J. Phys. Chem. 1995, 99, 12704-12710
12704
ARTICLES Nascent State Distribution of the HCO Photoproduct Arising from the 309 nm Photolysis of Propionaldehyde Andrew C. Terentis, Pamela T. Knepp, and Scott H. Kable* Division of Physical and Theoretical Chemistry, Universio of Sydney, NSW, 2006, Australia Received: January 31, 1995; In Final Form: May 26, 1995@
The photodissociation dynamics of jet-cooled propionaldehyde have been investigated at a wavelength of 309.1 nm by monitoring the resultant nascent HCO fragments by laser induced fluorescence spectroscopy. HCO was formed only in the %(O,O,O) state. The population distribution of different rotational states characterized by N a n d K, is reasonably described by a Boltzmann distribution at a temperature of 480 f 50 K, which corresponds to an average energy in rotation of 6.0 f 0.6 kJ mol-'. Careful measurement of the width of individual K, = 0 lines in the LIF spectrum revealed that the average translational energy of the fragments is 23 f 4 kJ mol-' of HCO. These measurements have allowed us to estimate that the ethyl radical sibling fragment is born with almost no internal energy. The observed energy partitioning in the fragments is consistent with a model in which the HCO rotational and translational excitation is determined mostly by the fixed energy in the exit channel. By analogy with acetaldehyde and considering the lack of vibrational excitation, the barrier to dissociation is predicted to lie around 15 kJ mol-' below the photon energy.
Introduction Aliphatic aldehydes are a class of organic compounds that have been the subject of much investigation in the field of photochemistry over the past few decades.',2 One reason is that these molecules are known to be important constituents in various atmospheric chemical cycles, especially in the formation of photochemical ~ m o g . ~Unimolecular .~ photodissociation constitutes one of the major reactions that these molecules undergo in the atmosphere. The saturated aliphatic aldehydes all possess a weak absorption band in the region 240-360 nm as a result of an electric dipole forbidden but vibronically allowed (n,n*) transition localized on the CO chr~mophore.'*~ Acetaldehyde, propionaldehyde, and the other higher aldehydes exhibit a broad absorption spectrum centered around 290 nm in contrast to the highly structured absorption band of formaldehyde centered around 310 nm. Much of the early work on the photophysics of these molecules involved measurements of the (n,n*) upper state radiative and nonradiative rate constants for varying extents of vibrational excitation and varying pressures and temperatures. These measurements were aimed largely at ascertaining the relative importance of the radiative processes and the competing nonradiative processes such as intemal conversion (IC), intersystem crossing (ISC), and decay through available reaction channels. It has been found in general that, for unconjugated aliphatic aldehydes with relatively low vibrational excitation in the S I state under "isolated" molecule conditions, the radiative rate is about 3 orders of magnitude less than the total rate of nonradiative processes. The nonradiative processes are dominated by the competition between IC to the ground SO state and ISC to the first excited triplet state, T I , The relative importance of the two is somewhat dependent on the size of the molecule
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, July 1, 1995.
0022-3654/95/2099-12704$09.00/0
and the excitation wavelength employed. For example, formaldehyde is the only molecule in the series in which IC dominates over ISC across a large range of excitation energies, in contrast to propionaldehyde which exhibits large-molecule or statistical-limit behavior and is almost completely dominated by ISC, certainly at lower excitation energies.' Acetaldehyde is in the intermediate regime, exhibiting almost 100% ISC at very low excitation energies (e330 nm), about 80% ISC near 315 nm and almost no ISC near 250 nm excitation. Four primary decomposition processes have been postulated to exist for propionaldehyde:6 CH,CH,CHO
+ hv - C2H, + HCO
-
+ HCHO CH, + CH2CH0 C2H4
(1)
(3)
(4)
Quantum yields (4) for processes 1 and 2 have been measured as a function of wavelength, pressure, and temperature by two group^.^.^ At low excitation energy the radical channel 1 is highly favored and molecular channel 2 is relatively unimportant. At higher energies, reaction 2 becomes increasingly important and for I = 254 nm the quantum yields for (1) and (2) are comparable. Reaction 4 is very minor except at very high energies, and reaction 3 is very minor at all wavelengths. Extrapolation of 41 and $9 to zero pressure' provides an estimate of 41 42 e 1 for all wavelengths, which is consistent with the low fluorescence and large ISC and IC quantum yields discussed above. The correlation of ISC and the production of radical products and the increasing importance of IC and molecular products with shorter wavelengths make it clear that the radical species are produced from the triplet state of
+
0 1995 American Chemical Society
The Photolysis of Propionaldehyde at 309 nm propionaldehyde. The molecular products are formed mainly from highly vibrationally excited states in So following IC. A number of theoretical calculations have been performed on the ground state structure and conformations of propionaldehyde.*-' There are two stable conformations-a cis conformation where the methyl group is eclipsed with the oxygen atom and a skew conformation where the OCCC angle is approximately 128". The cis conformer is the most stable; the skew conformer is approximately 5 kJ mol-' higher in energy with a barrier from cis to skew of approximately 9 kJ The structure of propionaldehyde in the S I state has been probed through UV absorption spectroscopy and to a lesser extent theoretically by Godunov and c o - ~ o r k e r s ' ~and . ' ~ remains the only substantive information on the excited electronic states for this molecule. Recently we have published an investigation of the photodissociation dynamics of acetaldehyde in which we reported the nascent ( N , Ka) rotational state distribution and an average translational energy of the HCO photoproduct following 308 nm ex~itati0n.l~In this paper we report the nascent (N, Ka) rotational distribution and average translational energy of the HCO photoproduct following 309.1 nm excitation of propionaldehyde cooled in a supersonic free-jet expansion. We compare the results for propionaldehyde and those obtained previously for acetaldehyde to highlight similarities and differences in these prototypal aldehydes. Despite the current lack of theoretical backing for propionaldehyde, especially the transition-state structure and barrier heights, we then make some inferences about the photodissociation mechanism by comparing with the more detailed information available for acetaldehyde.
Experimental Section This experiment is almost identical to the experiment described in detail for our study of a~eta1dehyde.I~ Propionaldehyde vapor, seeded in helium as a carrier gas, is expanded into a vacuum chamber through a pulsed nozzle. The supersonic free jet expansion is intersected 8 mm downstream (X/D = 16) by the photolysis and probe laser beams. The resulting HCO fragments are probed approximately 100 ns later by laser induced fluorescence excitation spectroscopy. The experimental configuration and equipment were the same as that used previously except for the photolysis laser system employed. The photolysis laser system consisted of the frequency tripled output of a Continuum Surelite 1-20 Nd:YAG laser (355 nm) which was passed through a Quanta-Ray RS-1 Raman shifter (H2 gas, 200 psi). The first anti-Stokes line of the 355 nm beam, corresponding to 309.1 nm, was used as the photolysis beam providing 0.5 mJ pulses. Propionaldehyde (Aldrich, > 99%, no further purification) was seeded in the supersonic free jet by passing approximately 1 atm of the helium over the propionaldehyde liquid immersed in a salt/ice/water bath at approximately - 15 "C. The resulting seed ratio was about 5%. Following photolysis of propionaldehyde, nascent HCO fragments were probed by an excimer-pumped, frequencydoubled dye laser (Lambda Physik Lextra 2OOLPD-300l-CES/ coumarin 503 dye / BBO crystal) with an intracavity etalon providing 100 pJ pulses at around 258 nm. The probe laser was overlapped with the pump laser using a 250 nm dichroic reflector and copropagated through the' chamber. Both beams were focussed (500 mm lens) to a spot size of 1 mm diameter (probe) and 3 mm (pump) in the middle of the chamber. Probe laser power was measured after exiting the vacuum chamber, and after separation from the photolysis laser by a 308 nm dichroic reflector, by monitoring with a photodiode the fluo-
J. Phys. Chem., Vol. 99, No. 34, 1995 12705 rescence from a cuvette filled with rhodamine 6G dye in ethylene glycol. HCO fluorescence was detected through a Spex Minimate monochromator set with 20 nm bandpass centered at 355 nm. This wavelength enabled discrimination against pump beam scattered light and fluorescence from propionaldehyde excited by the pump laser. HCO signal disappeared completely when either laser was blocked. Fluorescence was detected with an EM1 9789-QB photomultiplier, the signal passed to an SRS280 boxcar averager and thence to a personal computer (80486DX). Timing of the nozzle, both lasers and electronics for the detection system was provided by an SRS DG-535 digital delay generator.
Results
(a) Assignment of the HCO Spectrum. A portion of the laser induced fluorescence spectrum of nascent HCO fragments from propionaldehyde is shown in Figure 1. The spectrum shows the rotational structure over most of the B(O,O,O) %(O,O,O) vibronic transition. For this work we have used exactly the same spectral assignments as those used in our recently published work on acetaldehyde. For details of the spectroscopy and our method of peak assignment the reader is referred to that paper.I4 Briefly, the spectrum is assigned as a predominantly parallel, near-prolate, A-type with the most prominent branches being the qR, and SP branches for Ka = 0, 1, and 2. Each rotational transition is split by the spin-rotation interaction induced by the unpaired electron. Additionally, for transitions with Ka > 0, each transition is asymmetry split. Some of the assignments for each of the six major branches are shown in Figure 1 with both the spin-rotation and asymmetry splitting indicated. Where resolved, the splitting of rotational transitions does not always appear to be regular. The spin-rotation and asymmetry splitting have different dependencies of N and Ka which makes the transitions appear as either single peaks, doublets, triplets, or quartets. Even so, there remains evidence of small perturbations affecting the peak positions. Figure 1 shows the limit of our more confident assignments except for the q R 0 branch which has been assigned to N = 27. In addition to the B(O,O,O) %(O,O,O) band we attempted but were unable to observe any vibrational hot bands, most notably the B(O,O,O) %(O,O,l) (v3 = bending vibration) hot band known to lie in the spectral region near 265.5 (b) Rotational Distribution. The relative population of each (N, Ka) state was calculated in the same manner as that employed in the acetaldehyde work. Briefly, the areas of all assigned, nonoverlapped peaks in the spectrum were corrected for variation in line strengths and the B-state fluorescence quantum In the event of peaks being split, the areas were simply added together to make up a total integrated intensity for the transition. The final nascent rotational state distribution is shown as a Boltzmann plot in Figure 2. Where independent measurements of the same ( N , Ka) state are available (by way of the separate P and R branches), the different values for the population were averaged. The data in Figure 2 have been plotted as different Ka data sets. There seems to be no dependence of the population with Ka outside the scatter of the data. The overall rotational distribution is characterized reasonably by a temperature of 480 f 50 K (shown in the figure). From the temperature of the distribution and using the classical equipartition formula for the average energy in rotation, we estimate that the average energy deposited as rotation in the HCO fragment is 6.0 f 0.6 kJ mol-'. (c) Doppler Line Widths. The peaks in the nascent HCO spectrum from the photolysis of propionaldehyde appear to be
-
-
-
Terentis et al.
12706 J. Phys. Chem., Vol. 99, No. 34, 1995
I
I
38600
I
38620
I
I
38640
1
1
I
I
38660
I
38680
38700
Wavenumbers Figure 1. Portion of the LIF spectrum of nascent HCO fragments showing the rotational contour of the B(0,O.O)
38720
-
X(0,O.O) transition. Only the more confident assignments within the six major branches are shown. The assignments for the bandhead regions of the ~ R and I 4R2 branches are not shown due to congestion. Spin-rotation and asymmetry splitting are indicated, where relevant. I
1.0
-
I
I
I
I
0.8 n
2
0.6 -
v
.-3 %
-
0.4
.
-70\1 T = 480 i 50 K
-8
200
400
Energy 600 (cm-') 800
1000
1204
Figure 2. Boltzmann plot of the distribution of nascent rotational energy in the HCO fragment. The solid line drawn through the data represents a temperature of 480 K.
significantly Doppler-broadened. A quantitative measurement of the width of several nonoverlapped peaks in the spectrum
was made by scanning very slowly over the peak several times and averaging the scans to get a final peak profile. Measurements were performed for qRo(7), q&(lO), q&(12), q&(13), qRo( El),qR0(20), qPo(3), and qPo(5) transitions. The splitting of transitions corresponding to K, # 0 prevented us from making meaningful sub-Doppler measurements as there were no cases for which both the spin-rotation and asymmetry splitting were fully resolved. The peak profile for qRo(13) is shown in Figure 3 and is fairly typical of those obtained for these experiments. The observed width of a rotational line is determined by the convolution of the laser line width, the Doppler width due to the transverse velocity distribution of the parent molecules and the Doppler shift due to the recoil velocity of the fragments
I
I
I
I
4
1
-0.4
-0.2
0.0
0.2
0.4
I
Relative cm" Figure 3. Doppler profile of the q h ( l 3 ) line in the spectrum. A Gaussian curve with fwhm = 0.26 cm-' is fit through the data. The instrument function, represented as a 0.1 cm-I Gaussian, is derived from an experimental measure of the laser line width and the transverse velocity component of the parent molecule in the free jet expansion (see text).
induced by the reaction. This last component is the quantity that we seek to determine. We have estimated the contribution of the laser and the parent velocity by measuring single rotational lines of other species in our apparatus under a variety of conditions. In particular, LIF spectra of jet-cooled CF2 radicals produced from a pyrolysis nozzleI9 and NO molecules in a free-jet expansion both provided a line width of 0.1 cm-I . This is slightly broader than the specified line width of the doubled dye laser (0.06 cm-I) and is of the expected magnitude for transverse velocity broadening. We therefore expect the parent propionaldehyde molecules to have a similar transverse velocity distribution in these experiments. Each peak measured
The Photolysis of Propionaldehyde at 309 nm
J. Phys. Chem., Vol. 99, No. 34, 1995 12707
TABLE 1: Summary of Thermochemical Data (Values Used in This Work Shown in Bold) species CH3CH2CHO CH3CH2 HCO
A&'(kJ mol-') -185.6 f 0.9 -187.4 & 1.7
ref
year
20
1986
21
1991
117
21
1991
119f4 107.5 41.8 f 0.6 45f4 43.5 43.1
22
23
1990 1982
24
1987
22 21
1990 1991 1982
23
was fit to a Gaussian profile which is characteristic of a Maxwell-Boltzmann distribution of speeds. The full width at half-maximum (fwhm) for the peak shown in the figure is 0.260 f 0.026 cm-I. For all the peaks measured (Ka = 0, N = 3-20) there was no systematic change in the measured width with N , outside the scatter of the data. The range quoted above covers the entire range of measured peak widths, including the worstlooking peaks. The peak profiles were deconvolved with the instrument function to estimate the recoil velocity of the fragments. Deconvolution of the experimental width of the peaks with this function provides an estimated final Doppler width of 0.24 f 0.03 cm-I. This corresponds to a translational temperature of 2000 f 400 K or an average translational energy for these rovibrational states of 25 f 5 kJ mol-'. We stress, however, that as a one-dimensional projection of a three-dimensional velocity distribution, sub-Doppler line shapes are not the ideal method by which to obtain translational energy distributions. We have fit the Doppler-broadened peaks in our spectrum to a Gaussian line shape (thereby insinuating a Maxwell-Boltzmann distribution of speeds), which in general seems to provide a satisfactory description of the peak shape. There is, of course, no requirement that the nascent translational energy distribution is described by a Maxwell-Boltzmann distribution. In fact, we see evidence in many of the peaks that the experimental peak profile is overestimated by the Maxwell-Boltzmann distribution (a Gaussian line shape) at high translational energy (the edges of the profile) and overestimated in the center of the profile. These effects are subtle, however, and could be caused by a number of factors (e.g., speed distribution, angular distribution, or vector correlations), and we prefer not to attempt to obtain a translational energy distribution from these peak shapes. The Gaussian fit to the data, however, usually provides a fairly robust estimate of the average translational energy.
Discussion
(a) Thermochemistry of Propionaldehyde and Products. To analyze the disposal of energy into the degrees of freedom of the products, we must first have an estimate of the total available energy in the reaction, i.e., hv - AHRaction.Knowledge of the electronic states of the reactant and calculations of transition states and barriers all provide useful information with which to understand the photodynamical data. Unfortunately, for the present reaction, these either are not available or are known with inadequate precision for our purposes. To estimate the available energy in this system, we have surveyed the thermochemical literature, including the main compilations, for estimates of the heats of formation, w,for CH~CHZCHO, HCO, and CH3CH2. Some of the more recent values are summarised in Table l.20-24The values that we have used in this work are shown in bold. Estimates of the enthalpy of formation for propionaldehyde itself have remained unchanged for many years. Our preferred value is from the 1986
400
375
t
1
...... ...........................
4
III
,I((
111 (11
(372)d
T.S.
- (358)' (350)'
350
--
.-
(344)C
h
CH3CH, + HCO (-326)b
I
1
- I
CH3CH2CH0
Figure 4. Energy level diagram showing the electronic states of
propionaldehyde and products relevant to this work: (a) spectroscopic value from ref 13; (b) spectroscopically determined value for triplet acetaldehyde from ref 25; (c) value for the heat of reaction using the numbers in bold from Table 1; (d) estimate of the banier height as outlined in the text (T.S. = transition state); (e) estimate of the barrier height from data in ref 7. publication of Pedley et aLZo The (apparently) more recent value from the 1991 NIST compilation2' is in fact derived from an older reference, again from Pedley et al. The formyl and ethyl values are somewhat more uncertain; we favor the value of Moore and c o - ~ o r k e r s(a~ spectroscopic ~ rather than thermochemical measurement) for formyl and the most recent data for ethyl from the 1991 NIST tables.2' These values are used in the production of Figure 4, a schematic showing the relative energies of the reactant, the products and the photon. A 309.1 nm photon provides 387 kJ mol-' of energy. Our best estimate of the residual available energy is consequently 43 f 4 kJ mol-' to be partitioned into the product degrees of freedom. Figure 4 also shows the zeropoint levels of the SI and TI states of propionaldehyde. The SI state is known accurately from spectroscopic measurement^,'^ the TIvalue we have estimated only from the equivalent triplet state in a~etaldehyde.~~ The figure also indicates two estimates of the barrier height along the triplet surface over which the reaction must progress, obtained from this work and from the data of Shepson and Heicklen7 (vide infra). (b) Energy Partitioning in the Fragments. The way in which the excess energy in a photochemical reaction is partitioned into the final energy of the fragments provides an important clue in elucidation of the photodissociation dynamics. The simplest way to gauge the energy partitioning is to measure the average energy in each of the degrees of freedom in the products. Experimentally, we have measured the distribution of energy in rotation and vibration of the HCO photofragment, and estimated the average translational energy. The average energy in HCO translation measured above (25 f 5 kJ mol-') was measured only for selected rotational states ( K , = 0, N = 3-20). For this Ka = 0 subset of nascent rotational states, conservation of linear momentum dictates that the sibling ethyl fragment must also possess 25 f 5 kJ mol-' of translational energy (they have the same mass). No trend of translational energy with N was observed, and as noted above, measurement of sub-Doppler profiles was not possible for Ka
Terentis et al.
12708 J. Phys. Chem., Vol. 99, No. 34, 1995
TABLE 2: Estimated Average Energy Deposited in Each Degree of Freedom of the Photofragments from Experiment and Two Model# energy disposalkl mol-’ (% total energy) propionaldehyde statistical impulsive average K, = 0 states degree of freedom fragment 1.6 (4) 12.5 (29) HCO vibration 0 0 (0) 2 6.0 f 0.6 (12) 8.2 (19) rotation translation 4.1 (9.5) 9 (21) 25 23 f 5 (44) ? (-0) 2s (59) 12.5 (29) CH3CH2 vibration rotation ? (-0) translation 25 23 f 5 (44) 4.1 (9.5) 9 (21) total energy 52 52 If 10 43 43 a
The values in parentheses are the average energies expressed as a percentage of the total energy at the bottom of the column.
> 0 because of splitting of the transitions. For these HCO fragments produced in v = 0, Ka = 0, and N = 3-20 the excess reaction energy which has been accounted for is summarised in Table 2 under the heading of “Ka = 0 states”. Notice that we have accounted for 52 f 10 kJ mol-’, already in excess of the estimated available energy for this reaction (43 f 4 kJ mol-’), although the error limits comfortably overlap. A consequence, however, is that the intemal (vibrational rotational) excitation of the concomitant ethyl fragment must be minimal for these low energy HCO fragments. The average HCO rotational excitation is somewhat higher than the typical value for the Ka = 0 peaks for which we measured the HCO translational energy. The average translational energy is therefore probably marginally lower than the value that we have measured. If our inference about the low intemal energy of the ethyl fragment is maintained in general, then we can estimate the average translational energy. This is shown in Table 2 in the “average” energy disposal column. Note that the bulk of the excess energy appears to end up as translation of the photofragments, the HCO has moderate rotational excitation, and it seems neither fragment receives much vibrational energy. (c) Models of the Dissociation Event. There are numerous general models of photodissociation dynamics which seem to fall into two broad classes: (i) the sfufisticalmodels, which include the prior,26RRKM,27 and phase space theory28models. These models all have as a basic tenet that the available energy is uniformly spread throughout the whole molecule prior to the breaking of the bond. As a result they are most often (and most successfully) applied to “large” molecules, where intramolecular redistribution is both rapid and complete; (ii) the other extreme in simple dynamical models is the impulsive models which include the simple impulsive model of Busch and Wilson29(and its variants30) and the Franck-Condon model.3’ These models have in common that the distribution of energy in the products is determined by the dissociation event rather than the distribution of energy in the parent. Propionaldehyde is a large molecule with many vibrational degrees of freedom. As such, and in common with other molecules of a similar size, it might be expected that one of the statistical models might reproduce the experimental distribution of energy, or at least the average energies. In fact none of the statistical models do at all well in predicting the observed energy in the fragments. The results of one such model, the prior model, are shown in Table 2. Even allowing for the uncertainty in the inferred intemal energy of the ethyl fragment, all statistical models place too much energy in the intemal degrees of freedom of both fragments and too little into translation. Impulsive models have been popular and sometimes successful models applied to a variety of state-resolved photodissociation experiments over the year^.^^.^^ Indeed the model has
+
been applied to acetaldehyde photodissociation with some ~ u c c e s s .However, ~ ~ ~ ~ ~for the current experiment on propionaldehyde, the impulsive calculation does not give good overall agreement (Table 2). The impulsive model also comprehensively underestimates the amount of energy in translation and (probably) overestimates the amount of intemal energy in the ethyl fragment. None of the simple models even sketch an appropriate picture of the propionaldehyde dissociation event. We could modify one or more of the models and perhaps do a better job, however there is little joy in constructing models that are applicable for only one molecular system. Instead, we have noticed many similarities between these data and the results for the smaller acetaldehyde molecule for which much more substantive experimental and theoretical data exist. We spend the rest of the discussion comparing the propionaldehyde data with acetaldehyde. (d) Comparison with Acetaldehyde Photochemistry. The spectroscopy and photochemistry of propionaldehyde have remained relatively unexplored, at least in comparison with its two smaller counterparts, formaldehyde and acetaldehyde. Those studies that have been carried out-theoretically, spectroscopically, and photochemically-indicate that there is a high degree of similarity in the series of aliphatic aldehydes from acetaldehyde to longer chain lengths. The photochemistry of formaldehyde is somewhat different from its larger counterparts. Theoretical calculations on the ground-state structure of acetaldehyde, propionaldehyde, and butyraldehyde reveal that the structural parameters (bond lengths and angles) of the aldehyde group are quantitatively uniform.33 Furthermore, Godunov et al., in their analysis of the vibrational structure of the SI SO electronic transition of cis-propionaldehyde by W absorption spectroscopy, have assigned progressions related to the CO stretch, CCO angle deformation, and CCHO plane puckering def~rmation.’~ It is evident that the same structural changes upon electronic excitation are occurring in propionaldehyde as in acetaldehyde. Most notably, propionaldehyde, like acetaldehyde, undergoes a transformation from planar to pyramidal geometry in the aldehydic group. The spectroscopic and theoretical data that are available therefore indicate that the electronic potential energy surfaces for the aliphatic aldehydes are all similar, at least in the region of the aldehydic group. The photochemistry of acetaldehyde has received relatively much greater attention than has propionaldehyde. Excitation at wavelengths shorter than 320 nm produces formyl and methyl radicals. The excess energy dissipation at A = 308 nm produces rotationally but not vibrationally excited HCO and a large translational energy release. The methyl fragment is probably produced with little or no internal e~citati0n.I~A series of measurements for A = 320-280 nm showed little dependence of HCO rotational excitation with excess energy3*but showed that vibrational excitation of the HCO appeared when the photon
-
The Photolysis of Propionaldehyde at 309 nm energy exceeded the barrier height by an amount greater than the product vibrational energy. The threshold for the formation of g(O,O,l) HCO has been estimated to correspond to an excess energy above the barrier of 14-15 kJ mol-’.” The barrier, it seems, is acting like a bottleneck. As the barrier is approached following 308 nm excitation, the energy is channelled into the reaction coordinate. At the transition state, little energy is left in any other vibration that can manifest into vibrational energy in the free fragments-only the methyl umbrella mode is possible. Furthermore, the transition state was calculated to occur at a reasonably long C-C distance (2.1 A) where the .structure of the departing fragments is close to that of the isolated products.34 After the barrier, the fragments are too far apart to impart sufficient impulse to couple any of the remaining energy into vibrations of the products. The fixed energy of the exit channel, estimated to be 24 kJ mol-’,35 is shared between translation of the fragments and rotation of the HCO. The methyl fragment receives little rotational excitation because the reaction pathway proceeds along the methyl center-of-mass and hence no torque is exerted. Many of these features are reproduced in the distribution of energy found in the fragments of propionaldehyde dissociation. Vibrational energy in HCO: Both reactions at these photon energies produce HCO fragments that are found only in the zeropoint level, even though about 40 kJ mol-’ of energy is available in each reaction. For acetaldehyde this was explained in terms of the high, “late” bmier on the triplet surface, as outlined above. The propionaldehyde reaction also occurs along the triplet surface, and it seems reasonable to assume that a similar barrier will exist. The threshold for production of radical products has not been determined experimentally or theoretically for propionaldehyde. However, the lack of vibrational energy in the HCO fragment suggests that the photon energy is within 15 kJ mol-’ of the top of the barrier. The energy level schematic in Figure 4 indicates a barrier along the triplet surface 15 kJ mol-’ below the photon energy. Room temperature studies have found radical products at A = 334 nm.7 However, care must be taken in the interpretation of these data in terms of evaluating a threshold energy because the experiments were performed at room temperature, at moderately high pressures, and are collision affected. A threshold wavelength of 334 nm would correspond to a barrier of 358 kJ mol-’ (indicated in Figure 4), some 14 kJ mol-’ lower than our estimate. Given the difficulty in interpreting the room-temperature data, the indirect nature of our estimate and the uncertainty in the thermochemical data we consider this agreement to be fair, Rotational energy in HCO: Both reactions produce very similar HCO rotational distributions which are characterized by a Boltzmann temperature of about 500 K and little, if any, preference for any K, state. A lack of K, dependence indicates that no preferred axis of HCO rotation is produced by the reaction. This is consistent with a “loose” transition state with little geometrical constraint on the angle of the HCO moiety with respect to the rest of the molecule. The calculated structure of acetaldehyde at the transition state places the HCO out-ofplane angle at about 70°.34 The inversion barrier for triplet acetaldehyde is estimatedz5 to be 13 kJ mol-’, and because of the long, floppy C-C bond it is conceivable that an even lower barrier to inversion exists at the transition state. It seems that the behavior of propionaldehyde is similar with respect to the HCO rotational distribution, and therefore we would suspect that the transition state is similarly “floppy” in the HCO outof-plane bend coordinate with probably a shallow minimum at some large angle to the planar configuration. Translational energy of both fragments: Both reactions
J. Phys. Chem., Vol. 99, No. 34, 1995 12709
produce most of the excess energy located in kinetic energy of the fragments. In acetaldehyde, some 82% of the available energy is kinetic, HCO receiving 28% and CH3 54%. In propionaldehyde, again the bulk of the available energy, about 88%, is found in kinetic energy of the fragments with each fragment receiving the same. Although it may seem at first glance that the kinetic energy distribution is somewhat different in each system, with the average HCO translational energy from propionaldehyde being about twice that from acetaldehyde, this is mostly just the effect of the different mass of the concomitant fragment. In propionaldehyde the HCO recoils from a much heavier partner and in turn receives more of the kinetic energy. The important aspect is that in both systems 80-90% of the available energy goes into translation-the amount into each fragment is governed simply by conservation of linear momentum. The similarity of HCO rotation and translation from both reactions suggests that the potential experienced by the HCO is much the same in both cases. Internal energy of the partner fragment: This is probably the weakest link in our understanding of the photochemistry of both of these systems. The energy deposited into vibration and rotation of the methyl or ethyl fragment has not been measured. In acetaldehyde, the energetics are sufficiently well-known that the energy found in the methyl fragment can be inferred to be very low ( < 5 kJ mol-’). The interpretation of this result is also straightforward-the C-C bond cleavage occurs along the methyl center-of-mass, and hence no torque is exerted. The high, late barrier, along with the high vibrational frequencies of the methyl group ensure that little or no vibrational energy can be deposited into the methyl fragment for exactly the same reasons as for the HCO fragment discussed above. The argument is not so straightforward for the ethyl fragment however. The impulse of the breaking C-C bond is no longer directed along the center-of-mass and hence some ethyl rotation might be expected. Additionally, the ethyl radical has several low-frequency vibrations which might be expected to retain some population even when crossing the high barrier. There are probably three or four vibrations in propionaldehyde which could be populated at the transition state (given the estimated excess energy above the barrier) and could manifest into vibrational energy in the ethyl fragment. We were forced to conclude above however (due to the large amount of kinetic energy in the fragments) that the ethyl fragment probably has little intemal energy. Unfortunately, the energetics of the reaction are not quite as well-known, and the experimental error limits on the translational energy are quite large so that this inference must be treated cautiously. We might rationalize the result by supposing that although the low-frequency ethyl vibrations may be populated at the transition state, only the ethyl torsion is very low in frequency and therefore likely to be significantly populated. Also the rotational constants of the ethyl radical are smaller than the formyl radical and so conservation of angular momentum may result in less rotational energy (=Bp)in the ethyl fragment than the formyl fragment even though angular momentum (4) is similar. The only sure way to resolve this issue is to probe the ethyl fragment.
Conclusions The internal and translational energy of the nascent HCO fragment from the 309.1 nm photolysis of propionaldehyde has been measured. Similar results have been obtained in comparison to our previous_investigation of acetaldehyde photolysis at 308 nm: (i) only X(O,O,O) HCO fragments were detected; (ii) the (A’, Ka) rotational distribution shows no definite preference for any particular K, state and can be characterized
12710 J. Phys. Chem., Vol. 99, No. 34, 1995
by a temperature of around 500 K; (iii) a large proportion (8090%) of the excess energy in the reaction is deposited into translational energy of the fragments; (iv) there is very little internal energy in the concomitant alkyl fragment in each reaction. Hence almost all the excess energy of the reaction in both cases is being deposited as HCO rotation and relative translational energy of the fragments. These similarities point toward a similar photodissociation mechanism for acetaldehyde and propionaldehyde at these wavelengths. We have invoked our results and the available theoretical and spectroscopic data for these molecules (mostly on acetaldehyde) to build a picture of the photodissociation mechanism. After photon absorption, the molecule undergoes intersystem crossing to the lowest-lying triplet state. On the triplet surface the molecule encounters a high, late barrier to the formation of radical products. Most of the photon energy is channelled into the reaction coordinate at the transition state, leaving around 15 kJ mol-' of energy in the rest of the molecule. This excess energy above the barrier lies mostly with the lower frequency torsions and in-plane and out-of-plane bends or wags at the expense of the higher frequency stretches and leads to the broad (N, Ka) HCO rotational distribution observed. The late barrier not only ensures a very floppy transition state but also prevents the impulse of the bond breakage from coupling any of the remaining energy into vibration of the fragments.
Acknowledgment. We would like to acknowledge the Australian Research Council and the University of Sydney Research Grants Scheme for providing funds to support this project. References and Notes (1) Lee, E. K. C.; Lewis, R. S. Adv. Photochem. 1980, 12, 1. (2) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77, 793. (3) Campbell, I. M. Energy and the Atmosphere: a Physical and Chemical Approach, 2nd ed.; Wiley: New York, 1986. (4) Findlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986. ( 5 ) Hansen, D. A.; Lee, E. K. C. J. Chem. Phys. 1975, 63, 3272. (6) Blacet, F. E.; Pitts, J. N., Jr. J. Am. Chem. SOC.1952, 74, 3382.
Terentis et al. (7) Shepson, P. B.; Heicklen, J. J. Photochem. 1982, 19, 215. (8) Allinger, N. L.; Hickey, M. J. J. Mol. Struct. 1973, 17, 233. (9) Van Nuffel, P.; Van Den Enden, L.; Van Alsenoy, C.; Geise, H. J. J. Mol. Struct. 1984, 116, 99. (10) Gupta, V. P. Can. J. Chem. 1985, 63, 984. (1 1) Bell, S. J. Mol. Struct. 1994, 320, 125. (12) Godunov, I. A.; Yakovlev, N. N.; Belozerskii, I. S. Russ. J. Phys. Chem. 1994, 68, 954. (13) (a) Alekseev, V. N.: Godunov, I. A. Russ. J. Phys. Chem. 1993, 67, 89, (b) 448. (14) Terentis, A. C.; Stone, M.; Kable, S. H. J. Phys. Chem. 1994, 98, 10802. (15) Sappey, A. D.; Crosley, D. R. J. Chem. Phys. 1990, 93, 7601. (16) Adamson, G. W.; Zhao, X.; Field; R. W. J. Mol. Spectrosc. 1993, 160, 11. (17) Gejo, T.; Takayanagi, M.; Kono, T.; Hanazaki, I. Chem. Phys. Lett. 1994, 218, 343. (18) Shiu, Y. J.; Chen, 1.-C. Chem. Phys. Lett. 1994, 222, 245. (19) Cameron, M. R.; Kable, S. H.; Bacskay, G. B. J. Chem. Phys., in press. (20) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds; Chapman and Hall: London, 1986. (21) Stein, S. E.; Rukkers, J. M.; Brown, R. L. NIST Structures and Properties Database, version 1.2; NIST Standard Reference Data: Gaithersburg, MD, 1991. (22) Martinho-Sinnoes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (23) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H. J. Phys. Chem. Ret Data, Suppl. 1 1982, 11, 1. (24) Chuang, M.-C.; Foltz, M. F.; Moore, C. B. J. Chem. Phys. 1987, 7, 3855. (25) Yakovlev, N. N.; Godunov, I. A. Can. J. Chem. 1985, 70, 931. (26) Levine, R. D.; Bernstein, R. B. Acc. Chem. Res. 1974, 7, 393. (27) Truhlar, D. G.; Hase, W. L.; Hynes, J. T. J. Phys. Chem. 1983,87, 2664. (28) Pechukas, P.; Light, J. C.; Rankin, C. J. Chem. Phys. 1966, 44, 794. (29) Busch, G. E.; Wilson, K. R. J. Chem. Phys. 1972, 56, 3626. (30) Trentelman, K. A,; Kable, S. H.; Moss, D. B.; Houston, P. L. J. Chem. Phys. 1989, 91, 7498. (31) Shapiro, M.; Bersohn, R. Annu. Rev. Phys. Chem. 1982, 33, 409. (32) Kono, T.; Takayanagi, M.; Hanazaki, I. J. Phys. Chem. 1993, 97, 12793. (33) Klimkowski, V. J.; Van Nuffel, P.; Van Den Enden, L.; Van Elsenoy, C.; Geise, H. J.; Scarsdale, J. N.; Schafer, L. J. Comput. Chem. 1984, 5, 122. (34) Yadav, J. S.; Goddard, J. D. J. Chem. Phys. 1986, 84, 2682. (35) Kono, T.; Takayanagi, M.; Nishiya, T.; Hanazaki, I. Chem. Phys. Lett. 1993, 201, 166.
J P95 0328S
