A reinvestigation of the near-ultraviolet photodissociation dynamics of the methyl radical

16 September 1994

ELSEVIER

CHEMICAL PHYSICS LETTERS

Chemical Physics Letters 227 ( 1994) 456-460

A reinvestigation of the near-ultraviolet photodissociation dynamics of the methyl radical Steven H.S. Wilson, Jonathan D. Howe, Keith N. Rosser, Michael N.R. Ashfold, Richard N. Dixon School of Chemistry University ofBristol, Bristol BS8 ITS, UK Received 27 June 1994

The translational energy spectrum of H Rydberg atoms resulting from the photolysis of the methyl radical ( CH3) at 2 16.3 nm has been recorded. The use of pulsed supersonic jet flash pyrolysis of azomethane (CH3N2CHa) as a clean source of methyl radicals has allowed a clear analysis of the data, eliminating possible interference from other molecular species. The measured times-of-flight of the resulting H atoms indicate that the partner CHr fragments resulting from predissociation of CHs (8) radicals are formed in their low-lying H ‘A, excited state, as predicted by theory. An earlier contradictory report of this dissociation process, using methanethiol ( CHsSH) as a photolytic precursor to generate methyl radicals, has been reinterpreted in terms of secondary dissociation of ground-state thiomethoxy radicals (CHrS) yielding electronically excited thioformaldehyde CH,S(A’A,)+H(*S).

1. Introduction There have been many experimental and theoretical studies of the methyl ( CH3) radical, which is not surprising, given its considerable role in many areas, for example, combustion [ I] and astronomical [ 2 ] chemistry, and surface science [ 3 1. The first optical spectrum of the methyl radical was recorded by Herzberg and Shoosmith [4] in 1956, who reported an ultraviolet absorption system which they assigned to the B ‘A’,+% ‘AZ transition of planar CHS. This corresponds to excitation of the 2p, electron in the ground state to the 3s Rydberg orbital. The observed spectrum exhibited a strong, broad and diffuse origin band centred at 2 16.3 nm. Herzberg [ 5 ] rationalised the diffuse nature of this feature, and the observation of much clearer rotational structure in the corresponding band of CDs, by the suggestion that the B state of the CHj radical is strongly predisso-

ciated by tunnelling. Orbital correlation arguments predicted that the products of this predissociation shouldbeCH2(Z’AI)+H(2S), CH,(B2A;)+CH2(PA,)+H(2S).

(1)

A subsequent ab initio study by Yu et al. [ 6 ] reached the same conclusion but did not give full consideration to the possibility of the formation of ground state CH2 (2 3B1) radical products, CHS(B2A;)+CHZ(%

‘B,)+H(‘S)

.

(2)

Experimental progress had to await the development of pulsed supersonic jet flash pyrolysis by Chen and co-workers [7] as a clean source of internally cold methyl radicals. This allowed time-of-flight (TOF) mass spectrometric analysis of the dissociation products of CH3 (B 2Ai ) radicals produced by photoexcitation at 2 16.1 nm [ 8 1, and served to confirm the

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S.H.S. Wilson et al. /Chemical PhysicsLetters 227 (1994)456-460

photochemical production of CHz, though could not ascertain its spin-state. Our recently reported hydrogen Rydbergatom photofragment time-of-flight (TOF) study of the methyl radical [ 9 ] relied on the assumption that 2 16 nm photolysis of methanethiol (CH$H) provided a source of methyl radicals CH$H w

SH(g)+CH,(ji)

.

(3)

It is recognised that S-H bond fission is the dominant primary photochemical process for CH$H molecules following excitation at 216 nm at all longer wavelengths; indeed most of the features evident in our earlier hydrogen atom TOF spectra were attributable to H atoms formed in conjunction with CH$ (2 ‘E) fragments,

451

of methyl radicals. We have utilised a pulsed supersonic jet pyrolysis system, similar to that described by Chen and coworkers [ 71, to provide a clean source of internally cold methyl radicals. This has allowed a much clearer analysis of the data, eliminating possible interference from other molecular species. The new data clearly confirms the theoretical predications, that the major partner species to the detected hydrogen atoms are methylene fragments in their lowlying excited 2 ‘A, state (Eq. ( 1) ). We conclude by reinterpreting the H atom TOF spectrum obtained in the earlier methanethiol study in terms of another secondary photodissociation of a fragment arising from the methanethiol precursor CH,S(z)

a

CH,S(A’A,),+H(2S), (5)

CHsSH m

CH,S(x’E)+H(‘S).

(4)

However certain peaks were inconsistent with this pathway, and it was suggested that these were due to hydrogen atoms, arising from the secondary photolysis of CHS (R) radicals, themselves formed via Eq. ( 3). This analysis suggested that the major partner fragment to the detected hydrogen atoms were methylene fragments in their ground (2 3B, ) electronic state, i.e. that dissociation pathway (2) dominates channel ( 1) . As described above, this observation was in direct contradiction with the conclusions of all previous theoretical considerations of the CH3 (8) -CH2 + H dissociation process [ 4-6 1. In order to rationalise the experimental result, a mechanism was advanced in terms of surface crossing, via a seam of intersection between the potential energy surfaces leading to ground state CH,(g 3B1) fragments and the excited CH2(I ‘A,) products, fuelled by a variation in the H-C-H internal bond angle [ 9 1. However, more recently, wavepacket calculations on these surfaces [ lo] have failed to reproduce the experimental data in two ways. First, in order to achieve predominantly ground state CH2 (2 ‘B1) products, an improbably large coupling between the two surfaces was required and second, the calculation could not reproduce the observed vibrational energy distribution in the methylene fragments. Inspired by these difftculties, we report a second hydrogen Rydberg-atom photofmgment time-of-flight study of this system, using a far better defined source

i.e. specifically, S-H bond fission in the parent molecule (Eq. (4) ) and subsequent detection of hydrogen atoms produced by the secondary photodissociation of the resulting thiomethoxy (CH3S) radicals (Eq. (5)).

2. Experimental The design of our flash pyrolytic nozzle is loosely based on that described by Chen and co-workers [ 7 1. The pyrolysis assembly consists of a machineable ceramic base which holds a 45 mm quartz capillary, 0.6 mm inner diameter. The last 10 mm (furthest away from the base) of the capillary was resistively heated by an external coiled tantalum wire. This entire pyrolysis assembly was simply screwed onto the front of a pulsed solenoid valve (General Valve series 9 ) . The tube was typically heated to 1150 K, measured using a thermocouple embedded in the fire-cement surrounding the tantalum heating wire. Methyl radicals were thus generated by flash pyrolysis of either tert-butyl nitrite ( (CH3)3CONO) or azomethane (CH3N2CH3), typically 5O/bseeded in 95% Ar, at a total stagnation pressure of x900 Torr. In general, azomethane was the preferred precursor, since it allows the ‘cleanest’ pyrolysis, and there was no evidence for any hydrogen-atom background signal with the heated nozzle at room temperature. Azomethane was synthesised according to the method of Renaud

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and Leitch [ 111. Tert-butyl nitrite was obtained commercially (Aldrich). The photofragment translational spectrometer and lasers used to ‘tag’ the atomic hydrogen photofragments have been described in detail previously [ 12 1. In brief, a skimmed pulsed supersonic beam of methyl radicals is crossed at right angles by the output of three laser systems. The photolysis laser used in these experiments, operating at wavelengths x 2 16 nm, effects the photodissociation of the methyl radicals. The resulting hydrogen photoatoms are tagged by twocolour two-photon excitation to a high-n Rydberg state, resonantly enhanced at the n = 2 intermediate state. The recoil kinetic energy distribution of the tagged atoms is determined by measuring their collision-free times-of-flight to a detector (a Johnston multiplier, type MM l-SG), which is positioned along the third orthogonal axis, with its front face ~426 mm distant from the interaction region.

following past procedures [ 13-l 51, by first transforming the TOF spectrum into a spectrum of total fragment kinetic energy release, using the relationship &ill,tOtal=&in,u

+&~,cHz

(6) which assumes conservation of total energy and linear momentum. tH in this equation is the measured TGF, and the flight distance, d=426 mm. Taking the best literature value for the H&-H bond strengths, i.e. Dg(H,C-H)=4.69+0.05 eV (37830+400 cm-‘) [ 161, the spectrum of the total fragment kinetic energy release can be transformed into a spectrum of the CH2 fragment internal energy using the relationship Eint,CHz=Eint,CHJ+hV- [Ek,n,total+D8(H2C-H)]

,

(7) 3. Results and discussion Fig. 1 shows the experimental time-of-flight (TGF) spectrum resulting from the 2 16.3 nm photodissociation of methyl radicals generated by flash pyrolysis. The spectrum reveals a strong, single peak, centred at approximately 42 ps. In order to derive spectrometer independent information from this TGF spectrum, it is necessary to transform these data into a spectrum of the CH2 fragment internal energy. This is achieved, 160 140 120; 100'

lj

80:

60’ i

I\ Y

40’

20, n 'JL 30

40

50

I30

'~70'

80

'so

loo

110

time-of-flight I ps Fig. 1. H atom TOF spectrum from photolysis at 2 16.3 nm of a jet-cooled sample of CHs radicals generated by flash pyrolysis of azomethane.

where hv is the energy of the photolysis photon and Eint,cus is the internal energy of the parent molecule. In previous studies [ 12-15 1, this latter term has been neglected since the supersonic expansion of the sample gas achieved internal temperatures of ~20 K. Here however we acknowledge that the cooling is not as efficient as it would be without the pyrolytic assembly, even though the flash pyrolysis occurs within the supersonic expansion. In the transformation to generate a CH2 fragment internal energy spectrum (Fig. 2), we continue to neglect this term, but we note its effect in degrading the resolution of the resulting spectrum. The photolysis wavelength chosen (216.3 nm) coincides with the origin of the intense, predissociated, lowest energy B ‘A’,+-g ‘AT electronic absorption of the CH,(U=O) radical, first characterised by Herzberg and Shoosmith [ 4,5]. It is clear that the single dominating peak in the CHI internal energy spectrum (Fig. 2) cannot be attributed to the formation of CH2 in the vibrationless level of its ground (2 3B1) state (Eq. (2) ), since one would expect the onset of the peak to be close to 0 cm-’ on the CH2 internal energy scale. Fig. 2 also demonstrates that the peak is not consistent with the results of recent wavepacket calculations [ lo], which predicted that any CH;? fragments formed in their 2 3B1ground state would display a distribution of vibrational energy in the

S.H.S. Wilsonet al. / Chemical PhysicsLetters 227 (1994) 456-460

cl’-l’ 0

1000

‘,‘,,‘,,,“,,““&

2000 intemal

3000

4000

459

5000

energy / cm”

Fig. 2. Internal energy spectrum of the CHa fragments resulting from photolysis of the CHS radical at 216.3 nm. The internal energy spectrum is obtained by taking the experimentally measured TOF spectrum (Fig. 1), and applying the time to energy conversion outlined in Eqs. ( 6) and ( 7). The combs above the figure indicates the expected TOFs of H atoms formed, in association with CHr(2 3B, ) or CHa(H‘A,) radicals respectively as a function of vr, the vibrational quantum number for their respective bending mode [ 191.

bending mode, peaked at v2= 5. Taking the known value for the energy splitting between the ground ii: 3B, and low-lying excited Ti‘A, states of CH2 [ 171, we can predict that the threshold for forming CH2(g 1A,),,o fragments together with a hydrogen atom (Eq. ( 1) ) would be close to 3 147 cm-’ on the CH2 internal energy scale. This value is very close to the observed onset of the peak. The discrepancy is to be expected given the non-zero internal energy in the parent methyl radical. This result is in excellent agreement with the theoretical literature on this predissociation [ $6 1, including an ab initio study [ 6 1. This result now requires us to comment on the assignment of our previous study of the apparent photodissociation dynamics of the methyl radical at 2 16.3 nm [ 91. We believe that this misinterpretation arose because of the activity of another secondary photodissociation process. All the features in that study, hitherto ascribed to hydrogen atoms formed in conjunction with CH2( ii: ‘B1) fragments, are also entirely consistent with hydrogen atoms formed by the secondary photolysis of thiomethoxy ( CH3S) radicals (Eqs. (5 ) and (6) ), for which there is a known absorption around 216 nm [ 181. We presume that the products of this secondary photolysis are hydrogen atoms, in combination with thioformaldehyde (CH$), formed predominantly in its electronically excited A ‘A2 state. This reinterpretation is attrac-

tive, given the difficulties arising from the wavepacket calculations [ lo] based on the surface crossing model introduced in Ref. [ 91 and the weight of theoretical predictions which precluded the formation of CH2 (g 3B1) from planar CH3 [ 5,6 1. A full in depth discussion of this pathway may be found in Ref.

t121.

4. Conclusion

A pulsed solenoid valve has been modified by the addition of a pyrolysis assembly, allowing the generation of a supersonic beam of methyl radicals by flash pyrolysis. Using this source, in combination with the Rydberg variant of the technique of H atom photofragment translational spectroscopy, the 2 16 nm photodissociation of the methyl radical has been studied. Analysis of the resulting spectrum shows clearly that the products of this photodissociation are CH2( 2 ‘Al ) + H (‘S), in complete agreement with previous theoretical predictions. An earlier contradictory study, which presumed generation of methyl radicals by the 216.3 nm photolysis of CH,SH, has been successfully reinterpreted in terms of secondary photolysis of the CH,S(%) radical.

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S.H.S. Wilson et al. I Chemical Physics Letters 227 (1994) 456-460

Acknowledgement We are indebted to SERC for equipment grants and to the NERC and EPSRC respectively for research studentship to two of us (SHSW and JDH). We also thank our colleagues Dr. Colin Western, Dr. Lionel Hart, Dr. Roger Alder and Dr. David Mordaunt for their help and advice. SHSW thanks the B’nai Brith Scholarship Committee for additional financial support.

References [ I] M.T. Macpherson, M.J. Pilling and M.J.C. Smith, J. Phys. Chem. 89 (1985) 2268. [2] H. Okabe, Photochemistry of small molecules (Wiley, New York, 1966) p. 359, and references therein. [ 31 S.J. Harris and A.M. Weiner, J. Appl. Phys. 67 ( 1990) 6520. [4] G. He&erg and J. Shoosmith, Can. J. Phys. 34 (1956) 523. [5]G.Herzberg,Proc.RoySoc.A262(1961)291. [6] H.T. Yu, A. Sevin, E. Kassab and E.M. Evleth, J. Chem. Phys. 80 (1984) 2049.

[ 71 D.W. Kohn, H. Clauberg and P. Chen, Rev. Sci. Instr. 63 (1992) 4003. [ 8 ] P. Chen, S.D. Colson and W.A. Chupka, Chem. Phys. Letters 147 (1988) 466. [9]S.H.S. Wilson, M.N.R. Ashfold and R.N. Dixon, Chem. Phys. Letters 222 (1994) 457. [lo] R.N. Dixon, unpublished results. [ 111R. Renaud and L.C. Leitch, Can. J. Chem. 32 ( 1954) 545. [ 121S.H.S. Wilson, R.N. Dixon and M.N.R. Ashfold, J. Chem. Phys., in press, and references therein. [ 131 G.P. Morley, I.R. Lambert, M.N.R. Ashfold, K-N. Rosser andC.M. Western, J. Chem. Phys. 97 (1992) 3157. [ 141 D.H. Mordaunt, LR. Lambert, G.P. Morley, M.N.R. Ashfold, R.N. Dixon, CM. Western, L. Schnieder andK.H. Welge, J. Chem. Phys. 98 (1993) 2054. [ 151 G.P. Morley, LR. Lambert, D.H. Mordaunt, S.H.S. Wilson, M.N.R. Ashfold, R.N. Dixon and C.M. Western, J. Chem. Sot. Faraday Trans. 89 (1993) 3865. [ 161 H. Okabe, Photochemistry of small molecules (Wiley, New York, 1966) p. 295. [ 17 ] M.E. Jacox, J. Phys. Chem. Ref. Data 19 ( 1990) 1387. [ 181 C. Anastasi, M. Broomfield, O.J. Nielsen and P. Pagsberg, Chem. Phys. Letters 182 ( 1991) 643. [ 191A. Alijah and G. Duxbury, Mol. Phys. 70 ( 1990) 605.