Highly Excited Molecules - American Chemical Society

Chapter 8

Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch008

Unimolecular Decay of Azabenzene: A Mechanistic Model Based on Pyrazine's Photoreactive Relaxation Pathways 1

James D. Chesko and Yuan T. Lee

2

Department of Chemistry, University of California, and Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Relaxation processes inducing chemical reactions are effective dissipation mechanisms for azabenzenes. Photofragment translational spectroscopy has characterized a rich variety of photodissociation products whose evolution follows the amount of internal energy available and the degree of nitrogen substitution. Although a diverse range of reactivity is observed, a reaction model consistent with orbital symmetry, radiationless transition theory and energetic constraints suggests both ground and excited state processes, including isomerization, ring opening and extensive rearrangement is presented. The partitioning of translational energy into the primary reaction product, HCN, resultsfromseveral mechanisms including a retro-Diels -Alder concerted elimination and a stepwise, biradical pathway. Benzene and its nitrogen substituted derivatives, the azabenzenes, have historically been a fruitful source of studies in spectroscopy, radiative and non-radiative relaxation processes (1-6). The list of remarkable phenomena observed— the onset of fast radiationless processes (channel III behavior (7-8)), the role of geometric isomers in describing characterized ultraviolet photochemistry, the novel concerted triple dissociation of highly substituted triazine (9) and tetrazine (10) species, the large transfer of energy from a diazine during 'supercollisions' (1 l)~are consequences of dynamic energy flow through the aromatic system. Although the azabenzenes invite comparison based upon their structural and spectroscopic similarities (12-13), the reactive behavior they span may appear too broad to generalize. While benzene decomposes primarily by loss of hydrogen, the more heavily substituted s-triazine and s-tetrazine triple dissociate into three fragments such as H C N and molecular nitrogen (see Figure 1). The diazines form a bridge midway between these two reaction 'Current address: Pacific Northwest National Laboratory, Battelle, MS K2-14, 902 Battelle Boulevard, Richland, WA 99352 2

Current address: Academica Sinica, Taipei 11529, Taiwan

© 1997 American Chemical Society In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

107


108

HIGHLY EXCITED MOLECULES

benzene

Û pyridine

-J pyrazine

pyrimidine

pyridazine

1ST

s-triazine

II s-tetrazine

Figure 1. Benzene and the azabenzene (nitrogen derivatives) discussed in this paper.

In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


8. CHESKO & LEE

Unimolecular Decay ofAzabenzene

109

extremes. Despite the detailed measurements of electronic structure and relaxation which have been performed on 1,4-diazine (pyrazine), little is known about its reactive character beyond a small propensity to isomerize (14) and report of triple dissociation (15) at high photon energies (λ 100 mJ/cm ) the fast m/e=53 peak will dissociate into translationally fast fragments of H C N and C2H2. Because the species at m/e=53 cracks to daughter fragments primarily at m/e=26 we were unable to rule out the possibility of triple dissociation, except to note that all the H C N fragments could be accounted for by two body dissociations. 2

2

2

2

Concerted HCN Elimination. Accounting for the multiplicity in reaction channels required detailed consideration of the mechanisms involved. The large amount of translational energy released suggested a concerted process with a repulsive exit barrier that would bestow some stability to the C3H3N fragment. A retro-Diels-Alder reaction could be responsible, with the π bonds of H C N acting as the dienophile and C3H3N playing the role of the diene. Because of the high degree of unsaturation in this diene and the requirement of a cisoid configuration, a cyclic transition state structure would be preferable, for cis-fused cyclic dienes are very reactive in this manner, especially for smaller rings where the bonding distances of the diene ends are convenient. The dipolar character of H C N and inductive (electron withdrawing) effect of the nitrogen will both enhance the reactivity. The process is formally a [ 4 + 2 ] cycloreversion which is thermally allowed by orbital symmetry correlations (see Fig. 3 and (19)). The mechanism of Diels-Alder reactions, namely the simultaneous making (or breaking) of bond pairs, directly addresses the topological difficulty of extracting two atoms (carbon and nitrogen) intrinsic to a ring (i.e. disrupting at least two bonds) in a concerted manner. Arriving at a configuration conducive to cycloreversion can be achieved by examining the various valence isomers of the aromatic ring and considering how their structure and electronic configuration satisfy demands imposed by the reaction path. Of interest is the cis-bicyclo[2.2.0]-2,5-diene, also known as the 'Dewar' isomer. The conversion of the familiar, planar, aromatic structure to the Dewar form is a photoallowed [ 2 + 2 ] (disrotary) process. Once formed, the Dewar isomer cannot σ

π

8

π

8


5

π

8

8. CHESKO & LEE

Unimolecular Decay of Azabenzene

111


Pyrazine's HCN Elimination (248 nm)

100 200 300 Flight time (us)

400

P(E, translational)

o.o Translational Energy (kcals/mol)

Figure 2. Time-of-flight spectra taken for the photodissociation reaction of pyrazine into H C N and C3H3N. Three distinct translational energy distributions are seen and referred to as channels 27-A, 27-B and 27-C in order of decreasing average translational energy release.



112


π*

π*

8MB Ψ2

s

r ,

η

π

V π

A

""-

Ό

σ

2

Ψι σι Cyclobutadiene

Acetylene

Dewar Benzene

Figure 3. A orbital correlation diagram for a [ 4 + 2$1 retro-Diels-Alder cycloreversion, formally symmetry allowed in the ground state. The reverse reaction, a [ 4 + ^ 2 ^ cycloaddition, shares the same correlation by the principle of microscopic reversibility. σ

π

8

π

8



8. CHESKO & LEE


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readily convert to the ground state form because the thermally allowed conrotatory ring opening results in a highly strained cis,cis,trans triene. With the orbital symmetry correlations well established, the question of energetic accessibility became an issue. Computer ab initio methods are well-suited to answering this question, so MP2/6-311+G* calculations were run with Gaussian 92 and are summarized in Figure 4. The results confirm that the energy of the Dewar isomers decrease with increasing nitrogen substitution. Furthermore, the opposite trend is observed for the energy of the azete (nitrogen substituted cyclobutadiene) relative to the dissociation products HCN, acetylene and molecular nitrogen. Herein lies the thermodynamic reasons underlying the diverse range of reactive behavior: elimination of the 'first' H C N (or N2, but not C2H2) can occur through a concerted [4+2] elimination, but the remaining fragment must follow a [2+2] or biradical pathway to further dissociate. Thus when H C N is eliminated from pyridine the cyclobutadiene co-fragment must undergo an energetically unfavorable endothermic reaction to 'triple dissociate' into two additional acetylenes. Experimentally, this second dissocation is not seen (22). For pyrazine (or pyrimidine), the monoazete intermediate is still energetically favored over separated acetylene and H C N fragments and the orbital symmetry prohibition to dissociation provides enough of an effective barrier to greatly limit the amount of secondary dissociation. Alternatively, a [1,3] hydrogen migration can occur with ring opening to form the more stable acrylonitrile species. For striazine, the diazete intermediate is not energetically favored over two H C N molecules and can further dissociate, perhaps via a [ 2 + 2 ] concerted process. This mechanism strongly suggests that s-triazine and perhaps s-tetrazine undergo sequential dissociation steps. Recent experiments on s-triazine (17) have provided substantial evidence that stepwise elimination occurs, contrary to earlier reports (9). The large amount of bending excitation found in the H C N (21) is predicted by the large change in the C-H bond angle in going from Dewar compound to linear HCN. The case for stetrazine is weaker due to the absence of acetylene product (even from multiphoton processes), although one may note that the original theoretical work describing the triple dissociation (23) did not consider the possible influence of a Dewar intermediate and only a very weak polarization dependence was observed for the H C N product (10), a possible consequence of either the dissociation geometry or perhaps lifetime of a diazete intermediate. On the other end of the spectrum, elimination of acetylene from benzene is not as favored due to the greater endothermicity of the process and the unsubstituted groups that would be involved in the process. σ

8

a

a

Stepwise elimination of HCN through a biradical intermediate. The small amount of translational energy released in the slowest H C N produced is consistent with a stepwise elimination involving a biradical. One may compare the reaction to elimination of nitrogen from a cyclic azo compound, a facile process in which a biradical intermediate has been suggested (24). As one would predict, the diazine expressing this reaction channel the strongest is pyridazine (see Fig. 1). Production of translationally slow H C N from triazine (9,28) following 193 nm excitation has been observed and would be expected if a biradical intermediate were the initial product following ring opening.


114


Energies of Relevant Species Dewar


τ

ΔΕ

10d

_ 94

70

j _

•••

65

52

(kcals)

34

19.8

46

'

54

ΔΗ

0

-i—ι—iN=0

Λ N^N N^N U ν V M

Cyclo butadiene/ azetes 26

s

-id (kcals)

— τ

— χ 18(25) —i-S

14 —r-S

-3 (-8) 108.4

39

86.5 [4.6

N=0

Α

A H °

f

η

Aromatic hydrocarbons taken from J.A. Miller and C.F. Melius, Combustion and Flame, 91:21-39 (1992) S-T splittings from J. Michl et al., JACS, 111:6140-6146 Heats of formation from JANAF tables, Azete energies MP2/6-311+G* Numbers in parentheses from experiment

Figure 4. Some important valence isomers of benzene. Understanding the photochemistry of benzene has led to a recognition of the central role these species play in transformations such as exchange and isomerization processes. The processes which interconvert these various forms strong reflect the principle of conservation of orbital symmetry.



8. CHESKO & LEE

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Simple bond rupture and H atom elimination. Simple bond fission forming an H atom and a pyrazyl radical became an important pathway, especially at higher excitation energy. With typical pre-exponential (A) factors of 10*4, this reaction was expected to dominate at higher internal energies, especially since the strength of the C-H bond is likely to decrease due to nitrogen's inductive effects. While this assertion holds for benzene and to a lesser extent pyridine, for the diazines and especially s-triazine it is not followed. Although it may appear more facile to simply break one bond exocyclic to the aromatic ring system, the reactive pathways followed by the heavily substituted azabenzenes clearly suggest that the perturbation introduced by the nitrogens is drastic with respect to the reactive behavior, even though the spectroscopic transition energies and molecular geometries are quite similar. The heats of formation of the pyrazyl and triazyl radicals can be estimated from the maximum translational energy release of the ejected H atom: AR° (pyrazyl) = AR° (pyrazine) + hœ- ΑΗ}(Η,% ) f

f

2

- E

ljm

- E

mt

( 1 )

The standard enthalpies of formation for pyrazine and atomic hydrogen are well known, as well as the photon energy. If we assume that a negligible amount of internal energy was present in the molecular beam expansion before photoexcitation and that the fastest products recorded had very little internal excitation, a heat of formation for the radical species may be determined This measurement was difficult due to the presence of multiphoton effects, when combined with modest quantum yields and the background of fast H C N (fragmenting to H+), made this value uncertain to about 10 kcal/mol. The results are summarized in Table I. The pyrazyl radical does not survive electron bombardment very well, but its daughter fragments (m/e=52,40,26) could be traced, especially m/e=40 which had very small background levels.

Table I. Energetics of C-H Bond Fission Species Benzene Pyridine Pyrazine Triazine

Ab initio Dissociation Energy (kcals/mol) 114.6 107 104 102

Measured Dissociation Energy (kcals/mol) 115.2 101±10 98±10 96±10

Formation of Methylene Nitrile Radical Pairs. The ring opening process which is usually followed by elimination of H C N can follow a second path, namely dissociation into a pair of methylene nitrile radicals. Time-of-flight of spectra for this fragment are shown in Figure 5. Of interest is the speculation by Woodward and Hoffman (19) that the conversion of the Dewar isomer may occur through a biradical pathway due to symmetry prohibition of a concerted pathway. For example, the bridging bond breaks and is followed by a twisted boat distortion, thus creating two allyl radical systems which then separate. An upper limit for the standard enthalpy of formation can be


116



Methylene Nitrile Channel 2000 ο 500 c

400 100 200 300 400 Flight time (microseconds)

Probability (E,trans.)

0 3 6 9 12 Translational Energy (kcals/mol) Figure 5. Ab initio results verifying the energetic accessibility of several key reaction intermediates and products in azabenzene unimolecular decay. The number of nitrogens present in each cyclic species markedly influences the propensity to follow reactive (dissociative) pathways.


8. CHESKO & LEE

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estimated at 74±2 kcal/mole, about 8 kcal/mole less than that measured for the propargyl radical (25). This process likely occurs in a stepwise manner from the vibrationally excited open chain biradical. Methyl Radical Elimination. The loss of a methyl radical which has been reported as a minor channel for benzene (26) and pyridine (22) was only found for the diazine pyrimidine, suggesting that three contiguous carbons (CH groups) are necessary to supply the necessary hydrogen atoms. The existence of this channel is strong evidence for the process of ring opening, for the extent of H atom migration (sigmatropic rearrangements) is difficult to rationalize within a closed ring. In the case of pyridine and pyrimidine the 'initiation' step along this pathway, ring opening to a biradical, can be promoted by immediate formation of an energetically stable nitrile group coupled with a hydrogen [1,2] or [1,3] shift. Although this channel is quite minor, it reflects important similarities in the dynamics on the vibrationally hot SQ manifold. Comparison of Azabenzene Reactivity as a function of Excitation Energy. The branching ratios of the various product channels reflect the propensity of the relaxation processes down the available reaction paths. Table II summarizes the variation in product expression at 5.0 and 6.5 eV of excitation energy while Figure 6 describes the various paths. Two general features should be noted. First of all, at the

Table II. Branching Ratios for Unimolecular decay of Azabenzenes following 5.0 and 6.5 eV of photoexcitation Molecule(ref) Benzene(26) Pyridine(27) (22,27) Pyrazine(28) s-triazine(9,17) s-tetrazine(lO)

Energy 5.0 eV 6.5 5.0 6.5 5.0 6.5 5.0 6.5 5.0

H 0.00 0.80 0.05 0.75 0.08 0.24 0.00 0.05 0.00

0.96 0.16 0.00 0.00 0.02 0.01 0.00 0.00 0.00

0.04 0.04 0.01 0.02

Highly Excited Molecules - American Chemical Society

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