Quenching and Isomerizatioii in the Photolysis of trans-But-2-ene and cis-Pent-2-ene at 185 nm and 203 nm

BY PETER BORRELL AND A. CERVENKA * Dept. of Chemistry, University of Keele, Staffordshire ST5 5BG

Received 3 1st August, 1971

Additional results on the effect of additives on the product yields in the vapour-phase photolysis of trans-but-Zene and cis-pent-2-ene are reported. They confirm, extend and reconcile our previous results with those of Chesick.2 The main photoreaction could be quenched by all additives at both wavelengths ; at 203 nm, and with some additives at 185 nm, quenching was accompanied by photoisomerization. The effects can be correlated with the two states, Rydberg and V(n +n*), which are excited at the two wavelengths. Collisional deactivation of the V state results in photo- isomerization while when the R state is deactivated no isomerization occurs. Apparently there is no ready collisional interconversion of the R and V states.

The pressure effects recorded by us on the photolysis of the but-2-enes at 185 nm apparently conflicted with those noticed by Chesick in his work at 203 nm. At both wavelengths the photolysis could be quenched by the addition of inert gases. At 185 nm the small yield of the geometrical isomer was unaffected by additives but at 203nm Chesick found that the quenching was accompanied by a considerable increase in the yield of the isomer. Subsequently we have obtained similar quenching results at 185 nm with cis-pent-2-ene and several other olefins 4-6 and it seemed worthwhile to conclude our studies by trying to resolve the anomaly. This final paper reports new results which confirm and extend the previous work at both wave- lengths. The differences can be correlated with the electronic states excited.

EXPERIMENTAL using

vapour-phase chromatography to analyze the products. The lamps were a low-pressure mercury arc, fitted with a Spectrosil envelope, for 185 nm radiation and a Phillips zinc-arc lamp mounted close to the reaction vessel window to produce radiation at 203 nm and 206 nm. The absorption spectrum of the olefins ensures that only wavelengths below 210nm are absorbed.

RESULTS These systems have been studied in detail before to ascertain the quantum yields

of decomposition and the mechanism of primary breakdown so that here we con- centrate on the pressure effects only. The yields were recorded for several products but just three have been chosen to illustrate the effects. In the figures the yields are recorded as the ratio of the yield at the pressure of additive, p , R ( p ) to the yield with no additive, R(p = 0).

The photolyses were made in the static mercury-free system, described before,l*

T R A N S - B U T - 2 - E N E

The product distribution was the same at both wavelengths. At 185 nm a general decrease in product yield with pressure of added gas was noticed (fig. 1).

* on leave from the Institute of Macromolecular Chemistry, Academy of Sciences, Prague, 1968-69. 345

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346 PHOTOLYSIS OF TRANS-RUT-2-ENE AND CIS-PENT-2-ENE

3

V

2 0 40 6 0 additive pressure/kN m-2

FIG. 1.-Photolysis of trans-but-Zene at 185 nm; variation of yield of n-butane with added gas. 0, nitrogen; 0, hydrogen; 0, methane ; +, ethane; V, propane.

3.c

2.c

II s --- 9

1.c

0 2 0 40 6 0 additive pressure/kN m-2

FIG. 2.-Photolysis of trans-but-2-ene at 185 nm ; variation of yield of the cis isomer with added gas. 0 , nitrogen ; A, helium ; , hydrogen ; 0, methane ; 4, ethane ; V , propane.

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P. BORRELL AND A . C E R V E N K A 347

0 2 0 40 6 0

additive pressure/kN m-2

FIG. 3.-Photolysis of trans-but-2-ene at 203 nm; variation of yield of cis isomer with added gas. 0, nitrogen ; 0, hydrogen ; 0, methane.

additive presswe/kN m-2

0, nitrogen ; 0 , hydrogen ; 0, methane. FIG. 4.-Photolysis of trans-but-2-ene at 203 nm ; variation of yield of n-butane with added gas.

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348 PHOTOLYSIS OF TRANS-BUT-2-ENE AND CIS-PENT-2-ENE

Previously [ref. (l), fig. (6)], it was found that N,, Ar, and CO, had similar quenching effects and this lack of specificity has been noticed in the photolysis of ethylene.6 In fig. 1, despite the experimental scatter, a small gradation in effect can be seen, the effectiveness increasing with molecular weight. Fig. 2 records the effect on the yield of the cis-isomer. N, and He have little effect, as was found previously for several gasesY1 but H2 and the hydrocarbons increase the yield. The order of effectiveness is the same as that for quenching. The quantum yield of formation for the isomer at R(p = 0) is 0.1 so that the maximum quantum yield observed is - 0.3.

At 203 nm, the results (fig. 3 and 4) with added methane confirm the results of Chesick., N, and H, produce similar but smaller effects than CH,. The order of effectiveness, CH4 > N2 > H, also appears in the quenching reaction (fig. 4), although it is less apparent because of the scatter. The quantum yields at 203 nrn have not been measured but if that for the formation of the cis isomer is the same at 203 nm as at 185 nm then the limiting quantum yield at high CH4 pressures is - 0.5- 0.6.

CIS-PENT-2-ENE The principal difference between the product distribution at 203 nm from that

at 185nm is the absence of the major product buta-Iy3-diene. At 185nm, N2 quenched the yields of most products (fig. 5), but the yield of the trans isomer was nearly independent of Nz pressure. The yield was also independent of CH4 pressure

0 2 0 40 6 0

additive pressure/kN m-2 FIG. 5.-Photolysis of cis-pent-2-ene at 185 nm ; variation of yields of the trans isomer and 3- methylbut-l-ene with pressure of added gas. Trans isomer with methane, 0 ; nitrogen, @. 3-

methylbut-lene with methane, 0 ; nitrogen, +. but CH, diminished the yields of the other products more than either N2 (fig. 5), or Ar [ref. (3), fig. (4)]. At 203 nm both N2 and CH4 increase the yield of the isomer, (fig. 6), and diminish the yields of the other products. CH4 is more effective than N, and both gases have a greater effect in quenching at 203 nm than at 185 nm.

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P. B O R R E L L A N D A . C E R V E N K A 349

2 0 4 0 6 0

additive pressure/kN m-2

FIG. 6.-Photolysis of cis-pent-Zene at 203 nm ; variation of yields of the trans isomer and 3-methyl- but-l-ene with added gas. Trans isomer with methane, 0 ; nitrogen, 0. 3-methylbut-1-ene with

methane, 0 ; nitrogen, +. DISCUSSION

P R I M A R Y R E A C T I O N S

The similarity in product distribution at 203 nm and 185 nm indicates that the primary reactions for but-2-ene are the same at both wavelengths. For pent-2-ene one of the two main primary reactions is absent at 203 nm :

C5Hlo+ltv + C4H6+CH3+H, 4(185 nm) = 0.36, (buta-lY3-diene) $(203 nm) = 0.

The enthalpy change for this reaction is approximately 490 kJ. At 203 nm and 206 nm the photon energy available is 492 and 486 kJ mol-1 while at 185 nm it is 645 kJ mo1-I so that the difference in quantum yield is due to insufficient energy at the longer wavelengths.

M E C H A N I S M

As in our previous work, the decrease in yield with added gases, common to most products, can be attributed to the collisional deactivation of a common excited state intermediate. The increase in yield of the geometric isomer can be attributed to the same cause since, by inspection, the two effects occur over the same pressure range and the small differences observed between additives are the same for both

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350 PHOTOLY SIS OF TRANS-BUT-2-ENE A N D CIS-PENT-2-ENE

effects. The specific effect of hydrocarbons on the isomer yield from but-2-ene at 185 nm could indicate the presence of a hydrogen abstraction reaction but the effect of H2 at 185 nm, and the similar effect at 203 nm (fig. 3), where N2 as well as the hydrocarbons increases the isomer yield, rules out this possibility.

From a lunetic viewpoint the results could be fitted to several mechanisms: a reasonable one for cis-pent-2-ene would be :

cis-Ol+hv -+ 01* (1) 01" -+ trans-01 k2 01* + oz** k3 01** -+ products k4 (4) 01**+M -+ cis-01 k5xc (5)

-+ trans-OZ ksxt 01, OZ*, UZ** represent the olefin and two of its excited states, M is a collision partner, ki are the rate constants for the reactions, and x,, xt are the probabilities that reaction ( 5 ) will form either the cis or trans isomer ; xc+x, = 1. Reaction (2) is included to accoui;t for the absence of the effect of pressure on the isomer yields at 185 nm.

4.0

0 2 0 4 0 6 0 pressurelkN m-2

FIG. 7.-Photolysis of cis-pent-2-ene at 203 nm; effect of nitrogen on the yield of 3-methylbut-1-ene. A Stem-Volmer plot of points from fig. 6.

With this mechanism the differences in effect of wavelength and collision partner, and also between the two compounds, are located in the variation of k3/k2, k5/k4, x,/xt and in the nature of OZ**.

The normal steady-state approximation gives expressions for the rates of formation of a product, Rprod(p) and of the isomer, Rtrans(p) at concentration of additive, [MI :

(ii) Rprod(P) = k3kdk2 +kd(k4+k5[MI), (0 &a&) = ( l / (kz + k3))[k2 +k&5~t[MIl(k4 + k,[MI)I,

where I is the intensity of light absorbed.

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P . B O R R E L L A N D A . C E R V E N K A 35 1

These can be rearranged to give two expressions : (iii)

R,,,,,(p = co) is the asymtotic value of the trans yield at high pressures of M. When fitted to expressions (iii) and (iv), the results give straight lines (fig. 7), but the quality is not good enough for detailed analysis, particularly with (iv) ; here the least accurate points determine the slope and, when combined with the uncertain Rtrans ( p = 00) value, k5/k4 is subject to large errors.

The few quantitative results are summarized in table 1. The only definite value for x,/xt is determined from the approximate quantum yield of isomer formation at high methane pressures. The figures reflect the greater effect of CH4 than N, in quenching and isomerization, and the larger effects at 203 nm than at 185 nm.

N A T U R E OF T H E I N T E R M E D I A T E S

A potential-energy diagram for these systems is shown in fig. 8. It is slightly modified from the original diagram by Mulliken to take into account some recent calculations on the shapes of the excited states.8-10 The main features of interest are the existence of potential-energy barriers to isomerization in the Rydberg states and the minima in the T and V (n + n*) states.

The absorption spectrum l 1 below 200 nm in these compounds consists of over- lapping N + R and N + V bands. In ethylene the Rydberg band in displaced to shorter wavelengths but with the pentenes and butenes either the V or the R state, or both states, may be excited on absorption of light. The origin of the Rydberg band is not known but is thought l 1 to be close to 200 nm.

Most of our results can be explained if absorption at 185 nm excites the Rydberg state while at 203 nm the V state is populated. In our mechanism OZ** is the R state at 185 nm and the V state at 203 nm.

At low pressure, most molecules decompose from either state. At 185 nm, as the pressure is increased, the molecule is deactivated but maintains its geometrical identity (x,/x, = 0 for pentene) since in the Rydberg state it cannot isomerize. At 203 nm its identity is lost on excitation to the Y state and collisional quenching of the molecule results in isomerization.

An anomaly is the isomerization caused by H2 and the hydrocarbons in the photolysis of butene at 185 nm. N2, H2, COZY Ar and O2 cause no photoisomeri- ization. It is concluded that the hydrocarbons can react with the R state and cause crossover to V state with subsequent isomerization. This crossover does not occur when pentene is deactivated by methane.

The state OZ* was postulated to account for the small proportion of isomer formed at zero pressure of additive at either wavelength. This, with reaction (2), probably represents the fraction of excited molecules which proceed through some internal conversion of energy, through the V or T states and so to the ground state of either isomer.

The small difference in collision efficiency between various additives has been attributed previously 1 * to the large collision diameter l1 of the R state and the possible formation of a collision complex. Although collision partners with differing efficiencies have been found, the differences are still relatively small and need not modify the general picture. Small differences between molecules when deactivating excited singlet states have been observed by Parrnenter l 2 with benzene and by

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P. BORRELL AND A . CERVENKA 353 Yardley l 3 with glyoxal. In each case the deactivation was by crossover to the triplet state. If this occurs here, then the triplet states do not decompose, which is consistent with results from mercury photosensitization l4 where much of the reaction is isomer- ization. However, if our explanation is correct, ready collisional conversion between the Y and R states is not usual.

9

3

angle of twist FIG. 8.-A potential-energy diagram for the ground and excited states of ethylene plotted against the angle of twist about the central double bond. In butene and pentene the Rydberg, R, state is

displaced to lower energies so that the Y and R curves overlap.

The problem of the exact nature of the excited states and the details of the collision reactions are unlikely to be solved by steady-state photolysis although some additional information could be gained by experiments with deuterated compounds and work at different wavelengths. For proper elucidation, the system must await the develop- ment of rapid methods at these wavelengths so that the transient species may be observed directly.

We thank the University of Keele for a research fellowship for one of us (A. C.) and the S.R.C. for an apparatus grant.

P. Borrell and F. C. James, Trans. Faradzy Sac., 1966, 62,2452. J. P. Chesick, J. Chem. Phys., 1966, 45, 3935. P. Borrell and P. Cashmore, Trans. Faraday Soc., 1969, 65, 2412. P. Borrell and P. Cashmore, Trans. Farday Soc., 1969, 65, 15 95 ; Ber. Bunsenges., 1968, 72, 182. P. Borrell, P. Cashmore, A. Cervenka and F. C. James, 20th Reunion de la Societe' de Chemie Physique-Transitions non-radiatives dans les molecules, Paris, 1970, 229.

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354 PHOLOLYSIS OF T R A N S - B U T - 2 - E N E A N D C I S - P E N T - 2 - E N E

P. Borrell, A. Cervenka and J. W. Turner, J. Chem. SOC. B, 1971, 2293. ’ R. S. Mulliken, Reu. Modern Phys., 1942, 14,265. * R. S. Mulliken, Tetrahedron, 1959, 5, 253.

A. J. Merer and L. Schoonveld, J. Chem. Phys., 1968, 48, 522. M. B. Robin, H. Basch, W. A. Kuebler, B. E. Kaplan and J. Meinwald, J . Chein. Phys., 1968, 48, 5037. A. J. Merer and R. S. Mulliken, Chenz. Rev., 1969, 69, 639.

l2 C. S. Parmenter, personal communication. l3 J. T. Yardey, personal communication. l4 P. Kebarle and M. Avrahami, J. Chew. Phys., 1963,38, 700.

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