5240
J . Phys. Chem. 1990, 94, 5240-5243
TABLE VI: Scheme of Energy Decomposition total energy electrostatic energy exchange energy CT(B-+A) + PL(A+A) + EX
x
I )
CY
EEX
electronic structure immediately before TS, it is reasonable that FCTPLX yields stabilization. The stabilization energy at s = -1.5 is smaller than that at s = 0.0. This is probably because the two molecules SiH4 and NH3 approach more closely at s = 0.0 than
EFCTPLX
-1.5.
E O
EES
vacant MO's occupied MO's
,EMIX
TABLE VII: Results of the Energy Decomposition for Reaction 1
(4 s = -1.5
-0.01274
A P
AEES
1.30758
AEEX
~EFCTPLX AEMIX
-1.22690 -0.07019 -0.02323
5
= 0.0
-0.05466 1.29500
-1.15992 -0.1 1654
-0.07320
The residues are denoted as MIX. Results of the energy decomposition for reaction 1 at s = -1.5 and 0.0 are shown in Table VII. Here, the negative sign indicates that the interaction yields stabilization. At both points, the large repulsive energy due to ES is partly cancelled by EX, but still exceeds AEO. Stabilization due to FCTPLX finally gives the reasonable value for A E O . The stabilization energies of the system due to forward CT( B-A ) are -0.070 19 and - 0 . 1 16 54 at s = -1.5 and 0.0, respectively. Considering the result of configuration analysis that forward CT(B5-AIO) plays an important role in the
IV. Conclusion The mechanism of the formation of silicon nitride in the gas phase has been examined by using IRC as the unique reaction path. We have chosen reaction 1 as a candidate for the gas-phase elementary reaction. Our result for the activation energy of reaction 1, 55.4 kcal/mol, is in agreement with the experimentally ~ the other hand, the reactivity observed value 39 k ~ a l / m o l . ' On of the side reaction 3 is very low as compared with reaction I , and hence it can be predicted that it does not occur easily in the presence of NH3. The geometrical change along the IRC strongly suggests that the excitation of certain vibrational modes of SiH, effectively enhances reaction 1 . Once the SiH, is vibrationally activated, the successive reactivity is very high, because the charge-transfer interaction is very strong in this reaction system. This is consistent with the recent laser-driven synthesis of Si,N4 in the gas p h a ~ e . ~In, ~addition to that, the excitation of the vibrational modes of NH3 may lead to a increase in the reaction rate. This suggests a new experiment in such a way that a different source of laser with different wavelength which vibrationally activates NH, further promotes the reaction. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, for which the authors express their gratitude. The numerical calculations were carried out at the Data Processing Center of Kyoto University and the Computer Center of the Institute for Molecular Science (IMS) and we are grateful for their generous permission to use FACOM M-780 and VP-400, and HITAC M-680H and S-820 computer systems, respectively. Registry No. SiH4, 7803-62-5;NH,, 7664-41 -7: Si3N4,12033-89-5.
Photochemistry of 2-Butyne In Solid Xenon at 10 K Susan Collins Department of Chemistry, California State University, Northridge, California 91 330 (Received: September 25, 1989; I n Final Form: November 28, 1989)
--
-
The photochemistry of 2-butyne was studied by FTIR spectroscopy at 10 K in xenon matrices. The 2-butyne undergoes the following overall photochemical reactions: CH3-C=C-CH3 2 H - c ~ c - H + H,; 2CH3-C=C-CH3 CH4 + C H 3 m - H + 2 H - C x - H ; and 2CH3-C=C--CH3 CH3-CH2-CH=CH2 + ZH--C=C-H. The reactions are observed in xenon, but not in argon, when 2-butyne is excited between 200 and 230 nm for M / R = 1/50, 1/100, 1/200, and 1 /300. We postulate the lowest triplet excited state of 2-butyne becomes accessible in this region by spin-orbit coupling due to the external heavy-atom effect of xenon, resulting in triplet-state photochemistry.
Introduction
The ultraviolet photochemistry of 2-butyne in xenon matrices at 10 K has been examined by FTIR spectroscopy. In our experiments, we found evidence of photochemistry in xenon, but not in argon, when we excited the matrix in the spectral region between 200 and 240 nm. Like acetylene, 2-butyne is known to absorb strongly in the vacuum ultraviolet region and undergo photochemical reactions.' Singlet photochemistry due to 200-240-nm excitation is known to occur in low yield in the gas phase.2 Our observations indicate the possibility of matrix-induced singlet( I ) Deschenes, J.; Deslauriers, H.; Collin, G.J . Con. J . Chem. 1980, 58, 2108. ( 2 ) Whitten, D. G.;Berngruber, W. J . Am. Chem. SOC.1971, 93, 3204.
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triplet absorption in 2-butyne and subsequent triplet photochemistry. There are no literature reports of triplet photochemistry of 2-butyne for comparison, either by direct excitation or by mercury photosensitization. We report here an analysis of the infrared spectra of the photoproducts, including an analysis of the deuterium-labeled products. Experimental Section
The experiments were performed in two different laboratories. The vacuum and refrigeration techniques were essentially identical. In the early experiments, an IBM 9700 FTIR spectrometer and a 200-W medium-pressure mercury photolysis lamp were used. I n later experiments, a Nicolet 2ODXB FTIR spectrometer and 400-W high-pressure xenon photolysis lamp were used. In all 0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5241
Photochemistry of 2-Butyne in Solid Xenon
w
0
z a a K
0 v)
a a
WAVENUMBERS Figure I . The FTlR difference spectra illustrating the photolysis products (positive bands) of xenon matrices of (A) CH3-C=C-CH3 with M / R = 1 /50; (B) CH3-C=C-CD3 with M / R = 1/50. Both spectra are representative of approximately 10% photolysis of 2-butyne. Bands marked by asterisks are due to impurities.
experiments, infrared spectra were obtained in the spectral region of 4000-400 cm-I. Both instruments have a glowbar source, a Ge-coated KBr beam splitter, and a HgCdTe detector, cooled to 77 K. In the early experiments, spectra were routinely taken with 0.5 cm-l resolution. In the later experiments, spectra were obtained with 1 .O cm-’ resolution. Identical results were obtained in both laboratories. Argon and xenon matrices of 2-butyne were prepared by simultaneously depositing 1-2 mmol of premixed 2-butyne/rare gas at a rate of about 0.5-1 mmol/h. The samples were deposited onto a CsI window, held at IO K in the case of the argon mixtures and 20 K in the case of the xenon mixtures. The 20 K xenon matrix then was slowly cooled to IO K for spectroscopic and photochemical studies. The cryocoolers were Air Products Model CS202 closed cycle helium refrigerators. 2-Butyne (99%) was obtained from Aldrich Chemical Co. Oxygen and other impurities were removed from 2-butyne by subjecting it to freeze-pumpthaw cycles at 77 K before mixing the fourth fraction with the rare gas. 2-Buyne-d6 (97 atom % D) was obtained from MSD Isotopes and was used without further purification. 2-Butyne-d, (98010) was synthesized according to the methods of ref 3. This also was subjected to freeze-pump-thaw purification cycles under vacuum. Argon (Pacific Oxygen Sales, 99.998%) and xenon (99.995%) were used without further purification.
Results The photoreaction of 2-butyne has been studied at concentrations of M /R = 1 /50, 1/ 100, 1/200, and 1/300 in xenon matrices as well as with 2-butyne-d3 and 2-butyne-d,. In this section we will discuss the infrared spectra in terms of product growth, relative product yield, and the effects of isotopic substitution. Product Growth. Figure IA shows that the photolysis of 2butyne in Xe results in many new bands in the infrared spectrum. Decreases in the absorption of the parent 2-butyne at 2833.2 cm-I show that after 3 h of photolysis approximately 10% of the 2butyne had reacted. Table I lists the product band frequencies obtained after approximately 10% photolysis of 2-butyne with M / R = 1/50. When the band intensities are plotted as a function of time, two types of growth behavior, I and 11, are observed, indicating that at least two reaction pathways are followed as the reaction proceeds. Figure 2 illustrates the complete growth behavior of the bands appearing at 625.0 and 1300.5 cm-’from type ~
~~
~~~~
(3) Whitesides, G M.: Ehmann, W. J J . Am. Chem. SOC.1969, 91, 3806.
TIME
(min)
TIME ( m i d
Figure 2. The growth behavior of product bands of 2-butyne in Xe a t IO K at 625 cm-’ ( 0 )and 1300.5 cm-’ (*), type I; 729 cm-l (a) and 921.4 cm‘’ (*), type 11. Each band was normalized to the maximum products, attained intensity of that same band. (A) CH3-C=C-CH3 (B) CH3-C=C-CD3 products.
I and bands at 729.3 and 921.4 cm-l from type 11. In Figure 2, each band intensity has been normalized relative to the maximum attained intensity of that same band. The growth patterns lose their two-path character upon dilution and separate into four different growth curves. This behavior also is seen in the dilution studies of 2-butyne-d3. In all cases the band at 729.3 cm-I grows
5242 The Journal of Physical Chemistry, Voi. 94, No. 13, 1990 TABLE I: Band Frequencies of the Photoproducts of the 2-Butyres and Their Growth Behaviors, Types I and 11 ( M I R = 1/50) kinetic CD,-C= behavior CH,-C= CH,-C= C-CH, C-CD, C-CD, twe 33 15.4 3305.1 3305.1 I 3303.2 II 3269.2 3269.2 II 3255.0 3255.0 I 2589.3 2583.5 1 2583.5 I 2579.9 I1 2554.3 II 2550.0 II 2426.3 II 2414.3 2 132.0 21 33.4 2095. I 1927.7 I I 1881.9 1300.5 1 300.5 I I 1281.9 I 1235.0 1234.9 I 1151.4 I 998.3 988.2 I 988.2 1 967.2 I 951.1 921.4 II 92 1.4 I 913.3 I 883.1 I1 839.2 I 807.2 1 790.3 75 1 .O II 751 0 743.7 743.7 ir 11 737.0 737.0 II 729.3 729.3 707.6 11 707.6 II 683.7 I1 676.0 11 668.3 1 634.8 634.8 623.9 623.9 613.8 613.8 [I 543. I 538.2 I1 538.2 493.6 I 492.9 I 412.6
most rapidly. The existence of a unimolecular reaction could not be determined by dilution studies because the quantum yield of the reaction is too low; however, the existence of at least two pathways has been demonstrated. From Figure 3, it is apparent that several sites are populated during product formation. The 625-cm-' region exhibits three bands at 634.8, 623.9, and 613.8 cm-I, with the band at 623.9 cm-I being the major component at all concentrations. The 729-cm-' region exhibits four bands at 75 1 .O, 743.7, 737.0, and 729.3 an-'. Population of 737.0- and 729.3-cm-' bands is favored at higher concentrations. Relatiue Product Yield. As an example of the relative product yield after approximately 10% photolysis of 2-butyne, the ratio of the absorbance values of the bands at 1300.5,921.4, 729, and 625 cm-l is l:l:4:lO at M / R = 1/50 and 1:2:10:10 at M / R = 1 /300. For these measurements the peak height designated as 729 cm-l was taken from the sum of the peak heights at 751 .O, 743.7,737.0, and 729.3 cm-I. The measurement of the peak height designated as 623 cm-l was taken as the sum of peak heights at 634.8, 623.9, and 613.8 cm-'. Isotopic Labeling of2-Butyne. Figure 1 B shows the product absorptions due to 10%photolysis of CD3-C*-CH3. Table I lists the product band frequencies formed from photolysis of CH3--(3=--C-CH3, C D 3 - C = C 4 H 3 , and C D , m - - C D , , respectively, along with their growth behaviors at M / R = 1/50.
Collins
m D
1/50
WAVENUMBEAS
Figure 3. The FTIR product band absorptions of 2-butyne in Xe at I O K as a function of concentration, illustrating the concentration dependent site populations. ( A ) M / R = 1/50; (B) M/R = ]/loo; (C) M / R = 1 /300.
Discussion Product Identification. Kinetic behavior and the spectra of isotope-labeled molecules provide the basis of identitification. Type I product bands were seen at 3315.4, 3305.1, 2132.0, 1300.5, 1235.0, 634.8, 623.9, and 613.8 cm-l. All of the bands except for 1300.5 cm-' could be assigned to propyne. Comparison to an authentic sample of propyne in Xe at 10 K was the original basis of the assignment. Further verification came from the photolysis products of CD3-C=C-CH3 where, in addition to the bands mentioned above, other bands were identified as the CD stretch (2589.3, 2583.5, 2579.9 cm-I), the CD bend (951.1 cm-I), and the photolysis the CD bend overtone (492.9, 472.6 ~ m - ' ) . From ~ of CD3-C=C-CD3 bands at 2583.5 and 493.6 cm-' were observed, thus confirming the assignment. The remaining band at 1300.5 cm-l belonging to type I behavior can be assigned as being due to CH4.4 Photolysis of CD3-C= C-CD,.gave a counterpart band with similar behavior at 988.2 cm-I, which is assigned to CDq.4 Photolysis of C D 3 - C k C - C H 3 confirmed the above results and gave additional bands at 1 15 1.4 and 998.3 cm-'. These bands are attributed to CH3D and CD3H, re~pectively.~There was no evidence of CD2HZ,which will be an important link in establishing the mechanisms, as will be discussed later. Type I1 product bands were seen at 3269.2, 3255.3,921.4, 751.0, 743.7,737.0, and 729.3 cm-'. Of these all except the band at 921.4 cm-' can be assigned to a ~ e t y l e n e .The ~ observed splittings are consistent with studies which have shown that the presence of perturbing molecules can effect the splitting of acetylene bend and stretch bands5 The photolysis of C D , a - - C D 3 resulted in bands at 543.1, 538.2, 2426.3, and 2414.3 which are attributed to C,D2.4 The photolysis of CD3--C=C-CH3 resulted in the bands attributed to C2Hzand C2D2and gave additional bands at 2554.3, 2550.0, 683.7, 676.0, and 668.3 which are attributed t o C,HD.4 Thus all possible H / D combinations occur in the formation of the acetylenes. The remaining type I1 band at 921.4 cm-l was the most difficult to assign since no other bands appeared with it. Photolysis of CD3-C=C-CD3 resulted in the deuterium counterpart band at 707.6 cm-l, giving a C-H/C-D frequency ratio of 1.302. The frequency ratio of the C-H/C-D bend of ethylene in xenon is of the same magnitude, 940.0 cm-'/718.5 cm-l = 1.309.6 Thus the shift is indicative of a terminal double bond. Methylene, (4) Herzberg, G . Infrared and Raman Spectra; Van Nostrand Reinhold New York, 1945. ( 5 ) McDonald, S. A.; Johnson, G. L.; Keelan, B. W.; Andrews, L. J . Am.
Chem. SOC.1980, 102, 2892.
(6) Collins, S.; Pimentel, G.J . Phys. Chem. 1984, 88, 4258.
J . Phys. Chem. 1990, 94, 5243-5246 ethylene, propene, vinylacetylene, and butadiene all were ruled out after consultation with literature spectra. I-Butene in the gauche form was selected after comparison to its literature spectrum.' Of our possible unassigned product bands from the CH3-C=C-CD3 photolysis at 883.1, 839.2, 807.2, and 790.3 cm-I, the band at 839.2 cm-I surely must correspond to the R= C H D out-of-plane bend because the kinetic behavior is type 11. N o literature source was available for CH3CH2CH=CHD, but it compares favorably with the frequency for the corresponding mode in ethylene in xenon at 839.5 Kinetics. The structural assignments combined with the kinetic behavior now provide us with the information that methane and propene are formed simultaneously. I-Butene is formed along another pathway. Acetylene is formed most rapidly at all concentrations. This must be due to its formation in reactions I and 11, and possibly a third reaction. Relative Product Yield. The distribution of two acetylene molecules per reaction between the two bimolecular reactions can now be explained by considering relative absorbance ratios after approximately 10% photolysis of 2-butyne. The ratio of the band heights for methane:butene:acetylene:propyne is 1:I :4:10 at M / R = 1/50 and 1:2:10:10 at M / R = 1/300. It can be seen that an approximately 2-fold enhancement of the formation of butene and acetylene occurs upon dilution from M / R = 1/50 to 1/300. Conclusion
The ultraviolet-region singlet photochemistry of gas-phase 2-butyne is known to result from the primary step of C-H bond breakage.2 All of the reported products are due to recombination of the 2-butyne fragments and, presumably, the formation of molecular hydrogen.2 The formation of acetylene in our experiments may be considered to arise from the elimination of molecular hydrogen from a triplet-excited 2-butyne monomer unit according to CHj-CEC-CH3
+
CH,-CaCxH
+ H2
followed by CHj--C=CxH
+
[CH=CH-CH=CH]
-+
2C2H2
Alternatively, the photoreaction of two 2-butyne molecules or a higher aggrega,tion in a xenon cage would involve the concerted formation of [CH-CH-CH=CH], methane and propyne, or [CH=CH-CH=CH] and butene to give the overall reactions (7) Barnes, A. J.; Howells, J. D. R. J . Chem. SOC.,Faraday Trans. 1973, 69, 532.
5243
2CH3-CzC-CH3 CH, CH3--CrC-H 2CH3-CaC-CH3 CH3--CH2-CH=CH2
+
+
-
+ 2H-C=C-H
+ 2H-C=C-H
path I path I1
The idea that acetylene may be formed from monomer 2-butyne and from two or greater numbers of aggregated 2-butyne molecules is consistent with Figure 3, which shows acetylene in at least four different concentration-dependent sites. Furthermore, the isotope scrambling would be correct for all three reactions. Evidence that the mechanism described above does not occur from singlet excitation comes indirectly from the fact that we did not see a photoreaction from propyne/Xe ( M / R = 1/ 100) or from propyne/Ar ( M / R = 1/100) using the same experimental conditions. Reactions are known to occur in the gas phase, initiated by singlet-state C-H bond breakage.* Presumably the singlet mechanisms have lower quantum yield in the matrix, perhaps due to cage recombination and/or a lower singlet transition probability in xenon. Although lifetime studies on electronically excited molecules have demonstrated the external heavy atom spin-orbit coupling effect by xenon, relatively few studies have demonstrated the effect on the photochemical outcome. Studies by Warren et al.? Collins and Pimentel,6 Kelley and Rentzepis,Io Rasanen," and Laursen and Pimenteli2all demonstrate product differences. Presumably the different photoproducts form along triplet and singlet potential surfaces, which can be selected by choosing xenon or argon, respectively. Acknowledgment. I would like to dedicate this paper to the late Professor George Pimentel. His creative ideas and continued interest in this project and all my other projects will not be forgotten. I also would like to acknowledge Dr. Ed Orton for synthesizing CH3-C=C-CD3, and my students, Iva Milson, Joe Raffetto, and Terry Miles, for their assistance. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this project. This material also is based upon work supported partially by the National Science Foundation under Grant No. 8804804. Acknowledgment also is made to the Affirmative Action Program and Office of Research and Sponsored Projects at CSU, Northridge, for valuable release time grants. (8) Smith, R. N.; Leighton, P. A.; Leighton, W. G. J . Am. Chem. SOC. 1939,61, 2299. (9) Warren, J.; Smith, G. R.; Guillory, W. A. J . Photochem. 1977, 7,263. (IO) Kelly, D.; Rentzepis, P. J . Am. Chem. SOC.1983, 105, 1820. (1 1 ) Rasanen, M. Personal communication. (12) Laursen, S.; Pimentel, G.C. J . Phys. Chem. 1989, 93, 2328.
Period Doubling and Chaos in a Three-Variable Autocatalator Bo Peng,+Stephen K. Scott,***and Kenneth Showalter**+ Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045, and Department of Physical Chemistry, University of Leeds, Leeds LS2 9JT, U.K. (Received: October 20, 1989) A three-variableextension of the classical twevariable autocatalator is presented as a prototype for complex dynamical behavior
in isothermal chemical reactions. The model is closely related to recent extensions of the autocatalator involving temperature feedback. Period doubling and chaos with periodic windows are found on varying a bifurcation parameter. The bifurcation sequence is a remerging Feigenbaum tree, with period doubling leading to chaos followed by a reverse sequence leading back to period 1. Introduction
The two-variable autocatalator model for isothermal reaction in a thermodynamically closed system has been used successfully
to reproduce many typical features of chemical oscillation^.^-^ This scheme considers the conversion of a chemical precursor P ( I ) Gray, P.; Scott, S . K. Ber. Bunsen-Ges. Phys. Chem. 1986, 90,985. (2) Merkin, J. H.; Needham, D. J.; Scott, S. K . Proc. R. SOC.London 1986, A406, 299.
'West Virginia University. f University of Leeds.
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0 1990 American Chemical Society
