- Email: [email protected]
Rovibrational state distributions for H2 photoeliminated from 1,3-butadiene. Evidence for a symmetric transition state
Volume 194, number 3
CHEMICALPHYSICSLETTERS
26 June 1992
Rovibrational state distributions for H2 photoeliminated from 1,3-butadiene. Evidence for a symmetric transition state B.K. Venkataraman and J.J. Valentini Department of Chemistry, Columbia University, New York. N Y 10027. USA
Received 20 March 1992
The rovibrational distributions of H2 photoeliminated from 1,3-butadiene upon excitation at 212.8 nm have been measured via 1+ 1 resonance-enhancedtwo-photonionization. The measured rotational distributions are in reasonable agreement with a prior distribution, whilethe experimental vibrational distribution is substantiallyhotter than the prior. These results indicate that the photoeliminationoccursvia a concertedmechanismthrougha symmetrictransition state in whichthe H-H distance is greater than the equilibrium H2 value.
1. Introduction The photochemistry and spectroscopy of 1,3-butadiene has provided a challenge for both experimental and theoretical studies. The spectroscopy of the first electronic absorption band, which peaks near 210 nm, is still being analyzed and interpreted [ 13 ]. This band shows only diffuse structure, due to rapid internal conversion to another excited state. The internal conversion is apparently the gateway for the photochemistry, which in the gas phase leads to three different primary product channels [4,5 ]: H2 C=CH-CH--CH2 + h v--, H2 C=CH2 + H C - C H ,
(1) H2 C=CH-CH=CH2 + h p--*H2 C=C=CH-CH3,
(2) H2 C=CH-CH=CH2 + h/P--~H2 + H2 C = C H - C - C H .
(3) The results ofab initio theoretical calculations [69 ] make it easy to see why the photochemistry is complex. These calculations show that the ~Bu excited electronic state accessed by the absorption and Correspondence to: J.J. Valentini, Department of Chemistry, Columbia University, New York, NY 10027, USA.
the lower lying 2 ~Ag excited state each have several distinct geometrical conformations corresponding to local minima on the global potential energy surface. In fact, in the 2 nAg excited state rotation about any of the three C-C bonds is predicted to be only slightly hindered. In the ground electronic state rotation about the C:-C3 single bond is hindered with a consequent production of cis and trans isomers, the latter of which is more stable by about 2.5 kcal mol[11. Of interest to us here is the molecular hydrogen photoelimination channel in the photodissociation of 1,3-butadiene, process (3). Isotope substitution studies show this occurs via 1,1 a n d / o r 1,4 elimination, as well as by 1,2 and 2,3 elimination [5 ]. We address here two questions about this process: Is it concerted? Does it involve a symmetric transition state? Measuring the internal state distributions of the H 2 fragment will provide information on the nature of the transition state geometry. For example, if the mechanism of elimination is concerted through a symmetric transition state, then there can be no torque exerted on the HE as it is eliminated, and the H2 should not be highly rotationaUy excited. However, if the H - H bond that is forming has at the transition state an internuclear separation that is sub-
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
191
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
stantially longer than the equilibrium H - H distance, the H 2 should be vibrationally excited. These experiments are also designed to test a new molecular beam apparatus that we have designed for studies of photodissociation and reaction dynamics. It uses 1 + 1 resonant two-photon ionization detection of HE [10]. This is an appealing ionization scheme, for it is possible to saturate the ionization step, effectively reducing the process to a one-photon spectroscopic probe with consequent simplification in the extraction of populations from the spectral peak intensities. Our experimental approach and motivation are very similar to the methods used in and the rationale for two recent studies of HE photoelimination from 1,4-cyclohexadiene [ 11 ] and ethylene [ 12 ]. The ethylene results provide an interesting comparison with the measurements we report here.
26 June 1992
pumped chambers. The pulse energy of the 300 nm light is sufficiently high that the photoionization step in the two-photon process is saturated, so the signal intensities depend only on the one-photon transition to the C intermediate state. To determine the wavelength dependence of the VUV intensity we measured the mass spectrometer signal from one-photon ionization of propylene by the VUV light. Since the ionization cross-section of propylene is wavelength independent over the 99102 nm wavelength region [ 15] we scanned for the 1 + 1 photoionization of H2, the propylene ion signal is a good indicator of the VUV intensity. The adjustable delay between the photolysis and probe laser pulses was set for 10 ns in these experiments. The short delay guarantees collision-free detection conditions and eliminates potential problems due to fast photoproducts leaving the viewing region before being detected.
2. Experiment These experiments were performed in a differentially pumped multi-chambered dual molecular beam apparatus. Once chamber houses a slit geometry pulsed free-jet expansion nozzle, from which a beam of 1,3-butadiene is formed by expansion of the neat sample at ~ 10 Torr. The skimmed beam propagates into the photodissociation/detection interaction region of the time-of-flight detector chamber. Laser beams for photolysis of the butadiene and for photoionization of the H2 photoproduct counterpropagate through the detector chamber at right angles to the molecular beam axis. The photolysis beam at 212.8 nm, from the fifth harmonic of a Nd:YAG laser, with about 4 mJ in 6 ns pulses, is weakly focussed to a diameter of ~ 2 mm. The Hz detection laser beam has both ~ 100 nm light for excitation of the Hz in the X IZ~ to C q-lu transition and ~ 300 nm light for ionization of H2 molecules in the excited C ~rI, state. The 100 nm light is produced by frequency-tripling [ 13,14 ] the 300 nm output of a doubled pulsed dye laser in a pulsednozzle free-jet expansion of argon, located in a separate chamber of the apparatus. The 300 nm, 6 ns pulse with ~ 20 mJ produces 109-10 ]1 photons at 100 nm. The 10 nm/300 nm beam passes the short distance into the detector region through differentially 192
3. Results and discussion Fig. 1 show a part of the 1 + 1 ionization spectrum of the H2 photoproduct resulting from 212.8 nm photolysis of 1,3-butadiene. The peaks were assigned using the transition frequencies of Dabrowski [ 16 ]. 2.2
L8 1.6 1.4 1.2 1 0.8 0.6 I I
o.41 0.2'
10tl .4
1011.5
I 101.6
I 101.7
wavelength
t 101.8
10tl .9
I
102
(nm)
Fig. 1. A portion of the 1 + 1 two-photon ionization spectrum of the H2 photoproduct. The peaks are due to transitions from v---0 in the X Zs state to both C tHu and B ' + intermediate states.
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
The H2 population in a particular rovibrational state v, j is given by
OE+O-1-
l(v,j)C, N(v'J)= f ( v ' , j ' ; v , j ) C z '
(4)
where I(v, j ) is the integrated peak area. The factor C1 corrects for different detector settings, while C2 corrects for the wavelength dependence of the VUV intensity determined by the propylene ionization yields measurements. The factorf(v', j'; v, j ) is the line-strength for the one-photon transition from X (v, j ) to C(v', j ' ), which we take from the theoretical calculations of Roueff [ 17 ]. Plots of In [N(v, j ) / ( 2 j + 1 )gU) ], where gU) is the nuclear spin degeneracy of H2, versus rotational energy for the v=0 and v= 1 states are shown in figs. 2 and 3. The error bars on the populations derive mostly from shot-to-shot fluctuations in the VUV intensity. Summation of the N(v, j) over j yields the H2 vibrational state distribution that is presented in table 1. Only data for v = 0 and 1 are shown. Although we have not completed an exhaustive search for all possible spectral peaks due to H2 (v>~ 2), it is clear that the yield of product in the higher v is substantially smaller than in v~< 1.
~}
.2 2
+
g
-4 ]
-5 z
-6 -7 -8 -9 OE+O
2
4
6
8
Rotational Energy
10
12
14
16
(kcal/mol)
Fig. 3. Plots of In [N(v,j)/(2j+ 1)g(j) ] versus rotational energy for H2 v= 1 photoproduct. The solid circles are the experimental points. Curve (1) is the prior distribution calculated for the H 2+ vinylacetylene product channel and curve (2) the prior distribution for the H2 + diradical product channel. Table 1 Comparison of the experimental H~ vibrational distribution with the prior distributions for the H2+ vinylacetylene and H2 + diradical product channels
OE+O-
-2-
26 June 1992
v=0 v= 1
Exp.
Prior for H2 + vinylacetylene
Prior for H2 + diradical
0.89+0.16 O.11 + 0.02
0.95 0.05
0.996 0.004
-4-
-6+
-8~"
-10-12-14 -16 . . . . . . OE+O 2
r. . . . 4 6
, 8
, ............... 10 12 14
16
18
Rotational Energy (kcal/mol)
Fig. 2. Plots ofln[N(v,j)/(2j+ l )g(j) ] versus rotational energy for H2 v - 0 photoproduct. The solid circles are the experimental points. Curve (1) is the prior distribution calculated for the H2 +vinylacetylen¢ product channel and curve (2) the prior distribution for the H2 + diradical product channel.
There is considerable evidence that the photochemistry of 1,3-butadiene proceeds through an excited intermediate state with a lifetime much longer than a rotational period [ 1-5 ]. Thus, it is reasonable to use a statistical prior H2 photoproduct state distribution as a benchmark against which to evaluate the measured rovibrational distributions. The solid curves in fig. 2 and 3 show the prior rotational distributions. There are actually two prior distributions, one calculated assuming that the H2 is formed along with vinyl acetylene (CH---C-CH=CH2) the other assuming that the H2 elimination leaves behind a radical (:C=CH-CH=CH2 or -CH=CH-~=CH2 or •CH=CH-CH=CH. ), which rearranges by an H atom shift after the H2 elimination to vinylacetylene. The 193
Volume 194, number 3
CHEMICALPHYSICSLETTERS
known heat of formation of vinyl acetylene (72.80 kcal m o l - t [ 18 ] ) allows determination of the available energy for the vinyl acetylene channel. For the diradical channels the thermochemistry is not known, so we estimated the stability of the radicals relative to vinyl acetylene using typical values [ 19 ] for C=C and C-=C, bond energies. In both cases the density of states of the polyatomic fragment was calculated using the Whitten-Rabinovitch approximation [ 20 ], assuming that the unknown vibrational frequencies of the diradical are the same as those of vinylacetylene. The measured rotational state distributions follow the prior distributions reasonably well, suggesting that the energy disposal is nearly statistical. The average H2rotational surprisal [ 21 ] is - 0.125 (0.11 ) in v = 0 and - 0 . 0 1 4 (0.20) in v= 1 for the H2+vinylacetylene (H2+radical) products, indicating the absence of any prominent dynamical bias for or against product rotational excitation. The experimental rotational distributions for v= 0 and v= 1 do show more structure than the prior distributions. This may be dynamically meaningful, but it may also simply reflect errors in the theoretical linestrength factors [ 17 ] used to extract populations from the spectral intensities via eq. (4). We have seen small discrepancies in populations derived from different bands using the theoretical line-strength factors, indicating that there may be errors in them. The only way to resolve this issue is to calibrate the 1 + 1 two-photon ionization spectra against a known H2 state distribution. We suggest a way to effect such a calibration in the last section of this paper. In contrast to the H2 photoproduct rotation, the vibrational distribution is far from and very much hotter than statistical, as the data in table I show. The vibrational surprisal parameter is calculated to be - 5.84 ( - 23.46) for the vinyl acetylene (radical) channel, the negative sign indicating that the experimental vibrational distribution has more energy than would be expected for statistical energy partitioning. These H2 state distributions indicate that the dominant contribution to the photoelimination proceeds through a transition state that is sufficiently symmetric that the forces imposed on the two departing H atoms are equivalent. That is, there is no torque acting on the nascent H2 as it departs. The H - H dis194
26 June 1992
tance in the transition state must be significantly longer than the equilibrium internuclear separation to give the very hot non-statistical energy release in this degree of freedom. Such an elongated H - H bond is intuitively sensible, and fully expected based on theoretical calculations of the geometries of 1,3-butadiene in the electronic states involved in the photoelimination. The symmetric transition state for photoelimination from 1,3-butadiene state implied by our H2 rotational distributions provides an interesting contrast to the photoelimination of H2 from ethylene, for which the recently measured H2 rotational distributions [ 12 ] are interpreted to indicate an asymmetric transition state.
4. Conclusions At 212.8 nm the H2 photoeliminated from 1,3-butadiene has a rotational distribution that agrees with a prior distribution but a vibrational distribution that has more population in the higher vibrational states than predicted by a prior distribution. These results suggest that the H2 is eliminated via a concerted mechanism with the two C - H bonds breaking simultaneously as the formation of the H - H bond. The rotational distributions suggest a symmetric transition state. The eliminated H2 is formed at an internuclear distance that is longer than the equilibrium bond length of H2, resulting in a vibrationally excited product. More definite conclusions and more extensive analysis of the results is precluded now by uncertainties in the accuracy of the theoretical line strength factors needed to analyze the H2 1 + l ionization spectra. We plan to establish the accuracy of these and correct them if necessary by repeating the experiments described here, using coherent anti-Stokes Raman scattering (CARS) for H2 photoproduct state detection. The v, j dependence of the Raman cross sections is known to a high degree of accuracy, so very reliable and accurate quantum state populations can be extracted from the CARS spectra. These will provide calibration of the two-photon ionization detection results and establish any corrections for the theoretical line-strength factors. CARS has sensitivity adequate for the proposed calibration experi-
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
ment, but not adequate for most molecular beam experiments, so it provides a calibration of but not a substitute for the two-photon ionization method.
Acknowledgement T h i s w o r k is s u p p o r t e d by the N a t i o n a l Science F o u n d a t i o n t h r o u g h g r a n t C H E - 9 1 - 1 4 5 4 4 . T h e authors t h a n k Dr. E v e l y n e R o u e f f for p r o v i d i n g the H E X ~ C t h e o r e t i c a l line strength factors.
References [ 1 ] B.S. Hudson, B.E. Kohler and K. Schulten, in: Excited states, Vol. 6, ed. E.C. Lim (Academic Press, New York, 1982) p. 1. [2 ] R. McDiarmid and A. Sheybani, J. Chem. Phys. 89 ( 1988 ) 1255. [ 3 ] D.G. Leopold, R.D. Pendley, J.L. Roebber, R.J. Hemley and V. Vaida, J. Chem. Phys. 81 (1994) 4218. [ 4 ] R. Srinivasan, Advan. Photochem. 5 (1964) 113. [5] I. Hailer and R. Srinivasan, J. Am. Chem, Soc. 88 (1966) 3694. [6] R.J. Cave and E.R. Davidson, J. Phys. Chem. 91 (1987) 4481.
26 June 1992
[ 7 ] O. Kitao and H. Nakatsuji, Chem. Phys. Letters 143 (1988) 528. [ 8 ] M. Aoyagi and Y. Osamura, J. Am. Chem. Soc. l 11 (1989) 470. [9] P.G. Szalay, A. Karpfen and H. Lischka, Chem. Phys. 141 (1990) 355. [ 10 ] A.H. Kung, T. Tricld, N.A. Gershenfeld and Y.T. Lee, Chem. Phys. Letters 144 (1988) 427. [ l 1 ] E.F. Cromwell, D.-J. Liu, M.J.J. Vrakking, A.H. Kung and Y.T. Lee, J. Chem. Phys. 95 (1991) 297. [12]A. Stolow, B.A. Balko, E.F. Cromwell and Y.T. Lee, J. Photochem. Photobiol. A 62 (1992) 285. [ 13] R. Mahon, T.J. McIlrath, V.P. Myerscough and D.W. Koopman, IEEE J. Quantum Electron. QE-I 5 (1979) 444. [ 14 ] R. Hilbig and R. Wallenstein, Opt. Commun. 44 ( 1983 ) 283. [ 15] J.A.R. Samson, F.F. Marmo and K. Watanabe, J. Chem. Phys. 36 (1962) 783. [16] I. Dabrowski, Can. J. Phys. 62 (1984) 1639. [ 17 ] E. Roueff, private communication. [ 18] D.R. Stull and E.F. Westrum, G.C. Sinke, The chemical thermodynamics of organic compounds (Wiley, New York, 1969). [ 19] R.C. Weast, ed., CRC handbook of chemistry and physics (CRC Press, Boca Raton, 1986-1987). [ 20] P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York, 1972). [21 ] R.B. Bernstein and R.D. Levine, Advan. At. Mol. Phys. 11 (1975) 215.
195
CHEMICALPHYSICSLETTERS
26 June 1992
Rovibrational state distributions for H2 photoeliminated from 1,3-butadiene. Evidence for a symmetric transition state B.K. Venkataraman and J.J. Valentini Department of Chemistry, Columbia University, New York. N Y 10027. USA
Received 20 March 1992
The rovibrational distributions of H2 photoeliminated from 1,3-butadiene upon excitation at 212.8 nm have been measured via 1+ 1 resonance-enhancedtwo-photonionization. The measured rotational distributions are in reasonable agreement with a prior distribution, whilethe experimental vibrational distribution is substantiallyhotter than the prior. These results indicate that the photoeliminationoccursvia a concertedmechanismthrougha symmetrictransition state in whichthe H-H distance is greater than the equilibrium H2 value.
1. Introduction The photochemistry and spectroscopy of 1,3-butadiene has provided a challenge for both experimental and theoretical studies. The spectroscopy of the first electronic absorption band, which peaks near 210 nm, is still being analyzed and interpreted [ 13 ]. This band shows only diffuse structure, due to rapid internal conversion to another excited state. The internal conversion is apparently the gateway for the photochemistry, which in the gas phase leads to three different primary product channels [4,5 ]: H2 C=CH-CH--CH2 + h v--, H2 C=CH2 + H C - C H ,
(1) H2 C=CH-CH=CH2 + h p--*H2 C=C=CH-CH3,
(2) H2 C=CH-CH=CH2 + h/P--~H2 + H2 C = C H - C - C H .
(3) The results ofab initio theoretical calculations [69 ] make it easy to see why the photochemistry is complex. These calculations show that the ~Bu excited electronic state accessed by the absorption and Correspondence to: J.J. Valentini, Department of Chemistry, Columbia University, New York, NY 10027, USA.
the lower lying 2 ~Ag excited state each have several distinct geometrical conformations corresponding to local minima on the global potential energy surface. In fact, in the 2 nAg excited state rotation about any of the three C-C bonds is predicted to be only slightly hindered. In the ground electronic state rotation about the C:-C3 single bond is hindered with a consequent production of cis and trans isomers, the latter of which is more stable by about 2.5 kcal mol[11. Of interest to us here is the molecular hydrogen photoelimination channel in the photodissociation of 1,3-butadiene, process (3). Isotope substitution studies show this occurs via 1,1 a n d / o r 1,4 elimination, as well as by 1,2 and 2,3 elimination [5 ]. We address here two questions about this process: Is it concerted? Does it involve a symmetric transition state? Measuring the internal state distributions of the H 2 fragment will provide information on the nature of the transition state geometry. For example, if the mechanism of elimination is concerted through a symmetric transition state, then there can be no torque exerted on the HE as it is eliminated, and the H2 should not be highly rotationaUy excited. However, if the H - H bond that is forming has at the transition state an internuclear separation that is sub-
0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
191
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
stantially longer than the equilibrium H - H distance, the H 2 should be vibrationally excited. These experiments are also designed to test a new molecular beam apparatus that we have designed for studies of photodissociation and reaction dynamics. It uses 1 + 1 resonant two-photon ionization detection of HE [10]. This is an appealing ionization scheme, for it is possible to saturate the ionization step, effectively reducing the process to a one-photon spectroscopic probe with consequent simplification in the extraction of populations from the spectral peak intensities. Our experimental approach and motivation are very similar to the methods used in and the rationale for two recent studies of HE photoelimination from 1,4-cyclohexadiene [ 11 ] and ethylene [ 12 ]. The ethylene results provide an interesting comparison with the measurements we report here.
26 June 1992
pumped chambers. The pulse energy of the 300 nm light is sufficiently high that the photoionization step in the two-photon process is saturated, so the signal intensities depend only on the one-photon transition to the C intermediate state. To determine the wavelength dependence of the VUV intensity we measured the mass spectrometer signal from one-photon ionization of propylene by the VUV light. Since the ionization cross-section of propylene is wavelength independent over the 99102 nm wavelength region [ 15] we scanned for the 1 + 1 photoionization of H2, the propylene ion signal is a good indicator of the VUV intensity. The adjustable delay between the photolysis and probe laser pulses was set for 10 ns in these experiments. The short delay guarantees collision-free detection conditions and eliminates potential problems due to fast photoproducts leaving the viewing region before being detected.
2. Experiment These experiments were performed in a differentially pumped multi-chambered dual molecular beam apparatus. Once chamber houses a slit geometry pulsed free-jet expansion nozzle, from which a beam of 1,3-butadiene is formed by expansion of the neat sample at ~ 10 Torr. The skimmed beam propagates into the photodissociation/detection interaction region of the time-of-flight detector chamber. Laser beams for photolysis of the butadiene and for photoionization of the H2 photoproduct counterpropagate through the detector chamber at right angles to the molecular beam axis. The photolysis beam at 212.8 nm, from the fifth harmonic of a Nd:YAG laser, with about 4 mJ in 6 ns pulses, is weakly focussed to a diameter of ~ 2 mm. The Hz detection laser beam has both ~ 100 nm light for excitation of the Hz in the X IZ~ to C q-lu transition and ~ 300 nm light for ionization of H2 molecules in the excited C ~rI, state. The 100 nm light is produced by frequency-tripling [ 13,14 ] the 300 nm output of a doubled pulsed dye laser in a pulsednozzle free-jet expansion of argon, located in a separate chamber of the apparatus. The 300 nm, 6 ns pulse with ~ 20 mJ produces 109-10 ]1 photons at 100 nm. The 10 nm/300 nm beam passes the short distance into the detector region through differentially 192
3. Results and discussion Fig. 1 show a part of the 1 + 1 ionization spectrum of the H2 photoproduct resulting from 212.8 nm photolysis of 1,3-butadiene. The peaks were assigned using the transition frequencies of Dabrowski [ 16 ]. 2.2
L8 1.6 1.4 1.2 1 0.8 0.6 I I
o.41 0.2'
10tl .4
1011.5
I 101.6
I 101.7
wavelength
t 101.8
10tl .9
I
102
(nm)
Fig. 1. A portion of the 1 + 1 two-photon ionization spectrum of the H2 photoproduct. The peaks are due to transitions from v---0 in the X Zs state to both C tHu and B ' + intermediate states.
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
The H2 population in a particular rovibrational state v, j is given by
OE+O-1-
l(v,j)C, N(v'J)= f ( v ' , j ' ; v , j ) C z '
(4)
where I(v, j ) is the integrated peak area. The factor C1 corrects for different detector settings, while C2 corrects for the wavelength dependence of the VUV intensity determined by the propylene ionization yields measurements. The factorf(v', j'; v, j ) is the line-strength for the one-photon transition from X (v, j ) to C(v', j ' ), which we take from the theoretical calculations of Roueff [ 17 ]. Plots of In [N(v, j ) / ( 2 j + 1 )gU) ], where gU) is the nuclear spin degeneracy of H2, versus rotational energy for the v=0 and v= 1 states are shown in figs. 2 and 3. The error bars on the populations derive mostly from shot-to-shot fluctuations in the VUV intensity. Summation of the N(v, j) over j yields the H2 vibrational state distribution that is presented in table 1. Only data for v = 0 and 1 are shown. Although we have not completed an exhaustive search for all possible spectral peaks due to H2 (v>~ 2), it is clear that the yield of product in the higher v is substantially smaller than in v~< 1.
~}
.2 2
+
g
-4 ]
-5 z
-6 -7 -8 -9 OE+O
2
4
6
8
Rotational Energy
10
12
14
16
(kcal/mol)
Fig. 3. Plots of In [N(v,j)/(2j+ 1)g(j) ] versus rotational energy for H2 v= 1 photoproduct. The solid circles are the experimental points. Curve (1) is the prior distribution calculated for the H 2+ vinylacetylene product channel and curve (2) the prior distribution for the H2 + diradical product channel. Table 1 Comparison of the experimental H~ vibrational distribution with the prior distributions for the H2+ vinylacetylene and H2 + diradical product channels
OE+O-
-2-
26 June 1992
v=0 v= 1
Exp.
Prior for H2 + vinylacetylene
Prior for H2 + diradical
0.89+0.16 O.11 + 0.02
0.95 0.05
0.996 0.004
-4-
-6+
-8~"
-10-12-14 -16 . . . . . . OE+O 2
r. . . . 4 6
, 8
, ............... 10 12 14
16
18
Rotational Energy (kcal/mol)
Fig. 2. Plots ofln[N(v,j)/(2j+ l )g(j) ] versus rotational energy for H2 v - 0 photoproduct. The solid circles are the experimental points. Curve (1) is the prior distribution calculated for the H2 +vinylacetylen¢ product channel and curve (2) the prior distribution for the H2 + diradical product channel.
There is considerable evidence that the photochemistry of 1,3-butadiene proceeds through an excited intermediate state with a lifetime much longer than a rotational period [ 1-5 ]. Thus, it is reasonable to use a statistical prior H2 photoproduct state distribution as a benchmark against which to evaluate the measured rovibrational distributions. The solid curves in fig. 2 and 3 show the prior rotational distributions. There are actually two prior distributions, one calculated assuming that the H2 is formed along with vinyl acetylene (CH---C-CH=CH2) the other assuming that the H2 elimination leaves behind a radical (:C=CH-CH=CH2 or -CH=CH-~=CH2 or •CH=CH-CH=CH. ), which rearranges by an H atom shift after the H2 elimination to vinylacetylene. The 193
Volume 194, number 3
CHEMICALPHYSICSLETTERS
known heat of formation of vinyl acetylene (72.80 kcal m o l - t [ 18 ] ) allows determination of the available energy for the vinyl acetylene channel. For the diradical channels the thermochemistry is not known, so we estimated the stability of the radicals relative to vinyl acetylene using typical values [ 19 ] for C=C and C-=C, bond energies. In both cases the density of states of the polyatomic fragment was calculated using the Whitten-Rabinovitch approximation [ 20 ], assuming that the unknown vibrational frequencies of the diradical are the same as those of vinylacetylene. The measured rotational state distributions follow the prior distributions reasonably well, suggesting that the energy disposal is nearly statistical. The average H2rotational surprisal [ 21 ] is - 0.125 (0.11 ) in v = 0 and - 0 . 0 1 4 (0.20) in v= 1 for the H2+vinylacetylene (H2+radical) products, indicating the absence of any prominent dynamical bias for or against product rotational excitation. The experimental rotational distributions for v= 0 and v= 1 do show more structure than the prior distributions. This may be dynamically meaningful, but it may also simply reflect errors in the theoretical linestrength factors [ 17 ] used to extract populations from the spectral intensities via eq. (4). We have seen small discrepancies in populations derived from different bands using the theoretical line-strength factors, indicating that there may be errors in them. The only way to resolve this issue is to calibrate the 1 + 1 two-photon ionization spectra against a known H2 state distribution. We suggest a way to effect such a calibration in the last section of this paper. In contrast to the H2 photoproduct rotation, the vibrational distribution is far from and very much hotter than statistical, as the data in table I show. The vibrational surprisal parameter is calculated to be - 5.84 ( - 23.46) for the vinyl acetylene (radical) channel, the negative sign indicating that the experimental vibrational distribution has more energy than would be expected for statistical energy partitioning. These H2 state distributions indicate that the dominant contribution to the photoelimination proceeds through a transition state that is sufficiently symmetric that the forces imposed on the two departing H atoms are equivalent. That is, there is no torque acting on the nascent H2 as it departs. The H - H dis194
26 June 1992
tance in the transition state must be significantly longer than the equilibrium internuclear separation to give the very hot non-statistical energy release in this degree of freedom. Such an elongated H - H bond is intuitively sensible, and fully expected based on theoretical calculations of the geometries of 1,3-butadiene in the electronic states involved in the photoelimination. The symmetric transition state for photoelimination from 1,3-butadiene state implied by our H2 rotational distributions provides an interesting contrast to the photoelimination of H2 from ethylene, for which the recently measured H2 rotational distributions [ 12 ] are interpreted to indicate an asymmetric transition state.
4. Conclusions At 212.8 nm the H2 photoeliminated from 1,3-butadiene has a rotational distribution that agrees with a prior distribution but a vibrational distribution that has more population in the higher vibrational states than predicted by a prior distribution. These results suggest that the H2 is eliminated via a concerted mechanism with the two C - H bonds breaking simultaneously as the formation of the H - H bond. The rotational distributions suggest a symmetric transition state. The eliminated H2 is formed at an internuclear distance that is longer than the equilibrium bond length of H2, resulting in a vibrationally excited product. More definite conclusions and more extensive analysis of the results is precluded now by uncertainties in the accuracy of the theoretical line strength factors needed to analyze the H2 1 + l ionization spectra. We plan to establish the accuracy of these and correct them if necessary by repeating the experiments described here, using coherent anti-Stokes Raman scattering (CARS) for H2 photoproduct state detection. The v, j dependence of the Raman cross sections is known to a high degree of accuracy, so very reliable and accurate quantum state populations can be extracted from the CARS spectra. These will provide calibration of the two-photon ionization detection results and establish any corrections for the theoretical line-strength factors. CARS has sensitivity adequate for the proposed calibration experi-
Volume 194, number 3
CHEMICAL PHYSICS LETTERS
ment, but not adequate for most molecular beam experiments, so it provides a calibration of but not a substitute for the two-photon ionization method.
Acknowledgement T h i s w o r k is s u p p o r t e d by the N a t i o n a l Science F o u n d a t i o n t h r o u g h g r a n t C H E - 9 1 - 1 4 5 4 4 . T h e authors t h a n k Dr. E v e l y n e R o u e f f for p r o v i d i n g the H E X ~ C t h e o r e t i c a l line strength factors.
References [ 1 ] B.S. Hudson, B.E. Kohler and K. Schulten, in: Excited states, Vol. 6, ed. E.C. Lim (Academic Press, New York, 1982) p. 1. [2 ] R. McDiarmid and A. Sheybani, J. Chem. Phys. 89 ( 1988 ) 1255. [ 3 ] D.G. Leopold, R.D. Pendley, J.L. Roebber, R.J. Hemley and V. Vaida, J. Chem. Phys. 81 (1994) 4218. [ 4 ] R. Srinivasan, Advan. Photochem. 5 (1964) 113. [5] I. Hailer and R. Srinivasan, J. Am. Chem, Soc. 88 (1966) 3694. [6] R.J. Cave and E.R. Davidson, J. Phys. Chem. 91 (1987) 4481.
26 June 1992
[ 7 ] O. Kitao and H. Nakatsuji, Chem. Phys. Letters 143 (1988) 528. [ 8 ] M. Aoyagi and Y. Osamura, J. Am. Chem. Soc. l 11 (1989) 470. [9] P.G. Szalay, A. Karpfen and H. Lischka, Chem. Phys. 141 (1990) 355. [ 10 ] A.H. Kung, T. Tricld, N.A. Gershenfeld and Y.T. Lee, Chem. Phys. Letters 144 (1988) 427. [ l 1 ] E.F. Cromwell, D.-J. Liu, M.J.J. Vrakking, A.H. Kung and Y.T. Lee, J. Chem. Phys. 95 (1991) 297. [12]A. Stolow, B.A. Balko, E.F. Cromwell and Y.T. Lee, J. Photochem. Photobiol. A 62 (1992) 285. [ 13] R. Mahon, T.J. McIlrath, V.P. Myerscough and D.W. Koopman, IEEE J. Quantum Electron. QE-I 5 (1979) 444. [ 14 ] R. Hilbig and R. Wallenstein, Opt. Commun. 44 ( 1983 ) 283. [ 15] J.A.R. Samson, F.F. Marmo and K. Watanabe, J. Chem. Phys. 36 (1962) 783. [16] I. Dabrowski, Can. J. Phys. 62 (1984) 1639. [ 17 ] E. Roueff, private communication. [ 18] D.R. Stull and E.F. Westrum, G.C. Sinke, The chemical thermodynamics of organic compounds (Wiley, New York, 1969). [ 19] R.C. Weast, ed., CRC handbook of chemistry and physics (CRC Press, Boca Raton, 1986-1987). [ 20] P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York, 1972). [21 ] R.B. Bernstein and R.D. Levine, Advan. At. Mol. Phys. 11 (1975) 215.
195