J . Am. Chem. SOC.1982, 104, 5518-5519
5518
Selective Trans to Cis Isomerization of Alkenes via Infrared Multiphoton Excitation
Table I. Kinetic and Thermodynamic Parameters for Cis 'Trans Isomerization
logA
Ea, kcay mol
13.6 13.2 13.6
62 58 53
Peter P. Teng, Eric Weitz,+ and Frederick D. Lewis* Department of Chemistry, Northwestern University Evanston, Illinois 60201 Received April 5, 1982 It is now well established that infrared multiphoton excitation can effect the isomerization of thermodynamically more stable to less stable isomers in cases where the more stable isomer can be selectively excited.'-3 Among the reported examples of such isomerization reactions are the conversions of hexafluorocyclobutene' to hexafluorobutadiene and methylbutadienes to methylcycl~butenes.~Infrared multiphoton excitation might be expected to result in more selective isomerization than ultraviolet single-photon excitation in cases where both isomers absorb at the excitation frequency. In the case of ultraviolet excitation, reversible isomerization (A + B) leads to a photostationary state governed by extinction coefficients and quantum yields (eq 1). For isomers with similar electronic absorption spectra and quantum yields such as cis and trans alkenes, photostationary states near unity are ~bserved.~ In the case of multiphoton infrared excitation, reversible isomerization should yield a photostationary state governed by the absorption cross section (g) for each step in the up-pumping process and cross sections for stimulated emission, collisional deactivation rate constants, and rates of is~merization.~~ In cases where isomerization can be described by a master equation model, small differences in single-photon absorption cross sections may be amplified in the absorption process. Thus modest differences in u may lead to photostationary states with [A]/[B] >> uB/uA. We report here the preliminary results of our investigation of the multiphoton infrared isomerization of several unconjugated and conjugated alkenes. Kinetic and thermodynamic parameters for cis trans isomerization of 2 - b ~ t e n ecrotononitrile,6 ,~ and 1 , 3 - ~ e n t a d i e n eare ~~%~ given in Table I. Under conditions of thermal equilibrium [cis]/[trans] < 1 for 2-butene and 1,3-pentadiene, whereas [cis]/[trans] 1.4 f 0.2 for crotononitrile over the temperature range 300-560 'C. Direct and/or triplet-sensitized photoisomerization of all three alkenes results in photostationary states Infrared multiphoton reactions of trans[cis]/ [trans] 5 1 2-butene have been previously investigated by Yogev and Lowenstein-Benamair." With a focused laser beam the yield of fragmentation was found to exceed that of trans cis isomerization. We have investigated the infrared multiphoton irradiation of several alkenes by employing a collimated laser beam under conditions previously found to effect the unimolecular isomeri-
alkene 2-but m e a crotononitrileb 1,3-pentadienec a Data from ref
5.
Data from ref 6.
-
Alfred P. Sloan fellow. (1) Yogev, A.; Benmair, R. M. J. Chem. Phys. Lett. 1977,46, 290-294. (2) . . (a) . . Buechele, J. L.; Weitz, E.; Lewis, F. D. J . Am. Chem. SOC.1979,
101, 3700-3701. (b) Ibid., submitted for publication. (3) (a) Buechele, J. L.; Weitz, E.; Lewis, F. D. J . Am. Chem. SOC.1981, 103, 3588-3589. (b) Buechele, J. L.; Weitz, E.; Lewis, F. D. J . Chem. Phys., in press. (4) For some exceptions and ways of altering photostationary states see: Lewis, F. D.; Oxman, J. D. J. Am. Chem. SOC.1981, 103, 7345-7347. (5) (a) Kistiakowsky, G . B.; Smith, W. R. J . Am. Chem. SOC.1936, 58, 766-768. (b) Rabinovitch, B. S.; Michel, K.-W. Ibid. 1959,81, 5056-5071. (c) Jeffers, P. M.; Shaub, W. Ibid. 1969, 91, 7706-7709. (6) (a) Butler, J. N.; McAlpine, R. D. Can. J . Chem. 1963.41, 2487-2491. (b) Marley, W. M.; Jeffers, P. M. J . Phys. Chem. 1975, 79, 2085-2087. (7) Frey, H. M.; Lamont, A. M.; Walsh, R. J . Chem. SOC.A 1971, 2642-2646. . .- -.. . .
(8) Yarwood, A. J.; DeMare, G. R.; Termonia, M. NBS Spec. Publ. 1978, 526, 323-325; Chem. Abstr. 1979, 90, 953340. (9) Gale, D. M. J . Org. Chem. 1970, 35, 970-972.
(IO) Saltiel, J.; Metts, L.; Wrighton, M. J . Am. Chem. SOC.1970, 92, 3227-3229. (1 1) Yogev, A,; Loewenstein-Benmair, R. M. J. J . Am. Chem. SOC.1973, 95, 8487-8489.
kcaymol
cay (mol K)
-0.75 0.2
-0.4
-1.2
-0.5 Data from ref 7.
LOO 7
\
;I
' 1
L
,
90
1200
do0
1100
900
800
wavenumbers loo
I--7 I
\
-
-
AS,
AH,
t t t l
94
1 2 b O
1100
1000
900
800
wavenumbers Figure 1. FTIR spectra of trans- and cis-1,3-pentadiene (5 torr). Arrows indicate laser excitation frequencies (Table 11).
zation reactions of hexadienes and butadienes without significant competing f r a g m e n t a t i ~ n . * ~Reported ~ ~ ~ * in Table I1 are laser excitation frequency and fluence, single-photon absorption cross sections, per pulse yields for trans cis and cis trans isomerization, and the percent cis isomer content of the steady state, obtained from the ratio of per pulse yields or by G C analysis following prolonged irradiation. Values obtained by these two methods are in good agreement. The same stationary state is obtained upon 931- or 953-cm-' irradiation of either trans- or cis- 1,3-pentadiene. Some fragmentation (ca. 20%) was observed for 2-butene but was not observed for the other alkenes under the conditions reported in Table 11. Reported single-photon absorption cross iections, u, for the butenes, pentenes, and pentadienes were obtained from gas-phase (5-20 torr) FTIR spectra. The spectra of trans- and cis-1,3-pentadiene are shown in Figure 1. In the case of crotononitrile, multiphoton absorption cross sections were determined both by calorimetric measurement of the attenuation of the laser beam intensity and by photoacoustic measurements. Values obtained by both methods are in reasonable agreement
-
-
(12) Samples (0.1 torr) contained in a 1-dm path length Pyrex cell with NaCl windows were irradiated with the collimated, unfocused output of a Lumonics K203-2 C 0 2 TEA laser. Laser lines were determined with an Optical Engineering C 0 2 laser spectrum analyzer and energies with a Scientech calorimeter. Irradiated samples were analyzed by FID GC. The mass balance for isomerization and/or fragmentation products was quantitative (&lo%) as determined by GC analysis. When ethyl acetate was included as a transparent thermometer, it did not undergo fragmentation (E, = 48 kcal/mol).
0002-7863/82/1504-5S18$01.25/0 0 1982 American Chemical Society
J. Am. Chem. SOC.1982, 104, 5519-5521
5519
Table 11. Frequency and Fluence Dependence of per Pulse Yields and Stationary States
1O5( y i e l d l p ~ l s e ) ~
fluence, alkene
u e x , cm-'
10'' ot, cm2 10'9a,,n cmz
2-butene 2-pentene
975 949
1.4 0.17
0.5 0.15
crotononitrile
931 95 1 978 93 1 95 3
0.64 1.0 0.45 2.7 2.4
0.09 0.41 0.04 2.4 2.1
98 2
3.2
2.1
1,3-pentadiene
J/cmZ 4.5 4.5 6.0 3.2 5.5 3.4 2.7 2.7 2.9 3.1 2.7
t'c 30 1.0 7.0 70 90
80 310 41 145 310 300
steady state,c % cis
c't
