Triplet state, acetylene

Figure 2.42. (Top) DFT optimized structure of gas phase ground-state singlet and excited triplet state acetylene. (Bottom) Optimized structures of acetylene on cluster models of Cu(100) (left) and Cu(110) (right). From Ref. [84].

The triplet states of maleic anhydrides and maleimides add to olefins and to acetylenes to yield cyclobutane and cyclobutene derivatives.436,437 A host of sensitizers are effective in promoting the reaction,436 and in some cases the reaction does not proceed at all unless sensitized. Interestingly enough, many such additions have been reported to be stereospecific.438... 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

The first naphtho[a]cyclopropene 68d was also obtained by that route. 3H-indazole was proposed as an intermediate in this reaction 78>. The reaction proceeds equally well from the triplet state of 7. However with acetone light capture from the sensitizer may not have been complete. A trapping of the diradical intermediate 67 to give the known indene 69 by photolysing 7 in dimethyl-acetylene-dicarboxylate (ADC) afforded only a trace of 69. However the yield of 68a was reduced to 40%. Either trapping of 67 was not effective enough or ADC may have acted as quencher. Trapping with furan or cyclopentadiene was also not effective 78a). 

This feature of dinitrogen is an intrinsic characteristic of the molecule, rather than a reflection of the nature of species reacting with it. Thus, the energy of the N2 triplet state (which can be regarded as a Ni molecule with one of 7t-bonds cleaved) is much higher (6.17 eV) than the corresponding triplet state of acetylene... 

The photolysis of trapped diazomethane (21, 45) yields a complicated mixture of mostly unidentified products, one of which has been definitely identified as CH and another tentatively identified as CH2 in its groimd triplet state. One strong, relatively sharp feature at 4240 A. occurs in the photolysis of diazomethane and deuterated diazomethane, and also in discharges of acetylene plus a rare gas possibly contaminated with a trace of air. This feature remains unidentified. 

A laser flash photolytic study of the reaction between 2,2 -dipyridyl and tryptophan has been described. The primary photochemical step has been demonstrated to be pH independent and involves an electron transfer from the tryptophan to the dipyridyl triplet state. The triplet excited state of some peptide conjugates is produced on irradiation by a nanosecond laser flash. C-C Bond cleavage is the result of irradiation of the pinacols (214) in chloroform. This yields the corresponding aldehydes. The mechanism of the cleavage process has been shown to involve single electron transfer with chloroform as the electron acceptor. A study of intramolecular charge separation in aminophenyl(phenyl)acetylene and A, A-dimethylaminophenyl(phenyl)-acetylene has been reported. ... 

Acetaldehyde, 119-20, 380-82 Acetone, chemical titration. 428 oxetane formation, 318, 427 singlet and triplet states, 382. 428 Acetophenone. 407. 467 Acetylene, 203. 348 cycloaddition, 415, 423 excited state geometry. 45-46 2-Acetylnaphthalene, 398 6-Acetyloxycyclohexadienones. 463 Acidity, excited states, 48-52 Acrolein, 34, 382-83, 433 Acrylonitrile, 328, 414-15, 417 Activation energy, 382-83, 400 Acyl radical. 3S2-S5, 380-82, 460 Adiabatic. See Potential energy surface ... 

One typical example is given by G. Kaupp with the [2+2] cycloaddition of diphenyl-acetylene to 1,4-dioxene leading to a bicyclic cyclobutene in good yield (cf. iv of this chapter). Another example is presented by T. Miyashi starting from naphthoquinonor-bomadiene. In an intramolecular reaction a quadricyclane derivative is quantitatively formed upon irradiation at X > 400 nm. It is assumed that this process involves the diketo-norbomadiene in its triplet state. Although the spin multiplicity of intermediates have not... 

The increasing reactivity with alkyl substitution in the acetylenes is very pronounced something in excess of a factor of 100 in relative rates at 298°K. This, it may be observed, is substantially greater than the factor of 20 between the reactivities of ethylene and 2-butene. Such large relative reactivity differences are rare in free radical reactions, and one realizes that the triplet sulphur atom must be very selective in its reaction with different olefins. A comparison of the behaviour of the triplet state atoms of oxygen, sulphur and selenium will be deferred until the end of the discussion of this whole group of atom additions. 

Vibration-vibration energy transfer in SOa has also been discussed.390 The SOa-sensitized cis-trans isomerization of but-2-ene is a complex reaction, which at low pressures can be explained as involving only the ZBX state of S02, but which at higher pressures requires the intervention of some triplet state other than the phosphorescent SBX level.391 The photolysis of S02 in the presence of acetylene and allene also involves these non-phosphorescent triplet states, yielding CO as product, and giving rise to aerosol formation.392 Aerosol formation in S02-N2-02 mixtures has been experimentally investigated,393 and the effects of N02, propylene, and water on SOa reactions have been reported.884 Interactions between S02 and atmospheric particulate matter,395 photochemical removal of SOa from the lower atmosphere,396 the use of SOa to trap 1,4-biradicals generated photo-chemically,397 and the photochlorination of S02 398 have all been discussed in recent publications. 

The irradiation (light filtered through naphthalene in n-hexane) of methyl pyruvate in the presence of methyl-substituted olefins affords ethers, oxetans (from a quenchable triplet state), or hydroxyolefins from a singlet state, depending upon the nature of the olefin used (Scheme 24).144 145 Bos and his co-workers146 have reported further additions of 1,2-dicarbonyl compounds to acetylenes. These reactions involve photochemical addition of benzil, biacetyl, 1-phenyl-propane-1,2-dione, and phenylglyoxal to the propynes (246), and afford the diones (247), presumably via the oxeten, e.g. (248). The photoenolization of the dione... 

Francis JT, Enkvist C, Lunell S, Hitchcock AP (1994) Studies of X Is to n triplet states of carbon monoxide, benzene, ethylene, and acetylene. Can J Phys 72 879-884 Frank P, Hedman B, Carlson RMK, Hodgson (1994) Interaction of vanadium and sulfate in blood cells from the tunicate Ascidia ceratodes Observations using X-ray absorption edge structure and EPR spectroscopies. Inorg Chem 33 3794-3803... 

Absolute rate coefficients and Arrhenius parameters have been obtained for the cycloaddition reaction of S( F2,1,0) atoms with a representative series of olefins and acetylenes. The activation energies are small, and they exhibit a trend with molecular structure which is expected for an electro-philic reagent The A-factors show a definite trend which can be attributed to steric repulsions and a generalized secondary a-isotope effect explained by activated complex theory. Secondary a-H/D kinetic isotope effects have been measured and their origin discussed. Hartree-Fock type MO calculations indicate that the primary product of the S( F) + olefin reaction is a ring-distorted, triplet state thi-irane, with a considerable energy barrier with respect to rotation around the C-C bond. 

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