Triplet benzene

As shown, for example, by calculations of the structural index A, the aromaticity of alumobenzene (112) is still lower than that in borabenzene. Calculations (MNDO) have shown the ground state of this molecule to be, in contrast to benzene, triplet (3B,). Even so, the 67r-electron structure of (112) is more stable than the 47r-electronic one. However, the difference between their energies (36.6 kcal/mol) is nearly twice as small as that for the corresponding structures of borabenzene. 

The earliest estimates of G (triplet) from y irradiation of benzene were approximately 1.0133,134 but it is now thought that the olefin concentrations used were too low to quench all triplets. Several different workers have now measured G values around 5 for production of olefin triplets.237-238 A G value as high as 10 has been estimated from the benzene radiosensitized isomerization of the stilbenes.240 It is possible that the stilbenes are better quenchers than are simple olefins, so that they quench benzene triplets formed in a spur. A large amount of T-T annihilation within the spur may normally lower the yield of scavenge-able triplets.240 Alternately, direct radioionization at high stilbene concentrations could increase G (isomerization).239... 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

The benzene triplet has not been observed to phosphoresce in either the gas or liquid phase, as noted previously but in view of the lifetime measured In glassy media at low temperature this is not surprising, since very small amounts of quenching impurities would reduce emission to undetectable levels. Some detailed investigations have shown that there is an intramolecular process which Induces radiationless decay of the benzene triplet state, other than impurity quenching. 

A new experimental approach to the problem has been used by Hunter and Stock (231) involving measurement of the frequency dependence of the acoustic signal resulting from transformation of electronic excitation from vibrational to translational energy, consequent to excitation of the B2u state. This technique was used to measure the decay of the benzene triplet over the range 0 to 6 torrs, and it was found, in agreement with what could be inferred from some of the earlier work, that decay of the triplet depends upon benzene concentration. The pressure-independent decay process was found to have a rate constant of (1.7 0.1)... 

The self-quenching mechanism which has been invoked for the gas phase system applies also to the liquid phase. Order of magnitude agreement between data from the various techniques for measuring tx seems to have been achieved. Cundall and Griffiths (233), obtained the first indications from studies on the benzene/ butene-2 system, that the triplet state lifetime was very short. The value for kgx tx was 21.2M at 25°C where kgx was the overall rate constant for triplet excitation energy transfer from benzene to butene-2, and Tx the lifetime of the benzene triplet. Assuming a value of about 5 x 10 for kgx> x appears of the... 

The decrease of GFe2. However, the known reactivity of stilbene (and of the other additives studied, e.g. diethyl ether,... 

Ferrocene does not react with iron carbonyls under irradiation. Its low ionization potential [ 7—7.5 eV 387>] and its high triplet quenching efficiency 185> would cause a strong decrease of G pea(co)9 and G-[Fe(C0)4]3 if benzene triplets or any ions were involved in their formation. This is not the case. 0.54 mole/1 ferrocene causes Gj e2(co)9 to decrease by only 20%, and has no effect on G[Fe(CO)4]3 [0-21 mole Fe(CO)s/l benzene] 289>. This observation suggests that energy transfer from excited benzene singlets could play a role. 

The vibrational species and ei are observed in the phosphorescence spectrum of benzene. The possibilities for the spatial symmetry of the benzene triplet stales are Biu, and Bi . To what extent does the observation of both bt, and ei, vibrations aid the assignment of the lowest triplet level ... 

A benzene triplet state might be generated in the latter reaction which may account for the very negative entropy of activation in a cleavage reaction. This retro 2 + 2 reaction is entropically worse than that of the anti dimer which actually has a higher activation enthalpy. 

Resonance structures proposed to account for the formation of pris-mane and Dewar benzene from benzene triplets. (Adapted from reference 195.)... 

The kinetics and thermochemistry of the quantitative rearrangement of benzvalene (228) and bisdeuteriobenzvalene (229) into benzene and 1,2-dideuteriobenzene have been investigated. " Reactions are first order in the temperature range 313—330 K, with AH = 25.9 0.2kcalmol and AS = 1.6 + 0.7calK mol the heat of reaction for the Ag -ion catalysed process in chlorobenzene solution was estimated at 67.54 0.66 kcalmol" (i.e. more exothermic than the Dewar-benzene to benzene valence isomerization). In the presence of 9,10-diphenylanthracene or 9,10-dibromoanthracene (traps for benzene triplet states) the yield of excited benzene... 

Although identification of benzene triplet formation in solid media is a classic story, its vapor phase history is rather recent. The triplet stale does not emit in the vapor and several early efforts to detect it in absorj)-tion were not successful. Since 1960, however, abundant evidence has... 

The scheme can be made extremely simple because it is possible to use large amounts of butene-2, which pick up all the benzene triplets while leaving the excited singlet unperturbed. This point has been examined in detail by Lee, Denschlag, and Haningcr, who show that approximately... 

With complete scavenging of triplet benzene, a measurement of the yield of triplet butene provides a direct measure of in benzene. Triplet butene yields are assessed by monitoring the resulting ds-trans isomerization, and it is at this point that uncertainties in the mechanism arise. Little is known about the triplet state of butene-2. Various kinetic schemes have been proposed that distinguish but yet allow interconversion between different butene triplets formed from the cis and trans ground state. However, no data arc available on this proposition, and the more detailed schemes always reduce to that of a common triplet in actual treatment of the data. Accordingly, the relaxation of the triplet butene (B ) is characterized by the simple kinetic steps... 

If the mechanism is correct, two measurements then define the triplet yield of benzene. When ci j-butene is added at a pressure sufficient to scavenge all benzene triplets, the yield for conversion of cis to irons-butene at low conversions is given by... 

Thus measurements of two initial isomerization yields when butene pressures are high enough (a few torr) to scavenge all benzene triplets gives [Pg.399]

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