Photolytic precursors

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

Reactions 9 and 10 are thought to dominate HO production in the cleanest regions of the troposphere. Thus any approach that utilizes intense laser radiation at wavelengths shorter than about 320 nm should be undertaken with some care. In addition to ozone, H202 and HONO photolyze in the UV to produce HO and must be considered photolytic precursors to spurious HO. Nevertheless, 03 has been the only significant known photolytic parent of spurious HO to date. The earliest measurements of tropospheric HO contained significant contributions from spurious HO for which corrections were applied (71). 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

A number of product studies have been carried out on reaction (1) under atmospheric conditions (4-9.20). These studies used NOx containing precursors of OH in their steady photolysis systems, which Barnes et al. (20) have now demonstrated leads to complications in the subsequent kinetics. Barnes et al. (20) have utilized H202 as an alternative photolytic precursor of OH, and observed 70% production of SO7 at room temperature. However, they lso observed about a 30% yield of DMSO which is inconsistent with a recent study by Hynes and Wine (this volume). The major products that have been observed in these product studies (4-9.20) have included S02. MSA, CH20 and DMSO. It is still not clear whether each of the initial reaction pathways (la) or (lb) leads to a single product or a mixture of these products. Hynes and Wine (this volume) discuss the likely products arising from the addition mechanism (lb)... 

Barnes, et al. (22) have studied reaction (3) in the presence of O2 using a competitive rate technique which utilizes H2O2 as the OH photolytic precursor. This avoids the use of NOx containing precursors which nave been shown to produce unreliable results in studies of OH reactions with organo-sulfiir compounds (24.251. They find up to 30% yields of dimethylsulfone, (013)2802, and suggest that reaction (4b) is important If reaction (4b) is an elementary reaction and OH is regenerated, then our study of reaction (3) would have obtained values of k s which were too low and, hence, underestimated the importance of the addition channel. One apparent inconsistency between the recent work of Barnes, et al. (231 and our study (2) is that Barnes, et al. observed only DMSO2 as a product of reaction (4) neither DMSO nor MSA was observed. Since we would not have observed an O rate enhancement if the adduct + O2 reaction proceeded entirely via channel (4b), our results imply that an addition channel other than (4b) must be important The only addition pathways which seem reasonable are (4a), (4b), and... 

Behnke, W., Zetzsch, C. Production of a photolytic precursor of atomic Q liom aerosols and CP in the presence of O3. In Grimvall, A., de Leer, E.W.B. (eds.) Naturally-Produced Organohalogens, pp. 375-384. Kluwer Acad, Norwell (1995)... 

Photolytic nitrogen extrusion from tetrazoline derivatives (283) and (284) permits further syntheses of diaziridin-ones (285a) and -imines (285b) respectively (75AG(E)428). Thermal carbon dioxide extrusion from a five-membered precursor can also give a diaziridinimine (75JOC3112). 

Photolytic [4+2] cycloreversion of a disilabicyclo[2.2.2]octadiene precursor is considered to be a general method for the synthesis of disilenes of varying stability. Several examples of this method have been reported (Eq. 9).27-31 First used in reactions producing an unstable disilene, Me2 Si=SiMe2,27 this method is also successful for synthesizing the marginally stable -Bu2Si=Si(t-Bu)2,28 However, it has not yet been applied to the synthesis of disilenes that are fully stable at room temperature, probably because the appropriate precursors are inaccessible. 

Metal carbonyl compounds are other suitable precursors for the synthesis of NPs by thermal decomposition. The main advantage is the formation of CO that is expelled from the IL phase due to its poor solubility. However, high temperatures are commonly used to decompose such precursors. Metal NPs of Cr(0), Mo(0), and W(0) were prepared by thermal or photolytic decomposition of their respective monometallic carbonyl compounds [M(CO)6] dispersed in ILs [52]. Similarly, the precursors [Fe2(CO)9], [Ru3(CO)i2], and [Os3(CO)12] were employed in order to obtain stable metal NPs (1.5-2.5 nm) in BMI.BF4 [53]. The same procedure was extended to the preparation of lr(0), Rh(0), and Co(0) NPs in ILs [54]. 

In order to isolate a single isomer of diphosphathienoquinone, two bromo groups were attached to 3,4-positions to avoid formation of the ( )-form by steric repulsion between Br and Mes groups. Analogous dehydrochlorination of the precursor afforded (Z,Z)-diphosphathienoquinone 4b as a single isomer (Scheme 11) [11], Diphosphathienoquinone 4b did not undergo iiZ-isomerization either under thermal or under photolytical condition. 

In the absence of direct evidence for CACs other second intermediates have been proposed to rationalize the curvature observed in correlations of addn/rearr vs. [alkene], A particularly viable candidate is an excited state (or species derived therefrom) of the carbene precursor, a suggestion that is quite apposite when the precursor is a photolytically decomposed diazirine. 

Photolytic generation of carbene 19 from a precursor diazirine afforded cyclobutenes 27 (68%) and 28 (14%) via CH2 and CMe2 migrations, respectively cf. Eq. 15.45... 

It is suggested that the real carbene, generated by thermolysis of the diazo or diazirine precursors or photolysis of 40, gives mostly 1,3-insertion, whereas photolysis of either the diazoalkane or diazirine yields much 1,2-Me migration directly from precursor excited states.15 1 An analogous intervention of 1,2-Me migration via RIES was also observed in the photolytic decomposition of f-butylchlorodiazirine (24) to f-butylchlorocarbene (18) cf. Eq. 14.27... 

When, however, carbenes are directly generated from diazoalkanes, RIES becomes significant.56 Photolytic generation of carbene 45 from diazoalkane precursors in the presence of >1.5 M pyridine gave values for the derived pyridinium ylides. 

In the same vein is the observation that the lifetime of dipropylcarbene (59) in CH2C12 or cyclohexane is 0.3 ns,84 which, after statistical correction is 48 times less than the lifetime ( 21 ns) of Me2C in pentane.22 This reflects promotion by the propyl bystander groups of 59 of the 1,2-H shift to Z- and E-3-heptene.84 (Dipropylcarbene can be photolytically generated from either an oxadiazoline (diazoalkane)84 or diazirine85 precursor, but RIES lowers the efficiency of carbene production in either case.) Recently reported LFP lifetimes for Et2C and MeCEt in cyclohexane or benzene are 0.6-3 ns (cyclohexane) or 1-5 ns (benzene),14 in accord with the lifetimes of S822 and S9.84 The rate constants for carbene disappearance in cyclohexane ( 3 x 108 to 2 x 109 s 1) are presumably limited by 1,2-H shifts.14... 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

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