Quantum yields, of biacetyl

WAVELENGTH DEPENDENCE OF THE PRIMARY QUANTUM YIELD OF BIACETYL PHOTOLYSIS, CALCULATED ON THE BASIS OF RELATIONS (33) AND (34)... 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

The enolic form has been found to be formed from the second singlet state in dilute solutions of biacetyl in water, methanol, and n-heptane.77 In aqueous solutions, the enol is stabilized and its quantum yield of formation is 0.10 0.02 and varies slightly with the wavelengths. 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

The primary quantum yield was then shown to decrease from 0.59 to a steady value of 0.20 for 50 mm. acetone as up to 0.5 mm. biaeet.yl was added. The quantum yield of carbon monoxide decreased correspondingly from 0.16 to 0.05. It is clear that under such conditions deactivation of the triplet state by biacetyl causes a major decrease in the decomposition of acetone. The ratio of the total acetone emission intensity to the absorbed intensity is small (0.02 at 40°C.5li). 

Porter108 has made a careful study of the photooxidation at 3650 A. at both room temperature and at 160°C. At room temperature the quantum yield of products were very small (e.g., fao = 0.03) and the product ratios were rather different from those given when 2700 A. radiation was used. It seems that, in contrast to biacetyl photooxidation, the main role of oxygen was in deactivating ketene molecules but there was some direct reaction with the electronically excited ketene molecules. At 160°C. there was a chain oxidation similar to that when 2700 A. radiation was used, but the ethylene forming step appeared to be different. 

The formation of the triplet state can be directly followed in time through the observation of the transient triplet-triplet (T-T) absorption in flash or modulated photolysis or by the observation of the phosphorescence emission. A typical radiative lifetime of phosphorescence for simple carbonyls is 10 3 s. Therefore, it is extremely difficult to observe the "unrelaxed" phosphorescence emission without collisional relaxation, unless the triplet lifetime is significantly shortened by a competing radiationless process. Under these conditions, the correspondingly low quantum yield of phosphorescence makes such measurement rather difficult. Usually, "relaxed" phosphorescence from a molecule such as biacetyl is observed. Therefore, only the transient T-T absorption can provide useful data in the gas phase, although a determination of the absolute yield is rather involved and difficult. 

Above 100 C the stationary-state concentration of CHbCO is probably sufficiently small that reactions 6 and 6 are both negligible. This is indicated by the fact that the quantum yield of CO formation is very close to unity above 100 C (i.e., no recombination of CHj CHjCO) and the biacetyl yield falls to zero. 

The quantum yields of acetone consumption and ethane formation decrease with increasing intensity. The dependence of the C2H6/CO ratio on light intensity can be demonstrated in a very wide intensity range if the results of the mercury-sensitized photolysis are also taken into consideration (Table 16). An increase in the quantmn yield of biacetyl formation with increasing intensity is apparent from the results of Herr and Noyes, and has also been confirmed by the direct measurements of Howland and Noyes. 

In order to increase the quantum yield of cis -> trans isomerization, it is proposed to use aromatic hydrocarbons such as anthracene as a carrier of the quantum chain process. (Z,E)-(cis-18) and (E,E)-l,4-di(3,5-di-tert-butylstyryl) benzene (trans-18) underwent mutual isomerization on biacetyl sensitization with a quantum chain process to afford a 4>c >t value of 1.2-1.3 in a cis isomer concentration of 1 X 10 M [96]. The quantum chain process proceeds through the energy transfer from the trans triplet (H ) to the ground state cis isomer ( c). On addition of anthracene (AN) as a quencher or a carrier of the excited state, the quantum... 

Figure 12. Relative quantum yield of sensitized phosphorescence of biacetyl (top), and nonradiative decay rate of naphthalene vapor (bottom), as a function of the excess vibrational energy in Sl naphthalene. (From ref. [40] with permission.)...

High chemical but low quantum yields of the two products (233) and (234) are obtained on irradiation (X > 500 nm) of the tetraketones (235). The primary photochemical step is the conversion of the tetraketones into carbon monoxide and the ketenes (236). The reaction process occurs from the singlet state and is thought to be concerted. The final products are formed by the addition of these ketenes (236) to ground state tetraketone. The ketene (236b) was studied in a little more detail and it was shown that irradiation in benzene or toluene at room temperature results in the formation of the dimer (237). Addition of the same ketene to diketones such as biacetyl was also reported. 

On biacetyl sensitization 22 undergoes adiabatic one-way ZZ ZE and ZZ - EE isomerizations in the triplet manifold (Scheme 8) [98]. The ZE isomer also isomerizes to EE. The quantum yield of ZE - EE isomerization (0ze->ee) nd the photostationary state isomer ratios, ([EE]/[ZE])p , increase with increasing diene concentration. The ze->ee value is about 14 at... 

The absolute fluorescence yields for QHj derive with one exception from excitation at 2536 A. Two of these are based on calibration against the phosphorescence yield of biacetyl,which has been a gas phase standard for many years. The first reports a yield of about 0.23 0.05 at 11 torr, but this was subsequently remeasured by Poole as 0.17 in further evolution of that technique in the same laboratory. A later measurement independent of the biacetyl standard is reported by Noyes, Mulac, and Harter. They observe (in their Method B) that f = 0.18 0.04 at 11.5 torr. Further support for this value comes from its use in calibration of singlet relaxation quantum yields in fluorobenzene and toluene in which the sum of observed radiative and nonradiative yields is unity within experimental error. A higher benzene fluorescence yield would push that sum paradoxically above unity. 

While the factors resulting here in a given distortion are intramolecular, the conclusions we can draw may be applied to cases where an isolated molecule is distorted by its environment. We would like to examine this approach as starting point for treating the effect of the 11-cw retinal-opsin interaction on the photoisomerization quantum yield of 11-cw to aW-trans retinal. This quantum yield increases from 0.2 for the free chromophore [9] to 0.66 in rhodopsin [10]. This value is close to that of the biacetyl sensitized reaction of the free chromophore, 0.75 [11], and indicates that the triplet state is involved to a large extent in the photoisomerization of ll-cis retinal and that intersystem crossing is inefficient. 

Alcoholic solvents can be an alternative medium for photosubstitution reactions. As an example chlorobenzene yielded anisole (with a quantum yield of 0.049) when irradiated in MeOH [251] and phenetole in aqueous ethanolic solutions, though in the latter case phenol was the main photoproduct [252]. Analogously, monochlorotoluenes underwent photosubstitution in MeOH forming methylanisoles [253,254] and the quantum yield of decomposition ranged from 0.1 to 0.4 with the smallest values applying to the para and ortho isomers. Biacetyl sensitized experiments confirmed... 

Photochemistry. The cw-isomer of [PtCl2(py)2] undergoes two primary photoprocesses, isomerization and dissociation, with quantum yields of ca. 0.04 and 0.025 respectively (313 nm irradiation). /ra 5-(PtCl2(py)2] also undergoes parallel photoisomerization and dissociation, but with very much lower quantum yields of ca. 10" . The effect of acidity on the photochemistry of cw-[Pt(gly)2] has been examined-the balance between photoisomerization and photodissociation is pH-dependent. Photochemical cis trans isomerization of [Pt(gly)2] is sensitized by pyrazine or by xanthone, but not by thioxanthone, quinoline, naphthalene, or biacetyl. Ni + quenches the photoisomerization, but Mn + does not. The sensitization process was shown to involve triplet-triplet intermolecular energy transfer. ... 

Primary dissociation is followed by thermal dissociation of the acetyl radicals (secondary dissociation), so that ethane and carbon monoxide are the main products of the vapor phase photolyses. When the reaction is carried out at low temperature, with higher energy light filtered out, the acetyl radicals which are produced are sufficiently long lived so that their participation in coupling and disproportionation reaction competes with decarbonylation. Under such conditions formation of biacetyl, acetone, methane, and formaldehyde follow primary dissociation and the quantum yield of carbon monoxide decreases ( co = 1 at 2537 A and about 0.75 at 3130 A Noyes et al., 1956). 

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