J . Phys. Chem. 1985, 89, 5489-5495
5489
Elementary Processes in the Gas-Phase Photodecomposition of Singlet and Triplet 3-Chloro-3-methyldiazlrine Mada J. Avila, Departamento de Qufmica Inorgcinica, Facultad de Ciencias, Uniuersidad a Distancia, Cuidad Uniuersitaria, 28003 Madrid, Spain
Rosa Becerra, Juan M. Figuera,* Juan C. Rodriguez, Aurora Tobar, Departament of Photochemistry, Instituto de Qufmica Ffsica, C.S.I.C.,Serrano, 1 1 9, 28006 Madrid, Spain
and Roberto Martinez-Utrilla Instituto de Orgrinica, C.S.I.C.,Juan de la Cierua. 3, 28006 Madrid, Spain (Received: December 28, 1984; In Final Form: August 6, 1985)
3-Chloro-3-methyldiazirine has been photolyzed in the gas phase at 416, 365, and 337 nm. With these wavelengths, b * and Int* states could be prepared, the last one with different amounts of rovibrational energy. The elementary processes occurring after the optical excitation have been analyzed with the help of algorithms recently developed. The triplet leads to “hot” vinyl chloride through the intermediacy of a diradical. In the case of the singlet, two competitive pathways take place. The first one switches to the triplet route after the initial excited diazirine undergoes an intersystem crossing process. The second one produces the “hot” vinyl chloride through adiabatic isomerization of the diazirine to the isomeric diazo compound. In both routes, the intermediate diradical and diazo compound decompose to singlet chloromethylcarbenes differing only in their rotovibrational energy content. Subsequent 1,2-hydrogenshift gives rise to the final vinyl chlorides containing different amounts of internal energy. The reaction mechanism put forward agrees with all the data known to date about this photodecomposition. We should emphasize that the purpose of this work was the study of the unimolecular processes occurring in the photodissociation. Minor subsequent or parallel bimolecular reactions possibly occurring (formation of dichloroethanes, for example) have been purposely excluded. However, some quenching experiments have been used as probes of the reaction mechanism.
Introduction The gas-phase thermal and photochemical decomposition of 3-chloro-3-methyldiazirine (CMD) have been reported’s2to yield vinyl chloride (VC) and acetylene as the major organic products. Recent work has shown that relatively small amounts of 1,l-dichloroethane are also formed, although its mechanism of formation is contr~versial.~,~ The photolysis of C M D in solution has been rationalized by the intermediacy of a singlet carbene which partially rearranges to V C 5 Evidence of the participation of two different reaction channels giving identical products has been p~blished.~,’The first h a * excited electronic state from which decomposition occurs is not purely dissociative, since the spectrum shows resolved rotational structure.* On the other hand, the C M D triplet spectrum shows diffuse rotational contoursg that may be indicative of predissociation. However, photodecomposition from the singlet is effective and occurs with quantum yields close to one.1° The general subject of the thermolysis and photolysis of diazirines has been recently reviewed.” The exposition above shows clearly that the primary processes are not well understood. The purpose of this work was to clarify some of the more obscure points, especially the role that triplet surfaces can play in the mechanism. We have attempted the initial “preparation” of the excited CMD in electronic states of different multiplicities and with different rovibrational internal energies. (1) Bridge, M. R.; Frey, H. M.; Liu, M. T. H. J . Chem. SOC.A . 1969,91. (2)Cadman, P.; Engelbretch, W. J.; Lotz, S.; Van der Merwe, S. W. J. J . S . Afr. Chem. Inst. 1974, 27, 149. (3) Frey, H.M.; Penny, D. E. J . Chem. SOC.1977, 73, 2010. (4) Jones, W. E.;Wason, J. S.; Liu, M. T. H. J. Photochem. 1976,5, 31 1. (5) Moss, R. A,; Mamantov, A. J . Am. Chem. SOC.1970, 92, 6951. (6)Figuera, J. M.; Perez, J. M.; Tobar, A. J. Chem. Soc., Faraday Tram. I 1978, 74, 809. (7) Perez, J. M. J . Chem. SOC.,Faraday Trans. I 1982, 78, 3509. (8)Robertson, L. C.;Merrit, J. A. J. Mol. Spectrosc. 1967, 24, 44. (9) Robertson, L. C.; Merrit, J. A. J . Chem. Phys. 1972, 57, 316. (10) Figuera, J. M.; Tobar, A. J . Photochem. 1979, IO, 473. (11) (a) Liu, M. T. H. Chem. SOC.Reu. 1982, 11, 127. (b) Liu, M. T. H.; Palmer, G. E.; Chishtj, N. H. J . Chem. SOC.,Perkin Trans. 2 1981, 53.
The influence of these variables on the photofragments yield and on the partitioning of energy has been studied. Effects of added potential triplet quenchers have also been investigated. Experimental Section Synthesis and Analysis. 3-Chloro-3-methyldiazirine was prepared by halogenation of acetamidine following the procedure of Graham.12 The volatile contents were swept with nitrogen through a reflux condenser, an ice-cooled spiral trap, a U-tube packed with NaOH pellets and finally condensed in a trap cooled with a slush of dry ice + acetone. Some of the minor components found among the CMD photolysis products are formed spontaneously, in the dark. For this reason, the diazirine has to be distilled immediately prior to use in a grease-free vacuum line by using a methylcyclohexane slush bath (-126 “C) and its purity checked by GLC and UV spectroscopy. The hazards involved in freezing diazirines at 77 K should be remembered.I3 The diazirine was 99% pure (GLC) and was identified by its IR, UV, and mass spectra, which have been previously r e p ~ r t e d . * * ’ ~ J ~ ~ ’ ~ Solution spectra of CMD were recorded on a Perkin-Elmer Hitachi 200 spectrophotometer using 1-cm-path cuvettes. The systematic analyses of major products were done by gas chromatography (25% dimethylsulfolane on Chromosorb P); mass spectrometry and W spectrophotometry were used when required. Analyses of photolyzed samples were always done in duplicate. Photolysis with Added SF,. Gas-phase photolyses were performed in externally silvered Pyrex cells with cemented front quartz windows; stainless steel cell bodies with 0-ring-fitted quartz windows were used at the higher pressures. In all experiments with SF6 the initial pressure of diazirine was 5.5 torr and the buffer gas pressure was at least twice as much. The product ratio a t low pressures is practically identical with those found on photolysis of pure CMD; therefore, the use of (12) Graham, W. H.J . Am. Chem. SOC.1965, 87, 4396. (13) Liu, M. T. H. Chem. Eng. News 1974, 52, 3. (14) Mitchell, R. W.; Merrit, J. A. J . Mol. Spectrosc. 1967, 22, 165. (15) Engelbrecht, W. J.; Loubser, G. J.; J . S . Afr. Chem. Inst. 1975, 28, 191.
0022-3654/85/2089-5489$01.50/0 0 1985 American Chemical Society
5490
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The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
0.0:
w
002
0
0.0
f
S
1T 3
9
1 0 Log
0
n
2 rd
0
11
w
Figure 2. CMD photodecomposition results in SF,. Plot of data in the form D/(D+ S) where D is acetylene and S stabilized VC vs. collision frequency. (a) (0)X = 416 nm; (b) ( # ) X = 365 nm; (c) (+) X = 337 nm, the curve shown has been fitted to the experimental points by standard procedures; and (d) (*) data for the photolysis of pure CMD at X = 334 nm; included for comparison purposes (from ref 6)
001
380
420
-
460
500
wavelengthjnv
Figure 1. Singlet-triplet n T* absorption spectra of 3-chloro-3methyldiazirine in different solvents, showing the external “heavy atom” CMD in cyclohexane (3.3 mol/L); (b) (---) CMD enhancement. (a) in ethanol (3.1 mol/L); and (c) (-) CMD in methyl iodide (3.3 mol/L). (e..)
relatively low ratios SF6/CMD at low pressure should not introduce distortions in the calculated data. No dependence of product ratios on radiation dose was detected, but conversions above 1.O% were avoided. Cells filled with CMD and maintained in the dark for times similar to those of photolyses showed negligible decomposition and negligible products. Irradiation wavelengths were selected with the following sources and filters. 416 nm: A high-pressure mercury arc in combination with a “high intensity” B & L monochromator and a 1-cm-path filter of BiC1, (3.9 X lo-’ M) were used. The bandwidth (fwhm) was -5 kcal/mol. The absence of singlet photodecomposition by absorption of stray light was checked. 365 nm: A medium-pressure mercury arc in combination with the monochromator, mentioned earlier, was used. The bandwidth (fwhm) was -2 kcal/mol. 337 nm: A home-made pulsed nitrogen laser, of -5 mJ/pulse at a repetition rate of 1 H was used. The bandwidth (fwhm) was -2.5 kcal/mol. The bandwidth in the last two cases was mainly a consequence of the thermal energy distributions at room temperature. Photolyses with Added Oxygen and Nitrogen. Photolyses of CMD were carried out alternatively in the presence of oxygen and nitrogen under identical experimental conditions. Results Preparation of the CMD Triplet nr* State. The spin forbidden character of the transition at 416 nm was probed by using the external heavy atom effect. Spectra of CMD in different solvents, taken in the range 385-421 nm, are plotted in Figure 1. A clear enhancement of the light absorption in the presence of CH31can be observed. This hyperchromic effect cannot be due to solvent polarity, because the ethanol and cyclohexane curves are similar but different from CHJ. Therefore, the enhanced absorption must be attributed to the influence of the heavy iodine atom,I6 and the (16) (a) Turro, N. J. ‘Modern Molecular Photochemistry”; W. A. Benjamin: New York, 1978; p 125. (b) McGlynn, S. P.; Azumi, T.; Kasha, M. J. Chem. Phys. 1964, 40, 507.
feasibility of 3nn*electronic-state preparation by direct irradiation of the CMD ground state at 416 nm is confirmed. The problems associated with irradiation at an absorption band relatively close to other band hundreds of times stronger have been taken into account. It was checked that the filter used (see Experimental Section) effectively rejected any unwanted light eventually able to induce decomposition. Selecting the light corresponding to the closest 365-nm singlet absorption line with our monochromator and using the filter mentioned, we detected no decomposition. The uncontaminated preparation of the 3nr* state was thus confirmed and the possibility of photolysis by scattered light at other wavelengths discarded. Oxygen Effects. The participation of triplet 1-chloro- 1methylcarbene was investigated next. It has been repetitively assumed and confirmed by theoretical calculation^'^ that triplet carbenes should face energy barriers in their isomerization to the corresponding olefin, in contrast with singlet carbenes where this reaction is facile. The presence of such a barrier should confer to triplet carbenes a relatively long life and therefore make them easily trapped by triplet and radical quenchers, as, for example, 02.
Alternative photolyses of equal pressures (within 2%) of CMD in the presence of very close amounts of either N2 or O2 (very similar collides), in ratios gas “buffer”/CMD = 6, were performed at 296.7, 313, and 415 nm. It was observed that the amounts of vinyl chloride obtained at a given “buffer” pressure (Le., collision frequency) were independent of the gas used, N, or 02, for any of the wavelengths used. Therefore, the participation of triplet 1-chloro-1-methylcarbenecan be ruled out. Thus, at some point along the reaction path an efficient intersystem crossing to a singlet surface must occur when the initially prepared state is the n r * triplet. Decomposition Results and Vinyl Chloride Energy Distribution. Experimental photodecomposition data were analyzed assuming, as is indicated in the following section, that the final product distribution is a consequence of the unimolecular reaction of “hot” VC, whose rovibrational energy should be, in principle, dependent on its way of formation in each particular case: CMD -%. CMD*
- Ju -N2
VC’
k
C2H2(D) + HC1
VCW Implicit in this scheme is also the assumption that all the primary and secondary steps leading to hot vinyl chloride VC’ can be considered as collision-free processes. Thus, the experimental data ( 1 7 ) (a) Martin, M. An. Quim. (Madrid) 1978, 74, 1456. (b) Conrad, M. P.; Schaefer 111, H. F. J. Am. Chem. SOC.1978, 100, 7820.
Photodecomposition of 3-Chloro-3-methyldiazirine
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5491
++
Ene r g y / K c a 1 mo 1
-1
I
Figure 3. Photodecomposition of CMD. Plot of percentage of VC molecules vs. their energy. (a) (*) X = 416 nm; (b) (0)X = 365 nm (connecting curve has been drawn between points for clarity); (c) (+) X = 337 nm.
for the photolysis at 337, 365, and 416 nm were cast in the form D / ( D + S) vs. w, where D is the decomposed product and S the stabilized VC at a particular collision frequency w (i.e., pressure) and have been plotted in Figure 2. It can be observed that the curve corresponding to laser irradiation at 337 nm looks rather different from the other two at 365 and 416 nm where conventional irradiation sources (see Results) were used. Possible laser nonlinear effects were checked. Changes in laser focussing did not introduce observable changes in the product distribution. Moreover, previous experimental results obtained by conventional irradiation of pure C M D (a similar collider) a t a close wavelength (334 nm) gave rise to a similar pattern as can be observed in Figure 2. Therefore, laser coherent effects were discarded and the data obtained with the nitrogen laser treated on the same basis as the rest. The goal looked for in these experiments was the determination of the nascent vibrational energy distribution of the hot VC in each particular case. It has been demonstrated18 that the mathematical deconvolution of the D / ( D + S) vs. w data allows this determination. The process requires that the “hardest” possible collider (Le., average energy transferred per collisions 25 kcal/mol) should be used as buffer if “reliable” results are desired. Otherwise, distribution function maxima are shifted to higher energies (see supplementary material). According to work reported3 SF, seems to be adequate in the sense that (A,??)is greater than 5 kcal. The convenience of extending the data to high pressures has also been s u g g e ~ t e d .The ~~~ VC ~ ~internal energy distributions obtained from the data corresponding to the three irradiation wavelengths used are shown in Figure 3. It should be pointed out that the numerical method used provides different results for f(E)depending on the singular values rejected, and some results lack physical sense (great negativef(E) values, for example), these artifacts being due to effects introduced ~~
~~
(18)(a) Figuera, J. M.;Menendez, V.; Roddguez, J. C. An. Quim. (Madrid) 1984.80.490. (b) Figuera, J. M.; Menendez, V.;Rodriguez, J. C. Int. J . Chem. Kinet. 1985, 17, 583. (c) Nash, J. C. “Compact Numerical Methods for Computers”; Adam Hilger: Bristol, U.K., 1979;Chapter 3. (d) Rice, J. R., Ed. ”Mathematical Software”; Academic Press: London, J971; pp 347-356. (e) Twomey, S.J . Franklin Inst. 1965,279,95. (0 Rodriguez, J. C. An. Quim. (Madrid), in press. (19) Becerra, R.;Castillejo, M.; Crespo, M. T.; Figuera, J. M. Chem. Phys. Lett. 1981,83, 573. (20) Becerra, R.; Figuera, J. M.; Martin, M.; Rodriguez, J. C. Int. J . Chem. Kinet. 1984, 26,483.
e2
I
I
CMD
Diradical
I
I
REACTION COORDINATE Diazo' Carbened
Cve
Figure 4. Photodecomposition of CMD. Interconnectingpotential surfaces. General features of curves taken from ref 21, location of stationary points estimated from available data. Figures within parentheses are heats of formation (estimated or taken from bibliography). ( a l ) CMD So, ref 2; (a2) CMD )n?r*, ref 9;(a3) CMD ’n?r*, ref 8; (b) diradical 1,3Dv,v estimated from the activation energy for the thermal unimolecular decomposition of CMD assuming that the transition state is close in shape and energy to the radical; (cl) ground-state heat of formation of 1chlorodiazoethane estimated from M O results for similar diazo comp o u n d ~ (c2) ; ~ ~ diazo states “r*and Inn from ref 25; (d) l-chloro-lmethylcarbene energy estimated from M O calculations for similar carbenes:s singlet-triplet splitting estimated as being very (el) ground-state heat of formation of VC from ref 22; (e2) excited states of VC from ref 23.
+
by D / ( D S) experimental errors. However (see supplementary material), these effects are unable, within reasonable limits, to significantly change the main features of the deconvoluted distribution functions (number and positions of maxima, total area, and weight of the peaks, etc.). See supplementary material for computational details. It can be observed that the irradiation at 416 nm, i.e., preparation of the CMD 3 n ~ state, * produces a single maximum in the VC energy distribution. On the other hand, the 365- and 337-nm irradiations, which lead to vibronic states of the first n?r* singlet differing in rotovibrational energy content, present two well-defined maxima. One of them appears at a similar energy to that observed
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The Journal of Physical Chemistry, Vol, 89, No. 25, 1985
in the case of the triplet and does not change when the energy of the absorbed photon goes from 365 to 337 nm. In contrast, the peak located in the high energy region shifts to higher energy when irradiation is carried out at 337 nm. Furthermore, it is observed that the area of the higher energy peak increases at the expense of the lower one as the input photon energy is increased. Discussion The discussion of the results obtained will be done with the help of the energy potential curves of Figure 4. The main frame of this diagram has been built based on theoretical data for the photodecomposition and photoisomerization pathways of diazirine (CH2N2),obtained by ab initio MO calculation.2' The levels corresponding to equilibrium configurations have been located in the diagram according to the experimental thermodynamic and spectroscopicreported data for 3-chloro-3-methyldiazirine (CMD)2 and its singlet* and triplet9 electronically excited states, as well as for vinyl chloride and its ground2*and excited states.*3 The energies of 1-chlorodiazoethane and chloromethylcarbene ground-state singlets were estimated from MINDO-2 MO semiempirical calculations for a series of corresponding alkyldiazo compounds, diazirines and ~ a r b e n e s . The ~ ~ energies of the 1chlorodiazoethane higher states were estimated from other diazoalkanes spectroscopic data.2s The S-T carbene splitting was assumed to be very and finally the ground-state energy of the diradical was roughly evaluated by adding to the CMD ground-state energy the activation energy of its unimolecular decomposition.' This approach assumes that the transition state of this reaction is very close in structure and energy to the diradical. It should be remarked that only the gross features of the hypersurface are relevant for the following discussions, and therefore errors in energies that do not disturb these features can be neglected. Anyhow, the surface presented is probably the best that can be constructed with the available data and should provide a framework, reasonably accurate, for discussion purposes. Collisional Effects. In the following discussion we will assume that the reaction steps leading to vibrationally excited vinyl chloride are so fast that collisional effects, induced by the gases used in the experiments, can be neglected. This supposition is sustained by the following experimental observations. The shape of the energy distribution functions (see Figure 3) are well-defined and relatively narrow. These two facts are incompatible with the existence of collisional deactivation processes actuating, at some point along the reaction path, between the initial excited diazirine and the hot vinyl chloride. Even oxygen reactive collisions have to be discarded taking into account the similar VC yields obtained in the presence of oxygen or nitrogen. Photodecomposition f r o m the CMD 3na*State. The CMD 3 n ~ state * correlates with the lowest 1 -chlorodiazoethane triplet state. From this state formation of nitrogen and triplet chloromethylcarbene is easy as the lack nf significant energy barrier indicates (see ref 21 and Figure 4). However, the triplet carbene should face a barrier in its isomerization pathway toward triplet VC (the path to singlet VC is spin forbidden)." This situation should confer to the triplet carbene a lifetime relatively long which would make it readily quenchable by oxygen. The already mentioned equality between the VC yields obtained in the photolysis of CMD, with either N2or 02, provides evidence against the formation of the triplet carbene and, therefore, of its path of formation. The crosses ( X ) on this path in Figure 4 indicate the absence of this theoretically allowed path in the (21) Bigot, B,; Pons, R,;&"in, A,; Devaquet, A. J , Am, Chem, Sw,1978, 100, 6575. (22) Benson, S. W. "Thermochemical Kinetics";Wiley: New York, 1968; p 202. (23) Berry, M. J. J . Chem. Phys. 1974, 61, 3114. (24) Figuera, J . M.; Perez, J. M.; Tobar, A . An. Quim.(Madrid) 1976,
-.(25) Avila, M. J.; Figuera, J. M.; MenCndez, V.; Perez. J. M. J . Chem.
72 717 -
SOC.,Faraday Trans. 1 1976, 72, 422.
(26) Kirmse, W. "Carbene Chemistry";Academic Press: New York, 1971; p 187.
Avila et al. experimental photodecomposition from the CMD 3nr*. The "dissociative" character of this 3 n r * state is reflected in the UV spectrum of CMD, where the absorption at 416 nm is rather d i f f ~ s esuggesting ,~ a rapid crossing to the Is3D diradical surface. At this point the weakening of the spin-spin interaction may facilitate enormously the intersystem crossing. The final extrusion of nitrogen from the singlet diradical will produce the ' n r * state of the carbene that, without significant energy barrier, would lead to the hot VC. Another possible reaction path, the spin allowed extrusion of triplet nitrogen from the triplet D , , diradical to give singlet carbene, has to be excluded on energetic grounds. No nitrogen triplet states of energy low enough are known.27 According to the above discussion, the single maximum of the energy distribution curve, Figure 3, corresponding to photodecomposition from the triplet state reflects the existence of an unique pathway of fragmentation. The following elementary steps compose this route: formation of triplet diradical, intersystem crossing of this intermediate to the singlet state, extrusion of nitrogen giving singlet carbene, and final isomerization to the hot vinyl chloride. It is worthwhile to remark that the energy partitioning process occurs really during carbene formation (Le., nitrogen extrusion), the internal energy content of the final hot VC being determined by that process. Photodecomposition from the CMD ' n r * State. The CMD 'na* electronically excited state has been prepared by using two different wavelengths, 365 and 337 nm. The irradiation at the first corresponds to a hot band leading to the Inr* state in its 0 vibrational level8 Therefore, this route should prepare the Inr* state with very little vibration-rotation energy. On the other hand, the irradiation at 337 nm corresponds to a transition that introduces one quantum of vibrational energy (-4 kcal/mol) in the symmetric N=N stretching of the Inr* state. However, it should be remembered that both the relatively broad bandwidth of the irradiation sources used in our experiments and the continuum superimposed on the CMD spectrum preclude the preparation of a well-defined and individualized vibronic state. The two internal energy distribution curves pertaining to photodecomposition from the singlet state show the appearance of a second peak at higher energy. At 365 nm, it is only a small shoulder of the peak at around 83 kcal, but at 337 nm it is the predominant feature of the distribution. The opening of this new reaction path, whose quantitative participation is clearly energy dependent (see areas (i.e., number of molecules) under each maximum and their change with photon energy in Figure 3), will be interpreted with the help of the diagram in Figure 4. The entrance surface from the Inr* state intersects the dissociative curve at a point where efficient intersystem crossing may occur since the flipping of the electron spin is counterbalanced by a 90' rotation of the electron location in space (from n to T ) . ~ ' The relatively heavy chlorine should facilitate the crossing possibility through the mentioned spin-orbit coupling mechanism. Once this first surface crossing has occurred the molecules can follow the diradical path toward VC previously described for the 3nr* state decomposition. The molecules which do not intersystem cross can follow their way toward decomposition through the allowed structural isomerization to 1-chlorodiazoethane Inr* state, which subsequently should easily extrude nitrogen to give singlet l-chloro-lmethylcarbene. The resulting carbene should undergo finally a facile hydrogen shift that will generate the vibrationally excited
vc.
The isomerization of diazirines to diazo compounds in solution but its Occurrence in gas phase, has been although theoretically supported, has not been clearly confirmed e ~ p e r i m e n t a l l y . ~It~ should . ~ ~ be realized that ground-state diazo compound lifetimes at the high excess energies resulting from documented95"'
(27) Herzberg, G . "Spectra of Diatomic Molecules"; Van Nostrand: New York, 1950. (28) Amrich, M . J.; Bell, J. A. J . AM. Chem. SOC.1964, 86, 292. (29) Moore, C. B.; Pimentel, G . C. J . Chem. Phys. 1964, 41, 3504.
Photodecomposition of 3-Chloro-3-methyldiazirine
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5493
TABLE I: Energy Distributions wavelength/nrh energy of maxima, E,,"/kcal/mol
diradical diazo SI diradical diazo S ,
excess energies, EE,available for partitioningb/kcal/mol maximum energies, ET,available for partitioningc/kcal/mol percentage of excess available energies found on VC viad
diradical diazo SI CMD via diazo SI other diazoalkanes
percentage of total energies via diazo found on VCe same on equivalent olefins in other diazoalkaned
416 84 31.6 33.8 118.6 8.0 none
365s
331
82.5 95 50 46.0 128.3 3.2 21.9 14 7 3-8 3
83 103 54 49.9 134.8 3.8 36.3 16
"Experimental results: see Figure 3. According to our interpretation the areas under the curves with maxima at lower and higher more probable energies represent the molecules of VC formed via triplet (Le., diradical) and first singlet 2-chlorodiazoethane (Le., diazo SI),respectively. The maxima correspond to the more probably energy E,, on the VC molecule originated by each pathway. *Excess energies, EE,on photodissociating species (diradical or diazo SI)calculated as EE = Ehv AiYfo(CMD) - AHfo(diradical/diazo S,). 'Maximum excess energy that can appear on the vinyl chloride, ET, calculated as ET = E l , + M f 0 ( C M D ) - AH?(VC). dPercentage of excess energy EE of the photodissociating fragment appearing on vinyl chloride E , (excluding the part that comes from the potential to vibrational energy conversion) calculated as E,(diradical/diazo SI) = lOO[E,,(diradical/diazo SI) - Q]/[EE(diradical/diazo SI)]. The reaction heat Q is defined as Q = AHfn(diradical/diazo SI) - AHfo(VC). 'Calculated as ET(%) = EMpX 100/(Eh, AHfo(CMD) - AHfo(VC)). /See ref 24. gThis transition corresponds to a hot band;* this fact has been taken into account in the calculation.
+
+
CHART I: Mechanism of Reaction
H'
D l u o 91
ii
Cl H.
". Cl
4 "+ H
H
%J.
4
/'-
H
H
photoactivation processes should be extremely short6 Also, the clearly dissociative character of their hr* statez5should make the lifetimes of these states even shorter. Our conclusion is that the 'nr* state of 1-chlorodiazoethane is an intermediate in CMD decomposition and its inclusion in the reaction mechanism helps to explain the experimentally determined energy distribution found in VC. The elusive character of this intermediate agrees with our previous assumption that collisional effects could be discarded on the whole series of elementary processes leading to VC formation. The maximum vibrational energy that can appear in VC would equal the exoergicity of the reaction C M D VC + Nz(50 kcal/mol) plus the absorbed photon energy. The percentage of this maximum energy that appears in the VC, taken as the energy of the higher energy maximum in Figure 3, is around 75%. Values for diazoalkanes of similar complexity photodecomposed from their first 'nr* excited electronic state range from 73 to 83%24(see also Table I). This agreement supports our previous proposal that the adiabatic isomerization of excited C M D opens a new route for its fragmentation. In our mechanistic scheme, the pathway via diazo compound produces vinyl chloride with an internal energy
-
content greater than that generated through the diradical route. Moreover, the effect of the initial energy level is important from another point of view. An increase of -6 kcal/mol in the photon energy dramatically increases the number of molecules which follow the path involving the diazo compound intermediacy. The presence of a small energetic barrier in the entrance to this pathway (see Figure 4) would allow this path to compete with the intersystem crossing path going to the diradical, in an energy-dependent fashion. Energy Partitioning. As the N2 breaks away in either path toward the carbene the molecular system contains two major types of energy to be distributed. One is the vibrational excess energy of dynamic character and associated with the motion of the atoms. The other is the static (i.e., potential) energy of a molecular system, where the atoms are initially located in places far from the equilibrium situation corresponding to the final products. I t is clear that as the reaction proceeds this static potential energy will be rapidly converted in vibrational, rotational, or translational energy (i.e., motion of the atoms around their final equilibrium positions).
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The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
Both initially different energies should appear jointly as rotovibrational energy of the hot fragments and therefore memory of its origin will be lost. However, each energy type can be partitioned with different efficiency. We may assume that in all cases static potential energy is going to appear predominantly in the fragment with more atom coordinates out of their equilibrium. Both extrusion paths are, within this context, similar. They have a large number of molecular coordinates out of equilibrium in the hydrocarbon fragment but only one, the N-N stretch, in nitrogen. Accordingly we may assume that potential energy would appear almost quantitatively in all cases as vibrational on the hot carbene. There is another potential energy associated with the exoergicity of the reaction carbene VC that has to appear quantitatively as vibrational on VC and has to be taken into account in the calculations. Therefore, if we assume that the total exoergicity of the decomposition is channeled almost quantitatively to the hot vinyl chloride, the results obtained can be analyzed in the following way. Subtracting from the energy experimentally found on vinyl chloride, estimated from the corresponding distribution peak in Figure 3, the reaction exoergicity, we can evaluate the percentages of the initial rotovibrational energy which are channeled to vinyl chloride (Le., to carbene) (see Table I). The results show that the path through the intermediate diradical directs only 3 4 % of its rotovibrational energy to VC. A reasonable explanation of these results may be the existence of a weak coupling between the receding fragments during the dissociation process that would allow energy transfer between the different degrees of freedom of the fragments. Thus, the energy could flow to the rotational and translational degrees of freedom created as a consequence of the broken bonds. The high number of energy levels of this degrees of freedom should favor the energy flow toward them. A tortuous fragment exit channel with multiple collisions between the fragments may be responsible of the mentioned weak coupling. The higher energy path (i.e., the diazo compound route) behaves differently: about 30% of the diazo excess energy appears on the VC. In this case the energy distribution shifts toward higher energies as the input energy on CMD increases (see Figure 3 and Table I). This observation may indicate some contribution of statistical factors in the diazo excess energy partitioning process. The higher width of the distributions supports this interpretation. A more detailed discussion of nitrogen extrusion dynamics is out of the scope of this work. Reaction Mechanism. The primary mechanism of 3-chloro3-methyldiazirine gas-phase photodecomposition can be written, according to the evidence previously presented, in a simplified manner as shown in Chart I. The two detected reaction paths are ascribed to singlet and triplet na* CMD, respectively. The dual participation of the singlet in both paths is explained by efficient intersystem crossing between the two states. The increased participation of the singlet surface as the input energy on CMD augments is what can be expected as the result of two competing processes, one of chemical nature and, therefore, of rate very dependent on energy, and the other, the intersystem crossing of physical nature with a lesser dependence on energy. We think that the reported mechanism offers a satisfactory explanation of the known data about the reaction studied.
-
Conclusions A description of the elementary processes occurring in the gas-phase photodecomposition of 3-chloro-3-methyldiazirine that rationalizes the known data is presented and the relevance that the determination of the energy distribution function can have in connection with reaction mechanism elucidation is stressed. Possible applications of the methods of energy distribution determination that we have used here may extend the molecular dynamics field to more complex situations and, therefore, be more interesting from the point of view of the chemical reactivity. Acknowledgment. We thank CAICYT for financial support. R.B., J.C.R.. and A.T. thank C.S.I.C. for their scholarships.
Avila et a]. Fruitful discussions with Prof. J. Santamaria and Dr. V. Menindez are gratefully acknowledged. Appendix 1 Calculation of Collision rate^.^^,^' Collision rates were calculated by the standard kinetic relationship
where S is the effective collision diameter, M the molecular weight, and NB the concentration of B molecules. The S A B values are estimated from Lennard-Jones hard-spheres diameters uABby SA,’ = uAB2[n2’z(T*)]. The collision integral is a function of the reduced temperature P.The reduced temperature is calculated by T* = kT[(€/k)~(€/k)e]-”’,where elk is the force constant and the other symbols have their usual meanings. The parameters used are listed below. molecule CMD SF,
u,
cm
X
lo8
7.1 5.01 3.1
vc
elk, K 400 25 9 400
estimated values ref 31 ref 31
The calculated collision rates w / p for VC in CMD and for VC in SF6 were w / p = 2.25 X lo7 and 1.15 X lo7 s-l T-I. R R K M Calculations. Microscopic Rate Constants. Microscopic rate constants k ( E ) for monoenergetic vinyl chloride decomposition were calculated by using RRKM t h e ~ r y .The ~ ~ ~ ~ ~ vibrational wavenumbers (in cm-’) of the molecule and activated c ~ m p l e are x ~ given ~ ~ ~ below. molecule 3030 3130 7 24 1370 1280 895
3080 1610 3 95 1030 622 940
complex 3030 1800 7 24 1030 35 940
3080 956 1183 1280 895
The vibration at 395 cm-I was taken as reaction coordinate. The calculated ratio of rotational partition functions was calculated to be 1.99 and the critical energy E, was 73.6 kcal/mol. Sums and densities of states were determined by using the WhittenRabinovitch approximation^.^^ At low energies, an exact count algorithm was used. Energy Distribution Determination. A mathematical deconvolution method’* has been used for obtaining the energy distribution function, f(E),of the excited vinyl chloride. The method requires the knowledge of (a) the unimolecular decomposition data for a set of pressures and (b) unimolecular microscopic rate constants k ( E ) . This method is based on the direct mathematical resolution of the equation
This equation was solved numerically, transforming the integral into a summation with an adequate quadrature. (30) Hisrchfelder, J. 0.;Curtiss, C. F.; Bird, R. B. “Molecular Theory of Gases and Liquids”; Wiley: New York, 1954. (31) Setser, D. W.; Rabinovitch, B. S. Can. J. Chem. 1962, 40, 1425. (32) (a) Marcus, R. A.; Rice, 0. K. J. Phys. Chem. 1951, 55, 894. (b) Marcus, R. A. J. Chem. Phys. 1952, 20, 352. (33) (a) Wieder, M. G.; Marcus, R. A. J. Chem. Phys. 1962, 37, 1835. (b) Waage, E. V.; Rabinovitch, B. S.Chem. Rev. 1970, 70, 377. (34) (a) Torkington, P.; Thompson, H. W. J. Chem. SOC.1944, 303. (b) Herzberg, G. ”ElectronicSpectra of Polyatomic Moleculas”;D. Van Nostrand: Princeton, NJ, 1966. (35) (a) Whitten, G. 2.;Rabinovitch, B. S. J. Chem. Phys. 1963, 38,2466. (b) Whitten, G. 2.;Rabinovitch, B. S. J. Chem. Phys. 1964, 41, 1883. (c) Tardy, D. C.; Rabinovitch, B. S.; Whitten, G . Z. J . Chem. Phys. 1968, 48, 1427.
J . Phys. Chem. 1985, 89, 5495-5499 If we have enough D / ( D + S ) data we may write a system of m equations with n unknowns ( m 1 n) that in matrix representation is reduced to 6 = 21 where the elements of
b are
bi = D / ( D + S )
The coefficients of the matrix 0 . .=
'
2 are
k(Ej)
k ( E j ) + wi
and the unknowns of matrix
AE
1 are
5495
Solving the system of linear equations, we can find the values of the energy distribution function. Registry No. CMD, 4222-21-3; VC, 75-01-4; SF6, 2551-62-4; CHJ, 74-88-4; 0,, 7782-44-7; N2, 7727-37-9; 1-chlorodiazoethane, 59712-39-9; 1-chloro-1-methylcarbene,3 1304-5 1-5.
Supplementary Material Available: Detailed outlines of the mathematical deconvolution method used. The influence of experimental errors and collision softness on the results of the deconvolution method application is also estimated (8 pages including Figures 5-7). Ordering information is given on any current masthead page.
Bistability and Bromide-Controlled Oscillation during Bromate Oxidation of Ferroin in a Continuous Flow Stirred Tank Reactor V. Giispiir, G . Bazsa, and M. T. Beck* Department of Physical Chemistry, Institute of Chemistry, Kossuth Lajos University, Debrecen, P j 7 . , 401 0, Hungary (Received: April 15, 1985)
Autocatalytic oxidation of Fe(phen)?+ (ferroin) by bromate has been investigated in a continuous flow stirred tank reactor (CSTR). A closed region of bistability exists in the plot of input stream concentrations of ferroin and bromate. The bromate-ferroin-bromide system exhibits bistability and high amplitude oscillations over a broad range of concentrations. As a first approximation a revised Oregonator type model extended by reactions of ferroin and ferriin describes the system. However, further reactions have to be taken into consideration for a quantitative description.
Introduction Discovery of small-amplitude oscillations in a CSTR containing bromate, bromide, and cerous or manganeous ions has been reported by Orban et al.' and G e i ~ e l e r . ~ The , ~ search for these so-called minimal bromate oscillators was stimulated by compuon the Noyes-Field-Thompson (NFT) modeL5 t a t i o n ~based ~ Existence of a very narrow region of small-amplitude oscillations was predicted near the critical point, at which the bistability of the system disappears. Although ferroin is a well-known catalyst of the Belousov6Zhabotinsky' (BZ) reaction,8 there are no reports on either the dynamics of the simple ferroin-bromate autocatalytic reaction+l2 or the existence of a ferroin-catalyzed minimal bromate oscillator in a CSTR. The necessity for such investigations, however, is strongly supported by a number of experimental observations8J3-18 (1) Orbin, M.; De Kepper, P.; Epstein, I. R. J. Am. Chem. Soc. 1982,104, 2657. (2) Geiseler, W. Ber. Bunsenges. Phys. Chem. 1982, 86, 721. (3) Geiseler, W. J . Phys. Chem. 1982, 86, 4394. (4) Bar-Eli, K. In "Nonlinear Phenomena in Chemical Dynamics", Vidal, C., Pacault, A., Eds.; Springer-Verlag: West Berlin, 1981; Vol. 12, p 228. (5) Noyes, R. M.; Field, R. J.; Thompson, R. C. J. Am. Chem. SOC.1971, 93, 7315. (6) Belousov, B. P. ReJ Radiat. Med. 1959, 1958, 145. (7) Zhabotinsky, A. M. Dokl. Akad. Nauk. S S S R 1964, 157, 392. (8) Vavilin, V. A.; Gulak, P. V.; Zhabotinsky, A. M.; Zaikin, A. N. Izu. Akad. Nauk. S S S R Ser. Chim. 1969, 11, 2618. (9) K W s , E.; Burger, M.; Kis, A. React. Kinet. Catal. Lett. 1974, I , 475. (10) Rovinsky, A. B.; Zhabotinsky, A. M. Teor. Eksp. Khim. 1979, 15, 25. (1 1) Rovinsky, A. B.; Zhabotinsky, A. M. React. Kinet. Cafal.Left. 1979, 11, 205. (12) Yoshida, T.; Ushiki, Y. Bull. Chem. SOC.Jpn. 1982, 55, 1772.
which indicate differences in behavior and possibly in mechanism of metal-ion- and ferroin-catalyzed BZ oscillator^.'^^^^ The higher level of complexity in ferroin-catalyzed systems is demonstrated by the experiments performed in our laboratory earlier.19 It was found, surprisingly, that ferroin can react with bromate alone in an oscillatory manner. However, the continuous and partial removal of bromine and high concentrations of both reactants are the necessary conditions of oscillation. This observation strongly indicates that, at least at high concentrations, there must be basic differences in the chemistry of bromate-ferroin and bromate-metal ion (Ce3+,MnZ+)systems. Then in the BZ systems, in which the concentrations of catalysts are much lower, these differences can also result in some modifications. The bromate-ferroin batch oscillatory system shows similarities to the bromate-cerous-oxalic acid system.21 Therefore, at a very early stage of our study the differences mentioned earlier were attributed also to the possibility of a kind of bromine-controlled (13) (a) Viradi, Z. Ph.D. Thesis, Debrecen, Hungary, 1972. (b) Viradi, Z.; Beck, M. T. J . Chem. SOC.,Chem. Commun. 1973, 30. (14) Kijrb, E.; Burger, M.; Friedrich, V.; Ladinyi, L.; Nagy, Zs.; Orbin, M. Faraday Symp. Chem. SOC.1974, 9, 28. (15) Smoes, M. L. J . Chem. Phys. 1979, 71, 4669. (16) Ganapathisubramanian, N.; Noyes, R. M. J . Phys. Chem. 1982,86, 5158. (17) Rovinsky, A. B.; Zhabotinsky, A. M. J. Phys. Chem. 1984,88,6081. (18) Noyes, R. M. J . A m . Chem. SOC.1980, 102,4644. (19) (a) Beck, M. T.; Bazsa, G.; Hauck, K. In "Kinetics of Physicochemical Oscillations",Discussion Meeting at Aachen, 1979, Preprints of Submitted Papers, Vol. 1, p 123. (b) Beck, M. T.; Bazsa, G.; Hauck, K.Ber. Bunsenges. Phys. Chem. 1980, 84, 408. (20) "Oscillations and Travelling Waves in Chemical Systems", Field, R. J., Burger, M., Eds.; Wilcy: New York, 1984, Chapter 2. (21) Noszticzius, Z.; Bdiss, J. J . Am. Chem. SOC.1979, 101, 3177.
0022-3654/85/2089-5495$01.50/00 1985 American Chemical Society
