J. Phys. Chem. 1982, 86, 2646-2651
2646
taining compounds were observed here. On the basis of the proposed model of catalytic sites,3J4J5 we infer that a given catalytic site is potentially capable of catalyzing both the hydrogenation and hydrogenolysis reactions in ratios that depend upon the extent of interconversion of the two structures, 2 and 2a in Figure 7, and are both inhibited by adsorption of H2S and aromatic amines as shown in Figure 8. Hydrogenation sites may be directly removed from operation by adsorption of a basic amine as shown in 2b in Figure 8. Hydrogenolysis is inhibited when the anionic vacancy of a hydrogenolysis site (2a) is vitiated through coordination of the latter with an (unreduced) aromatic amine inhibitor (13, Figure 8). This can be seen to account for the inhibitory effects of aromatic nitrogen on both the hydrogenation and the overall hydrodesulfurization and hydrodenitrogenation reactions of the catalyst. The potential inhibitory effect of H2S on hydrogenation can be attributed to formation of a multicoordinate complex 10 in which H2S becomes a ligand of the hydrogenation site 2 (see also a +6 molybdenum in Figure 1). In much the same manner, a >C=C< 7 bond is complex coordinated, before its reduction, as depicted in 11 12 in Figure 8. A recent articlelgdescribing inhibition by various amines under mild hydrodesulfurization conditions reports that aromatic nitrogen heterocycles containing alkyl substitu-
-
(19)Gutberlet, L.C.; Bertolacini, R. J. Reprinta of the Conference of the Catalysis Society, Boston, MA, Sept 1981;ARticle F-10.
ents which strongly shield the lone pair do not inhibit the HDS reaction under catalytic conditions which minimize hydrogenation activity. This very interesting observation suggests that the sp2lone pair on (for example) a pyridine nitrogen is normally responsible for inhibition of HDS through coordination of the anionic vacancy in 2a as discussed above. Alkyl substituents in these cases apparently obstruct such coordination at 2a and thus 2,6-dimethylpyridine (for example) is foundla to be ineffective as an HDS inhibitor. Coordination of the sp2unshared pair in pyridine which projects from the plane of the ring is thus seen to result in an end-on configuration of the ring relative to the active center. It is thus configuration which cannot be achieved when sterically hindering alkyl groups are present. However, it must be recognized that 2,6-dimethylpyridine is readily hydrogenated under ordinary conditions, indicating that the hydrogenation site 2 easily binds the 7 electron cloud projecting perpendicular to the ring and holds the ring in the planar configuration necessary for the ensuing cis-hydrogen addition step. The resulting 2,6-dimethylpiperidine is indeed an effective inhibitor of the HDS reaction because the sp3lone pair in the puckered structure is not completely obstructed by the 2,6 alkyl substituents. Moreover, we would expect that, subsequent to coordination of this piperidine, the hydrodenitrogenation reaction steps would proceed as found for other aliphatic amines (e.g., benzylamine) by Gutberlet and Bert01acini.l~
Vacuum-Ultraviolet Photolysls of Methylaminet Edward P. Gardnert and James R. McNesby' Depertment of Chemism, University of Meryland, Collsge Park, Meryland 20742 (Received: September 1, 1981; I n Final Form: February 2, 1982)
Photolyses of methylamine have been carried out at 184.9, 147.0 and 123.6 nm. Quantum yields of hydrogen, hydrocarbons, and nitrogen have been measured. Evidence has also been obtained for the formation of HCN and CN in primary processes at the shorter wavelengths. Photolyses of CD3NHzin the presence of oxygen and NO provide evidence that H and D atom production dominates at 184.9 nm while Hz and Dz elimination occurs to an important extent at 147.0 and 123.6 nm. The primary process of molecular hydrogen elimination is partly terminal and partly nonterminal.
Introduction The synthesis of organic compounds (e.g., HCN and H2CO)by electric discharges in mixtures of methane, ammonia, and water was first achieved in the laboratory by Miller.'S2 Among the compounds produced were several of biological interest causing, in the mid-l950s, a revival of interest in prebiotic ~hemistry3.~ to explain the origin of simple life forms. These early experiments spawned a series of successful efforts to produce similar results using It was not until ultraviolet light as the energy 1979, however, that the molecule responsible for the establishment of the C-N bond in photolysis of CH4/NH3 mixtures was identified experimentally as methylamine.l1J8 This observation has been confirmed by Bossard and Toupance.lg The photochemistry of methylamine has, From the Ph.D. Disseration of E. P. Gardner, May 1981.
* Department of Planetary Science, California Institute of Tech-
nology, Pasadena, CA 91125.
0022-385418212086-2646$01.25/0
therefore, assumed new relevance to prebiotic photochemical synthesis. (1)S.L. Miller, Science, 117,528-9 (1953). 77,2351-61 (1955). (2)S.L. Miller, J. Am. Chem. SOC., (3)A. I. Oparin, Proischogdenie Zhizni Zzd. Moskousky Robotchii, (1924). (4)J. S. B.Haldane, Rationalist Annual, 148,3-10 (1928). (5)W. Groth, Agnew. Chem., 69,681 (1957). (6)W. Groth and H. Von Wesyenhoff,Natunuissenschoften, 44,510-1 (1957). (7)W. Groth and H. Von Wesyenhoff, Planet. Space Sci., 2, 79-85 (1960). (8) N.Y.Dodonova and A. I. Sidorova, Biophysics (Engl. Transl.), 6, 14-20 (1961). (9)C. Sagan and B. N. Khare, Science, 173,417-20 (1971). (10)K. Hong, J. Hong, and R. S. Becker, Science, 184,984-7 (1974). (11)K. Hong, J. Hong,and R. S. Becker In 'Cosmochemical Evolution and the Origin of Life", Vol. 11, J. Oro, S.L. Miller, C. Ponnamperuma, and R. S. Young, Eds., Reidel, Dordrecht, Holland, 1974,pp 287-94. (12)R. S.Becker, K. Hong, and J. H. Hong, J.Mol. Euol., 4,157-72 (1974). (13)J. P. Ferris and C. T. Chen, J. Am. Chem. SOC.,97,2962-7 (1975). (14)J. P. Ferris and C. T. Chen, Nature (London),258,587-8(1975).
0 1982 American Chemical Society
Vacuum-Ultraviolet Photolysis of Methylamine
A number of studies on the near-ultraviolet photolysis of methylamine have been p ~ b l i s h e din~which ~ ~ ~ the expulsion of a single atom of hydrogen was thought to be the major primary process. Hadley and V ~ l m a showed n~~ that, in a solid matrix at 184.9 nm, the major process was reaction 1 based on the ESR spectrum of the reaction CH3NH2 H + CH3NH (1) products. The first study of the photolysis of methylamine using the xenon resonance line at 147.0 nm was that of Magenheimer,= who concluded that the quantum yield for molecular elimination of hydrogen was 20% of the total hydrogen quantum yield. CH3NH2 H2 + CNH3 (2) No absolute quantum yields were measured in this work. The only other vacuum-ultraviolet study of methylamine photolysis is that of Kawasaki and Tanaka2' at 123.6 nm in which emission from the clr state of NH was observed. On the basis of the thermochemistryand the known energy levels of NH, they concluded that their observation was the result of processes 3 and 4. Scavenging experiments CH3NH2 CH4 + NH(clr) (3) NH(clr) NH(al.rr) + hv (4) suggested that about 10% of the methane observed was due to reaction 3. As we shall show, the total quantum yield of methane is only about 0.05 and the quantum yield for reaction 3 must, therefore, be no greater than about 0.005. Basic unanswered questions in methylamine photolysis which were addressed in the present work were the following: (1) absolute quantum yields of hydrogen, (2) quantum yields of hydrogen as a function of the wavelength of the incident radiation, (3) quantum yields of hydrogen as a function of pressure and light intensity, and (4) the detailed mechanism of hydrogen formation as a function of wavelength.
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2647
-
-
--
,
Experimental Section Chemicals. Commercial methylamine of 98.0% stated purity was distilled repeatedly from bulb to bulb until no impurity peaks appeared in either gas chromatograms or mass spectra. Final purity is estimated at 99.9%. The hydrochloride of CH315NH2was obtained from Merck Sharp and Dohme of Canada, and the free amine was released by the procedure of Gray and Lord.% The compound CD3NH2was obtained as the free amine from the same source. Chemical purity was 99.9% and D/(D + H) = 0.99 in the indicated positions as confirmed by mass spectrometry. Nitrous oxide (15N15NO)was obtained (15) J. P. Ferris, C. Nakagawa, and C. T. Chen, "Life Sciences and Space Research", Vol. XV, R. Holmquist, Ed., Pergamon Press, New York, 1977, pp 95-9. (16) G. Toupance, A. Bossard, and F. Raulin, Origins Life, 9,259-66 (1977). (17) E. P. Gardner and J. R. McNesby, NASA 2nd PAP1 Meeting, Boulder, CO, 1979. (18) E. P. Gardner and J. R. McNesby, J. Photochem., 13, 353-6 (1980). (19) A. Bossard and G. Toupance,Nature (London),288,243-6 (1980). (20) H. J. Emeleus and H. S. Taylor, J . Am. Chem. SOC.,53, 3370-7 (1931). (21) H. J. Emeleus and L. J. Jolley, J. Chem. SOC.,147,1612-7 (1935). (22) 0. C. Wetmore and H. A. Taylor, J. Chem. Phys., 12,61-8 (1944). (23) C. I. Johnson and H. A. Taylor, J. Chem. Phys., 19,613-7 (1951). (24) J. V. Michael and W. A. Noyes, Jr., J . Am. Chem. SOC.,85, 1228-33 (1963). (25) S. G. Hadley and D. H. Volman,J. Am. Chem. SOC.,89,1053-5 (1967). (26) J. J. Magenheimer,R. E. Varnerin, and R. B. Timmons, J. Phys. Chem., 73,3904-9 (1969). (27) M. Kawasaki and I. Tanaka, J. Chem. Phys., 78,1784-9 (1974). (28) A. P. Gray and R. C. Lord, J . Chem. Phys., 26,690-705 (1957).
\'s front
side
Figure 1. Reaction vessel for photolysis.
courtesy of Dr. Pierre Ausloos of NBS. Other commercial research-grade gases used were N2, 02,NO (further purified by distillation), NH3, argon, krypton, xenon, D2,HD, H2,CH4, C2H4, C2H6,C3H8. Azomethane was synthesized by the procedure of Renaud and L e i t ~ hand , ~ ~HCN was synthesized on the vacuum line from sulfuric acid and KCN. All other standards were commercial materials. Titanium getter wire was obtained from Ventron, Alpha Division. Column-chromatographicmaterials was obtained from Supelco, Inc. Gas Chromatography and Mass Spectrometry. A commercial gas chromatograph with flame ionization detection was used. Poropak Q, 3 m in length, was used to separate hydrocarbons and nitriles at 100 "C. A second column, Carbopak B, was used to measure the purity of methylamine and to search for nitrogen-containing reaction products. Peak areas were measured by planimetry. A Consolidated Electrodynamics Corp. mass spectrometer, Model 21-620A, was used. It was equipped with a capacitance manometer which facilitated the measurement of sensitivities of standard gases. All mass spectrometry was done on a rigidly controlled time schedule to minimize errors due to varying leak rates into the analyzer. Light Sources and Experimental Configuration. Resonance lamps at 123.6 nm (Kr) and 147.0 nm (Xe) were fabricated by techniques similar to those described by Ausloos and Lias.30 No other wavelengths were transmitted through the MgF2 windows up to 320 nm except the much weaker resonance lines at 116.5 and 129.5 nm, respectively. A commercial mercury resonance lamp was employed for the 184.9-nm line. All intensities were in the range 1014-1015quanta s-l cm-2 in the configuration employed. Figure 1 shows the physical relationship of the lamp/reaction cell/scanning monochromator in which all reactions were investigated. The space between the lamp window (LiF) and the window labeled 4 in Figure 1 constituted the reaction cell. The dimensions were closely identical for all three light sources which were, therefore, interchangeable. Actinometry Procedure. The actinometer gas was first admitted to the side arm (6 in Figure 1) at a known pressure, all relevant volumes having been previously calibrated. The lamp/reaction cell was connected to the monochromator assembly and the reaction cell pumped to a high vacuum. The lamp was turned on and the monochromator tuned to the resonance line. The signal generated by a sodium salicylate coated photomultiplier at the exit slit was measured. This signal, do, is independent of the wavelength31 and is proportional to the (29) R. Renaud and L. C. Leitch, Can. J. Chem., 32, 545-9 (1954). (30) P. Ausloos and S. G. Lias, Radiat. Res. Rev., 1, 75-107 (1968).
2648
The Journal of Physical Chemistry, Vol. 86,No. 14, 1982
number of photons per unit time emerging from the reaction cell at window 4 and is also proportional to the number of photons per second entering the reaction cell. The actinometer gas containing a known quantity of pure argon (usually 1%) is then admitted to the reaction cell generated by where its pressure is known. The signal, .It, the photomultiplier drops because of absorption by the actinometer gas. This immediately measures the absorption coefficient, E , since the path length, I , and the pressure, P, are known.
zt/zo = e-cp1
Gardner and McNesby UV Absorption Spectrum of CH,NH, 2000 loo0
.
,
~
,
.
,
.
,
.
(
.
/
.
,
~
,
.
,
~
,
.
,
.
,
~
,
T m n n h m . CoWln, md Horrlsm (19531
---.\
(5)
The number of photons absorbed per second, I,, is given by eq 6, where azois the magnitude of the signal on the .I, = azo - azt (6) chart paper. The photolysis of the actinometer gas is continued for a given time, and the reaction cell/lamp assembly is disconnected and coupled to the sampling system of a masa spectrometer. The Dewar trap that forms an integral part of the mass spectrometer sampling system is filled with liquid nitrogen and the valve (5) opened to admit the entire volatile fraction into the mass spectrometer analyzer. The volatile fraction consists mainly of Hz and argon for the ethylene actinometer and 15N15N,Oz, 15N0,and argon for the 15J5N20actinometer. To determine NA, the number of molecules of H2 (or NJ that were generated by the photolysis, it is necessary to multiply the mass-spectrometricaly measured ratio NH20rN2/NArtimes the number of molecules of argon originally contained in the reaction cell.
0.01
I
"
"
"
220
"
240
260
Wovelerqlh, nanxneters
Figure 2. Ultraviolet absorption spectrum of methylamine. The blackened circles are the measured absortion coefficients.
TABLE I: Absolute Quantum Yields of Methylamine Photolysis Productsa a)
product
Hl CH.4 CH,CN
C*H, ClH,
The known quantum yield of the actinometer is related to the absorbed intensity by eq 8. In this way, I, is ob-
'
Mo
C,H*
184.9 nm
1.5 0.029 < 0.0003 0.013 2.2 x 10-4
-
10-4
147.0 nm 123.6 nm 2.0 0.056 0.032 0.024 0.002 0.003
2.1 0.070 0.026 0.038
0.005 0.004
Mean values of 5.0-min irradiations. TABLE 11: Nitrogen Quantum Yields in Methylamine Photolysis with 5-min Exposures
tained. The factor a,which is a property of the invariant experimental configuration, is the ratio of the chart paper deflection, aZ,, to Z,. Since the chart paper deflection is independent of wavelength and proportional to intensity, then, for any wavelength, and any gas in the cell, dividing the deflection by a gives directly the number of photons per second that enter the cell (cell empty) and/or the number absorbed per second, Zo - Zt (cell filled). Photolysis of Methylamine. The same procedure is followed as with actinometry except that, first, a small sample of the contents of the reaction cell after photolysis is subjected to gas-chromatographic analysis. The produds measured chromatographically are determined relative to methane. The absolute numbers of molecules of methane and hydrogen were measured by a procedure exactly analogous to that employed in ethylene actinometry, using a trace of argon as an internal standard. The quantum yields are obtained from an expression just like eq 8. The possibility of accidental admission of mercury to the reaction cell and resulting mercury photosensitization at 253.7 nm was dismissed by running a blank experiment in which the 184.9-nm line was removed with a Vycor fiiter. No reaction occurred. Results Absorption Spectrum of C H P H , . The absorption spectrum of methylamine has been measured by Tannenba~m from ~ ~ 245-160 nm. The results of this study (31) K.Watanabe, M. Zelikoff, and E. C. Y. Inn, 'Air Force Cambridge Research Center Technical Report 53, 1953.
h , nm
184.9 147.0 123.6
@NZ 0.0084 0.0093 0.014
are shown in Figure 2. Also shown are single point determinations made in the present work at 253.7, 184.9, 147.0, 129.5, 123.6, and 116.5 nm. Although the intensity of the 253.7-nm line may be nearly 10 times that of the 184.9-nm line, the absorption coefficient at 253.7 nm is smaller than that of 184.9 nm by a factor of lo4. The 253.7-nm line is, therefore, ineffective in decomposing methylamine. At the pressures and path lengths used in the present work, complete absorption by methylamine and all actinometric gases was attained. Quantum Yields as Functions of Wavelength. Mean values of quantum yields as functions of wavelength are presented in Table I. The quantum yields of nitrogen production were measured in separate experiments since the values were small and were otherwise plagued by nitrogen contaminant peaks in the mass spectrometer. The measurements were made by using CH25NHzto avoid this difficulty. The results are presented in Table 11. Since the quantum yields of NHz and CH, must be equal and the latter is about 0.10, this also is the quantum yield of NHz, and one may estimate an upper limit to the quantum (32) E. Tannenbaum, E. M. Coffin, and A. J. Harrison, J. Chem. P h p . , 21, 311-8 (1953). (33) R. F. Hampson, Jr., J. R. McNesby, H. Akimoto, and I. Tanaka, J. Chem. Phys., 40, 1099 (1964).
.
,
,
The Journal of Physical Chemlstty, Vol. 86, No. 14, 1982 2649
Vacuum-Uttravioiet Photolysis of Methylamine
TABLE 111: Relative Amounts of Hydrogen Isotopes in Photolysis of 10.0 torr of CD,NH, at h = 184.9 nm in the Presence and the Absence of 0, and NO 2
0
h
n o scavenger present scavenger
D,
HD
H,
D,
HD
8.5 torrof 0, 17.0 torrof 0, 8.5 t o r r o f N O 17.0 t o r r o f N O
3.38 3.04 3.34 3.85 3.27
8.19 7.32 7.14 9.30 7.94
5.24 4.75 4.09 5.95 5.10
0 0 0 0 0
0.45 0.39 0.50 0.51 0.44
8.0 torrof
,
0 0.00
5
IO 15 20 25 30 35 40 45 50 55 60 65 70
IRRADIATION TIME, MINUTES
Flgure 3. Quantum yield of H, as a function of irradiation time. The decrease in @'HZ was determined to be due to the attentuation of light intensity by polymer formation on the lamp window. 2.5L
t
2
scavenger added
0,
ZP
H,
0.77 3.97 0.72 3.92 0.81 4.02 0.88 3.96 0.80 4.00
a K = [ H D ] z / ( [ H , ] [ D , ] ) where the concentrations are differences between those without and those with added scavenger.
TABLE IV: Photolysis of 10.0 torr of CD,NH, at h = 147.0 nm in the Presence and the Absence of NO NO scavenger, torr
D,
5.0 8.5 17.0
10.06 10.04 12.05
n o scavenger presentb
HD
scavenger added
H,
D,
HD
H,
Ka
13.13 5.84 3.04 5.23 3.57 3.92 12.91 5.75 3.04 5.14 3.52 3.88 15.62 6.95 3.63 6.19 4.22 3.87
K = [HD],/( [H,][D,]) where the concentrations are differences between those without and those with added scavenger. The quantum yield of H, t HD t D, = 1.9. TABLE V: Photolysis of 10.0 torr of CD,NH, at h = 123.6 nm in the Presence of the Absence of 0,
i-
b
-b
b
-
b
@H2
15-
IO
scavenger added
D,
HD
H,
D,
5.0 8.5 17.0
9.26 1.20 7.43
10.23 7.94 8.19
6.57 5.11 5.29
4.79 3.75 3.85
HD
H,
2.58 3.25 1.93 2.48 1.98 2.64
Ka 3.94 3.98 4.06
a K = [HD],/( [H,][D,]) where the concentrations are differences between those without and those with added scavenger.
2520
no scavenger present
venger, torr
b
-
-
> t
00 O 5 IO 2 0 30 40 50 60 70 80 90 100 110 intensity x ~ ~ quanta 1 4 SeC'
5. Quantvn yield of & as a functkm of ii@ intensity (A = 147.0 nm) at constant pressure and irradiatlon time.
yield of hydrazine of about 0.07. Secondary photolysis of hydrazine is probably responsible for the nitrogen in the reaction products. Quantum Yield of H2 at 147.0 nm. Polymer formation on the lamp window was found to decrease the lamp intensity with photolysis time. The original intensity could be restored by cleaning the window with white rouge and methanol. Because of this difficulty, the apparent quantum yields of hydrogen were measured as a function of time. Figure 3 presents the data and indicates an initial value for cPHz a t 147.0 nm of 2.2. In order to provide information on the mechanism of formtion of hydrogen, we
measured the quantum yield at 147.0 nm as a function of methylamine pressure. The data in Figure 4 show that the quantum yield for 5-min exposures is independent of the pressure. Additional data on this question are shown in Figure 5, in which the quantum yields of H2were measured at various intenities and 5-min irradiation times. Again, no effect is observed over an order of magnitude range in intensity. Photolysis of CDflH2. The mechanism of hydrogen and methane formation was investigated by photolyzing CD3NH2 at three wavelengths in the absence and the presence of hydrogen atom and methyl radical scavengers, NO and OF The quantum yield of H2 + HD + D2at 147.0 nm, 10 torr of CDaNH2, and 10-min photolysis time was found to be 1.9 in the unscavenged reaction, a value indistinguishable from the H2quantum yield in CH3NH2 photolysis (Figure 3). Under the conditions of the experiments, absorption by the scavenger was negligible. Table I11 summarizes the results at 184.9 nm and Tables IV and V present the results at 147.0 and 123.6 nm. Experiments were done in pairs, one without and one with added scavenger, making certain that all other parameters (light intensity, methylamine concentration, irradiation time) were held constant. It is immediately obvious from the data in Table I11 that nearly all of the hydrogen is scavengeable at 184.9 nm and that atomic mechanisms dominate. Entries for D2 of zero indicate only that the
2850
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
Gardner and McNesby
TABLE VI: Threshold Energies and Enthalpies of Formation in Decomposition of Methylaminea A H “ , kcal mo1-I
ATH, nm
species
A H ; , kcal mol-’
35.0 37.8 94.6 102.6 139.2 142.6 142.0 158.6 262.8 206.9 246.2 120.8 225.0 329.2 79.5 367 73.6
811.8 751.7 302.2 276.9 204.1 201.0 200.1 179.2 108.1 138.2 116.1 236.7 127.1 86.8 359.6 77.9 388.4
CH,NH, CH,NH, CH,NH CH,=NH HCN CN H CH3
-5.5 33.5 45 29.5 32.3 101 52.1 34.1 40.0 115.3 - 17.9 85
CH,NH, -+ H, + CH,=NH CH,NH, + 2H, + HCN CH,NH, + H + CH,NH, CH,NH, + H + CH,NH CH,NH, -+ 2H + CH,=NH CH,NH, + H + H, + CH=NH CH,NH, -+ 2H + H, + HCN CH,NH, + 2H, + H + CN CH,NH, -+ 3H + H, + CN CH,NH, + CH,NH,+ + e CH,NH, -+ 4 H + HCN CH,NH, + 2H, + HNC CH,NH, -+ 2H + H, + HNC CH,NH, -+ 4H + HNC CH,NH, + CH, + NH, CH,NH, -+ 5 H + CN CH,NH, CH, + NH -+
S. W. Benson, “Thermochemical Kinetics”. Wiley. New York, 1968. be 1 0 8 kcal mol-’ as in ethylene. 35
k 30
i
::L I
00 6
7
!
.
L
i
8 9 1 0 1 1 ENERGY, eV
Figure 8. &, /@m, as a function of photon energy (eV) in the photolysis of Cl$NH,.
amount is less than 0.1. Tables IV and V show a quite different result for photolysis at 123.6 and 147.0 nm, with about 40% of hydrogen being nonscavengeable and, therefore, arising from molecular elimination processes. The amount of scavengeable hydrogen should represent hydrogen formed in atomic mechanisms and, as such, should give a ratio K = [HD]2/([H2][D2])= 4.0. Tables 111-V show that the observed values of K are all 4.0 within experimental error. Several experiments were done at each of the three wavelengths to determine the mechanism of methane formation. Figure 6 shows the data. The formation of CD4 is a clear indication that methyl radicals are involved. The methane yield drops sharply in the presence of scavengers. Specifically, the formation of CD3H in the presence of free-radical scavengers strongly dominates CD4 although we are not able to make a quantitative statement on this point. Other Product Quantum Yields. Table I presents absolute quantum yield data for several hydrocarbons and CH3CN. The molecule HCN is also formed, but it was not possible to assess the quantum yield because of its high reactivity and adsorptivity. Discussion Judging by the absorption spectrum of methylamine (Figure 11, at 184.9 nm, the excited state may be different from that involved in the photolysis studied by Michael and Noyes.*O A second state appears to be involved at 147.0 nm, and still a third at 123.6 nm. Thus, the photochemistry reported here may be quite different from that encountered by most earlier workers. Because of the very large energies involved in the initiating photons, it is in-
Ek
CH, CH=NH~
Assuming the C-H
-
bond strength in CH,=NH t o
structive to ask which primary processes are thermodynamically possible. Table VI lists standard enthalpies for the various processes and the corresponding threshold wavelengths. Of particular interest is that it is not possible to produce CN a t the 184.9-nm wavelength, but is quite possible at 147.0 and 123.6 nm. Production of CH&N as primary product is observed readily at 147.0 and 123.6 nm, but not at all (less than 1% of the quantum yield at 147.0 nm) at 184.9 nm. We have not been able to rationalize CH3CN production without involving the production of CN in a primary event. Once formed, CN associates with CH3to form the nitrile. Photolysis at X = 184.9 nm. In respect to hydrogen production at 184.9 nm, the quantum yield of 1.5 is not possible with the expulsion of a single hydrogen atom from methylamine. I t is necessary for a second atom to be expelled with some substantial probability. Since most of the hydrogen arises from atoms produced in primary events, we assume a sequence based on the primary process 9, demonstrated by Hadley and V ~ l m a n ~ CH3NH2 CH3NH* + H (9) with a quantum yield of unity. The excited free radical can then decompose in reaction 10 with a probability which CH,NH* H + CH2=NH* (10) may depend upon the pressure. Reaction 10 is thermodynamically slightly more favorable than the expulsion of molecular hydrogen from CH3NH,but the possibility that reaction 11 occurs cannot be excluded. For the quantum CH3NH* H2 CH=NH (11) yield of 1.5 to be rationalized, it must be assumed that all hydrogen atoms (both thermal and kinetically “hot”) abstract hydrogen from methylamine and the nearly complete disappearance of H2 in the presence of scavengers suggests that all H atoms are scavengeable with added O2 or NO. Photolysis at X = 147.0 nm. Since the quantum yield of H2 at 184.9 nm can only be explained if the H atoms are assumed to abstract (rather than recombine), it is likely that H atoms formed at 147.0 nm also abstract. The failure of scavengers to reduce the hydrogen quantum yield below 0.41 suggests a second primary process involving expulsion of molecular hydrogen from the parent. CH3NH2 --* Hz + CNH3* (12) Since this process is only 35 kcal mol-’ endothermic (Table
-
-
-
+
J. Phys. Chem. 1882, 86, 2651-2656
Scheme I
hydrogen results from an atomic mechanism. Scheme I emerges as the most appealing possibility. The equations that result from Scheme I are as follows: @'Hz = 2 p(1 - a) = 2.2 (15)
CH3NH2
rL
+
(1-a)
H
+
CH3NH*
H2
+
CH2NH*
H
+
CH2NH
H2
+
HCN*
@HpU
+
+
4 HCN
Ctl
VI), CNH3* is likely to be highly excited and may decompose with a finite probability. CNH3*
+
H2 + HCN*
(13)
The excited HCN* may, according to the threshold data in Table VI, decompose further to give the CN radical. HCN*
-. + H
-
@PH~(M) + @H,(A) 2
I
H
265 1
CN
(14)
The mechanisms of the photolysis must be consistent with the basic observations that the hydrogen quantum yield is 2.2 and is independent of pressure and that 59% of the
2(1- a)
+ P(1 - a) = 0.41
(16)
The notation (M) signifies that the hydrogen is formed in molecular elimination processes, and (A) signifies the hydrogen is formed from atomic hydrogen abstracting hydrogen from methylamine. The solutions are a = 0.55 and /3 = 0.44. The detailed mechanisms by which the isotopic hydrogen molecules are formed in molecular eliminations require that H2,HD, and D2be produced by elimination from the parent as well as in sequential processes. Further discussion of the details of the mechanism is not warranted at the present time. Photolysis at X = 123.6 nm. It is somewhat surprising that, at X = 123.6 nm, the fraction of hydrogen from molecular elimination processes is the same as at 147.0 nm. Also surprising is why the quantum yield does not rise as the exciting wavelength is decreased from 147.0 to 123.6 nm. Until detailed studies are done on energy transfer and intensity effects, speculation on the reasons for the invariance of the hydrogen quantum yield with wavelength at 123.6 and 147.0 nm is not likely to be profitable. It is, nevertheless, unmistakable that terminal elimination of H2 and D2 from CD3NH2as well as 1,2 elimination of HD are important molecular elimination processes.
A Theoretical Study of the Low-Frequency Normal Modes of all-trans 1,4-DIphenyl- 1,&butadiene
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Brlan M. Pierce and Robert R. Blrge' Department of Chemistry, University of Califom&, Rivers&, Callforn& 92521 (Rwehmi: October 13, 7981; In Final Form: March 4, 1982)
Equilibrium conformations and normal-mode frequencies are calculated for the ground and l'B,*+ AT* states of aZl-tralts-1,4diphenyl-1,3-butadiene by using the QCFF/PI method. The eleven lowest energy normal modes of the ground state are investigated in detail. The lowest frequency (56 cm-') b, mode involves an in-plane bend of the entire butadiene moiety in all-trans-l,4-diphenyl-l,3-butadiene and is calculated to have an energy equal to the observed energy difference (40 f 20 cm-') [J.A. Bennett and R. R. Birge, J. Chem. Phys., 73,4234 (1980)l between the system origin of the first one-photon forbidden AT* state and the highest energy vibronic band in the fluorescence spectrum. The possibility that this vibration is a promoting mode responsible for inducing fluorescence from the lAg*- mr state into the IA; ground state is discussed.
Introduction A recent two-photon excitation study of 1,4-diphenyll,&butadiene (hereafter abbreviated diphenylbutadiene) by Bennett and Birge established that the system origin of a forbidden l$*- m* state lies only 130 cm-' below the system origin of the strongly allowed lBU*+m* state in EPA solvent at 77 K.l One curious feature of the spectroscopy of this compound, however, remained unresolved. The system origin of the IA,*- m* state was observed to (1) J. A. Bennett and R. R. Birge, J. Chem. Phys., 73,4234 (1980). 0022-3654/82/2086-2651~01.25/0
be 40 f 20 cm-' higher in energy than the highest energy vibronic peak in the fluorescence spectrum. The low resolution of the one- and two-photon spectra and the ability of EPA glass to break down the inversion symmetry of diphenylbutadiene2 made it difficult to analyze with certainty the identity of this peak. As a result, it is not clear whether the 40 f 20 cm-' separation reflects exper(2) The ability of solvent glasses to reduce the symmetry of the solute molecule has been observed in high-resolutionspectra of other polyenes. See M. F. Granville, G. R. Holton, and B. E. Kohler, Proc. Natl. Acad. Sci. U.S.A., 77,31 (1980), and ref 4, 5, and 14.
0 1982 American Chemical Society
