of the Hydrogen + Oxygen Reaction by Ethylene. Trans. Faraday Soc. 62, 2486-2492 Becket, R.S. (1975). Hot hydrogen in prebiological and interstellar chemistry. Science 188, 72 Becker, R.S., Hong, K.Y., Hong, J.H. (1974). Hot Hydrogen atoms: Initiators of reactions of interest interstellar chemistry. J. Mol. Evol. 4, 157-172 Campaigne, E., Chamberlain, N.F., Edwards, B.E. (1962). Nuclear magnetic resonance spectra of 1,3,5-trithiane and 2,4,6-Substituted 1,3,5-trithianes. J. Org. Chem. 27, 135-138 Cashion, J.K., LeRoy, D.J. (1954). Free radical mechanisms in the Mercury photosensitized reaction of hydrogen with acetylene. Can. J. Chem. 32,906-917 Combes, M., Enerenaz, Th., Vapillon, L., Zeau, Y. (1974). Conformation and the identification of C2H 2 and C2H 6 in the Jovian atmosphere. Astron. and Astrophys. 34, 33-35 Cowfer, J.A., Keil, D.G., Michael, J.V., Yeh, C. (1971). J. Phys. Chem. 75, 1584-1592 Darwent, B.deB., Roberts, R. (1953). The reactions of hydrogen atoms with hydrocarbons. Discuss. Faraday Soc. 14, 55-63 Dingle, J.R., LeRoy, D.J. (1950). Kinetics of the reaction of atomic hydrogen with acetylene. J. Chem. Phys. 18, 1632 Dzantiev, B.G., Shishkov, A.V. (1967). Photolysis of hydrogen sulfide. Effect of ethylene addition. Khim. Vys. Energ. 1(3), 192-196 Ferrin, I. (1974). University of Colorado, Private Communication Hellner, L., Masanet, J., Vermeil, C. (1971). Reactions of hydrogen and deuterium a atoms formed in the photolysis of methane and perdeuterated methane at 123.6 nm. J. Chem. Phys. 55, 1022-1028 Hunter, D.M. (1969). The upper atmosphere of Jupiter. J. Atmos. Sci. 26,826-834 Hong, K.Y., Hong, J.H., Becker, R.S. (1974). Hot hydrogen atoms: initiators of reactions of interest in interstellar chemistry and evolution. Science 184, 984-987 Lee, J .H., Michael, J.V., Payne, W.A., Stief, L.J. (1978). Absolute rate of the reaction of atomic hydrogen with ethylene from 198 to 320K at high pressure. J. Chem. Phys. 69, 1817-1820 Leonard, N.J., Conrow, K., Yethon, A.E. (1962). Eight-membered ring heterocycles from primary amines, hydrogen sulfide, and formaldehyde. J. Org. Chem. 27, 2019-2021 Mains, G.J., Niki, H., Wijnen, M.H. (1963). The formation of benzene in the radiolysis of acetylene. J. Phys. Chem. 67, 11-16
26
J.H. Hong and R.S. Becker
Mass Spectrometry Data Center, (1974). Eight Peak Index of Mass Spectra, 2nd Ed. United Kingdom NcNesby, J.R. (1969). The photochemistry of Jupiter above 1200 A. J. Atmos. Sci. 26,594-599 Payne, W.A., Stief, L.J. (1976). Absolute rate constant for the reaction of atomic hydrogen with acetylene over an extended pressure and temperature range. J. Phys. Chem. 64, 1150-1155 Penzhorn, R.D., Lissi, E., Rivas, I., Soto, H. (1973). The reaction of hot hydrogen atoms with ethylene. Zeitschrift fur Physikalische Chemie Neue Folge, Bd. 83,200-204 Rabinovitch, B.S., Dills, D.H., McLain, W.H., Current, J.H. (1960). Unimolecular decomposition of chemically activated ethyl-d 2 radicals. J. Chem. Phys. 32,493-498 Richter, F. (editor)(1955). Beilsteins Handbuch Der Organischen Chemie XXXVI I, pp. 525. Berlin, G/Sttingen, Heidelberg: Springer Ridgeway, S.T. (1974). Jupiter: Identification of ethane and acetylene. The Astrophys. J. 187, L41-L43 Roberts, C.W. (editor)(1969). Handbook of Chemistry and Physics, p. B-88, 50th Ed. The Chemical Rubber Co. Ohio Sagan, C., Khare, B.N. (1971a). Long Wavelength ultraviolet photoproduction of amino acids on the primitive earth. Science 173, 417420 Sagan, C., Khare, B.N. (1971b). Experimental Jovian photochemistry: initial results. The Astrophys. J. 168,563-569 Sagan, C., Khare, B.N. (1973). Molecules in the Galectic Environment (M.A. Gordon and L.E. Snyder, eds.) pp. 399-408. New York: Wiley Shida, S., Tsukada, M. (1970). Mercury-photosensitized reaction of acetylene. Bull. Chem. Soc. Japan 43, 2740-2745 Steacie, E.W.R. (1954). Atomic and Free Radical Reactions Vol. 1, pp. 514-515, 456-467. New York: Reinhold Publishing Corp. Strausz, O.P., Hikida, T., Gunning, H.E. (1965). Photochemical synthesis of vinylthiols. Can. J. Chem. 43,717-721 Stroebel, D.F. (1969). The photochemistry of methane in the Jovian atomosphere. J. Atmos. Scie. 26, 906-911 Stroebel, D.F. (1973). The photochemistry of hydrocarbons in the Jovian atmosphere. J. Atmos. SCk 30,489-498 Tsukada, M., Takefumi, O., Shida, S. (1972). Photochemical and radiation - Induced reactions of acetylene and hydrogen sulfide mixture. Synthesis of thiopehen. Chem. Lett. 437440 Tsunashima, S., Sato, S. (1968). The reactions of acetylene photosensitized by Cd. (3P1) Bull. Chem. Soc. Japan 41,2281-2284 Tuller, W.N. (Editor)(1954). The Sulphur Data Book, p. 67. New York: McGraw-Hill Book Co., Inc. Volpi, G.G., Zocchi, F. (1966). Mass spectrometric investigation of reactions of atomic hydrogen with acetylene. J. Chem. Phys. 44, 40104014 Wolley, G.R., Ovetanovic, R.J. (1969). Production of hydrogen atoms by photolysis of H2S and the rates of their addition of olefins. J. Chem. Phys. 50, 5697-4704 Received May 17, 1978~Revised November 9, 1978
Journal of Molecular Evolution © by Springer-Verlag. 1979
Hydrogen Atom Initiated Chemistry Jane Huey Hong and Ralph S. Becket Department of Chemistry, University of Houston, Houston, Texas 77004, U.S.A.
Summary. H atoms have been created by the photolysis of H2S. These then initiated reactions in mixtures involving acetylene-ammonia-water and ethylene-ammonia-water. In the case of the acetylene system, the products consisted of two amino acids, ethylene and a group of primarily cyclic thio-compounds, but no free sulfur. In the case of the ethylene systems, seven amino acids, including an aromatic one, ethane, free sulfur, and a group of solely linear thio-compounds were produced. Total quantum yields for the production of amino acids were % 3 x 10 -5 and % 2 x 10 -4 with ethylene and acetylene respectively as carbon substrates. Consideration is given of the mechanism for the formation of some of the products and implications regarding planetary atmosphere chemistry, particularly that of Jupiter, are explored.
Key words: H atom reactions - Acetylene - Ethylene - Amino acids - Thiocompounds - Jovian atmosphere. Introduction Some simple hydrocarbons such as methane (CH4), ethane (C2H 6) and acetylene (C2H 2) as well as carbon monoxide (CO) have been identified in the atmosphere of Jupiter (Combes et al., 1974; Ridgeway, 1974). There is also indication that ethylene exists (C2H 4) (Ridgeway, 1974). The presence of C2H6, C2H 2 and CO (as well as C2H 4) is an indication of non-equilibrium processes with irreversible conversion of CH 4 into larger hydrocarbons. A steady state must involve downward movement of the heavier hydrocarbons and upward movement of CH 4 (Hunten, 1969). Also, it is estimated that C2-type hydrocarbons produced from the photolysis of CH 4 have a downward flux of 3 x 10-14g cm "2 s -1 (Strobel, 1973). Based on the photolysis of CH 4 (at 123.6 nm) in the presence of deuterium it has been concluded that the photodissociation CH 4 + hv
-~ CH 3 + H
0022-2844/79/0013/0015/~ 02.40
16
J.H. Hong and R.S. Becker
has an almost zero probability of occurrence (Hellner et al., 1971). Appreciable quantum yields of C2H6, C2H 4 and C2H 2 were obtained. The principal reactions appear to be (Hellner et al., 1971) CH 4 + hv -+ 1CH2 + H 2 3CH2 + 2H -+ CH + H 2 + H In the presence of inert gases such as Ar, the yields of C2H 4 and C2H 2 were increased. The possible contribution of hot and thermalized H atoms to the reactions occurring in the Jovian atmosphere is complex. The most important argument is that any hot H atoms produced by photodissociation would normally be expected to rapidly thermalize by collisions with the large excesses of both H 2 and He which are present (compared to any other components, even CH4). In such a case initially hot H atoms would loose their potential for initiating secondary reactions where higher activation energies are required, such as H abstraction reactions. However, thermalized H atoms can undergo addition reactions where low activation energies are involved. This problem appears to be an important one and we have considered it in our later discussion. Hot and thermalized H atoms are produced in both the atmospheres of Venus and Mars (Ferrin, 1974). Furthermore, several of the molecules included in this study have been identified in interstellar space such as NH 3, H 2 0 and C2H 2 (NH 3 and H 2 0 are also present in the Jovian atmosphere). Of all the simulated syntheses of biologically meaningful organic compounds, the carbon sources were frequently acquired from saturated hydrocarbons such as methane or ethane (Hong et al., 1974; Becket et al., 1974; Becker, 1975; Sagan and Khare, 1971a and b, 1973). Little or no effort has been dedicated to the exploration of unsaturated hydrocarbons as carbon sources. One of the primary objectives of this work is to explore the nature and mechanism of H atom initiated chemistry as related to molecules important in chemical evolution, and present in planetary atmospheres and interstellar space.
Experimental All gases used were c.p. grade (Matheson) and were further purified by low temperature distillation and degassing and collected at dry ice-acetone slush temperature to remove acetone, CO 2 and water. GC-MS analysis indicated that these impurities were removed. Deionized and distilled water was used on all occasions where water was involved. The experimental apparatus used in this study and the detailed procedures for product analysis have been described previously (Hong et al., 1974; Becker et al., 1974; Becker, 1975). The volume of the apparatus was carefully calibrated in order to be able to estimate the quantum yields of some of the photochemical products. For each set of reactions, three individual reactions were carried out. Two reactions involved photodissociation of H2S in the presence of substrates and one was a blank thermal reaction involving H2S plus substrates. None of the products identified in this study were found in the thermal blank reaction. Gas mixtures of C2H4-NH3-H20-H2S (1:1:0.1:2) and C2H2-NH3-H20-H2S (1:1:0.1:2.7), where the number in parenthesis stands for the pressure ratio (at one
Hydrogen A t o m Chemistry
17
atmosphere total pressure and total volume of 7212 ml) were irradated respectively for approximately 150 h with a 500 W high pressure mercury lamp from Illumination Industry Inc. A filter system was used for which the band pass was 2 2 0 - 2 8 0 nm with a maximum transmission of 54% at 252 nm. The product identification was based mainly on mass spectral data obtained from a CG/MS/data system (Hewlett-Packard gas chromatograph Model 57 lOA/mass spectrometer Model 5930A/Data System Model 5933A). The gas chromatograph was modified by the installation of a gas sampling valve (Carle Inst. Co.) with provided for fast transfer time from a loop to a column, prevented introduction of air into the system and increased sensitivity. The identity of the amino acids obtained from a reaction was established by direct comparison of the GC retention times and mass spectra with those of authentic samples measured under the same conditions. All other products except thialdin and the trisulfide, were identified by direct comparison of the mass spectra with those given in the literature (Mass Spectrometry Data Center 1974). Thialdin was identified based on mass spectra, NMR, IR and elemental analysis, vida infra, and the structure of trisulfide was assigned from its mass spectrum. Columns used for gas sample analyses include Porapak P, Q, R, N, and Tenax GC. For derivatized amino acid (N-trifluoroacetyl isopropyl esters) analyses, a 6 ft. EGA (0.65% on AW Chromosorb W) packed glass column was used. Supplementary IR (Perkin Elmer Model 237 B) and NMR (Varian T60) data were used occasionally to assist the identification of comparatively more complicated products. Elemental analysis of a solid product was done by Spang Microanalytical Laboratory. A conventional vacuum system with an oil diffusion pump and greaseless stopcocks was used for gas handling. A Matheson type 316 stainless steel pressure gauge was used for pressure measurement. In general, quantum yields of formation of products are calculated in the following way. Liquid nitrogen was used to trap all products except H 2 (not condensable at 77OK). The pressure of H 2 was measured and assuming the ideal gas law was valid, the amount of H 2 was determined. The amount of H 2 is equal to the number of photons absorbed b y H2S since each photon gives one hot H atom which then abstracts another H atom from a substrate to give H 2. The approximations regarding the sources of H 2 are that the reactions H + R" + M ~ RH + M H + HS" + M ~ H2S + M H+ H+M ~ H2 + M
(i)
where M is a third body are not important. This is an extremely good approximation since the concentration of all of the gases are much, much greater than that of the sum of R', HS', and H and furthermore, three b o d y collisions are highly unlikely; therefore, the rates of the above reactions, (I), are vanishing small by comparison. In addition, knowing the amount of products produced permits calculation of the quantum yields for those products. The amount of an amino acid produced was estimated by the GC peak area (as measured by the half-width times height). Calibration was previously done with an amino acid standard of known concentration. However because of the complication of ethylene and acetylene acting as H atom scavengers, in addition to H2S , certain corrections were necessary as described in the next section on Results.
18
J.H. Hong and R.S. Becker
Because of the large excess of H2S compared to any products formed and the wavelengths of light used, contribution from secondary photolysis of products to produce H 2 can be considered to be negligible. Results
A summary of results obtained from this study together with the results of a saturated hydrocarbon substrate is listed in Table 1. It should be noted that light was absorbed by H2S only (initially producing hot H atoms) in all reactions considered. In the case where ethylene was the carbon source, thio-compounds solely of a linear type were found, eight in all, Table 1. Ethane was also found. There were six aliphatic amino acids and one aromatic amino acid found of which glycine was the most abundant. Others such as aspartic acid, glutamic acid, alanine, valine,/~-alanine and phenylalanine were found in decreasing order of their abundances. The individual quantum yields are listed in Table 1. It is very unique that phenylalanine was formed since it was not detected using saturated hydrocarbon substrate counterparts nor in the acetylene system. The overall quantum yield of amino acids with ethylene as a substrate is estimated to be ~ 3 x 10 "5. The quantum yield estimation was complicated by the fact that the photolysis of H2S in the presence of C2H 4 will reduce the hydrogen yield (Wolley and Cvetanovic, 1969; Dzantiev and Shishkov, 1967; Darwent and Roberts, 1953). The latter can be explained by the competition between reactions (1) and (2) with those of (3) and (4): H* + C2H 4 -+ C2H5"* (v) -+ H + C2H4(v)
(1)
H + C2H 4 -+ C2H5" (v)
(2)
H* + H2S -+ H 2 + HS
(3)
H + H2S -+ H 2 + HS
(4)
where H* is a hot hydrogen atom, H is a thermalized hydrogen atom, and C2H5" (v) and C2H5"* (v) are hot and super hot ethyl radicals respectively. It has been suggested that the only fate of the H* when colliding with C2H4, reaction (1), is thermalization (Penzhorn et al., 1973). At sufficiently high pressures, C2H5" (v) from reaction (2) involving thermal H atom is efficiently deactivated and therefore, the reverse of reaction (2) does not occur (Wolley and Cvetanovic, 1969). The ratio k2/k 4 has been measured (Wolley and Cvetanovic, 1969) as 1.2. Also data (Wolley and Cvetanovic, 1969) suggest that the hot H* atoms exhibit even less discrimination in their reaction with olefins and H2S; therefore, the ratio k l / k 3 is much nearer to unity (Wolley and Cvetanovic, 1969) than is k2/k 4. It has also been reported (Wolley and Cvetanovic, 1969) that H 2 formation in the presence of C2H 4 was reduced to 65% of that of H2S photolyzed alone and remained practically constant up to total pressure of 800 torr under the condition that the pressure ratio of H2S to C2H 4 was 1 to 1. In our system although the initial pressure of H2S was twice as much as that of C2H 4, the NH 3 (equal to the C2H 4 pressure) reacted with one-half of the H2S to form NH4HS, which has a dissociation equilibrium constant of 0.1 (25oc) (Tuller, 1956). Also, NH4HS has a
Hydrogen Atom Chemistry
19
.I---t
"G II
b
v ~
~
b
g
~q
V
~2
I
~9 eq II
~
"~
"o
v
b
b o "O
o
o
e~
i ©
20
J.H. Hong and R.S. Becker
solubility in H 2 0 of 128 g/100 ml at 0 o c (Roberts, 1969). A combination of these two factors brings the ratio of H2S to C2H 4 to approximately one (the H2S pressure is probably greater than that of C2H 4 by approximately 25%-40%). On account of this fact, the total light absorbed by H2S based on the amount of H 2 formed from H2S is subject to a correction. This was done by dividing the total yield o f H 2 obtained b y 0.65 (the quantum yield of H 2 formation in the photolysis of H2S alone is unity) which in turn was used to estimate the quantum yield of formation of the amino acids, Table 1. The quantum yields may then be lower limits because the factor 0.65 could be larger by +25 to +40%. In the case of C2H2, the rate constant for the addition of H is % 1.6 x 10 -13 cc mo1-1 s"1 (Payne and Steif, 1976), which for C2H 4 the value is % 10 x 10 "13 cc mo1-1 s -1 (Lee et al., t 9 7 8 ; Cowfer et al., 1971) at approximately 1 arm total pressure and 298OK (our conditions). Therefore, the H 2 formation in the presence of C2H 2 should be reduced to % 90% of that of H2S photolyzed alone. As in the case of C2H4, the total light absorbed by H2S is subject to a correction by dividing the total H 2 obtained by 0.9, which in turn was used to estimate the quantum yield of formation of the amino acids, Table 1. The total quantum yield of amino acids is % 2 x 10 -4. The results with acetylene as a substrate is interesting by contrast in several ways relative to o t h e r c a r b o n sources we have studied. In the acetylene system there is a strong tendency to form cyclic thio-compounds while the ethylene system forms solely linear thio-compounds, Table 1. In addition with acetylene as a substrate three linear thio-compounds were formed, as well as ethylene and an alcohol, Table 1. Sulfur was not found in the acetylene system but was in the alkane and ethylene cases. Where acetylene was the substrate, only two amino acids were found but these were present in relatively high yields. The most abundant of all the products obtained appeared at first as a viscous oil that covered every cool surface of the reaction vessel. It had a peculiar odor characteristic of a sulfur compound, was sparingly soluble in water and had a density greater than water. The oily material was separated from water by separator funnel onto a molecular Sieve (Type 3A) to remove traces of water. This was followed by vacuum sublimation into a collector which gave white crystals with a m.p. of 42oc. Elemental analysis indicated that the empirical formula was C6H 13NS2 (formula weight of 163). Mass spectra showed the highest mass peak was at 163 with high intensity (94% of the base peak, 44) with weak isotope peaks at 163 and 165. Thus, the formula weight was assumed to be the molecular weight. The infrared spectrum (in chloroform) showed no absorption in the C=C, - O H , and - S H stretching regions. A weak absorption at 3300 cm -1 was attributed to an - N H stretching frequency. CH 3 bending absorptions at 1450 and 1375 cm "1 were observed. NMR (in Chloroform-d) showed two doublets at ~ values of 1.4 and 1.6 and multiplets were centered at ~ = 4.2 (which is the result of overlap of two quarters arising from two different C--H hydrogens) with relative areas of 6 : 3 : 3. The NMR spectrum of 5-methyl-5,6-dihydro-1,3,5, dithiazane (Leonard et al., 1962) which has the moiety
s~s
Hydrogen A t o m Chemistry
21
has multiplets centered at 6 = 3.43. The compound 2,4,6 trimethyl-l,3,5 trithiane (Campaigne et al., 1962) which has the moiety
C.H3
s/l-..s has two doublets at a 6 of 1.5 and 1.4 (Campaigne et al., 1962). Based upon all the various data obtained, the compound is assigned as thialdin with the structure of
c 3 N cH3 Discussion Perhaps the most important impact of this work is relative to potential reactions in the Jovian atmosphere. Of first importance is that acetylene and ethylene would appear to be formed by radical recombination reactions of the type "CH2 + "CH2 -+ C2H 4
(5)
-CH +-CH ~ C2H 2
(6)
The radicals originate from the photolysis of CH 4. There is a large enough excess of particularly H 2 and He and at a sufficiently high pressure that they can act as a third body. The photolysis of methane results in a near zero probability for the formation of CH 3 radical (Hellner et al., 1971). It has been proposed that ethane is produced as follows (in the absence of other gases) (Hellner et al., 1971). C H . + C H 4 ~ C2H 5.
(7)
C2H5 - + CH 4 -~ C2H 6 + CH 3-
(8)
The last reaction has an activation energy estimated at 15-20 kcal mo1-1 (Hellner et al., 1971). The C2H 5 radical formed by insertion of CH into the methane presumably has about 100 kcal tool "1 energy in excess of that required to rupture a C-H bond (Hellner et al., 1971); this can be then considered as a super hot radical. This fact would seem to permit reaction (8) to proceed even in the presence of H 2 and He since
22
J.H. Hong and R.S. Becker
rapid thermalization of the C2H5 radical by the light atoms (H 2 and He) would be relatively inefficient. Also, in view of the large excess energy in C2H 5 from reaction (7), dissociation of the super hot C2H5 radical into C2H 4 and H should certainly be able to occur as it does in reaction (1). This would provide a potential additional path for the formation of C2H4 . The photolysis of CH 4 (at 150 torr) leads to the formation of C2H6, C2H4 and C2H2 (among others) with quantum yields of 0.95, 0.14 and 0.05 respectively (Hellner et al., 1971). It is also possible for CH and CH 2 radicals to react with H 2 ultimately to produce methane (McNesby, 1969; Stroebel, 1969; Hunten, 1969). -CH + H 2 ~ CH 3-
(9)
•CH 3 + H 2 -+ CH 4 + H
(lO)
-CH 2 + H 2 + M
(11)
-+ CH 4 + M
where M is a third body. Presumably some radical recombination of CH 3 radical could occur to produce C2H 6. Despite the inability to be quantitative, C2H2, C2H 4 and C2H6 are produced in the Jovian atmosphere and are available for further reactions at some altitude and temperature. We shall now proceed to consider the significance of our experiments involving C2H 2 and C2H4 as well as H2S. In the lower Jovian atmosphere, it appears that a mixture of several gases including H2, He, CH4, C2H2, C2H4, C2H6, NH3, H2S and H 2 0 exist at total pressures in the 1-several arm pressure range and 250-400OK temperature range. Again H 2 and He are present in large excess compared to the other constituents. Photolysis of H2S will lead to the formation of H and HS radicals. In view of the large excess of H 2 and He over the other constituents, hot H atoms will very likely be thermalized before reaction. Despite this fact, the addition reactions of thermalized H atoms to acetylene and ethylene should be able to proceed since the activation energies for such additions are low. The addition of H (thermalized) to C2H 4 to produce C2H5 has an activation energy of 1.77 kcal mo1-1 and only 0.98 kcal/mol in the presence of H 2 as a third b o d y (Baldwin et al., 1966). The addition of H (thermalized) to C2H 2 to produce C2H3 has an activation energy of 1.5 kcal mo1-1 (Dingle and LeRoy, 1950). The activation energy in t h e presence of a third b o d y would appear to be still lower since the rate constant for the addition is larger in the presence of a third b o d y (Volpi and Zoeehi, 1966). The rate constants for the addition of H to C2H2 and C2H4 are % 1.6 x 10 -13 cc mol-1 s-1 (Payne and Steif, 1976) and "v 10 x 10 "13 cc mo1-1 s-1 (Lee et al., 1978; Cowfer et al., 1971) at approximately 1 atm total pressure and 298OK and are generally considered as fast reactions. Furthermore, in the case of the H + C2H2 reaction, the rate constant is both temperature and pressure dependent (Payne and Steif, 1976) although the pressure dependence tends to be quite small at the lower temperatures (193OK). At the upper temperatures (400OK), the high pressure limiting value is ~, 4 x 10-13 cc tool-1 s-1. At the limiting high pressure value, the rate constant varies from 5-43 x 10-13 cc mol-1 s-1 from 228OK to 400OK. In all cases the diluent gas was He. In the case of the C2H 4 + H reaction, the rate constant is also pressure dependent with a high pressure limiting value of % 10 x 10 -13 cc mo1-1 s-1 at 300OK (Cowfer et al.,
Hydrogen Atom Chemistry
23
1971 ; Lee et al., 1978). The rate constant is somewhat temperature dependent being larger at higher temperature. Although no data could be found regarding the activation energy for the addition of SH to C2H4, it is expected to be low. Furthermore, in the photolysis of H2S in the presence of C2H 4, the formation of C2HsSH was considered as necessary to explain the formation of C2H5-S-S-C2H 5 (Arthur and Bell, 1962). We also expect that the activation energy for the addition of SH to C2H 2 would be low. The irradiation of a mixture of H2S and acetylene at -78oc (photolysis of H2S assumed) led to the formation of vinyl mercaptan with minor yields of the saturated mercaptan, polymer, CS2 and possibly some olefins and dithiols (Straus et al., 1965). In the region of the Jovian atmosphere where principally H2S would exist, H2S would be the species undergoing photochemical decomposition (relative to C2H 2, C2H4, H 2 0 , and NH3). This would occur since the shorter ultraviolet wavelengths would have been removed by upper atmosphere molecules such as He, H2, CH 4 and NH 3 . Also, because of particularly the high pressures of He and H2, any hot H atoms generated would likely be rapidly thermalized before undergoing secondary H atom abstraction reactions with C2H2, C2H4, H 2 0 or NH 3. Thus the principal secondary reactions expected would be the additions of H and SH to C2H2 and C2H4 (or other unsaturated organic molecules). We shall first consider C2H4 . The addition of H and HS would proceed as C2H4 + H ~ C2H5 • C 2 H 4 + S H ~ C2H4SH • followed b y reactions of the type C2H 5. + H2S ~ C2H6 + SHC2H4SH • + H2S -+ C2H5SH + SHOnce this has occurred, photolysis of molecules like C2H5SH could occur such as C2H5SH ~ C2H5 S' + H followed by C2G5 S" + C2H5S" ~ C2G5-S-S-C2H 5 to produce more complex organic thio-compounds. These types of reactions can account for many of the thio-compounds, as well as C2H6, found in the reaction involving the photolysis of H2S in the presence of C2H4, NH3, H 2 0 , see Table 1, reaction II. The trace amount of CH3SH found in interesting since it indicates the presence of CH 3 radical. The biradical reaction of C2H5 and H is well known (Rabinowiteh et al., 1960; Steacie, 1954). C 2 H 5 . + H ~ C2H6" ~ 2CH3"
24
J.H. Hong and R.S. Becker
The mechanism(s) of the formation of the amino acids with C2H4 as the carbon substrated cannot be put forward. Nonetheless, the presence of odd number carbon atom amino acids also indicates that the decomposition of C2H5 occurred. Whether or not the formation of such amino acids could occur in a region of the atmosphere where CH4, NH3, and H 2 0 coexist is uncertain. However, it should be pointed out that it is not necessary for H2S to be present to supply a hot H atom for secondary reactions. Furthermore, it is also worth pointing out that we have observed the formation of a number of amino acids b y the photolysis of H2S (only) in the presence of CH4, H 2 0 and NH 3 (Becker et al., 1974). In any event it is worth repeating that it would appear that the formation of organic thio-compounds in the Jovian atmosphere would primarily occur b y addition reactions to unsaturated organic molecules, vide supra, rather than being initiated by secondary reactions involving the H atom abstraction from a saturated organic molecule. In the case of C2H2, we account for the formation of C2H 4 and C2H5SH by the addition of H and SH to C2H2 . Thiophene was produced in relatively large amounts. Its formation has been reported as a photochemical products from the ultraviolet irradiation of hydrogen sulfide in the presence of acetylene (Tsukada et al., 1972). Although it is known that the reaction of H atoms with acetylene forms benzene as one of the products (Cashion et al., 1954; Mains et al., 1963 ; Tsunashima et al., 19681 Shida et al., 1970), it was not found in this study. A small amount of H2S has been shown to decrease the yield of benzene and increase the yield of thiophene (Tsukada et al., 1972). The high concentration of H2S used in our system may have totally suppressed the formation of benzene. Thialdin has been synthesized by boiling of acetaldehyde and ammonium sulfide in water (Bruni et al., 1955). No CH3CHO was detected as part of our reaction products and it therefore appears that a different path involving free radical gas phase processes is involved in the mechanism. The mechanisms responsible for the formation of all of the cyclic thio-compounds are not clear. Nonetheless, it seems quite apparent that the CH2 = CH: and the vinyl mercaptan (-CH = CH - SH) radicals are involved and probably the C2H5 S. radical. ~-aminobutyric acid and glycine were produced with acetylene as a substrate. No other amino acids were detected. Both amino acids formed possess an even number of carbon atoms (2 and 4 carbons). This is not surprising in view of the strong carbon-carbon triplet bond and the large amount of energy that would be required to break it or double bonded radicals formed from acetylene. The absence of amino acids with higher carbon numbers than 4 may be due to the high chemical reactivity of vinyl radical which can be produced and then polymerized or cyclized to form products other than amino acids. A comparison of the results utilizing C2H6, C2H4, and C2H2 as substrates demonstrate that C2H 4 may be the most interesting hydrocarbon substrate for amino acid production especially based on the variety of amino acids produced and the formation of an aromatic amino acid. Nonetheless, the total quantum yield of the two amino acids produced with C2H 2 as a carbon source is ~ 7 times greater than that for C2H4 . In addition to the amino acids, a large number of thio-compounds are produced being principally cyclic beginning with C2H2 and linear beginning with C2H4 . The majority of the C2H 2 ultimately reacts to form thialdin and thiophene. Perhaps a change in the hot H atom source would prevent the formation of thialdin and thiophene
Hydrogen Atom Chemistry
25
and result in the formation of more variety and possibly higher yields of amino acids including aromatic ones (as well as benzene).
Acknowledgement. This work was supported by the National Aeronautics and Space Administration, grant NGR 44-005-O91. References Arthur, N.L., Bell, T.N. (1962). Photochemical addition of hydrogen sulphide to C 2 Olefins. J. Chem. Soc. 48664870 Baldwin, R.R., Simmons, R.F., Walker, R.W. (1966). Inhibition of the Hydrogen + Oxygen Reaction by Ethylene. Trans. Faraday Soc. 62, 2486-2492 Becket, R.S. (1975). Hot hydrogen in prebiological and interstellar chemistry. Science 188, 72 Becker, R.S., Hong, K.Y., Hong, J.H. (1974). Hot Hydrogen atoms: Initiators of reactions of interest interstellar chemistry. J. Mol. Evol. 4, 157-172 Campaigne, E., Chamberlain, N.F., Edwards, B.E. (1962). Nuclear magnetic resonance spectra of 1,3,5-trithiane and 2,4,6-Substituted 1,3,5-trithianes. J. Org. Chem. 27, 135-138 Cashion, J.K., LeRoy, D.J. (1954). Free radical mechanisms in the Mercury photosensitized reaction of hydrogen with acetylene. Can. J. Chem. 32,906-917 Combes, M., Enerenaz, Th., Vapillon, L., Zeau, Y. (1974). Conformation and the identification of C2H 2 and C2H 6 in the Jovian atmosphere. Astron. and Astrophys. 34, 33-35 Cowfer, J.A., Keil, D.G., Michael, J.V., Yeh, C. (1971). J. Phys. Chem. 75, 1584-1592 Darwent, B.deB., Roberts, R. (1953). The reactions of hydrogen atoms with hydrocarbons. Discuss. Faraday Soc. 14, 55-63 Dingle, J.R., LeRoy, D.J. (1950). Kinetics of the reaction of atomic hydrogen with acetylene. J. Chem. Phys. 18, 1632 Dzantiev, B.G., Shishkov, A.V. (1967). Photolysis of hydrogen sulfide. Effect of ethylene addition. Khim. Vys. Energ. 1(3), 192-196 Ferrin, I. (1974). University of Colorado, Private Communication Hellner, L., Masanet, J., Vermeil, C. (1971). Reactions of hydrogen and deuterium a atoms formed in the photolysis of methane and perdeuterated methane at 123.6 nm. J. Chem. Phys. 55, 1022-1028 Hunter, D.M. (1969). The upper atmosphere of Jupiter. J. Atmos. Sci. 26,826-834 Hong, K.Y., Hong, J.H., Becker, R.S. (1974). Hot hydrogen atoms: initiators of reactions of interest in interstellar chemistry and evolution. Science 184, 984-987 Lee, J .H., Michael, J.V., Payne, W.A., Stief, L.J. (1978). Absolute rate of the reaction of atomic hydrogen with ethylene from 198 to 320K at high pressure. J. Chem. Phys. 69, 1817-1820 Leonard, N.J., Conrow, K., Yethon, A.E. (1962). Eight-membered ring heterocycles from primary amines, hydrogen sulfide, and formaldehyde. J. Org. Chem. 27, 2019-2021 Mains, G.J., Niki, H., Wijnen, M.H. (1963). The formation of benzene in the radiolysis of acetylene. J. Phys. Chem. 67, 11-16
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