Photodissociation of acrylonitrile at 193 nm: the CN-producing channel

CHEMICAL

26 January 1996

PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 249 (1996) 40-45

Photodissociation of acrylonitrile at 193 nm: the CN-producing channel Cathy A. Bird, D.J. Donaldson * Department of Chemistry and Scarborough College, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S IAI

Received 7 August 1995; in final form 17 November 1995

Abstract

We report the rotational and vibrational quantum state distributions of the CN (X 2,~ +) product formed in the 193 nm photolysis of acrylonitrile (vinyl cyanide). The CN product is formed in v = 0 and v = I with relative populations of 0.86 and 0.14, respectively. The rotational distributions in both v-levels are well fit to a temperature of approximately 1450 K. The results are consistent with a prompt dissociation on the excited electronic potential surface.

1. I n t r o d u c t i o n Ethylene and its substituted analogs represent the simplest possible olefinic systems. Consequently, the electronic spectroscopy, photochemistry and photophysics of these molecules has received considerable attention [1]. In ethylene the first observed transition, often labelled N - V , is from the planar ground state to a twisted excited state valence level. In the chlorihated ethylenes, the corresponding valence state is mixed (to varying degrees) with the lowest lying Rydberg levels. Parent fluorescence is not observed following excitation to the valence or the mixed valence-Rydberg levels in any of these molecules. Photodissociation experiments have established two principal product channels following excitation of substituted ethylenes. The first is a simple bond cleavage, yielding H from ethylene, and X (or H) from the halogenated ethylenes, C 2 H 3 X (X = Cl,Br).

* Corresponding author. E-mail: [email protected], utoronto.ca

In C2H4, this process is believed to occur after internal conversion to the ground electronic state [2-5]. In the halogenated species, the X-producing channel has received attention; it is thought to occur promptly on an excited electronic surface, perhaps even the initially excited one [6-8]. The second product channel is a molecular elimination, giving H 2 from ethylene and HX from halogenated ethylenes [2-8]. Both 3-centre and 4-centre elimination are energetically possible; the results to date favour the 3-centre mechanism [7], though both are inferred from the measurements. The electronic spectroscopy and photodissociation of acrylonitrile (vinyl cyanide) have not received much previous attention. Mullen and Orloff [9] measured the vapour phase absorption spectrum of acrylonitrile in the vacuum UV region down to 125 nm. They assigned three electronic transitions: n ~ 7r *, -rr ~ Tr *, and (r-~ cr *, whose origins lie at 210.7, 203.0 and 172.5 nm, respectively. The absorption spectrum of jet-cooled acrylonitrile near 190 nm [10] is shown in Fig. 1. It consists of a continuum upon

0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)01336-9

41

CA. Bird, D.I. Donaldson/Chemical Physics Letters 249 (1996) 40-45

which is superimposed some resolvable vibrational structure. A comparison of Fig. 1 with the spectrum shown by Mullen and Orloff [9] shows that no new structure or significant narrowing of features is observed upon jet-cooling the sample. This strong absorption feature was assigned [9] to an ethylenic

Table 1

~ w * transition, with some ~x ~ ty * character. The vibrational bands associated with the transition were assigned [9] as combinations of the CCN bend, trans CH wag, C = C stretch, and CN stretch, with the most intense bands being those containing the C = C stretch and to a slightly lesser degree, the CN stretch. This assignment is reproduced in Table 1. Gandini and Hackett [11] investigated the photodissociation of acrylonitrile at 213.9 nm, in the region of the first excited n ~ -rr * electronic transition assigned by Ref. [9]. Their results are consistent with the existence of two molecular elimination channels only, one yielding C2H 2 and HCN, and the other yielding C H C C N and H 2. At shorter wavelengths, near 206 nm, evidence for radical-producing channels was obtained [I 1]. However, Fahr and Laufer [12], in a broad-band UV flash photolysis experiment, did not detect any CN or C2H 3 product, In contrast, Nishi et al. [13] did report CN and C2H 3 product from the 193 nm photodissociation of jet-cooled acrylonitrile. The photofragments from the photolysis were detected using time-of-flight (TOF) mass spectrometry and the centre-of-mass translational energy distribution fit to a composite of two

500o0 50710 51390 51730 52550

lOOO 1710 2390 2730 3550

5525o 5540o 5618o 57140

6250 64o0 6920 7880

.~ £ .i o "~

170

180

190

Wavelengl:h

~'00 (nm)

I

210

Fig. I. The direct absorption spectrum of jet-cooled acrylonitrile [10]. See Ref. [28] for a complete discussion of the technique,

Assignment of 203 nm absorption (from Ref. [9])

Wavenumber Distancef r o m 0-0 band (cm -~) 49000 0 49310

53620 54200

57470

310

4620 5200

8210

Assignment origin of -n-w* transition CCN bend trans CH wag (A) c = c stretch (B)

CN stretch (C) A+B (2×B) (2 X C) CCN bend + (3 × B)

A+(3 ×B) (3xc) (2xA)+(3×B) (2xB)+(2XC) (2 x B) + (2 × C)

Maxwell-Boltzmann distributions with temperatures of 2500 K and 6000 K. The low energy component was attributed to a channel forming C2H 3 and CN, and the high translational energy component to the molecular elimination process, forming C2H 2 and HCN. The angular distributions of the TOF signals at masses 26 and 27 were isotropic within the experimental uncertainty of 10%. This observation suggests that the timescale for dissociation is longer than the rotational period of the molecule. Nishi et al. [13] suggested that the excited molecule undergoes internal conversion from the initially excited (rr, "rr * ) state to highly excited vibrational levels of the ground electronic state, from which either the radical or the molecular elimination channel is open. In the present study, we investigate the CN quantum state distributions following the 193 nm photolysis of room temperature acrylonitrile. The results indicate that CN is formed in this process and show a non-statistical partitioning of the available energy among the CN degrees of freedom, suggestive of an excited state dissociation process.

2. Experimental The experimental setup is essentially the s a m e as that used in previous studies in this laboratory [ 1 4,15].

42

C.A. Bird, D.J. Donaldson/Chemical Physics Letters 249 (1996) 40-45

A 1 inch diameter cylindrical pyrex flow cell, equipped with suprasil windows, was evacuated by a 5 cfm mechanical pump, through a trap held at 77 K. Samples of freshly degassed acrylonitrile (Aldrich, 99 + %) were introduced into the flowcell through a needle valve. For some experiments, the sample was vacuum-distilled prior to use; there was no difference in the results between these samples and the non-distilled ones. Pressures of sample were controlled and were in the 10-100 reTort range, as measured by a capacitance manometer mounted on the cell. Under our standard operating conditions, the residence time in the cell is always less than the time between laser shots, The unfocused output of a 193 nm ArF excimer laser (the photolysis laser) was attenuated to < 1 mJ/pulse and introduced into the cell. The frequency-doubled output of a Nd:YAG-pumped dye laser (the probe laser) counterpropogated with the excimer beam. This laser was attenuated to pulse energies < 10 IXJ/pulse prior to entering the cell. The relative timing between the two laser pulses was controlled using a digital delay generator; typical delay times used here were in the range 20-100 ns. The combination of sample low pressures and short photolysis-probe delay times ensured that the CN state distributions being measured are those formed in the photodissociation, prior to collisional relaxation. Laser-induced fluorescence spectra of the CN (B 2y ,_ X 2~) transition excited by the probe laser were measured over the range 380-390 nm. The total emission signal was collected perpendicular to the laser axis through an optical low-pass filter which attenuated wavelengths < 380 nm, and imaged onto a photomultiplier tube (PMT). The PMT signal was averaged over 30 laser shots by a boxcar averager and stored on a laboratory computer. Both photolysis and probe laser energies were contiuously measured at the same time as the excitation spectrum; they remained constant (to within 10%) during the course of each spectral scan. The CN excitation spectra were assigned using the molecular constants given in Ref. [16]. Relative populations were extracted using the appropriate f a c t o r s from Ref. [17] and FranckCondon factors from Ref. [ 18].

Honl-London

3. Results Fig. 2 shows a portion of the excitation spectrum of CN (X 2~) obtained 100 ns after the 193 nm photolysis of l0 mTorr of room temperature acrylonitrile. Six scans are co-added to produce the spectram displayed. No evidence of optical saturation in the spectrum is observed for probe laser energies less than about l0 IxJ/pulse; the results in the figure were obtained at energies of 1-4 txJ/pulse. Shortening the photolysis-probe delay to 25 ns does not change the measured spectrum, or the derived populations, by an amount greater than the 10% uncertainty estimated on the measurements. Therefore we take the spectrum shown in Fig. 2 to be that of the initially produced CN fragments. In the present experiment, only the strong diagohal transitions in the B - X band were measured. There is evidence of population in v = 0 and v = 1 of CN (X 2 ~ ) over a range of rotational states. No signal arising from v = 2 or higher is observed. Fig. 3 shows Boltzmann plots for v = 0 and v = 1. For rotational states N">~5, each is reasonably well described by a single rotational temperature of approximately 1450 K. A simulation of the spectrum, using this rotational temperature and treating the v = l/v = 0 population ratio as an adjustable parameter [14,15], yields a relative normalized population 100 ~ -,

80

t-

.~-

6o

_= 40 .~ ~ 2o

|

,.~. 0382

383

. , 384

385

386

387

388

389

Wavelength (nm) Fig. 2. The excitation spectrum of CN (X 25~ ÷) obtained 100 ns after the photolysis of 10 mTorr of acrylonitrile. Transitions from v" = 0 and v" = l only are observed. The arrow shows the bandhead of the (1-1) transition P-branch.

Ca4. Bird, DJ. Donaldson/Chemical Physics Letters 249 (1996) 40-45 1

,

43

product would provide a more precise picture of the total energy release. 4.2. Vibration in the CN(X)

v

-2 z, -3

"

4 -500

, 0

•

•

, , , , . . . . 5oo 1000 lSOO ax~o 2soo 3ooo 350o 4o0o RotationalEnergy(errr')

Fig. 3. Boltzmann plots of the rotational populations in v = 0 and v = 1 extracted from spectra such as that shown in Fig. 2. Didmonds represent v = 0; circles show v = I. The solid lines show fits to the data, yielding rotational 'temperatures' of approximately 1450 K in each vibrational level,

in v = 1 of 0.14 _+ 0.05. Our experimental signal-tonoise ratio sets a limit on the relative population of v = 2 of < 0.03.

4. Discussion 4.1. Total energy disposal in the photolysis The total energy available to the products is given by hv-AH°n =h~,-[AHf°(CN)+AH°(C2H3)

Eavai I =

-AH°(CEHaCN)]. The earlier uncertainty in the value for AHf°(CEH3 ) [8,19,20] seems to be resolved in favour of a value of 300 k J / m o l [19,21,22]. Taking this value of AH°(CeH3 ), Eavai I = 76.7 kJ/mol. The vibrational and rotational distributions given above correspond to 23.6 k J / m o l being deposited into internal energy in the CN fragment. Combining this with the centreof-mass translational energy release of 31.4 k J / m o l reported by Nishi et al. [13], leaves 21.7 k J / m o l for internal excitation in the vinyl fragment. It should be noted that these energetics represent mean values of internal and translational energy; both distributions are quite broad. A measurement of the translational energy distribution in each rotational state of the CN

The relative population of vibrationally excited CN is considerably greater than a statistical outcome would predict [23]. The predicted population in v = 1 is < 0.005, a natural consequence of the high CN vibrational frequency. This suggests either that the dissociation does not take place from the highly excited ground electronic level, or, if it does, that there are severe exit channel effects which deposit vibrational excitation into the nascent CN fragment. S u c h effects a r e n o t expected in a simple bond cleavage reaction; the present result s e e m s m o s t consistent with dissociation taking place from a n excited electronic state. In photodissociation studies of NO-containing molecules, Reisler and co-workers [24] have noted a degree of ' vibrational adiabaticity'. Excitation of the parent molecule into excited vibronic levels containing some NO stretch character leads to production of vibrationally excited NO photoproduct [24]. A similar effect may be operative in the present case as well. The vibrational analysis of Mullen and Orloff [9], reproduced in Table 1, assigns a CN stretch vibration at 194 nm, as well as a combination band near 193 nm. The lack of high quality calculations on the structure and vibrational frequencies in the electronically excited state precludes a proper Franck-Condon analysis at this time. However, it could be that some fraction of the parent molecules are excited by the photolysis laser into vibronic levels containing CN stretch character, and dissociate adiabatically, without losing this character, giving rise to the observed fragments in v = 1. High quality ab initio calculations of the excited state and transition state for the dissociation would help to determine the source of the vibrational excitation.

4.3. Rotation in the CN Since the experiment was performed at room temperature, rather than with a jet-cooled sample, the CN rotational distribution could have some contribution from parent rotation. We estimate this using the

44

C.A. Bird, D.I. Donaldson / Chemical Physics Letters 249 (1996) 40-45

method outlined by Hanazaki [25] and the ground state structure of C2H3CN given by Costain and Stoicheff [26]. The result is that < 10% of the parent rotational energy can appear in CN rotation; for a 295 K parent this gives a maximum contribution of 0.25 k J / m o l , much less than the energy measured in CN rotation, A Boltzmann distribution over fragment rotational levels is not necessarily indicative of the breakup of a long-lived intermediate species. The previous section argues that the amount of energy appearing in CN vibration is more easily understood as the result of a prompt dissociation from an excited state, rather than the breakup of a long lived intermediate complex. It is interesting in this respect that the rotational distributions are the same in each of the vibrational levels populated. Since population of v = 1 requires on the order of 2 5 % - 3 0 % of the available energy, one might well expect a considerably reduced level of rotational excitation in this level, if the entire available energy were partitioned statistically among all product degrees of freedom. To ascertain whether the observed rotational exci-

where M = total mass, /z a = mcHmc/(mcH + mc), /xf = mCNmC2H3/M, rc is the distance between the C atom and the CN centre of mass, and ICN represents the CN moment of inertia. The pure impulsive model predicts that approximately half the available energy will appear as product relative translation. This is similar to the mean value of translational energy release observed by Nishi et al. [13]. Using our measured Erot(CN) we can solve for 0, the CCN angle in the impulsive model. The result is that this angle is 1300°-1400 °, somewhat bent away from its value of 180° in ground state acrylonitrile. In the excited electronic state, the Mullen and Orloff analysis shows activity in the (low energy) CCN bend [9]. Thus a dissociation from a bent CCN geometry in that state does not seem unreasonable. The rotational distribution in the CN product is therefore consistent with a prompt, excited state dissociation.

tation is also consistent with the picture of a prompt dissociation, we can estimate the rotational energy expected to appear in CN as a result of this process. The impulsive dissociation of a polyatomic ( >

The 193 nm photodissociation of acrylonitrile produces CN in v = 0 and v = 1, with relative popu-

triatomic) into a diatomic and another fragment was treated explicitly by Tuck [27]. Fig. 4 shows the co-ordinate system we use in the case of acrylonitrile. The translational and rotational energies imparted to the CN fragment are given in this model by E t (CN) = ( m c 2HJ M ) ( ~La//]£f ) Eavai I = 0.24Eavait ' Erot(CN)

= [ rncH m c / ( mc H + mc )] (r~/iCN)(sin20 ) Eavail = (0.28

sin20) Eavail,

5. Conclusions

lations 0.86 and 0.14. No o = 2 is observed, setting an upper limit of 0.03 for its relative population. The rotational distributions in both vibrational levels are reasonably well fit by a Boltzmann distribution with temperature 1450 K. The degree of vibrational excitation, and the degree of rotational excitation, especially in v = l, are consistent with a prompt dissociation of acrylonitrile in an electronically excited state. This may be the state initially accessed, or another populated via an electronic predissociation process. Further spectroscopic work and high quality ab initio calculations are needed for a fuller understanding of this interesting molecule.

Acknowledgements HzC--

CH o ~C~N

Fig. 4. Cartoon diagram of an acrylonitrile molecule, illustrating the co-ordinate system used in the impulsive dissociation model calculation. See the text for details,

This work was financially supported by NSERC and CEMAID, a Canadian federal NCE. DJD thanks NSERC for a University Research Fellowship. We a r e grateful to Professor J.A. Guest for many helpful discussions.

C.A. Bird, DJ. Donaldson/Chemical Physics Letters 249 (1996) 40-45

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