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The dynamics of CO production from the reaction of O(3P) with 1-and 2-butyne
Chemical Physics 25 (1977) 353-359 D North-Holland Publish_mg Company
:
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THE DYNAMICS OF CO PRODUCTIOh FROM THE REACTION OF O(3P) WITH l- and ZBUTYNE
M.E. UMSTEAD andM.C. LIN Cfzemisw Division. Naval Research Laboratory. Washington. D.C. 20375, USA Received 25 April 1977
The reactio_ns of l- and 2-butyne with O(3P) were studied at 298 K by means of a CO laser resonant absorption technique. ‘IbeCO formed in the 1C4H6 reaction carried 0.78 r 0.03% of the total available reaction energy, while that from 2C4He carried 0.98 c 0.06% of the reaction ener,v. Reasonable agreement was obtained between the observed CO population distributions and those predicted by simple statistical models assuming that diradicals, and not propylene, were formed initially in the reactions. These results are consistent with the mechanisms: O(aP) +CH3CHzC=CH-,
CH3CH2CH
+ CO
ICsHs, O(jP) + CH3C=CCHa
-f CH3CCH3 +CO IC;He.
From the kineticmodeling of the observed rates ofC0 formation, the rates of these reactions were found to be 5.0 X‘LO” and 1.6 X lo’* ma mole-’ Br for l-and 2-butyne, respectively, based on previously reported values for the propyne reaction.
1. Introduction The reactions of I-butyne [1,2] and 2-butyne [2-41 with O(3P) both lead to CO and C3H6 as the principal products. The I-butyne reaction is believed to proceed primarily through an ethylketene intermediate which decomposes to form CO and the CH,CH,CH diradical [1,2], and 2-butyne, tbrougb a dimethyllcetene intermediate which breaks down to produce CO and the (CH3)2C diradicai [2,4] _Both diradicals rapidly isomerize to yield C3He-_Although recent mass spectroscopic e_vidence [2] supports the presence of these diradicals as intermediates in the reactions, they have not yet been detected unambiguously by chemical or spectroscopic methods. In investigating the dynamics of CO production in 0(3P)-alkyne reactions, we have employed a continuous wave (cw) CO laser which can be operated over a broad range of the CO vibrational spectrum [5,6] to measure the viimtional population of the nascent CO formed in -these reactions. With the help of simple statistical cali culations, we have previously shown quite conclusively
that in the 0 f propyne reaction, CO and the CH3CH diradical are produced directly from the decomposition of vibronically excited methylketene, and that the isomerization reaction, CH,CH + C,H,, takes place after the complete separation of CO and CH,CH [6,7] _ We have now extended our investigation to in&de l-. and 2-butyne and have shown that their reactions with 0 atoms also proceed through substituted ketene intermediates which subsequently break down to produce CO and diradicals. We have also obtained the rates of these reactions relative to CH3C2H by the computer modeling of the total production rate of CO, CNJ, for each alkyne.
2. Experimental
The flash photolysis and laser probing systems have been described previously in detail [5,6]. Briefly, a stabilized cw CO laser, preset at various vibrationalrotational lines accessible to the reaction, was directed through the axis of a Pyrex flash photolysis tube. The
M.E. Urnstead,M.C. LiniReactions of 013p) with I- and 2-&yne
~~e~p&&e of the flash tube could be controlled to + 1°C by circulating fluid from a constant temperature bit& thro$$ the outer jacket of the tube. O(3P) atoms were &tierated by the photodissociationbf NO, at waielengths >300 MI. Mixtures of NO,,SF,and I- or 2-butyne were flash photolyzed, and time-resolved CO abso&iGn curves were obtained for all vibrational levels populated by the reaction. The CO vibrational populations were determined by analyzing the initial portions of the absorption curves by means of a computer according to the gain equation, given in ref. [5]. The initial population distribution was evaluated by extrapolating the NV/No ratios to the earliest appearance time of absorption in order to eliminate both the effects of CO vibrational relaxation and any possible contributions from secondary reactions that might generate co. Stable products of the reactions were measured in a different flash photolysis apparatus by means of a Beckman CC4 gas chromatogriph equipped with hydrogen flame detectors. A Pyrex flash tube, sealed at one end and connected at the other to the chromatographic sampling system, was mounted coaxially in the center of a quartz flash lamp. After a mixture was flash photolyzed, it was expanded directly into the evacuated gas sampling loop of the chromatograph and analyzed. A 0.3 1%cm-o.d. Porapak-T colu’mn, 305 cm in length, was used for the hydrocarbon products, and a 0.635cm-o.d. SA Molecular Sieve column, 183 cm in length, was used for CO measurements. The CO was hydro- , genated over a Ni catalyst at ca. 300 C at the exit of the Molecular Sieve column and was detected as CH4 by the flame detector [8*9]. It was necessary to remove NO, and SF6 from the samples prior to analysis for CO. The NO, was found to react with organic compounds in the Molecular Sieve column to produce substantial quantities of CO, and the SF, rapidly poisoned the Ni catalyst. These substances were removed by the use of a Porapak-T column (0.318 cm o-d., 30 cm long) maintained at 195 K which was suitably valved so that it was put in series with the Molecular Sieve during sample injection, and then backflushed after the CO had passed through and entered the analytical column. The butynes were obtained from the Chemical Samples Company and were purified by trap-to-trap distiIlation before use. The NO2 and SF6 were obtained from the Matheson Gas Products Company. The NO, was purified by pumping at 195 K to remove traces of N20s,
-
--
_ _-
and the SF, was thoroughly degassed at 77 K before use.
3. Results. The hydrocarbon products found essentially confirmed the results of Herbrechtsmeiei and Wagner [1,4] and were not investigated extensively. The main difference in product cotiposition found in this work was the virtual absence of CH4, C,H, and C,H,, indicating that the free radical precursors of these products are effectively scavenged by the NO and NO2 in the mixtures. From the laser absorpti& measurements, the CO las found to be vibrationally excited to u =4 in the
d
0
IO
20
30
40
50
&/KCAL-MOLE-’
Fig. 1. VibrationaI energy distriiutions of theC0 formed in the 0(3P) f l- ar.d 2-butyne re&tions. Solid line: theoreti@ly predicted population distriiution based on-A@, foi (CHJ)~C of -68 kcal/mole, dotted limes: AI$, = -63 and -73 k&/mole. Dashed line: Theoretical distriiution based on randomization of full reaction energy in complex. Filled .ticles: TJw prior CO viirationaldistniutiqfi etiluat? by ta$ngE&= 52 kcaljmole and treating all vibrational-rotational modes of the’CHsCCH3 radical as active.
M.E. Urnstead, ML’. Lin/Reach*ons of OpP) with I- and 2-butyne
355
where f,=Nu/Zvsfluis the normalized population distribution and E, is the vibrational energy of CO at the uth level with the zero-point enemy excluded. The data shown m fig. 1 give rise to (EJ = 0.97 2 0.04 fcr tbe CO from I-butyne and LFJ = 1.14 f 0.07 k&/mole for that from 2-butyne. Fig. 2 shows the total production of CO, EN,, as a function of time over the f=st few microseconds of the reaction for the butyne isomers as well as that from propyne. All three mixtures were run alternately. To obtain the rates of the 0 + C4H6 reactions relative to C3H4, the CO production curves were modeled with a computer, using the fobowing reaction scheme: NO, +hvhO
+NO,
(1)
O+N02LNO+02,
(2)
O+NO+MkNO2+M,
(3)
0+C3H4hCO+C,H4,
t4a)
0 + C4H6 *CO
+C3H6,
(4b)
0 + C2H4 k5a
Products + CO,
@a)
0 f C,H,33+
Products + CO.
(5b)
VPS Fig. 2. Rates of formation of totaIC0 (EN,) from the 0c3P) f Cs&, 1C4H6 and 2-C4Hs reactions. Solid Lines: Computed rat& b&d on different values of k4;(1) 4.2 X 10”. (II)>.0 x lo’*, (III) 1.2 X lOI’, (IV) 1.6 X 10” and (V) 2.9 X lo’*, aII in units of ml mok? 3.
l-butyne reaction and to u = 6 in the 2-butyne reaction, with Boltzmann vibrational temperatures of 2000 + 150 K in both casks. The higher vibrational levels observed in the 2-butyne reaction reflect the faster rate: of this reaction compared with 1-butyne rather than the formation of more energetic CO, since the increased rate of CO production increases the detectability of CO by the absorption measurements. In fig_ 1 are plotted the population distributions of CO formed in these two reactions. Tire distributions are close to Boltzmann, and the results for both butynes are very similar; The points on the curves represent the averages of 3 sets of data for each hydrocarbon, obtamed from 7 torr of 1~2.17 C4H6 :N02 :SFs mixtures flashed at-an energy of 1 kJ at 298 K. The amount of vibrational energy channeled into CO was calculated from the expression: : G?,,)= fE
z
IGO
u “’
Avalue of 4.2 X 10” mQ mole-’ s-’ (298 K) was taken fork4,(averageof4.i+1 X 101r [IO] and4.4X IOr [l l]), and reactions (l-3), (4a) and (Sa) were first considered. The NO, photodissociation rate k, was determined as a function of time from the flash profile using Treanor’s modified fourth-order Rungs-Kutta integrations [ 121 of all coupled differential rate equations. Values for kl were varied until a computed curve was obtained that fit the CjH4 experimental points. Other rate constants used were: k2 = 5.5 X 1Ol2 mQ mole-’ s-’ [ 131, k, = 4.9 5 1016mQ2 moleW2sW1 [14], k, = 4.9 X lOI mQ mole-l se1 [ 151 and li5b = 2.2 X IOr mQ mole-’ s-l [15]. Once kl was established, reactions (l-3) (4b) and (Sb) were considered, and kab was varied until curves were obtained that fit the l- and 2-C4H6 experimental results. The solid lines in fig. 2 are the computed curves. Based upon rhe value of 4.2.X 10” mQ mole-’ s-l for C3H4, rate constants of 5.0 X 10” and 1.6 X 1012 were found for I- and 2-C4H6, respectively.
356
M.E. Urnstead,MC. LinjReactions of O[3P) with I- and 2-butyne
4. Discussion The mechanism of CO production from the reaction of O(3P) with 1-butyne is believed to proceed via the decomposition of an excited ethylketene intermediate to yield CO and the propylidene diradical [ 121: O(3P) + CH,CH,C=CH CH,CH,CH=C=Ot
* CH:CH&“H
+
+ CH,CH,CH: + CO+ I
A$
?
(6)
CH,CH=CH,,
= -120.8 kcal/rnole,
and the 2-butyne reaction through migration of a methyl radical to form a dimethylketene complex which breaks down to CO and 1-methylethylidene [2-41: ‘P O(3P) f CH3C=CCH3 + CH,C=CCH w3 (CH,),C=C=O+
-+ CH,il;CH, + COT I
A$
+ (7)
CH,CH=CH,,
= -116.5 kcal/mo!e.
The CH3CHZCH and (CH3),C diradicals rapidly isomerize to form propylene. Evidence for the formation of CH3 CH2CH and (CH3)2C in these reactions has been largely indirect. Herbrechtsmeier and Wagner [ 1,4] found I-hexene in the I-butyne reaction and observed a small peak at m/e = 84 in the 2-bdtyne reaction which they attributed to the recombination of these diradicals. Recently, Blumenberg et al. [2] investigated these reactions by means of mass spectroscopy with moIecular beam sampling. They found that the C3Hs initially formed in these reactions had different cracking patterns at various ionization energies from that of thermal admixed C,H,, and that the cracking patterns of the C,H, formed in the two reactions were also different from each other_ These differences were attributed to the initial formation of the CH,CH,CH and the (CH3).$ radicals. We have previouay investigated the reactions of 0(3P) with allene and propyne [6,7]. The energetics of both these reactions are similar, and both lead to CO and C2H4 as major products. From CO laser absorption
measurements,it was found that in the alIene reaction,
6.80 + 0.63 kcal/mole of reaction energy was channeled into CO vibrational excitation, while the CO from propyne was excited only to the extent of 2.28 + 0.28 k&/mole. The results were examined by means of some simple statisticalmodels [6]. The models ignore the possible effect of angular momentum conservation on the partitioning of reaction energy into various internal degrees of freedom. In the allene reaction, the total available energy, E,, = -AH0 + E, + 2.5 RT, wes randomized among the vibrational and internal rotational modes of a cyclopropanone complex prior to its dissociation into CO and C2H4. This model predicted 6.79 k&/mole of CO vibrational excitation energy. In the case of the propyne reaction, a methylketene complex was assumed which dissociated into CO and the ethylidene diradical which subsequently isomerized to ethylene. In this reaction, the heat of isomerization of CH,CH, AH: (calculated to be 68 k&/mole), is not available for CO excitation since it is not released until after the CO and CH,CH have corn letely separated. Thus randomization of.!& - AJ iso among the modes of a methylketene complex Followed by its dissociation into CO and CH,CH led to a calculated value of 2.20 kcal/mole of CO vibrational excitation. Both these calculated values are in excellent agreement with the experimental. These results provided convincing evidence that the CH3CH radical was indeed produced initially in the 0(3P) methylacetylene reaction_ The results obtained for the C4H6 isomers have also been examined in terms of simple statistical models in which reaction energy is statistically randomized in substituted ketene complexes which dissociate into CO and dlradicals. As in the case of propyne, it is also assumed that these diradicals do not isomerize until after they are completely separated from the CO, and thus their isomerization energies are not available for CO excitation. The heats of isomerization of CH3CH2CH and (CH,),C are not known, but can be estimated from bond dissociation energies. Thus A.$& for (CH3)2C corresponds to +he energy released in the formation of a C=C a-bond (Q,, =58 kcal/mole) and an ethylenic C-H bond [D (CCC-II) = 108 kcal/mole] less the dissociation energy of an aliphatic C-H bond [D (R-H) ~98 kcal/mole] , or -68 kcal/mole. In a similar manner AHL for CH,CH,CI% was estimated to be -71 kcal/ mole.
M.E. Urnstead, M.C. LinjReactions of O(‘P) with I- and 2-butyne
On the basis of the spin-conservation rule, it should be expected that triplet diradicals are produced in these reactions. Some theoretical values for the heats of formation or the heats of isomerization of these diradicals have been calculated quantum-mechanically_ For @H&J (T,), avalue of -27.8 kcal/mole for Al!& has been obtained [NJ, for (CH&C (So), -43.2 kcaI/mole [16], and for CH3CH2CH (So), -55.4 kcal/ mole [17]. In our earlier investigation of the 0(3P) f CH,C,H reaction, AE& for CH,CH + CzH4 was estimated to be -68 k&/mole from bond energy calculations [6]. A number of quantum-mechanically calculated values are also available for this isomerization (all in kcal/mole): -72.3 and -72.6, for So and Tt, respectively [18], -85 and -80 1191, -75 and -43 [20]; and for So, -75 [21], -53.5 [22], -72.1 [23] and -51 to -55 [24]. In view of the wide spread of the theoretically calculated values of A& for CH,CH and the disagreement as to the calculated separation of the Su - T, energy levels, the values for AH; obtained from bond energy calculations were taken to be reasonable estimates and were used for further computations. In the case of reaction (7), randomization of the total available energy for CO excitation, E,, = -(A@ - A@,) +E, f 2.5 RT= 52 kcal/mole (taking Nfk = -68 kcal/mole, E, = 1.8 keel/mole [4] and T = 298 K) among the vibrational modes [15] of a dimetbylketene complex according to the approximation of Whitten and Rabinovitch [26] predicted the CO vibrational population distribution illustrated by the soiid line in fig. 1. This population distribution leads to a calculated vaIue of L!?,) = 1.18 k&/mole of CO vibrational‘excitation, as compared to the experimental one, 1.14 f. 0.07. The two dotted curves lying on either side of the solid line in fig. 1 were obtained by varying E,,, by i- 5 kcal/mole and provide an indication of the sensitivity of the calculated population distributions to variations in Etet and thus also to AE$!$!.These curves lead to values for (EJ of 1.00 and 1.34 kcal/mole respectively for values of Et,, of47 and 57 kcal/mole. With the assumption that propylene rather than (CH&,C is the primary reaction product, a similar randomization of the total available reaction energy, Eta, = -120 kcal/mole, leads to the dashed curve in fig. 1 end a value for CEu)of 358 keel/mole. This model clearly predicts a much greater degree of CO vibrational
357
excitation than was found experimentally. The filled circles in fig. 1 represent a prior statistical distribution [27] evaluated by the method of Bogan and Setser [28], takingErot = 52 kcaI/mole and treating alI vibrational-rotational modes of the CH,CCH, radical as active. The vibrational frequencies of the radical were assigned according to those of CH,CD,CH, [29], excluding those associated with the D-atom vibrations. As expected, the prior distribution agrees closely with that predicted by our simple statistical model [6] which takes into account all internal degrees of freedom in the dissociating complex with only two modes excluded. One mode, vCzO, corresponds to the fmal diatomic product vibrational excitation and the other, ~c=CO, becomes the motion along the reaction coordinate that subsequently results in the product’s translational excitation. Some of the vibrational modes in the dissociating complex eventually degenerate into the rotational modes of both diatomic and polyatomic products. The observed CO vibrational distribution compares closely with the statistical ones, with noticeable deviation above u = 4. The concentration of molecules above u =4, however, amounts to less than 0.1% of the total CO formed; the accuracy of the experimental points at those levels is, therefore, subjected to greater uncertainties. These data will be reexamined in the future with an improved data-acquisition method. Statistical calculations were not made for reactton (6). Based upon bond energy calculations, the total enemy available for CO excitation in this reaction is only about 1 k&/mole different from *hat of reaction (7), and in view of the large uncertainties in ML for both diradicals, the results would not be significantly different from those of 2-butyne. The results obtained for the butynes are tabulated in table 1 along with those previously obtained for propyne and allene [6,7I. The amount of vibrational energy carried by CO from the C4H6 reactions is significantly less than that carried by CO from the CsH4 reaction. This is consistent with the presence of additional vibrational degrees of freedom for energy randomization in the substituted ketene complexes associated with _thebutynes. Rate constants for the reaction of O(3P) with I- and 2-C4H6 relative to C3H4 were obtained by the computer modeling of the rates of total CO production, &VU,from each alkyne according to reactions (l)-(5). By use of an average value for the C3H4 rate constant of4.2 X !O1l mQ mole-’ s-’ (298 IC) [10,11,30] _rate
358
M.E. Urnstead, M.C. Lin/Reactions
+ablel--.
Experimental and theoretical (statistical) average vibrational energiesof CO-from the reaction of 0(3P) with alkynes Reaction O(3P) +
L’.Q/kcal mole-’
‘Jv-%ot
of 0(3P) with I- and 2-butyne Table 2 Observed and calculated yields of CO based on different tion rates for 0 + lC4 H6 and 0 _+2C4 H6 Reaction
reac-
Vieid [CO]QH~/[CO]&~Q expt. a’
talc. b)
talc. c)
expt.
talc. a)
allene
6.80 5 0.63
6.79
0.055
O+ lC.+Hs
1.04
1.23
2x5
propyne
2.28 -L0.28
2.20
0.018
0 f 2C4Hs
2.41
3.25
5.07
l-butyne
0.97 + 0.04
-
0.0078
2-butyne
1.14 + 0.07
1.18
0.0098
a) Based on the model described irrref. [6]_
constants of 5.0 X 1011 and 1.6 X 1012 m!? mole-’ s-l were obtained for I- and 2-CqH6 respectively. These values are somewhat lower than ‘hose reported by Hetbrechtsmeier and Wagner [1,4], 13 X 1012
0 +NO2
f-ig. 3. Computed rates of major O-atom consuming and CO producing reactions in the 0(3P) + lC,H, reaction.
a) From flash photolysis (1 kJ) of 7 torr of 1:2: 17 alkyne: NOa rSF6 miutures. b, Based on rate constants derived from CO laser absorption experiments. ‘1 Based on Herbrechtsmeier and Wagner’s rate constants.
and 3-. 9 X 1012 mP mole-’ s-l (2-C4H6)-detennined by alkyne and 0-ztom disappearance in a flow system. CO production curves computed using their rate constants are also shown in fig. 2. The rates of the lC4H6 and the C3H4 reactions were found to be fairly similar by the CO laser absorption experiments, which should be expected as the additioual methyl group in I-C4Hs is far enough removed from the triple bond so that it should have little influence upon it. This is supported by the reactions of C3H6 and I-C4H8 with 0(3P): whose rates are essentially the same [1.5,31] _ Fig. 3 shows the computed rates of the major reactions for I-C4H6. At least over the early stages of the reaction where the CO laser absorption measurements were made, the rates of secondary reactions, such as 0 f C3H6, are much lower than that of reaction (4b). The reaction of 0 atoms with other major stable reaction products, such as C2H2 and C2H4, is considerably slower than with C3H6 [ 151 and should be insignificant as CO sources. Finally, the toti CO formed in the l-and 2-C4H6 reactions was measured by gas chromatography and compared with that from the C3H4 reaction. The ratios of the amount of CO formed in the C4H6 reactions reIatFve to that from C3H4 are compared in table 2 with the computed values for CO after completion of*be’reactions, using the two sets of rate constants. Altitough the measured values are undoubtedly influenced by secondary reactions, the lower relative rate constants measured in the CO laser absorption experiments predict CO raties more in line with the experimental data than those of Herbrechtsmeier and Wagner [ 1,4]: (l-C4H6)
ME. Urnstead, M.C. Lin/Reactians
5. Condusions The results of this study indicate that the previously proposed mechanisms represented by reactions (6) and (7) are the dominant primary steps producing CO in the 0(3P) + l- and Zbutyne reactions. The good agreement between the observed and the statistically predicted vibrational population distributions for the CO formed in these reactions indicates that complete randomization of the reaction energy takes place in the CH,CH,CHCO and the (CH,),CCO complexes and that the CH,CH,CH and the (CH3)2C diradicals are the other primary reaction products.
Acknowledgement The authors are grateful to Dr. Denis Bogan for the use of his computer program for surprisal analysis.
References (11 P. Herbrechtsmeier i2] [3] (41 IS] !6] [7] [S] [9] [lo]
and H.G. Wagner, Ber. Bunsenges. Physik. Chem. 79 (1975)461. N. Blumenberg, K. Hoyermann and R. Sievert. Proc. 16th Int. Combust. Syrnp., 1977, to be published. H.E. Avery and S.J. Heath, Faraday Trans. I3 (1972) 512. P. Herbrechtsmeier and H.G. Wagner, Ber. Bunsenges. Physik. Chem. 79 (197.5) 673. MC. Lin and R.G. Shortridge, Chem. Phys. Letters 29 (1974) 42. MC. Lin, R.G. Shortridge and M.E. Umstead, Chem. Phys. Letters 37 (1976) 279. ME. Umstead, R.G. Shortridge and MC. Lin, Chem. Phys. 20 (1977) 271. K. Porter and D.H.Volman, Anal. Chem. 34 (1962) 748. F.W. Witliams, F.J. Woods and M.E. Umstead, J. Chromat. Sci. 10 (1972) 570. J.M. Brown and B.A. Thrush, Trans. Faraday Sot. 63
(1967) 630.
of O13P)with I- and 2-bulyne
359
[ 1 l] P. Herbrechtsmeier and H.G. Wagner, Z. Physik. Chem. NF 93 (1974) 143. 1121 C.E. Treanor, Math. Computation 20 (1966) 39. [13] D. Garvin and R.F. Hampson, eds., Chemical kinetics data survey VII. Tables of rate and photochemical data for modelBig of the stratosphere (revised), NBSIR 74430 (1974) p. 12. [14] J. Heicklen and N. Cohen, Advances in Photochemistry, Vol. 5, eds. W.A. Noyes Jr., G.S. Hammond and J.N. Pitts Jr. (interscience. New York, i968) p. 157. [ 151 J.T. Herron and R.E. Huie, J. Phys. Chem. Ref. Data 2 (1973) 467. (161 N. Bodor, M.J.S. Dewar and J.S. Wasson, J. Am. Chem. sot. 94 (1972) 9095. [ 171 M. Martin, V. Mennendez and J.M. Figuera, Chem. Phys. Letters 27 (1974) 292. [ 181 J.A. Ahmann, LG. Csizmadia and K. Yates, J. Am. Chem. Sot. 96 (1974)4136. [19] V. Staemmler, Theoret. Chim. Acta. 35 (1974) 309. [20] W.A. Lathan, L.A. Curt&, W-T. Hehre. J.B. Lisle and J.A. Pople, Prog. Phys. Org.Chem. 11 (1974) 175. 1211 V. Menendez and J.M. Figuera, Chem. Phys. Letters 18 (1973)426. [22] N. Bodor and hf.J.S. Dewar, J. Am. Chem. Sot. 94 (1972) 9103. 1231J.A. Ahmann, LG. Csizmadiaand K. Yates, J. Am. Chem. sot. 97 (1975) 5217. [24] C. Trindle and J-K. George, Intern. J. Quantum Chem. 10 (1976) 21. 1251 W.H. Fletcher and W.B. Barish, Spectrochim. Acta 21 (1965) 1647. [26] G.Z. Whitten and B.S. Rabinovitch, I. Chem. Phys. 41 (1964) 1883. [27] R.D. Levine and R.B. Bernstein, AccountsChem. Res. 7 (1974) 393. [28] D.J. Bogan and D.W. Setser, J. Chem. Phys. 64 (1976) 586. [ZS] T. Shimanouchi, Tables of molecular vibrational frequencies, Vol. 1 (National Bureau of Standards, NSRDS-NBS39,1972). 1301 C.A. Arrington and D.J. Cos, J. Phys. Chem. 79 (1975) 2584. [31] R-l. Cvetanovic. in: Advances in photochemistry, Vol. 1, eds. W.A. Noyes Jr., G.S. Hammond and J.N. Pitts Jr. (Interscience, New York, 1963) p. 115.
:
.
THE DYNAMICS OF CO PRODUCTIOh FROM THE REACTION OF O(3P) WITH l- and ZBUTYNE
M.E. UMSTEAD andM.C. LIN Cfzemisw Division. Naval Research Laboratory. Washington. D.C. 20375, USA Received 25 April 1977
The reactio_ns of l- and 2-butyne with O(3P) were studied at 298 K by means of a CO laser resonant absorption technique. ‘IbeCO formed in the 1C4H6 reaction carried 0.78 r 0.03% of the total available reaction energy, while that from 2C4He carried 0.98 c 0.06% of the reaction ener,v. Reasonable agreement was obtained between the observed CO population distributions and those predicted by simple statistical models assuming that diradicals, and not propylene, were formed initially in the reactions. These results are consistent with the mechanisms: O(aP) +CH3CHzC=CH-,
CH3CH2CH
+ CO
ICsHs, O(jP) + CH3C=CCHa
-f CH3CCH3 +CO IC;He.
From the kineticmodeling of the observed rates ofC0 formation, the rates of these reactions were found to be 5.0 X‘LO” and 1.6 X lo’* ma mole-’ Br for l-and 2-butyne, respectively, based on previously reported values for the propyne reaction.
1. Introduction The reactions of I-butyne [1,2] and 2-butyne [2-41 with O(3P) both lead to CO and C3H6 as the principal products. The I-butyne reaction is believed to proceed primarily through an ethylketene intermediate which decomposes to form CO and the CH,CH,CH diradical [1,2], and 2-butyne, tbrougb a dimethyllcetene intermediate which breaks down to produce CO and the (CH3)2C diradicai [2,4] _Both diradicals rapidly isomerize to yield C3He-_Although recent mass spectroscopic e_vidence [2] supports the presence of these diradicals as intermediates in the reactions, they have not yet been detected unambiguously by chemical or spectroscopic methods. In investigating the dynamics of CO production in 0(3P)-alkyne reactions, we have employed a continuous wave (cw) CO laser which can be operated over a broad range of the CO vibrational spectrum [5,6] to measure the viimtional population of the nascent CO formed in -these reactions. With the help of simple statistical cali culations, we have previously shown quite conclusively
that in the 0 f propyne reaction, CO and the CH3CH diradical are produced directly from the decomposition of vibronically excited methylketene, and that the isomerization reaction, CH,CH + C,H,, takes place after the complete separation of CO and CH,CH [6,7] _ We have now extended our investigation to in&de l-. and 2-butyne and have shown that their reactions with 0 atoms also proceed through substituted ketene intermediates which subsequently break down to produce CO and diradicals. We have also obtained the rates of these reactions relative to CH3C2H by the computer modeling of the total production rate of CO, CNJ, for each alkyne.
2. Experimental
The flash photolysis and laser probing systems have been described previously in detail [5,6]. Briefly, a stabilized cw CO laser, preset at various vibrationalrotational lines accessible to the reaction, was directed through the axis of a Pyrex flash photolysis tube. The
M.E. Urnstead,M.C. LiniReactions of 013p) with I- and 2-&yne
~~e~p&&e of the flash tube could be controlled to + 1°C by circulating fluid from a constant temperature bit& thro$$ the outer jacket of the tube. O(3P) atoms were &tierated by the photodissociationbf NO, at waielengths >300 MI. Mixtures of NO,,SF,and I- or 2-butyne were flash photolyzed, and time-resolved CO abso&iGn curves were obtained for all vibrational levels populated by the reaction. The CO vibrational populations were determined by analyzing the initial portions of the absorption curves by means of a computer according to the gain equation, given in ref. [5]. The initial population distribution was evaluated by extrapolating the NV/No ratios to the earliest appearance time of absorption in order to eliminate both the effects of CO vibrational relaxation and any possible contributions from secondary reactions that might generate co. Stable products of the reactions were measured in a different flash photolysis apparatus by means of a Beckman CC4 gas chromatogriph equipped with hydrogen flame detectors. A Pyrex flash tube, sealed at one end and connected at the other to the chromatographic sampling system, was mounted coaxially in the center of a quartz flash lamp. After a mixture was flash photolyzed, it was expanded directly into the evacuated gas sampling loop of the chromatograph and analyzed. A 0.3 1%cm-o.d. Porapak-T colu’mn, 305 cm in length, was used for the hydrocarbon products, and a 0.635cm-o.d. SA Molecular Sieve column, 183 cm in length, was used for CO measurements. The CO was hydro- , genated over a Ni catalyst at ca. 300 C at the exit of the Molecular Sieve column and was detected as CH4 by the flame detector [8*9]. It was necessary to remove NO, and SF6 from the samples prior to analysis for CO. The NO, was found to react with organic compounds in the Molecular Sieve column to produce substantial quantities of CO, and the SF, rapidly poisoned the Ni catalyst. These substances were removed by the use of a Porapak-T column (0.318 cm o-d., 30 cm long) maintained at 195 K which was suitably valved so that it was put in series with the Molecular Sieve during sample injection, and then backflushed after the CO had passed through and entered the analytical column. The butynes were obtained from the Chemical Samples Company and were purified by trap-to-trap distiIlation before use. The NO2 and SF6 were obtained from the Matheson Gas Products Company. The NO, was purified by pumping at 195 K to remove traces of N20s,
-
--
_ _-
and the SF, was thoroughly degassed at 77 K before use.
3. Results. The hydrocarbon products found essentially confirmed the results of Herbrechtsmeiei and Wagner [1,4] and were not investigated extensively. The main difference in product cotiposition found in this work was the virtual absence of CH4, C,H, and C,H,, indicating that the free radical precursors of these products are effectively scavenged by the NO and NO2 in the mixtures. From the laser absorpti& measurements, the CO las found to be vibrationally excited to u =4 in the
d
0
IO
20
30
40
50
&/KCAL-MOLE-’
Fig. 1. VibrationaI energy distriiutions of theC0 formed in the 0(3P) f l- ar.d 2-butyne re&tions. Solid line: theoreti@ly predicted population distriiution based on-A@, foi (CHJ)~C of -68 kcal/mole, dotted limes: AI$, = -63 and -73 k&/mole. Dashed line: Theoretical distriiution based on randomization of full reaction energy in complex. Filled .ticles: TJw prior CO viirationaldistniutiqfi etiluat? by ta$ngE&= 52 kcaljmole and treating all vibrational-rotational modes of the’CHsCCH3 radical as active.
M.E. Urnstead, ML’. Lin/Reach*ons of OpP) with I- and 2-butyne
355
where f,=Nu/Zvsfluis the normalized population distribution and E, is the vibrational energy of CO at the uth level with the zero-point enemy excluded. The data shown m fig. 1 give rise to (EJ = 0.97 2 0.04 fcr tbe CO from I-butyne and LFJ = 1.14 f 0.07 k&/mole for that from 2-butyne. Fig. 2 shows the total production of CO, EN,, as a function of time over the f=st few microseconds of the reaction for the butyne isomers as well as that from propyne. All three mixtures were run alternately. To obtain the rates of the 0 + C4H6 reactions relative to C3H4, the CO production curves were modeled with a computer, using the fobowing reaction scheme: NO, +hvhO
+NO,
(1)
O+N02LNO+02,
(2)
O+NO+MkNO2+M,
(3)
0+C3H4hCO+C,H4,
t4a)
0 + C4H6 *CO
+C3H6,
(4b)
0 + C2H4 k5a
Products + CO,
@a)
0 f C,H,33+
Products + CO.
(5b)
VPS Fig. 2. Rates of formation of totaIC0 (EN,) from the 0c3P) f Cs&, 1C4H6 and 2-C4Hs reactions. Solid Lines: Computed rat& b&d on different values of k4;(1) 4.2 X 10”. (II)>.0 x lo’*, (III) 1.2 X lOI’, (IV) 1.6 X 10” and (V) 2.9 X lo’*, aII in units of ml mok? 3.
l-butyne reaction and to u = 6 in the 2-butyne reaction, with Boltzmann vibrational temperatures of 2000 + 150 K in both casks. The higher vibrational levels observed in the 2-butyne reaction reflect the faster rate: of this reaction compared with 1-butyne rather than the formation of more energetic CO, since the increased rate of CO production increases the detectability of CO by the absorption measurements. In fig_ 1 are plotted the population distributions of CO formed in these two reactions. Tire distributions are close to Boltzmann, and the results for both butynes are very similar; The points on the curves represent the averages of 3 sets of data for each hydrocarbon, obtamed from 7 torr of 1~2.17 C4H6 :N02 :SFs mixtures flashed at-an energy of 1 kJ at 298 K. The amount of vibrational energy channeled into CO was calculated from the expression: : G?,,)= fE
z
IGO
u “’
Avalue of 4.2 X 10” mQ mole-’ s-’ (298 K) was taken fork4,(averageof4.i+1 X 101r [IO] and4.4X IOr [l l]), and reactions (l-3), (4a) and (Sa) were first considered. The NO, photodissociation rate k, was determined as a function of time from the flash profile using Treanor’s modified fourth-order Rungs-Kutta integrations [ 121 of all coupled differential rate equations. Values for kl were varied until a computed curve was obtained that fit the CjH4 experimental points. Other rate constants used were: k2 = 5.5 X 1Ol2 mQ mole-’ s-’ [ 131, k, = 4.9 5 1016mQ2 moleW2sW1 [14], k, = 4.9 X lOI mQ mole-l se1 [ 151 and li5b = 2.2 X IOr mQ mole-’ s-l [15]. Once kl was established, reactions (l-3) (4b) and (Sb) were considered, and kab was varied until curves were obtained that fit the l- and 2-C4H6 experimental results. The solid lines in fig. 2 are the computed curves. Based upon rhe value of 4.2.X 10” mQ mole-’ s-l for C3H4, rate constants of 5.0 X 10” and 1.6 X 1012 were found for I- and 2-C4H6, respectively.
356
M.E. Urnstead,MC. LinjReactions of O[3P) with I- and 2-butyne
4. Discussion The mechanism of CO production from the reaction of O(3P) with 1-butyne is believed to proceed via the decomposition of an excited ethylketene intermediate to yield CO and the propylidene diradical [ 121: O(3P) + CH,CH,C=CH CH,CH,CH=C=Ot
* CH:CH&“H
+
+ CH,CH,CH: + CO+ I
A$
?
(6)
CH,CH=CH,,
= -120.8 kcal/rnole,
and the 2-butyne reaction through migration of a methyl radical to form a dimethylketene complex which breaks down to CO and 1-methylethylidene [2-41: ‘P O(3P) f CH3C=CCH3 + CH,C=CCH w3 (CH,),C=C=O+
-+ CH,il;CH, + COT I
A$
+ (7)
CH,CH=CH,,
= -116.5 kcal/mo!e.
The CH3CHZCH and (CH3),C diradicals rapidly isomerize to form propylene. Evidence for the formation of CH3 CH2CH and (CH3)2C in these reactions has been largely indirect. Herbrechtsmeier and Wagner [ 1,4] found I-hexene in the I-butyne reaction and observed a small peak at m/e = 84 in the 2-bdtyne reaction which they attributed to the recombination of these diradicals. Recently, Blumenberg et al. [2] investigated these reactions by means of mass spectroscopy with moIecular beam sampling. They found that the C3Hs initially formed in these reactions had different cracking patterns at various ionization energies from that of thermal admixed C,H,, and that the cracking patterns of the C,H, formed in the two reactions were also different from each other_ These differences were attributed to the initial formation of the CH,CH,CH and the (CH3).$ radicals. We have previouay investigated the reactions of 0(3P) with allene and propyne [6,7]. The energetics of both these reactions are similar, and both lead to CO and C2H4 as major products. From CO laser absorption
measurements,it was found that in the alIene reaction,
6.80 + 0.63 kcal/mole of reaction energy was channeled into CO vibrational excitation, while the CO from propyne was excited only to the extent of 2.28 + 0.28 k&/mole. The results were examined by means of some simple statisticalmodels [6]. The models ignore the possible effect of angular momentum conservation on the partitioning of reaction energy into various internal degrees of freedom. In the allene reaction, the total available energy, E,, = -AH0 + E, + 2.5 RT, wes randomized among the vibrational and internal rotational modes of a cyclopropanone complex prior to its dissociation into CO and C2H4. This model predicted 6.79 k&/mole of CO vibrational excitation energy. In the case of the propyne reaction, a methylketene complex was assumed which dissociated into CO and the ethylidene diradical which subsequently isomerized to ethylene. In this reaction, the heat of isomerization of CH,CH, AH: (calculated to be 68 k&/mole), is not available for CO excitation since it is not released until after the CO and CH,CH have corn letely separated. Thus randomization of.!& - AJ iso among the modes of a methylketene complex Followed by its dissociation into CO and CH,CH led to a calculated value of 2.20 kcal/mole of CO vibrational excitation. Both these calculated values are in excellent agreement with the experimental. These results provided convincing evidence that the CH3CH radical was indeed produced initially in the 0(3P) methylacetylene reaction_ The results obtained for the C4H6 isomers have also been examined in terms of simple statistical models in which reaction energy is statistically randomized in substituted ketene complexes which dissociate into CO and dlradicals. As in the case of propyne, it is also assumed that these diradicals do not isomerize until after they are completely separated from the CO, and thus their isomerization energies are not available for CO excitation. The heats of isomerization of CH3CH2CH and (CH,),C are not known, but can be estimated from bond dissociation energies. Thus A.$& for (CH3)2C corresponds to +he energy released in the formation of a C=C a-bond (Q,, =58 kcal/mole) and an ethylenic C-H bond [D (CCC-II) = 108 kcal/mole] less the dissociation energy of an aliphatic C-H bond [D (R-H) ~98 kcal/mole] , or -68 kcal/mole. In a similar manner AHL for CH,CH,CI% was estimated to be -71 kcal/ mole.
M.E. Urnstead, M.C. LinjReactions of O(‘P) with I- and 2-butyne
On the basis of the spin-conservation rule, it should be expected that triplet diradicals are produced in these reactions. Some theoretical values for the heats of formation or the heats of isomerization of these diradicals have been calculated quantum-mechanically_ For @H&J (T,), avalue of -27.8 kcal/mole for Al!& has been obtained [NJ, for (CH&C (So), -43.2 kcaI/mole [16], and for CH3CH2CH (So), -55.4 kcal/ mole [17]. In our earlier investigation of the 0(3P) f CH,C,H reaction, AE& for CH,CH + CzH4 was estimated to be -68 k&/mole from bond energy calculations [6]. A number of quantum-mechanically calculated values are also available for this isomerization (all in kcal/mole): -72.3 and -72.6, for So and Tt, respectively [18], -85 and -80 1191, -75 and -43 [20]; and for So, -75 [21], -53.5 [22], -72.1 [23] and -51 to -55 [24]. In view of the wide spread of the theoretically calculated values of A& for CH,CH and the disagreement as to the calculated separation of the Su - T, energy levels, the values for AH; obtained from bond energy calculations were taken to be reasonable estimates and were used for further computations. In the case of reaction (7), randomization of the total available energy for CO excitation, E,, = -(A@ - A@,) +E, f 2.5 RT= 52 kcal/mole (taking Nfk = -68 kcal/mole, E, = 1.8 keel/mole [4] and T = 298 K) among the vibrational modes [15] of a dimetbylketene complex according to the approximation of Whitten and Rabinovitch [26] predicted the CO vibrational population distribution illustrated by the soiid line in fig. 1. This population distribution leads to a calculated vaIue of L!?,) = 1.18 k&/mole of CO vibrational‘excitation, as compared to the experimental one, 1.14 f. 0.07. The two dotted curves lying on either side of the solid line in fig. 1 were obtained by varying E,,, by i- 5 kcal/mole and provide an indication of the sensitivity of the calculated population distributions to variations in Etet and thus also to AE$!$!.These curves lead to values for (EJ of 1.00 and 1.34 kcal/mole respectively for values of Et,, of47 and 57 kcal/mole. With the assumption that propylene rather than (CH&,C is the primary reaction product, a similar randomization of the total available reaction energy, Eta, = -120 kcal/mole, leads to the dashed curve in fig. 1 end a value for CEu)of 358 keel/mole. This model clearly predicts a much greater degree of CO vibrational
357
excitation than was found experimentally. The filled circles in fig. 1 represent a prior statistical distribution [27] evaluated by the method of Bogan and Setser [28], takingErot = 52 kcaI/mole and treating alI vibrational-rotational modes of the CH,CCH, radical as active. The vibrational frequencies of the radical were assigned according to those of CH,CD,CH, [29], excluding those associated with the D-atom vibrations. As expected, the prior distribution agrees closely with that predicted by our simple statistical model [6] which takes into account all internal degrees of freedom in the dissociating complex with only two modes excluded. One mode, vCzO, corresponds to the fmal diatomic product vibrational excitation and the other, ~c=CO, becomes the motion along the reaction coordinate that subsequently results in the product’s translational excitation. Some of the vibrational modes in the dissociating complex eventually degenerate into the rotational modes of both diatomic and polyatomic products. The observed CO vibrational distribution compares closely with the statistical ones, with noticeable deviation above u = 4. The concentration of molecules above u =4, however, amounts to less than 0.1% of the total CO formed; the accuracy of the experimental points at those levels is, therefore, subjected to greater uncertainties. These data will be reexamined in the future with an improved data-acquisition method. Statistical calculations were not made for reactton (6). Based upon bond energy calculations, the total enemy available for CO excitation in this reaction is only about 1 k&/mole different from *hat of reaction (7), and in view of the large uncertainties in ML for both diradicals, the results would not be significantly different from those of 2-butyne. The results obtained for the butynes are tabulated in table 1 along with those previously obtained for propyne and allene [6,7I. The amount of vibrational energy carried by CO from the C4H6 reactions is significantly less than that carried by CO from the CsH4 reaction. This is consistent with the presence of additional vibrational degrees of freedom for energy randomization in the substituted ketene complexes associated with _thebutynes. Rate constants for the reaction of O(3P) with I- and 2-C4H6 relative to C3H4 were obtained by the computer modeling of the rates of total CO production, &VU,from each alkyne according to reactions (l)-(5). By use of an average value for the C3H4 rate constant of4.2 X !O1l mQ mole-’ s-’ (298 IC) [10,11,30] _rate
358
M.E. Urnstead, M.C. Lin/Reactions
+ablel--.
Experimental and theoretical (statistical) average vibrational energiesof CO-from the reaction of 0(3P) with alkynes Reaction O(3P) +
L’.Q/kcal mole-’
‘Jv-%ot
of 0(3P) with I- and 2-butyne Table 2 Observed and calculated yields of CO based on different tion rates for 0 + lC4 H6 and 0 _+2C4 H6 Reaction
reac-
Vieid [CO]QH~/[CO]&~Q expt. a’
talc. b)
talc. c)
expt.
talc. a)
allene
6.80 5 0.63
6.79
0.055
O+ lC.+Hs
1.04
1.23
2x5
propyne
2.28 -L0.28
2.20
0.018
0 f 2C4Hs
2.41
3.25
5.07
l-butyne
0.97 + 0.04
-
0.0078
2-butyne
1.14 + 0.07
1.18
0.0098
a) Based on the model described irrref. [6]_
constants of 5.0 X 1011 and 1.6 X 1012 m!? mole-’ s-l were obtained for I- and 2-CqH6 respectively. These values are somewhat lower than ‘hose reported by Hetbrechtsmeier and Wagner [1,4], 13 X 1012
0 +NO2
f-ig. 3. Computed rates of major O-atom consuming and CO producing reactions in the 0(3P) + lC,H, reaction.
a) From flash photolysis (1 kJ) of 7 torr of 1:2: 17 alkyne: NOa rSF6 miutures. b, Based on rate constants derived from CO laser absorption experiments. ‘1 Based on Herbrechtsmeier and Wagner’s rate constants.
and 3-. 9 X 1012 mP mole-’ s-l (2-C4H6)-detennined by alkyne and 0-ztom disappearance in a flow system. CO production curves computed using their rate constants are also shown in fig. 2. The rates of the lC4H6 and the C3H4 reactions were found to be fairly similar by the CO laser absorption experiments, which should be expected as the additioual methyl group in I-C4Hs is far enough removed from the triple bond so that it should have little influence upon it. This is supported by the reactions of C3H6 and I-C4H8 with 0(3P): whose rates are essentially the same [1.5,31] _ Fig. 3 shows the computed rates of the major reactions for I-C4H6. At least over the early stages of the reaction where the CO laser absorption measurements were made, the rates of secondary reactions, such as 0 f C3H6, are much lower than that of reaction (4b). The reaction of 0 atoms with other major stable reaction products, such as C2H2 and C2H4, is considerably slower than with C3H6 [ 151 and should be insignificant as CO sources. Finally, the toti CO formed in the l-and 2-C4H6 reactions was measured by gas chromatography and compared with that from the C3H4 reaction. The ratios of the amount of CO formed in the C4H6 reactions reIatFve to that from C3H4 are compared in table 2 with the computed values for CO after completion of*be’reactions, using the two sets of rate constants. Altitough the measured values are undoubtedly influenced by secondary reactions, the lower relative rate constants measured in the CO laser absorption experiments predict CO raties more in line with the experimental data than those of Herbrechtsmeier and Wagner [ 1,4]: (l-C4H6)
ME. Urnstead, M.C. Lin/Reactians
5. Condusions The results of this study indicate that the previously proposed mechanisms represented by reactions (6) and (7) are the dominant primary steps producing CO in the 0(3P) + l- and Zbutyne reactions. The good agreement between the observed and the statistically predicted vibrational population distributions for the CO formed in these reactions indicates that complete randomization of the reaction energy takes place in the CH,CH,CHCO and the (CH,),CCO complexes and that the CH,CH,CH and the (CH3)2C diradicals are the other primary reaction products.
Acknowledgement The authors are grateful to Dr. Denis Bogan for the use of his computer program for surprisal analysis.
References (11 P. Herbrechtsmeier i2] [3] (41 IS] !6] [7] [S] [9] [lo]
and H.G. Wagner, Ber. Bunsenges. Physik. Chem. 79 (1975)461. N. Blumenberg, K. Hoyermann and R. Sievert. Proc. 16th Int. Combust. Syrnp., 1977, to be published. H.E. Avery and S.J. Heath, Faraday Trans. I3 (1972) 512. P. Herbrechtsmeier and H.G. Wagner, Ber. Bunsenges. Physik. Chem. 79 (197.5) 673. MC. Lin and R.G. Shortridge, Chem. Phys. Letters 29 (1974) 42. MC. Lin, R.G. Shortridge and M.E. Umstead, Chem. Phys. Letters 37 (1976) 279. ME. Umstead, R.G. Shortridge and MC. Lin, Chem. Phys. 20 (1977) 271. K. Porter and D.H.Volman, Anal. Chem. 34 (1962) 748. F.W. Witliams, F.J. Woods and M.E. Umstead, J. Chromat. Sci. 10 (1972) 570. J.M. Brown and B.A. Thrush, Trans. Faraday Sot. 63
(1967) 630.
of O13P)with I- and 2-bulyne
359
[ 1 l] P. Herbrechtsmeier and H.G. Wagner, Z. Physik. Chem. NF 93 (1974) 143. 1121 C.E. Treanor, Math. Computation 20 (1966) 39. [13] D. Garvin and R.F. Hampson, eds., Chemical kinetics data survey VII. Tables of rate and photochemical data for modelBig of the stratosphere (revised), NBSIR 74430 (1974) p. 12. [14] J. Heicklen and N. Cohen, Advances in Photochemistry, Vol. 5, eds. W.A. Noyes Jr., G.S. Hammond and J.N. Pitts Jr. (interscience. New York, i968) p. 157. [ 151 J.T. Herron and R.E. Huie, J. Phys. Chem. Ref. Data 2 (1973) 467. (161 N. Bodor, M.J.S. Dewar and J.S. Wasson, J. Am. Chem. sot. 94 (1972) 9095. [ 171 M. Martin, V. Mennendez and J.M. Figuera, Chem. Phys. Letters 27 (1974) 292. [ 181 J.A. Ahmann, LG. Csizmadia and K. Yates, J. Am. Chem. Sot. 96 (1974)4136. [19] V. Staemmler, Theoret. Chim. Acta. 35 (1974) 309. [20] W.A. Lathan, L.A. Curt&, W-T. Hehre. J.B. Lisle and J.A. Pople, Prog. Phys. Org.Chem. 11 (1974) 175. 1211 V. Menendez and J.M. Figuera, Chem. Phys. Letters 18 (1973)426. [22] N. Bodor and hf.J.S. Dewar, J. Am. Chem. Sot. 94 (1972) 9103. 1231J.A. Ahmann, LG. Csizmadiaand K. Yates, J. Am. Chem. sot. 97 (1975) 5217. [24] C. Trindle and J-K. George, Intern. J. Quantum Chem. 10 (1976) 21. 1251 W.H. Fletcher and W.B. Barish, Spectrochim. Acta 21 (1965) 1647. [26] G.Z. Whitten and B.S. Rabinovitch, I. Chem. Phys. 41 (1964) 1883. [27] R.D. Levine and R.B. Bernstein, AccountsChem. Res. 7 (1974) 393. [28] D.J. Bogan and D.W. Setser, J. Chem. Phys. 64 (1976) 586. [ZS] T. Shimanouchi, Tables of molecular vibrational frequencies, Vol. 1 (National Bureau of Standards, NSRDS-NBS39,1972). 1301 C.A. Arrington and D.J. Cos, J. Phys. Chem. 79 (1975) 2584. [31] R-l. Cvetanovic. in: Advances in photochemistry, Vol. 1, eds. W.A. Noyes Jr., G.S. Hammond and J.N. Pitts Jr. (Interscience, New York, 1963) p. 115.