Acetylene, photodissociation

Finally, we note that Figure 17.2 and the above brief discussion of acetylene photodissociation (Section 17.1) serve to illustrate the increasing complexity of PESs, which are already quite challenging even for molecules containing only four atoms. We shall discuss other possible ways of deciphering the complex dynamics involved in the photodissociation of large polyatomic molecules at the end of Chapter 19, and some further examples of polyatomic photodissociation dynamics will be discussed in Chapter 18, which is concerned with multiphoton excitation. 

Photochemistry. Vinyl chloride is subject to photodissociation. Photexcitation at 193 nm results in the elimination of HCl molecules and Cl atoms in an approximately 1.1 1 ratio (69). Both vinyUdene ( B2) [2143-69-3] and acetylene have been observed as photolysis products (70), as have H2 molecules (71) and H atoms [12385-13-6] (72). HCl and vinyUdene appear to be formed via a concerted 1,1 elimination from excited vinyl chloride (70). An adiabatic recoil mechanism seems likely for Cl atom elimination (73). As expected from the relative stabiUties of the 1- and 2-chlorovinyl radicals [50663-45-1 and 57095-76-8], H atoms are preferentially produced by detachment from the P carbon (72). Finally, a migration mechanism appears to play a significant role in H2 elimination (71). 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

Photolysis of H3NBH3 with 121.5 nm radiation yields imidoborane, HBNH, which has been of theoretical interest Spectral shifts observed for several isotopic species containing °B, N, and D show clearly that the spectrum is due to HNBH which is isoelectronic with HBO, HCN and HCCH. From the spectrum of the isolated species two of the and one of the tr-type vibration frequencies for a linear molecule have been obtained. The location of the missing S (B-H stretch) frequency has been calculated. A comparison of observed and calculated frequencies for HBNH is given in Table 7. Another isolated product observed in these experiments is identified as HNB. This radical may be generated by photodissociation of HNBH subsequent to its formation. In this respect the photolysis mechanism would be similar to the formation of C2H from acetylene. 

Photochemistry. Vinyl chlonde is subject to photodissociation. Pho-loexalation at 193 mu results in the elimination of HC1 fragments and Cl atoms in an approximately 1.1 1 ratio. Both vinylidene and acetylene have been observed as photolysis products, as have H2 molecules and H atoms. 

Preparation of C2 Fragments. The photodissociation of hexafluorobutyne-2 and acetylene yield a variety of products, many of which are in electronically excited states resulting in prompt fluorescence which is quenched by the presence of a buffer gas. 

It has been shown that acetylene when submitted to a high frequency electric discharge yields some diacetylene 75. The action of ultra-violet radiation of wavelength 2,378-2,100 A on acetylene also produced diacetylene, and Cherton considered that the primary step must be the photodissociation of acetylene to C2H and H. At sufficient pressure the absorption spectrum of acetylene shows a continuous absorption in this region superimposed in the band structure. This gives an upper limit of 121 kcal to D(HC C-H). 

Alkynes.- Coyle has reviewed the photochemical reactions of ethyne. The photodissociation of acetylene has been studied at 193.5 nm. The alkyne (44) is photochemically reactive and has been studied in a variety of solvents. ... 

The formation of addition compounds from photodissociated metal carbonyls and acetylenes has been postulated from flash spectroscopic studies of acetylene/02 explosions in the presence of metal carbonyls 144>. 

Acrylonitrile Photolysis Minor Channel. The major channel of acrylonitrile photodissociation at 193 nm forms HCN and either acetylene or vinylidene [72], The CN channel is a very minor one, measured to have a quantum yield of only (0.003 0.001) [73]. 

Another important application of DCN-sensitized photodissociation of strained ring compoimds has been demonstrated by Muller and Mattay [127] for synthesizing iV-substimted imidazoles (147) by the [3-l-2]-cycloaddition of the 2-azaallenyl radical cation (144), produced by the cleavage of corresponding radical cation from azirine (143), with imines. This strategy is further extended [128] for the synthesis of pyrrolophane 3,4-dimethyl ester (152) by the ring opening cycloaddition reaction of (148) with dimethyl acetylene dicarb-oxylate (Scheme 32). 

As expected, the IE for C2H2 determined here is in excellent accord with that of HC=CH (acetylene), indicating that acetylene is a major photo-product formed in the 193-nm photodissociation of thiophene. The rapid rise of the ionization threshold observed in Figure 32a has been observed in previous PIE studies of acetylene [181]. 

Douberly GE, Ricks AM, Itcknor BW, McKee WC, Schleyer PvR, Duncan MA (2008) Infrared photodissociation spectroscopy of protonated acetylene tmd its clusters. J Phys Chem A 112 1897-1906... 

Mordaunt DH, Ashfold MNR, Dixon RN et al (1998) Near threshold photodissociation of acetylene. J Chem Phys 108 519-526... 

Lee YP. 2003. State-resolved dynamics of photofragmentation . Ann. Rev. Phys. Chem. 54 215-244. Mordaunt DH, Ashfold MNR, Dixon R, Loffler P, Schnieder L, Welge KH. 1998. Near-threshold photodissociation of acetylene . J. Chem. Phys. 108(2) 519-526. 

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