Low temperature photochemistry

In these studies it appeared that low-temperature photoreduction of the bound iron-sulfur centers is not greatly decreased (about 2x) by the removal of phylloquinone (Setif et al, 1987). Low temperature photochemistry was measured by three methods the total amount of iron-sulfur centers A and B reduced by continuous illumination at 77K, the extent of reduction of these centers by saturating laser flashes at 77K, and the flash-induced formation of triplet P-700. These methods show that all the reaction centers are still able to oxidize P-700 and to reduce the iron-sulfur centers. This observation raises serious questions concerning the role of phylloquinone or the significance of low-temperature photochemistry. For the moment we consider that room temperature data are more reliable in indicating that A] is a phylloquinone. 

Studies of the low-temperature photochemistry of umbellone (288) established a dual pathway for its conversion to thymol (291). Besides the opening of (288) to ketene (289), its direct conversion to the cyclohexadienone (290) was also postulated. However, this reaction depends on the substitution pattern, and in the case of lumisantonin (292) only traces of cyclohexenone (294) were detected in the photolysis... 

Fig. 8. Photochemical and chemical reduction of the semiquinone-iron complex in PS-II membranes (a) sample frozen in the dark, (b) sample illuminated for 10 m at 5 K, (c) sample illuminated for 10 m at 77 K, (d) sample illuminated for 4 m at 200 K. and (e) sample frozen in the dark in the presence of dithionite (no mediators present). EPR spectra were measured at 4.8 K. Figure source Rutherford and Mathis (1983) A relationship between the midpoint potential of the primary acceptor and low temperature photochemistry in photosystem II. FEES Lett 154 332.

AW Rutherford and P Mathis (1983) A relationship between the midpoint potential of the primary acceptor and low temperature photochemistry in photosystem II. FEBS Lett 154 328-334... 

The low temperature photochemistry of a range of ting substituted half sandwich L-V(CX >)4 complexes, L = Cp. Cp, V-indenyl, and ii -CsQ has been studied and infra-red evidence is presented for reversible CO loss/ Photoelectton spectrosct ic, electrochemical and X-ray crystallographic studies have been carried out on [Nb(ii-C7H7)(Cp )(THF)] [PFJ and related compounds and the bonding in such compounds has been discussed in terms of the findings. The synthesis and structure of [V[2,4-(Me)2CsH5](Qp)Co] has been reported. ... 

Aryl substituents also activate benzyHc C-H bonds adjacent to a nitrene center. An interesting study on this topic has dealt with the atropoisomeric 3,5-dimethyl-2-(9-fluorenyl)phenylazides 49 and 52. ° The low-temperature photochemistry (77 K) of these rotameric azides proved to be fundamentally different Photolysis of 49 resulted in the formation of nitrene 50 in addition to azanorcaradiene 51, while photolysis of 52 gave nitrene 53 in addition to iminoquinone methide 54. This subsequently underwent intramolecular cycloaddition to yield dihydroindenocridine 55. Laser flash photolysis showed that the formation of iminoquinone methide 54 from azide 52 occurred in less than 10 ns. Deuteration of the 9-position of the fluorene chromophore resulted in a significantly diminished yield of 55. LFP of the rotameric azide 49 yielded the rate constant for the formation of azanorcaradiene 51 which was determined as k = 7.1 x 10 ... 

Harder, X, StoBer, R., Wessig, R, and Bendig, J., Low-temperature photochemistry of nitro-sub-stituted aromatic azides subsequent reactions of intermediates, /. Photochem. Photobiol. A Chem., 103, 105,1997. 

Mao, B., Tsuda, M., Ebrey, T.G., Akita, H., Balogh-Nair, V., and Nakanishi, K., Flash photolysis and low temperature photochemistry of bovine rhodopsin with a fixed 11 -ene, Biophys.., 35,543,... 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

The matrix experiments thus reveal some complex photochemistry of relevance to solution chemistry but the experiments do not provide information about kinetics. For this we need a fluid medium e.g. gas or liquid, and we consider such experiments in the next two sections. Flash photolysis suggests itself as the technique for detecting a species as reactive as Cr(C0)5 but before describing these experiments we show what can be achieved from low-temperature solutions. 

Another dinuclear carbonyl which presents interesting problems is ](ri C5H5)Fe(CO) ] 2 Does the photochemistry proceed exclusively through homolytic fission to produce two (13 -05 )Fe-(C0)2 radicals or by other possible routes The discussion of this reaction has involved mechanistic and synthetic studies (77), flash photolysis (78) and low-temperature photolysis (29) - the latter work, in THF or ethyl chloride at -78°C, invokes an intermediate in which the Fe-Fe direct bond is broken but the two halves of the molecule are held together by a CO bridge. Clearly such an intriguing problem merits more detailed investigations. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

Secondly, Leyva et al found that the solution phase photochemistry of phenylazide (PA) was temperature dependent. Photolysis of PA in the presence of diethylamine at ambient temperature yields azepine 2, first prepared by Doering and Odum. Lowering the temperature suppresses the yield of 2 and encomages the formation of azo compound. Thus, high temperatures favor reactions of singlet state intermediates, whilst low temperatures favor reactions associated with triplet phenylnitrene. 

Jacox and Milligan (520a) favor the three-membered ring structure with an O—C—O angle of 65 from the analysis of infrared spectra of isotopic species of C03 in low temperature matrices. A new broad and weak absorption band at 4060 A with an absorption coefficient of 1.1 +0.3 atm -1 cm -1 is also found in 03—C02 matrix (551). However, no corresponding infrared absorption bands have been found in the gas phase photolysis [DeMorc and Dcde (277)]. The photochemistry of C02 in this region may be summarized as follows. 

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